Rokiatou Diarra
email: rokiatou.diarra@u-psud.fr

Alain Merigot
email: alain.merigot@u-psud.fr

Bastien Vincke
email: bastien.vincke@u-psud.fr

RECKA and RPromF: two Frama-C Plug-ins for Optimizing Registers usage in CUDA, OpenACC and OpenMP Programs

Keywords: CUDA, OpenACC, OpenMP, scalar replacement, static analysis, alias analysis, Frama-C

Pointer aliasing still hinders compiler optimizations. The ISO C standard 99 has added the restrict keyword that allows programmer to specify non-aliasing as an aid to the compiler's optimizer. The task of annotating pointers with the restrict keyword is still left to the programmer and this task is, in general, tedious and prone to errors. Scalar replacement is an optimization widely used by compilers. In this paper, we present two new Frama-C plug-ins, RECKA for automatic annotation of CUDA kernels arguments with the restrict keyword, and RPromF for scalar replacement in OpenACC and OpenMP 4.0/4.5 codes for GPU. More specifically, RECKA works as follows: (i) an alias analysis is performed on CUDA kernels and their callers; (ii) if not found any alias then CUDA kernels are cloned, the clones are renamed and their arguments are annotated with the restrict qualifier; and (iii) instructions are added to kernels call sites to perform at runtime a less-than check analysis on kernel actuals parameters and determine if the clone must be called or the original one. RPromF includes five main steps: (i) OpenACC/OpenMP offloading regions are identified; (ii) functions containing these offloading codes and their callers are analyzed to check that there is no alias; (iii) if there is no alias then the offloading codes are cloned; (iv) clone's instructions are analyzed to retrieve data reuse information and perform scalar replacement; and instructions are added to be able to use the optimized clone whenever possible. We have evaluated the two plug-ins on PolyBench benchmark suite. The results show that both scalar replacement and the usage of restrict keyword are effective for improving the overall performance of OpenACC, OpenMP 4.0/4.5 and CUDA codes.

I. INTRODUCTION

Nowadays all computing systems are becoming heterogeneous for higher power efficiency and computation throughput. Such systems may include general purpose CPUs and accelerators, such as the Graphics Processing Unit (GPU). There are various parallel programming frameworks for GPU. Kernel-based languages, CUDA or OpenCL, offer a number of features for performance optimization as the architecture is directly accessible to the user but this adds complexities for application developers. Directive-based programming models (e.g.: OpenMP 4.0/4.5, OpenACC) may therefore become an interesting solution.

Variants of the C language (e.g., CUDA, OpenCL) are still used to program on GPU machines. One of the most important features of languages such as C is the existence of pointers. Pointer can hinder compiler optimization. Indeed, it is hard to know where pointers are pointing and compilers must be conservative in their presence. Consider the C version in the example provided in listing 1, without further knowledge or special hardware support, the compiler must assume that A, B, x, y and tmp may refer to the same memory region or overlapping regions so that the loop cannot be parallelized or software-pipelined because it has to be ensured that an update of tmp[i] is performed before the next value of x[j +1] is loaded for example. Therefore, because the compiler must conservatively assume the pointers alias, it will compile this code inefficiently. Thus on CPU, compilers typically generate various assembly codes for the C version of gesummv routine (shown in listing 1), depending on the arguments passed to gesummv on the call site. Unlike the CPU case, GPU codes compilers (e.g.: nvcc, pgcc, clang) generate a single PTX code for the kernel assuming that kernel arguments pointers might be aliased at runtime. Thus, it may be useful in the absence of aliasing to inform the compiler that kernel pointers arguments are not aliased, so that it can generate a more optimized code.

To ensure that the correct code is generated in the presence of aliases, compilers have to perform an alias analysis. Although the literature of alias analysis is abundant and much work (e.g, [START_REF] Andersen | Program analysis and specialization for the c programming language[END_REF]- [START_REF] Maalej | Pointer disambiguation via strict inequalities[END_REF]) has been done in the last few decades, the research community has not yet solved pointer alias analysis satisfactorily. Many alias analyzer are implemented in mainstream compilers but the results of these analyzers are often inaccurate. Pointer analysis imprecision prevents the compiler from optimizing some code where there is no aliasing. To mitigate the problem posed by pointers, the C programming language, since the C99 Standard, features the restrict keyword, that can be used by the programmer to give the compiler information about aliasing. For instance, if the arguments of the routine in listing 1 have been annotated with the restrict qualifier, thereby allowing the compiler to perform more aggressive optimization, such as instructions scheduling, register promotion, redundant load/store elimination, etc. Although the restrict keyword has been available for several years already, it remains less used by programmers and its insertion is left to the programmer. The task of inserting the restrict qualifier is, in general, tedious and prone to errors.

The goal of scalar replacement is to identify repeated accesses made to the same memory address and to remove the redundant accesses by keeping the data in registers. This is done by identifying sections of the code in which it is safe to place the redundantly accessed data in a register.

Since the release of the OpenACC and OpenMP 4.0 standards, many works (e.g., [START_REF] Martineau | Evaluating openmp 4.0s effectiveness as a heterogeneous parallel programming model[END_REF], [START_REF] Memeti | Benchmarking opencl, openacc, openmp, and cuda: Programming productivity, performance, and energy consumption[END_REF]) have been done to evaluate their performance against those of CUDA and OpenCL. Other works (e.g., [START_REF] Lashgar | Openacc cache directive: Opportunities and optimizations[END_REF], [START_REF] Hayashi | Exploring compiler optimization opportunities for the openmp 4.x accelerator model on a power8+gpu platform[END_REF]) suggested some optimizations based on the implementation of new directives. Moreover, many efforts have been done to improve OpenACC and OpenMP compilers in order to generate more optimized code. However, like other mainstream compilers, they still fail to recognize even the simplest opportunities for reuse of subscripted variables.

In this paper, we present RECKA and RPromF, two new Frama-C plug-ins we developed for automatic insertion of restrict keyword and scalar replacement in GPU codes. The remainder of this paper is organized as follows. Section II provides a brief introduction about CUDA, OpenACC and OpenMP 4.0/4.5 programing models. Section III reviews related work. Section IV describe briefly the Frama-C framework. Section V develops the design of different components of the RECKA and RPromF plug-ins. Section VI deals with the evaluation of the two plug-ins and analysis of the obtained results. Finally, Section VII presents concluding remarks and future works.

II. BACKGROUND

In this section, we provide a brief introduction about CUDA, OpenACC and OpenMP 4.0/4.5 programing models.

A. CUDA programming model

CUDA is a parallel computing programming model that fully utilizes hardware architecture and software algorithms to accelerate various types of computation. In CUDA, the programmer writes device code in functions called kernel. To obtain optimized code, the programmer must understand well GPU architecture and CUDA optimization strategies like memory-coalescing access, efficient usage of shared memory or tiling technology. Additionally, grid and block configurations, computing behaviors of each thread, and synchronization problems also need to be carefully tuned. Listing 2 shows a simple restricted CUDA implementation of the C version of gesumv kernel provided in listing 1. OpenMP 4.0 introduced new directives for programming accelerators such as GPUs and Intel Xeon Phi. In order to offload a region of code into device, OpenMP 4.0/4.5 uses the target construct. Various directives are provided to express the levels of parallelism. The teams construct, creates a league of thread teams where the master thread of each team executes the region. The distribute construct specifies loops which are executed by the thread teams. The target data construct handles data transfers between host and accelerators. An exhaustive description of OpenMP directives is provided in [START_REF] Board | Openmp application programming interface[END_REF]. Listing 3 shows OpenMP version of code provided in listing 1, when scalar replacement is performed.

1 v o i d gesummvMP ( f l o a t * A, f l o a t * B , f l o a t * tmp , 2 f l o a t * x , f l o a t * y ) { 3 i n t i

C. OpenACC programming model

OpenACC is another specification focused on directivebased ways to program accelerators. OpenACC has fewer constructs but most of them are analogous to those of OpenMP 4.0/4.5. The OpenACC parallel construct starts parallel execution on the current accelerator device by creating one or more gangs of workers. The kernels construct defines a region of the program that is to be compiled into a sequence of kernels for execution on the current accelerator device. The loop construct specifies the distribution of iterations. Detailed description of OpenACC directives is provided in [START_REF]The openacc application programming interface[END_REF]. Listing 4 shows OpenACC version of code showed in listing 1.

III. PREVIOUS WORK

Alias analysis is one of the most used techniques that aims to optimize languages with pointers. The literature of alias analysis is abundant. The most popular algorithm for context-insensitive, flow-insensitive, iterative and constrainsbased points-to analysis is known as inclusion-based or Andersen-style analysis [START_REF] Andersen | Program analysis and specialization for the c programming language[END_REF]. Alves et al. provided in [START_REF] Alves | Runtime pointer disambiguation[END_REF] two ways to determine at runtime when two memory locations can overlap. They concluded that the combination of cloning plus dynamic disambiguation of pointers is an effective way to make compiler optimizations more practical. Sperle et al. presented in [START_REF] Sperle Campos | Restrictification of function arguments[END_REF] three different techniques to disambiguate pointers used as arguments of functions. Their first technique relies on the static alias analysis already available in mainstream compilers to perform pointer disambiguation and the others combine static bound inference with code cloning, hence, extending the reach of pointer disambiguation. Maalej et al. introduced in [START_REF] Maalej | Pointer disambiguation via strict inequalities[END_REF] a new technique to disambiguate pointers, which relies on a less-than analysis. Their alias analysis uses the observation that if p1 and p2 are two pointers, such that p1 < p2, then they cannot alias. The original scalar replacement algorithm was proposed by Carr-Kennedy in [START_REF] Carr | Scalar replacement in the presence of conditional control flow[END_REF] more than 20 years ago. Since then, several works have been done to improve this algorithm in many aspects. Surendran et al. presented in [START_REF] Surendran | Inter-iteration scalar replacement using array ssa form[END_REF] new algorithms for scalar replacement and dead store elimination based on Array SSA form. Byoungro et al. described in [START_REF] So | Increasing the applicability of scalar replacement[END_REF] an algorithm for scalar replacement that can exploit reuse opportunities across multiple loops. Tian et al. presented in [START_REF] Tian | Optimizing gpu register usage: Extensions to openacc and compiler optimizations[END_REF] an extension to the classical scalar replacement algorithm for optimizing registers usage in OpenACC codes. Their approach is based on feedback information regarding register utilization and a memory latency-based cost model to select which array references should be replaced by scalar references.

IV. PRELIMINARIES: FRAMA-C

Frama-C 1 is a static analyzer for C code. It provides its users with a collection of plug-ins that perform static analysis, deductive verification, and testing, for safety-and securitycritical software [START_REF] Kirchner | Frama-c: A software analysis perspective[END_REF]. The platform is based on a common kernel, which hosts analyzers as collaborating plug-ins. Frama-C kernel is based on a modified version of CIL. CIL is a frontend for C that parses ISO C99 programs into a normalized representation. For instance, for loops are replaced by equivalent while loops , normalized expressions have no side-effects, ect. Frama-C extends CIL to support other features such as ACSL (ANSI/ISO-C Specification Language) annotations. 1 https://frama-c.com/ This modified CIL front-end produces the C + ACSL abstract syntax tree (AST). The AST assigns unique identifiers for statements and blocks that can be used for the program counter, it also keeps line numbers. In addition to the AST, the kernel provides several general services. For example, it provides a visitor mechanism to facilitate crawling through the AST. In general, writing a Frama-C plug-in requires to visit the AST to compute information for some C constructs.

V. DESIGN PRINCIPLES

This section presents alias analysis we implemented to identify on which pointers of CUDA kernel arguments must be added the restrict keyword, our scalar replacement algorithm implementation, and describes the design of the different modules composing the RECKA and RPromF tools.

A. The alias analysis module

Since the optimized version of a function by performing the scalar replacement is equivalent to its original version only if there is no alias and marking aliased pointers as restricted may result in undefined behavior, we run an alias analysis before making any changes in the source code. For that purpose, we have implemented in Frama-C a simple and fast alias analyzer inspired by Andersen-style analysis [START_REF] Andersen | Program analysis and specialization for the c programming language[END_REF] and the concepts of the basicaa pass of LLVM 2 . In our implementation, we only considered two pointers operations: taking the address of a variable (e.g.: p = &val) and assignments (e.g.: p = q, p = T ab where p and q are pointers and Tab a constant array).

B. Pre-processing module

Frama-C can't analyze CUDA codes or programs comprising OpenACC and OpenMP directives. On the other hand, Frama-C transform all for loops in while loops thus denaturing the source code. Thus, it is necessary to do some text processing tasks on the source code before and after the analysis with Frama-C. To this end, we developed two programs in Perl for the pre-processing operations. The first takes as input a CUDA program, copies it into a new file, hides all CUDA specific keywords, types or API functions such as global , cudaMalloc, etc. It also adds an identifier to the kernels names so that we can find them in next steps. The second takes as input a source file with OpenACC/OpenMP pragmas, copies it in a new file, replaces all OpenMP/OpenACC pragmas by flags that will allow us to find, in next steps, offloading regions and to know what directives has been used. In both cases, these two programs generate an intermediate C file that will be passed to Frama-C.

C. Post-processing module

According to the results of the analysis made with Frama-C on the file generated by the pre-processing step , changes must be made to the source file. We developed another program in Perl to make these modifications. This program takes as input the initial source file and the output file of our Frama-C plugins and modifies the initial source file. For example, if the source file is a CUDA program then the output file of the RECKA plug-in is used and the changes made to the source file are: (i) clone a CUDA kernel; (ii) change the clone's name and add the restrict keyword on its arguments; and (iii) change this kernel call sites by adding an if statement allowing to call either the clone or the original version depending on the runtime less-than check result.

D. RECKA: REstrictification of CUDA Kernel pointers Arguments

The RECKA plug-in performs some alias analysis on a CUDA program and adds the restrict qualifier to kernels pointers arguments, in the absence of aliases. It takes as input the intermediate file generated in the pre-processing step and return a text file. This file contain messages that will make it possible to change the initial source file during the post-processing step. The RECKA plug-in first starts by identifying all CUDA kernels definition and then performs an alias analysis in these kernels. Then for each kernel where there is no alias, he performs another alias analysis in the launcher function of this kernel. If no alias is found then it print messages to clone this kernel, rename it and add restrict keyword on its pointers arguments. It also saves in an hash table all device pointers and allocated arrays sizes. This table is then used to print messages that will make it possible to insert, in the source file, instructions to check on runtime if kernel arguments pointers do not actually point to overlapping regions. For generating messages for the runtime less-than check test, we implemented a module in Frama-C inspired from the methods described in [START_REF] Maalej | Pointer disambiguation via strict inequalities[END_REF]. Figure 1 shows an overview of this plug-in.

E. RPromF: Register Promotion with a Frama-C plug-in

The goal of this plug-in is to perform the scalar replacement in OpenACC/OpenMP programs. It takes as input an intermediate file generated by the pre-processing module. First, it identifies OpenACC/OpenMP offloading regions by using flags added by the preprocessing step. After that, an alias analysis is performed on functions containing these offloading codes as well as their callers if any. If alias analysis concludes that there is no alias involving any variables used in the offloading regions then generates messages that will make it possible to clone the offloading codes. Then try to identify repeated accesses made to the same memory address by using our simplified implementation of Carr-Kennedy algorithm [START_REF] Carr | Scalar replacement in the presence of conditional control flow[END_REF]. If found repeated accesses then prints messages that will make possible to perform the scalar replacement in offloading regions clones. Finally, it generates messages that will make it possible to insert in the source code an if statement allowing at runtime to use either the clone or the original version depending on the result of the runtime less-than check analysis. Figure 2 shows an overview of this plug-in.

VI. EVALUATION

We tested our tool on kernels taken from the PolyBench3 benchmark suite, to assess the ability of RECKA and RPromF plug-ins. We choose this benchmark because there are many potentials pointers aliasing situations in its kernels. For our experiments, performance data were collected on a NVIDIA GPU Quadro M2000M hosted in an Intel I7 CPU. LLVM/-Clang4 (4.0) was used to compile our OpenMP versions. For OpenACC, we used the PGI 17.10.0 compiler (pgcc). All performance data were collected with NVIDIA profiler, nvprof. For all arrays, we used float as data type and the extra large dataset value provided in PolyBench. All versions were compiled with -O3 flag.

Results analysis

Our preliminary results showed that RECKA was successfully able to annotate all pointers arguments of kernels with the restrict keyword without any misplacement. RPromF has also been able to place correctly all the data accessed redundantly in a register. Since our goal is to evaluate the impact of the restrict keyword and scalar replacement on application performances, we did not compare OpenACC or OpenMP against CUDA.

Figure 3 shows factors relative to the original codes for the CUDA kernels performance including compute time, executed instructions count, global load and L2 read transactions. We observed that the restrict qualifier reduces kernel compute time by 24% on average. However, this factor is more interesting for the correlation and covariance application where there are many potential pointer aliasing and redundant load/store. We found that annotating pointers arguments with the restrict reduced the executed instruction count by 6%s on average and the global load transaction only when there are many redundant load as it is the case for the adi, gesum, gramschmidt and mvt kernels. Regarding the L2 cache read transactions, we found that the restrict keyword reduced it by 18% on average. We can conclude that restrict keyword improve CUDA kernels overall performance, and that it must be used whenever possible to help nvcc to generate more optimized code.

Regarding the impact of scalar replacement on OpenACC and OpenMP codes for GPU, we compared the performance of the generated code, after running RPromF on PolyBench kernels, to that of the original versions to show that the scalar replacement, performed in source level, really contributes to improve performance. Figure 4 shows factors relative to the original code for the offloading regions compute time. We observed that with OpenACC, the scalar replacement reduced kernels compute time by 43% on average. While with OpenMP we found that scalar replacement reduce kernels compute time by 31% on average. We can show on both figures 4 and 3, the performance gain obtained on gesum kernel which was taken as an example in section II. In summary, we observed Third, instructions are added to kernels call sites to perform at runtime a less-than check analysis on kernel actuals parameters and determine if the clone must be called or the original one. On the other hand, RPromF includes five main steps. First, OpenACC/OpenMP offloading regions are identified. Second, functions containing these offloading codes and their callers are analyzed to verify if there is no alias implying any variables used in offloading regions. Third, if there is no alias then the offloading code are cloned. Fourth, clone's instructions are analyzed to retrieve data reuse information and perform scalar replacement. Fifth, instructions are added to be able to use the optimized clone whenever possible. We have assessed the two plug-ins on the PolyBench benchmark suite. The results show that both scalar replacement and the usage of restrict keyword are effective for improving the overall performance of OpenACC, OpenMP 4.0/4.5 and CUDA codes. In future work, we plan to evaluate both plug-ins on more complex benchmarks such as SPEC and Rodinia for example and trying to add a cost model to avoid registers spilling with scalar replacement. While Frama-C has two deductive verification plug-ins, we intend to use them in our tool in order to be able to prove the reliability of the generated code.

Fig. 3 .Fig. 4 .

 34 Fig. 3. Restrict keyword impact on CUDA kernels performance

https://llvm.org/docs/AliasAnalysis.html#the-basicaa-pass

https://sourceforge.net/projects/polybench/

https://github.com/clang-ykt

RPromF plug-in

VII. CONCLUSION AND FUTURE WORK In this paper, we have implemented two new Frama-C plug-ins, RECKA for automatic annotation of CUDA kernels arguments with the restrict keyword, and RPromF for scalar replacement in OpenACC and OpenMP 4.0/4.5 codes for GPU. RECKA works in three main steps. First, an alias analysis is performed on CUDA kernels and their callers to be sure that there is no alias and that kernels arguments can be marked as restricted pointers. Second, if not found any alias then CUDA kernels are cloned, the clones are renamed and


Bruno Darracq

Bastien Vincke

Pascal Aubert

A Platform For Remote Laboratories In Applied Physics

Keywords: Remote Laboratories, Remote actions, Applied Physics

Remote Laboratories (RLs) can be an efficient tool to acquire practical skills to complement face-to-face laboratories. The development of an RLs platform requires dedicated equipment and software tools to facilitate the access and control of the equipment. Other tools for communication between users and supervision of student activities by teachers are also necessary to support the learning process. This article presents a RLs platform used for practical activities in applied physics curricula at Paris-Saclay University. The platform combines a Guacamole server to access and share material resources and a Moodle environment for digital services, including virtual classrooms. We set up different remote actions to make RLs as close as possible to real laboratories. We decline these remote actions in three examples of RLs in electronics, acoustics, and material physics. .

Introduction

The number of tools for distance learning has increased dramatically over the last 20 years. Numerous technologies based on Learning Management Systems (LMS) now allow learners to follow distance courses in various topics. These platforms are dedicated to learning theoretical concepts in Science, Technology, Engineering, and Mathematics (STEM). More recently, the emergence of Remote Laboratories (RLs) has opened the way for distance learning practical work [START_REF] Cubillo | What remote labs can do for you[END_REF]. It is a critical contribution, particularly for curricula with a large laboratory load, such as technician or engineering education. However, some technological issues must be overcome to ensure the efficiency of a RL compared to a real one. For example, the software interface for accessing and controlling the lab equipment must be simple enough and not be an obstacle to learning. Managing lab resource allocation, guaranteeing the safety of the equipment, and implementing the possibility for teachers to communicate with students and supervise their remote laboratory work, are also crucial points. Several solutions have been

Contributions

During the COVID period, the Institute of Technology of Paris-Saclay University set up a RLs platform which has been enabled to ensure the continuity of education during the period of lockdown, mainly for practical activities with the implementation of a small number of RLs in STEM education [START_REF] Vincke | Cartable Distant : Un environnement numérique complet pour l'enseignement pratique à distance[END_REF]. Since students have returned to classrooms, this platform has evolved into a dedicated tool that turns out to be an efficient complement to face-to-face teaching. With the financial support of the Paris-Saclay University, we have increased the offer of RLs and set up a pedagogical service to assist teachers to introduce RLs in their courses. This article presents the main features of this RLs platform. We will start by defining the ideal remote lab (section 3). In section 4, we present the material and functional architecture of the platform. Section 5 focuses on the critical issue of remote actions to be implemented in RLs.

3

The ideal remote lab

Ideally, we would like students to do remote practical work in the same conditions as when they are physically present. We have therefore synthesized all the actions carried out by the students. The first need is to be able to communicate with a teacher but also with the students. Moreover, students often carry out discussions in small groups. Furthermore, they must have access to all available computer resources. During a practical course, they are required to handle materials. Specifically, they must set up their equipment, wire the manipulation, use instruments to generate signals and acquire measurements. They often have to make linear or 3-dimensional displacements. The result of a TP is usually visual. They must be able to react to the results obtained, modify parameters and observe all the consequences of the modifications made. We have made it possible for students to perform all these actions remotely.

4

Architecture of the RLs Platform 4.1

Material Architecture

The material infrastructure (see Fig. 1) is built around a dedicated LAN network. We have chosen to separate the remote network from our institution's network for security reasons.

Our remote laboratory is equipped with two kinds of workstations. We have 18 physical workstations and 16 virtual machines. Indeed, we want to make a set of materials available to students that are sometimes only accessible through a USB port. It is, therefore, necessary to provide physical stations allowing the USB connection of peripherals. However, we have added two USB/ethernet switches that will enable to deport of USB devices on the network. These switches only work with some USB devices, and you have to be very careful in terms of bandwidth usage. For example, connecting USB cameras to the switch is impossible because the bandwidth will be filled with only two video streams.

To carry out the practical work, students use instruments: function generator, multimeter, power supply, and oscilloscope. We use new instruments with an LXI connection to access the device remotely. This LXI bus allows access to a front panel of the instruments through an Ethernet bus. Each device is then accessible by all the workstations (virtual or physical). The students must carry out cabling to set up their manipulations during the practical work. We set up relay matrices to make a circuit between measurement equipment and circuit components to realize this remote cabling. The teacher can do this wiring before the practical session or be part of the experimental work.

According to their size, some equipment is placed in racks and others on large shelves (see Fig. 2). The university's IT service ensures the network's operation.

We quickly noticed that it was essential to have visual feedback on the manipulation of the room in which the TP is located. Indeed, the student must understand very quickly that it is not a simulation but that he is handling real material and instruments. We have therefore set up a video server that collects the streams from 50 webcams and makes them available on the Ethernet network. We chose this architecture for cost and simplicity reasons. We developed two pan-tilt cameras to observe the whole room.

The platform manages more than 100 instruments, including classical instruments such as oscilloscopes, function generators, multimeters, and power supplies, and more specific equipment such as NI-myDAQ and NI-ElvisII+ boards [START_REF] Klinger | Low-cost Remote Laboratory Concept based on NI myDAQ and NI ELVIS for Electronic Engineering Education[END_REF], XBee modules, microcontrollers, and robotic arm. These equipment is deployed in more than 20 RLs for the Institute courses in Electricity, Electronics, Acoustics, Material Physics, and Industrial computing. The platform is now flexible enough to add other equipment depending on the type of RLs teachers want to implement.

4.2

Software Architecture

The software architecture is described in Fig. 3. The main objective of the software part is to allow the student to access all the software resources available in our infrastructure and to offer him the same simplicity of access as if he were in our institute.

We have based our approach on two types of software. The first type is relatively classical: Moodle and BigBlueButton (BBB). Moodle could allow the student to find all the course documents, take MCQs, submit assignments, etc... The BBB software is a virtual classroom software we used during the lockdown. It allows all the possibilities of a virtual class: sharing documents, microphone, sound, polling, group work, etc... The second type is a remote desktop used via the Guacamole software [START_REF]Guacamole Homepage[END_REF]. It could allow students to access one of our 34 remotely accessible machines through the remote desktop. Thanks to Guacamole, students find exactly the same working session as when they physically connect to a computer of the institute. They can access all the software and documents stored in their network drive. It is a straightforward and secure provision of resources.

An exciting feature of Guacamole is the possibility of sharing computing sessions. This functionality has often been used during the Covid lockdown for collaborative, practical activities between students (students usually work in pairs) or for teacher supervision. Combined with a BBB virtual classroom, this resource-sharing feature is particularly efficient for distance learning. Using collaborative tools can also solve the problem of resource allocation for small student groups.

To simplify the platform's access to material and software resources, we gathered all information in a Wiki website [START_REF]Cartable Distant Homepage[END_REF]: how to access workstations, presentations, and instructions for using the RLs, links to Moodle resources and BBB virtual classroom, etc...

Remote actions

As mentioned above, in the ideal remote lab, users must be able to manage different types of remote actions. In this section, we illustrate the possibility offered by our platform to make remote wiring, moving, and controlling instruments.

Remote wiring

This wiring is sometimes made with flying wire, labdec plate, and soldered on a PCB. This wiring allows the connection of electronic components (resistor, capacitor, AOP...) and instruments allowing the generation and acquisition of signals. In order the students to do an electronic lab, they have to make a wire-based assembly. RLs do not allow this action to be performed remotely. They mainly provide a pre-wired circuit on which the students will conduct their experiments. We wanted to allow the students to have the freedom to achieve this wiring and also to be able to position the instruments in the assembly. We have reused the concept of switch matrix, which is very much used by industrials to test electronic boards. These matrices are organized in matrix form and composed of many relays to form a modular circuit and allow all possible configurations. To adapt these matrices to our use, we placed the instruments on the lines and the components of the circuit on the columns of the matrix. The number of rows and columns will have to be very important if we want to allow all the possible configurations. Indeed, if we want to use the usual instruments (power supply, multimeter, oscilloscope, etc.), we must have more than 10 to 15 lines for our matrix. For the columns, if we want to make a setup based on AOP, we will quickly need several columns (greater than 20).

There are commercially available arrays of this size. They often have excellent characteristics. In particular, they attach importance to the switching time, which must be as small as possible to allow many tests quickly. These arrays are costly, and the price rises with the number of relays. We do not need a low switching time for our lab, but we have specific needs. We want to maximize the number of rows and columns and have interfaces that we usually use: BNC or banana plug. To that end we chose to redevelop a relay matrix (see Fig. 4). This matrix is modular in every way: it can accommodate many relays depending on the need and adapt its interfaces using daughter boards. The cost is about 1.5€ per relay, which is very low compared to industrial relays.

Fig. 4. Our custom switch matrix

Different uses of the switch matrix can be considered. For example, in a 1st-year electronics course, we need to design and characterize electronic circuits. For that, we connected the I/O of a NI-myDAQ board [myDAQ] to columns and electronic components (resistors, capacitors, operational amplifiers, diodes) to lines. We developed a NI-Labview application to control the switch matrix and wire different electronic circuits (see Fig. 5). In the first tab of the application, complete control of the relay matrix is possible, so many circuits and measurements are possible. Note that we use lines for the connection between electronic components. The first column is used to activate each line. Virtual instruments (GBF, Oscilloscope) of the NI-myDAQ and Webcam can be launched directly from the interface. In the other tabs, a pre-configured matrix can be set to study classical electronic circuits (inverting/non-inverting AOP circuit, low-pass filter, etc.)

This application can be used by teachers (for example, during large or small class demonstrations) and students (for example, to revise a circuit for lab exams) by logging on one of the remote workstations of the platform. Fig. 5. Labview interface to study electronics circuits.

Remote moving and control instruments

Most real labs need to move objects. When the displacement is clearly defined, we can use a motorized axis. For example, in the Kundt experiment [START_REF] Kundt | On a new kind of acoustic dust figures and their application to determine the speed of sound in solid bodies and gases[END_REF], we need to move a microphone inside a tube to measure the level of stationary sound waves. We designed this remote lab using a 1D motorized axis (RCP3 Robo Cylinder) attached to a rail on which a ball-bearing plate supporting the microphone slides (See Fig. 6).

The slider is programmed through the Modbus protocol. We developed a Labview application to control the microphone's position and access the webcam and the instruments. Two operation modes are possible: a manual mode, where the user can set the microphone's position in the tube and control instruments using their Virtual Web front side, and an automatic mode. Note that the Virtual Web front side of the oscilloscope is very close to the real one (see Fig. 7). Students need to be in the same working environment as the real lab. Instrument manufacturers (such as Keysight) have made great efforts in recent years to implement a very realistic HTML5 Virtual Web front side on the new range of LXI Instruments.

Considering all the remote actions implemented in this experiment, students and professors can make the entire acoustic lab the same way as if they were in the real acoustic laboratory.

Acquisition of instruments programming skills is an important learning outcome in the second-year of Industrial Computing course. As almost all lxi/USB instruments of the platform are programmable, students can design their own applications to control instrument using the Labview environment installed in the platform's workstations.

Securing experiments

For some experiments, it is crucial to ensure the safety of the equipment, especially in remote mode. For example, when you heat materials to high temperatures, you must ensure that the heating element does not continue to operate if an incident occurs, like a LAN network cut-off or a crash of the software interface. In a second-year Material Physics lab, we heat Pb-Sn alloys around 300 °C and investigate the temperature cooling curves (with thermocouples) to determine the alloy composition (see Fig. 8).

To ensure the safety of the experiment, we implemented an active safety mechanism. The Software interface sends messages every 5 seconds to a micro controller that acquires data from thermocouples and controls a power relay between the heating plate and the electrical socket. In case of a software crash or a LAN network cut-off, the micro controller does not receive the messages and closes the power relay.

We can also turn off the lab's power from the software interface thanks to a Wifi connected plug (note that each lab can be individually turned on or off thanks to the Wifi network of connected plugs).

Conclusions and perspectives

The platform has been designed for a wide range of use cases, from simple access to personal working documents to the use of heavy industry software or full practical activities on RLs. The table 1 below provides a list of use cases from the feedback of early users since the platform became fully operational in early 2022. The analysis of Guacamole's logs revealed about 20 daily connections to the platform for an average time connection of 30 minutes. Most of these connections take place at night or at weekends, which means that the use of the platform is complementary to face-to-face teaching.

Table 1. List of uses cases (not exhaustive)

First feedback from students indicates that connections involved many 1st and 2nd-year courses: electricity, electronics, acoustics, mechanics, computer programming, instrumentation, computer-assisted design, projects, and even communication classes. Further developments are planned to improve the allocation and sharing of the resources (in particular RLs) and tools to supervise student lab works. Since September 2022, a training sessions has been offered to all teachers at the University of Paris-Saclay University to help them introduce remote practical activities in their courses and evaluate their added value.

Fig. 1 .

 1 Fig. 1. Material architecture of the remote lab.

Fig. 2 .

 2 Fig. 2. Our remote lab based on 19", 42U racks shelves.

Fig. 3 .

 3 Fig. 3. Software architecture of the remote lab.

Fig. 6 .Fig. 7 .

 67 Fig. 6. Kundt experiment (left) and and its Labview Interface (right)

Fig. 8 .

 8 Fig. 8. Remote lab using a heating plate

Acknowledgment

The authors would like to acknowledge the University Institute of Technology (IUT) of Orsay's support as well as the undergraduate school of Paris-Saclay University through the project "Hybridation des formations d'enseignement supérieur" (20-NCU-0014), and the French network Polytech Group. They also wish to thank Max Montillet, Yacine Yahi, Gabriel Poggia for their technical contributions and our colleagues of the IT service of IUT, Benoît Tonnerre and Olivier Guilleminot for their great assistance.


Matthieu Josse

Laure Patrier

Myriam Isnard

Cécile Turc-Baron

Antoine Grandperrin

Stéphane Nottin

Stéphane Mandigout

Jean-Paul Cristol

Claire Maufrais

Philippe Obert

Cardioprotective Effect of Acute Intradialytic Exercise: A Comprehensive Speckle-Tracking Echocardiography Analysis

Significance statement:

Hemodialysis (HD) can lead to acute left ventricular (LV) myocardial wall motion abnormalities (myocardial stunning) due to segmental hypoperfusion. Exercise during dialysis is associated with favorable effects on central hemodynamics and BP stability, factors considered in the etiology of HD-induced myocardial stunning. In a speckle-tracking echocardiography analysis, the authors explored effects of acute intradialytic exercise (IDE) on LV regional myocardial function in 60 patients undergoing HD. They found beneficial effects of IDE on LV longitudinal and circumferential function and on torsional mechanics, not accounted for by cardiac loading conditions or central hemodynamics. These findings support the implementation of IDE in people with ESKD, given that LV transient dysfunction imposed by repetitive HD may contribute to heart failure and increased risk of cardiac events in such patients.

Background:

Hemodialysis (HD) induces left ventricular (LV) transient myocardial dysfunction. A complex interplay between linear deformations and torsional mechanics underlies LV myocardial performance. Although intradialytic exercise (IDE) induces favorable effects on central hemodynamics, its effect on myocardial mechanics has never been comprehensively documented.

Methods:

To evaluate the effects of IDE on LV myocardial mechanics, assessed by speckle-tracking echocardiography, we conducted a prospective, open-label, two-center randomized crossover trial. We enrolled 60 individuals with ESKD receiving HD, who were assigned to participate in two sessions performed in a randomized order: standard HD and HD incorporating 30 minutes of aerobic exercise (HDEX). We measured global longitudinal strain (GLS) at baseline (T0), 90 minutes after HD onset (T1), and 30 minutes before ending HD (T2). At T0 and T2, we also measured circumferential strain and twist, calculated as the net difference between apical and basal rotations. Central hemodynamic data (BP, cardiac output) also were collected.

Results:

The decline in GLS observed during the HD procedure was attenuated in the HDEX sessions (estimated difference, -1.16%; 95% confidence interval [95% CI], -0.31 to -2.02; P = 0.008). Compared with HD, HDEX also demonstrated greater improvements from T0 to T2 in twist, an important component of LV myocardial function (estimated difference, 2.48°; 95% CI, 0.30 to 4.65; P = 0.02). Differences in changes from T0 to T2 for cardiac loading and intradialytic hemodynamics did not account for the beneficial effects of IDE on LV myocardial mechanics kinetics.

Conclusions:

IDE applied acutely during HD improves regional myocardial mechanics and might warrant consideration in the therapeutic approach for patients on HD.

INTRODUCTION

ESKD is a global public health issue, with nearly 2.6 million people worldwide on hemodialysis (HD).1 Individuals with ESKD have a substantially increased incidence of cardiovascular events andmortality.2 LV geometry and structure (e.g., fibrosis) abnormalities as well as both global systolic and diastolic dysfunctions are frequent components of ESKD. [START_REF] Foley | Impact of hypertension on cardiomyopathy, morbidity and mortality in end-stage renal disease[END_REF][START_REF] Cibulka | Metabolic disorders in patients with chronic kidney failure[END_REF][START_REF] Plawecki | sST2 as a new biomarker of chronic kidney disease-induced cardiac remodeling: impact on risk prediction[END_REF][START_REF] Fraser | The burden of comorbidity in people with chronic kidney disease stage 3: a cohort study[END_REF] The pathogenesis of heart disease in ESKD is not yet fully understood but remains multifactorial. Traditional cardiovascular risk factors and comorbidities (e.g., hypertension, diabetes, dyslipidemia) and factors related to ESKD itself, such as uremia and inflammation, contribute undoubtedly to the development and progression of ESKD-associated pathological cardiac remodeling. 7 HD is also recognized as an independent risk factor.8 It is associated with intradialytic hypotension (IDH) and disturbances in myocardial segmental perfusion,9,10 leading to acute myocardial wall motion abnormalities and LV transient dysfunction, well known as myocardial stunning.11,12 Repetitive episodes of myocardial stunning during HD may, in the long term, precipitate individuals with ESKD to heart failure and thus contribute to the high rate of cardiovascular mortality in this population. [START_REF] Burton | Hemodialysis-induced cardiac injury: determinants and associated outcomes[END_REF] Therapeutic strategies to mitigate cardiovascular disorders associated with HD are, therefore, mandatory. Acute IDE seems promising because it transiently increases central hemodynamics and BPs and can potentially reduce IDH, although this is not consensual.14 Data on its effect on myocardial function are, however, scarce. Two exploratory studies, performed on a small number of individuals with ESKD, documented a reduction in myocardial stunning identified by speckle-tracking echocardiography when IDE was applied acutely compared with standard HD.15,16 LV regional myocardial function results from a complex interplay between multidirectional (e.g., longitudinal and circumferential) deformations and torsional mechanics due to a complex interaction between myocardial fiber architecture and geometry. The LV twist motion in systole aids in ventricular ejection while early diastolic untwist generates suction and facilitates diastolic filling.17 Assessment of not only multidirectional deformations but also twist is, therefore, crucial for a comprehensive understanding of the effect of acute IDE on LV regional myocardial function in ESKD. To the best of our knowledge, no previous studies have made such measurements.

Accordingly, this study aims to explore the effect of acute IDE on LV linear deformations and torsional mechanics in HD individuals.

METHODS

Study Design and Participants

This pilot study was performed as a prelude to a larger study (EX-CHRODIAL for Chronic IDE: a Cardioprotective Role) designed to determine whether IDE improves long-term wellbeing and cardiovascular health. This study was a prospective, open-label, randomized trial performed in two HD units in France. It aimed to investigate the effect of acute IDE on LV myocardial mechanics over the course of HD. The ethics committee East II (clinicaltrials.gov NCT04697459) approved this study. All participants provided written informed consent. Adults, aged 20-79 years, with no regular exercise outside of dialysis, with no exposure to IDE before the study (,6 months), and undertaking maintenance HD for .3 months were eligible to participate. Exclusion criteria were poor echogenicity; contraindication to exercise; orthopaedic complications; severe heart (ischemic, valvular, etc.), vascular, or respiratory diseases; ejection fraction ,45%; body mass index .35; and pregnancy. Figure 1 shows a flowchart of patient enrollment. This study was a crossover design (Figure 2), in which all individuals participated in two HD sessions in a random order: a standard HD and a session incorporating 30 minutes of aerobic exercise (HDEX). Patients were randomized by a computer using IBM SPSS statistics version 26.0 (IBM, Armonk, NY). Randomization assignment was not provided to patients until completion of the baseline study session. Exercise was performed in a semi-recumbent position on a calibrated cycle ergometer (OxyCycle 3-PhysioMed) fixed to each patient's bed. The participants performed a 30minute cycling session (50-70 rpm), starting 30minutes after HD onset, at moderate intensity (11-14 on the Borg 20-point scale18). Echocardiography was performed three times during each session: immediately before HD (T0), after 90 minutes of HD (T1), and 30 minutes before ending HD (T2) while central hemodynamic data (BP; cardiac output [CO]) were collected before HD and every 30 minutes throughout dialysis (Figure 2). For consistency, HD and HDEX were performed 1-week apart from the first HD day of the week.

Clinical Examination and Dialysis Settings

In the first session, a comprehensive clinical examination was conducted and blood samples were collected from the arterial line of the dialysis circuit at the start of HD (Table 1). Biological data included hemoglobin, creatinine, calcium, potassium, sodium, high-sensitivity C-reactive protein, and urea levels. All participants were dialyzing three times weekly for 4-4.5 hours per session. HD parameters were collected at the end of each session (Table 2). Net ultrafiltration was determined clinically on the basis of ideal dry weight. For each participant, medication was not changed throughout the study period.

Echocardiography

Transthoracic echocardiography was performed using a Vivid Q system (GE Healthcare, Horten, Norway) with a 3.5-MHz transducer. Cine loops in apical 4, 2, and 3-chamber views were recorded at T0, T1, and T2, respectively, while loops in parasternal long-and short-axis (basal, papillary muscle and apical levels) views were only obtained at T0 and T2, respectively. The apical plane was acquired with the transductor in a caudal position (i.e., below the papillary muscles) to improve LV apical rotation measurement.19 Images were analyzed in cine loops triggered by the QRS complex and saved for blinded offline analysis, which was performed using dedicated software (EchoPAC 203TM-GE Healthcare, Chicago). Three cycles with the best image quality were used for analysis.

Standard Echocardiography

Standard acquisitions were recorded before the first session in accordance with the guidelines from the American Society of Echocardiography.20 LV internal diameters and wall thicknesses were measured from the parasternal long-axis view. LV mass was calculated by the Devereux formula and indexed to body surface area. LV volume and ejection fraction were measured using the Simpson biplane method. LV diastolic function was assessed using early (E) and atrial transmitral flow velocities, recorded in the apical 4-chamber view. Myocardial systolic, early diastolic (e'), and atrial velocities were measured with a colorcoded Doppler Tissue Imaging at the mitral annular level in the apical 4-chamber view. Peak e' (i.e., average recorded on the LV septal and lateral wall) and the E/e' ratio were used as indexes of LV relaxation and filling pressure, respectively.21

HD Echocardiography and Central Hemodynamics

LV volumes, ejection fraction, as well as internal dimensions and wall thicknesses were measured as aforementioned. Enddiastolic volumes (EDVs) were obtained as the preload index. The systolic meridional wall stress (ses) was calculated and used as an index of afterload.22 Analysis of linear strains and twist/untwist mechanics was conducted as previously described in our laboratory.23,24 Global longitudinal strain (GLS) was calculated using a 18-segment model from apical 4, 2, and 3-chamber views. Circumferential strain (CS, at the papillary muscle level) as well as apical and basal rotations were assessed from parasternal short-axis views. Increased negative values for GLS and CS indicate greater deformation. Data were processed with a specific toolbox (Scilab 4.1) and normalized to the percentage of systole (i.e., aortic valve closure representing 100% of systolic duration) using interpolations. Net LV twist was calculated as the instantaneous difference between apical and basal rotations. The LV untwisting rate was considered as an index of LV relaxation.25 Regular monitoring of cardiovascular hemodynamics was set up (Figure 2), with measurements staggered every 30 minutes from HD onset for aortic blood flow, heart rate (HR), and BP. Aortic blood flow velocity was measured in the ascending aorta using a 2.0-MHz continuous-wave Doppler transducer (Pedof P2D CW; GE Vingmed Ultrasound) placed at the suprasternal notch for stroke volume (SV) determination. 26 CO was calculated as SVxHR. Brachial BP was measured on the non-access arm during the HD session using an automated BP cuff integrated into the dialysis unit. Initial BP values were obtained at the start of HD. IDH was defined as a 20-mm Hg decrease in systolic BP (SBP) and/or a 10-mm Hg decrease in mean arterial pressure (MAP) associated with symptoms (e.g., abdominal discomfort, nausea, vomiting, muscle cramps, dizziness or fainting and anxiety) compared with initial BP values.27

Outcomes

The primary outcome was the GLS trajectory from T0 through T2 (T0, T1, and T2). Secondary outcomes included trajectories in all other speckle-tracking echocardiographyderived parameters and central hemodynamic variables as well as the number of IDH episodes per 100 hours of HD.

Statistical Analyses

Statistical analysis was performed using IBM SPSS statistics version 26.0 (IBM, Armonk, NY). Values are expressed as mean+/-SD. Statistical significance was defined as P value <0.05. No previous studies have examined the effect of IDE on GLS measured at baseline and at the end of HD (see Discussion). Sample size calculation was, therefore, based on the results from Penny et al. [START_REF] Penny | Intradialytic exercise preconditioning: an exploratory study on the effect on myocardial stunning[END_REF] on GLS obtained at the end of HD in the two conditions. We estimated that a total of 53 individuals would be required, considering an effect size of 0.34 with a statistical power of 0.80 and an (two-sided) of 0.05. All analyses regarding the effect of IDE were based on the intention-to-treat principle. The primary outcome was analyzed using a linear mixed-effects regression model, with condition (HD versus HDEX) as well as session sequence and time (T0, T1, T2) as a fixed effect and random effect, respectively, for patients. Analyses of prespecified secondary outcomes and HD parameters were undertaken using the same analysis methods as the primary outcome. A generalized linear mixed model (using a negative binomial regression model) was used to analyze the effect of IDE on the number of IDH episodes. Pearson correlations were used to investigate the association between differences in T0 to T2 changes between HD and HDEX of deformation imaging variables with indexes of cardiac loading conditions or the number of IDH.

RESULTS

Baseline Characteristics

The trial recruited from December 2020 to February 2022. A total of 200 people receiving HD were screened. Of the 147 individuals eligible and approached, 95 gave initial consent and 14 subsequently dropped out. After baseline assessments, 21 were also excluded for poor echogenicity (n=14) and ejection fraction lower than 45% (n=7). Sixty individuals completed both HD and HDEX sessions (Figure 1). Baseline characteristics are presented in Table 1. The mean age was 63+/-14 years, and 72% of the participants were male. Body weight before and after dialysis as well as all dialysis parameters were strictly similar between HD and HDEX (Table 2). Standard echocardiographic data are presented in Table 3.

Cardiac Function During HD and HDEX

Data are presented in detail in Table 4. For all myocardial mechanics and loading condition parameters, there was no condition by sequence interaction, highlighting the absence of carryover effect between HDEX and HD. For the primary outcome, IDE significantly affected the GLS trajectory from T0 through T2 (condition by time interaction P = 0.008). When compared with HD, HDEX attenuated the decline in GLS by 20. Regarding the secondary outcomes, IDE influenced also the trajectory from T0 to T2 in CS, apical rotation, and torsional mechanics, as evidenced by significant condition by time interactions (Table 4). Examination of estimated differences of T0 to T2 changes between HD and HDEX, and their 95% Cis clearly highlighted the beneficial effect of IDE on CS, apical rotation, and twisting motion. When compared with HD, HDEX yielded to greater improvement from T0 to T2 in twist, a very important component of LV myocardial function (estimated difference 2.48°, 95% CI, 0.30 to 4.65, P = 0.02). A modest effect of IDE on T0 to T2 changes was shown for basal rotation (estimated difference 20.94°, 95% CI, 22.39 to 0.51), with no significant condition by time interaction (P = 0.20).

No condition by time interaction and small estimated differences were obtained for loading conditions, but 95% CI revealed between-patient variations. However, when examining differences in T0 to T2 changes between HD and HDEX, no correlations were obtained between EDV with GLS (r = 20.17 P = 0.27), CS (r = 20.09 P = 0.59), apical rotation (r = 0.20 P = 0.22), and twist (r = 20.17 P = 0.30). Similar results were obtained for  es with GLS (r = 20.29 P = 0.08), CS (r =0.02 P = 0.90), apical rotation (r = 20.13 P = 0.43), and twist (r = 0.16 P = 0.35).

Hemodynamics

Figure 3 shows the time course of changes from baseline in HR, SV, CO, SBP, DBP, and MAP during both sessions. Significant condition by time interactions were noticed for SBP (P < 0.001), HR (P < 0.001), and CO (P = 0.02), underlining an effect of IDE. Estimated differences in changes from baseline between HD and HDEX were only significant at 60 minutes of HD (i.e., at the end of IDE) for SBP (P = 0.006) and CO (P50.003) and at 30 and 60 minutes of the procedure for HR (P = 0.01; P <0.001, respectively). IDE did not modify IDH episodes per 100 hours of HD (HD = 58+/-53 versus HDEX =51+/-56, P = 0.20; estimated difference: 2.2; 95% CI, 219 to 114). Furthermore, no correlations were found between HD and HDEX differences in the rate of IDH episodes with T0 to T2 changes in GLS (r = 0.18, P = 0.17), CS (r = 0.11, P = 0.51), apical rotation (r = 20.03, P = 0.85), and twist (r = 20.17, P = 0.29).

DISCUSSION

This study is, to the best of our knowledge, the first to explore in a large sample of people receiving HD the effect of acute IDE on LV linear deformations and torsional mechanics. In people with ESKD, conventional HD is mandatory to supply the failing renal function. However, the procedure can be detrimental to the myocardium. Acute reductions of LV myocardial perfusion, sufficient in magnitude to lead to transient LV myocardial dysfunction (i.e., myocardial stunning), have indeed been clearly documented using intradialytic positron emission tomography scanning or cardiac magnetic resonance imaging.10,12,28 Speckletracking echocardiography provides accurate and angle-independent measurements of LV linear deformations and twist,29,30 allowing a fully comprehensive evaluation of LV regional myocardial function. In our study, GLS and CS progressively declined during HD (Table 4), agreeing with previous studies using either 2D31 or 3D32,33 speckle-tracking echocardiography. Twist was, however, maintained (Table 4), in accordance with most of the available data, which may be viewed as a compensatory mechanism maintaining overall systolic function (i.e., ejection fraction) when longitudinal shortening (i.e., GLS) is depressed. [START_REF] Yip | Left ventricular twist before and after haemodialysis: an analysis using speckle-tracking echocardiography[END_REF][START_REF] Wu | Effect of preload reduction by hemodialysis on left ventricular mechanical parameters by three-dimensional speckle tracking echocardiography[END_REF] The repetitive nature of myocardial ischemic insults and transient dysfunction due to dialysis may, in the long term, yield to permanent regional myocardial dysfunction, conferring an increased risk of cardiac events and mortality,13 motivating, therefore, the application of countermeasures during HD. Acute IDE is an interesting prophylactic measure because it transiently increases central hemodynamics and BPs, factors considered in the etiology of HD-induced myocardial transient dysfunction,8 and can also potentially reduce IDH, although this is not consensual.14 However, the effects of acute IDE on LV regional myocardial mechanics remain extremely poorly documented. To the best of our knowledge, only longitudinal function has been investigated in two nonrandomized exploratory studies, performed on a small number (n,21) of individuals with ESKD.15,16 In these studies, despite a significant reduction in the number of myocardial stunned segments during the HD session with acute IDE compared with standard HD, a similar decrease in GLS during the HD procedure was reported in both conditions, results that clearly differ from ours where GLS changes from T0 through T2 were attenuated in HDEX compared with HD (Table 4). Several methodological aspects may potentially explain such discrepancies and also preclude any direct comparison between these and the current results: the low numbers of participants in both prior studies conferring weak statistical power and an increased risk of type II error, the last echocardiogram being performed 1.5 hours before the end of HD,16 or GLS changes being reported only from post-IDE to the end of HD as well as individuals with ESKD having undergone routine exercise participation during HD before the sessions during which echocardiography was performed. [START_REF] Penny | Intradialytic exercise preconditioning: an exploratory study on the effect on myocardial stunning[END_REF] No data are available in the literature regarding the circumferential function or torsional mechanics, despite their key roles in cardiac performance. Indeed, twist is a major component of LV mechanics reflecting the helical orientation of myocardial fibers. The LV twist motion induced by apical counterclockwise and basal clockwise rotations in systole aids in ventricular ejection while early diastolic untwist generates suction and facilitates diastolic filling.17 A salient feature, not previously demonstrated, was the cardioprotective benefit of IDE on circumferential function and torsional mechanics. The results from the mixed effects model and the one-sample t test clearly indicated the superiority of HDEX, by canceling the HD-induced deterioration of CS and apical rotation and enhancing twisting motion (Table 4). This study was not specifically designed to resolve the underlying mechanisms responsible for the beneficial effects of IDE on LV regional myocardial function, but several hypotheses may be discarded or put forward. The etiology of HD-induced transient LV dysfunction is incompletely understood, but hemodynamic instability has been presented as a potential contributor. By cardiac magnetic resonance imaging, Buchanan et al. reported indeed negative correlations between the number of dysfunctional LV segments and changes during dialysis in indexed SV and CO or minimum SBP.28 In our study, the time courses in SV and CO as well as SBP were similar during both HD and HDEX sessions, except during the 30-minute exercise, in agreement with previous data. [START_REF] Mcguire | Cardiac stunning during haemodialysis: the therapeutic effect of intra-dialytic exercise[END_REF] Owing to impaired baroreflex sensitivity, individuals with ESKD present with defective BP control and exhibit significant IDH during dialysis.14 IDH is independently associated with regional myocardial function. [START_REF] Burton | Hemodialysis-induced cardiac injury: determinants and associated outcomes[END_REF] In our study, we did not observe differences in the rate of IDH episodes per 100 hours of HD between HD and HDEX. However, we recognize that the study was not powered (i.e., too small sample size) to detect a statistically significant difference in this outcome. However, no correlations were found between differences in changes from T0 to T2 between HD and HDEX in deformation imaging variables with differences in IDH. Altogether, these results do not speak in favor of a role of hemodynamic factors in the explanation of the beneficial effects of IDE on LV regional myocardial function.

The load dependency of speckle-tracking echocardiography-derived parameters, including linear deformations and twisting mechanics, has been established. [START_REF] Park | The effect of loading alterations on left ventricular torsion: a simultaneous catheterization and twodimensional speckle tracking echocardiographic study[END_REF][START_REF] Ferferieva | The relative value of strain and strain rate for defining intrinsic myocardial function[END_REF][START_REF] Schlangen | Two-Dimensional global longitudinal strain rate is a preload independent index of systemic right ventricular contractility in hypoplastic left heart syndrome patients after fontan operation[END_REF][START_REF] Burns | Left ventricular torsion parameters are affected by acute changes in load: CME[END_REF] In our study, as expected, significant declines in both cardiac preload (EDV) and afterload (ses) during HD were found (Table 4). However, acute IDE did not affect the time course of loading indexes during dialysis. Despite small estimates, quite wide 95% Cis were noticed for the difference of change from T0 to T2 between HD and HDEX, revealing inter individual variations and a potential effect of IDE. However, no correlations were found between these differences for speckle-tracking echocardiography-derived parameters with those in loading conditions, arguing for an absence of a role of cardiac preload and afterload on the cardioprotective effect of IDE.

Speckle-tracking echocardiography allows quantifying the regional function of the stunned myocardium and is able to objectively detect segmental myocardial ischemia.22,29,40 Longitudinal function is mainly set by vertically oriented subendocardial fibers. The mid myocardial fibers run circumferentially and most significantly contribute to circumferential function while the subepicardial fibers, longitudinally oriented in an oblique counterclockwise direction compared with the subendocardial layer, are the predominant source of torsional motion. 41 Owing to its greater rotational radius, the epicardial layer dominates indeed the overall direction of rotation. A substantial body of evidence shows that subendocardial fibers are more sensitive to hemodynamic variations and acute ischemia than mid and subepicardial ones. [START_REF] Algranati | Why is the subendocardium more vulnerable to ischemia? A new paradigm[END_REF][START_REF] Toyota | Dynamics of flow velocities in endocardial and epicardial coronary arterioles[END_REF] In this context, it was not surprising that IDE attenuated but did not totally repress the HD-induced decline in GLS, while it preserved CS and improved twist mechanics. Considering that our speckle tracking echocardiography-derived parameters were controlled for loading conditions, our results obtained in stable individuals with ESKD free from severe epicardial coronary artery disease strongly support the possibility of an IDE cardioprotection through a reduction in HD-induced microvascular perfusion abnormalities. Mechanisms involving exercise-induced ischemic/nonischemic preconditioning44 or changes in wholeblood viscosity45 might be advanced to explain such an improvement in tissue perfusion. Future studies will be needed to determine the role of tissue perfusion in IDE-induced cardioprotection and to precise its underlying determinants.

Clinical Implication

GLS is currently the most widely used deformation imaging parameter in clinical practice. It provides independent and incremental prognostic information regarding long-term risk of cardiovascular events and mortality.46,47 The attenuation of GLS decline from T0 to T2 in HDEX compared with HD (estimated difference 21.16%, 95% CI, 20.31 to 22.02) gives preliminary evidence of cardiac protection provided by IDE. This finding may be clinically relevant because a reduction in GLS is associated with worse outcomes and predictive of poor prognosis among stable HD patients with preserved ejection fraction.48 Furthermore, in chronic kidney disease individuals with successful kidney transplant, the hazard ratio for cardiovascular events or death after adjusting for age, sex, and race ethnicity increased by 28% per 1% decrement in the absolute value of GLS. In patients with various cardiovascular risk factors or presenting with cardiovascular diseases, as seen in people with ESKD, each 1% decrease in GLS magnitude was also associated with an 11.3% increase in cardiovascular mortality risk. [START_REF] Medvedofsky | 2D and 3D echocardiographyderived indices of left ventricular function and shape[END_REF] The prognostic value of torsional mechanics indexes in the setting of kidney disease is unknown. Whether the transient improvement in myocardial mechanics conferred by IDE when repeated on each dialysis translates into improved clinical outcomes remains to be determined.

Limitations

Our study had limitations. Exercise intensity during IDE was based on the rate of perceived exertion rather than on the results of a previous triangular maximal cardiopulmonary exercise test. Not all the participants were, therefore, exercising at strictly the same intensity within the moderate-intensity zone expected. However, the rate of perceived exertion scores correlate relatively well with physiological measures of stress and arousal (e.g., HR, ventilatory thresholds).18 In addition, the prescription of exercise intensity on the basis of the perceived exertion rate is well accepted by people with ESKD and broadens the applicability of the protocol. Our very restrictive exclusion criteria (especially ejection fraction ,45%, severe heart disease and body mass index .35) helped control confounding factors, such as severe obesity or disease respiratory disorders, and minimize any potential cardiovascular risk from moderate exercise. Patients with orthopaedic complications affecting the lower limb, for whom completion of IDE was compromised, were also excluded. We recognize that these restrictions obviously affected the generalizability of our results. Only 28% of the patients were women. This gender bias constitutes a limitation regarding gender generalization of the results. A significant proportion of participants declined to participate in the study, so our findings cannot necessarily be generalized to all individuals on dialysis. However, most refused not because of IDE but because of repetitive echo measurements throughout dialysis. We believe, therefore, that the adoption of IDE in the dialysis population is not disputed. The intervention was delivered after the long interdialytic interval where patients tended to be more fluid overloaded and have higher cardiac events. Whether the effect of IDE on myocardial mechanics would have been different if applied after the short interdialytic interval is unknown. Only a continuous exercise of moderate intensity was applied in this study, and it remains unknown whether higher intensities on a continuous or interval mode would have produced greater benefits on cardiac function. Future work should attempt to investigate the optimum mode (continuous versus interval), intensity, timing, and duration of exercise to induce the greatest benefits on LV myocardial mechanics. Finally, we were unable to assess long-term outcomes because this was beyond the scope of our study. Future studies are needed to determine the cardiovascular effect of IDE when applied chronically and its effect on cardiovascular events and mortality in individuals with ESKD. This pilot study is the first to identify the beneficial effects of IDE on LV multidirectional deformation and torsional mechanics during dialysis in people with stable ESKD. Such cardioprotection conferred by IDE has significant clinical relevance because recurrent LV transient dysfunction imposed by repetitive HD can translate in the long term into overt cardiac dysfunction and an increased risk of cardiac events and mortality.

  59% per time point (95% confidence interval [95% CI], 20.15 to 21.01). Conducting the same statistical analysis with time in hours (T0 = 0 hour,T1 = 1.5 hours, T25HD duration 20.5 hour), the reduction in GLS decline in HDEX compared with HD was of 20.33% per hour of dialysis (95% CI, 20.08 to 20.57, condition by time interaction P = 0.009). A subsequent analysis at each time point (i.e., fromT0 to T2 and T0 to T1) reveals that the beneficial effect of HDEX on GLS change from baseline was more important at T2 (estimated difference 21.16%, 95% CI, 20.31 to 22.02, P=0.008) than at T1 (estimated difference 20.55%, 95% CI, 0.30 to 21.46, P = 0.20).

ACKNOWLEDGMENTS

We thank Axelife and the PACA region to support the scholarship of M. Josse. The authors express our sincere thanks to the volunteers involved in the study as well as the technical, administrative, and medical staff of AIDER Santé and ATIR Avignon.

FUNDING

This study was supported by the "Société Française de Cardiologie," grant number SFC21EXCRO.

Data curation:

Antoine Grandperrin, Matthieu Josse, Claire Maufrais, Stéphane Nottin, Philippe Obert.

Formal analysis:

Matthieu Josse, Claire Maufrais, Philippe Obert.

DATA SHARING STATEMENT

Anonymized data reported in this paper of type Analyzable Data have been deposited to Fig share ,10.6084/m9.figshare.21960083. The raw data supporting the conclusions of this article will be made available after publication by the authors, without undue reservation.

Conflict of interest statement

AUTHOR CONTRIBUTIONS

Conceptualization:

Claire Maufrais, Philippe Obert.

Funding acquisition:

Antoine Grandperrin, Matthieu Josse, Claire Maufrais, Philippe Obert.

Investigation:

Matthieu Josse, ClaireMaufrais, Stéphane Nottin, Philippe Obert.

Methodology:

Claire Maufrais, Philippe Obert.

Project administration:

Myriam Isnard, Matthieu Josse, Claire Maufrais, Philippe Obert, Laure Patrier, Cécile Turc-Baron.

Resources:

Jean-Paul Cristol, Myriam Isnard, Stéphane Mandigout, Claire Maufrais, Philippe Obert, Laure Patrier, Cécile Turc-Baron.

Software:

Claire Maufrais, Philippe Obert.

Supervision:

Jean-Paul Cristol, Myriam Isnard, Claire Maufrais, Philippe Obert, Laure Patrier.

Validation:

Jean-Paul Cristol,Myriam Isnard, ClaireMaufrais, PhilippeObert, Laure Patrier, Cécile Turc-Baron.

Visualization:

Claire Maufrais, Philippe Obert, Cécile Turc-Baron.

Writingoriginal draft:

Matthieu Josse, Claire Maufrais, Philippe Obert.

Writingreview & editing:

Matthieu Josse, Claire Maufrais, Philippe Obert.


Geological context from Dragonfly for determining Titan's habitability potential

Shannon M. MacKenzie 1 , Morgan L. Cable 2 , Jorge I. Núñez 1 , Alice Le Gall 3 , Elizabeth P. Turtle 1 , Melissa G.

Trainer 4 , Ralph D. Lorenz 1 , Jason W. Barnes 5 , Scott Murchie 1 , and the Dragonfly Team, 1 Johns Hopkins Applied Physics Laboratory, Laurel, MD (shannon.mackenzie@jhuapl.edu) 2 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, 3 LATMOS/IPSL, UVSQ Université Paris-Saclay, 4 NASA Goddard Space Flight Center, Greenbelt, MD, 5 Univ. Idaho, Moscow, ID Cassini-Huygens found that photochemistry in Titan's atmosphere produces haze particles that range from simple hydrocarbons like methane, ethane, and benzene (e.g. Vuitton et al., 2007;Cui et al. 2009) to more complex ion species up to 10,000 Da/q (Coates et al., 2007(Coates et al., , 2009)). Products that make their way to Titan's surface can then be reworked and redistributed by a variety of Earth-like physical and chemical processes. Titan is thus a unique, natural laboratory for studying prebiotic chemistry similar to that of early Earth (Trainer et al. 2006), potentially habitable niches in transient liquid-water environments (Neish et al. 2018), and potential habitable conditions for hydrocarbonbased life (McKay & Smith 2005).

Dragonfly is a mission concept selected for Phase A study designed that would determine the chemical composition of a variety of surface materials at different geological settings (Lorenz et al. 2018;Turtle et al. 2019). It is a rotorcraft lander equipped with a mass spectrometer (DraMS), a gamma-ray neutron spectrometer (DraGNS), a camera suite (DragonCam), and a meteorology/geophysics package (DraGMet). Due to the diversity of modification processes on Titan, geological context will be key to informing the astrobiological and/or habitability significance of the compositional analyses.

Reworking: On Titan's surface, transient liquidwater environments could facilitate aqueous chemistry and production of biologically relevant molecules like amino acids (e.g. Neish et al. 2008). Identifying whether a water-ice-rich surface sample is a cryovolcanic deposit or impact melt will require geological interpretation of images captured by DragonCam. Bulk elemental composition measurements by DraGNS will also provide information for discerning material history.

Redistribution:

The rates at which materials are deposited into and removed from an environment directly affect the habitability of the environment.

Across the surface: Wind and liquids redistribute sediment on Titan. Dunes formed by saltating organic sands dominate the equatorial region of Titan (e.g. Barnes et al. 2008, Le Gall et al., 2011). Like Curiosity (Bridges et al. 2017), Dragonfly will conduct passive monitoring for grain movement. However, since Dragonfly has the capability of creating its own wind, initiation of saltation induced by controlled rotor activation will also be monitored with DragonCam. This is especially important at Titan where winds above the saltation threshold may not be common at the time of year when Dragonfly arrives.

DragonCam is also equipped with an LED array for illumination at UV-visible wavelengths. Such spectral data provide microtextural, compositional context, creating a bridge between the sample-scale measurements of DraMS to lander-and landing-site-scale images taken by other DragonCam cameras on the surface and in flight.

In the near subsurface: Several lines of evidence from Cassini-Huygens (e.g. Lorenz et al. 2006;Turtle et al. 2018) suggest the presence of near-subsurface liquid hydrocarbons. Such a reservoir has an important effect on the methane cycle (e.g. Mitchell & Lora 2016). DraGMet will monitor the near surface methane humidity and constrain the porosity of the surface. Downward: It is not known whether organics from Titan's surface are transported to the subsurface ocean, providing an influx of organics. Some of the uncertainty lies in the poorly constrained thickness of Titan's crust (50-150km). Dragonfly will monitor ground motion with two skid-mounted geophones and a deployable seismometer (Lorenz et al. 2019); the direction and amplitude of observed waveforms provide information about the level of seismic activity and the ice crust thickness (e.g. Vance et al. 2018).


Frédéric Magnin
email: frederic.magnin@imbe.fr

A new Vitrea species from the Holocene of south-eastern France (Gastropoda: Eupulmonata: Pristilomatidae)

Keywords: Provence, Mediterranean, Quaternary, Zonitidae, biogeography, new species

Vitrea luberonensis n. sp. is described from south-eastern France. Shell attributes suggest that the new species is closely related to V. pygmaea, V. etrusca, and V. inae, in a "V. etrusca complex" which is characterised by very small size (the smallest of all Vitrea species) and a very wide umbilicus. Holocene subfossil shells were found in Manosque and Aix-en-Provence, in alluvial deposits between 7400 and 4200 BP. The species has not yet been found alive or in modern sediments. Associated malacofauna suggest a mosaic of riparian and open woodland habitats.

Introduction

Until now, 87 species have been assigned to the genus Vitrea (MolluscaBase 2023), of which 68 are extant in the European fauna (Welter-Schultes 2012, Georgiev & Dedov 2014, Pall-Gergely & Asami 2015). They are conchologically very diverse, although roughly similar to one another in their anatomy, except for the inner structure of the penis (Riedel 1992). Seven species are currently known from France, including 1 limited to Corsica (INPN 2023). I describe here a new species which is presently found only in Holocene deposits in Provence. Its shell resembles V. pygmaea (O. Boettger, 1880), Vi trea etrusca (Paulucci, 1878), and Vitrea inae de Winter & Ripken, 1991 in its very small size together with a large umbilicus. These characters differentiate them unambiguously not only from other living species but also from Pliocene fossil species, such as V. geisserti Schlickum, 1975 (Burgundy, Rhine region) and V. fau stinae (Sacco, 1884) (Piedmont), which exhibit a covered or minute umbilicus. For example, the latter species is extremely similar to the modern V. subrimata (Reinhardt, 1871) (Giusti & Mazzini 1971).

The shells of the new species were discovered during research on palaeoenvironments and Holocene land-snail communities in Provence (Ollivier et al. 2015, Magnin et al. 2022, in preparation). They come from 2 sedimentary sequences located in Manosque (Alpes-de-Haute-Provence) and Aix-en-Provence (Bouches-du-Rhône).

Materials and Methods

Samples of sediment, each 10 L, were collected in the different stratigraphic levels. The shells were extracted by the standard method (Evans 1972, Puisségur 1976), that is by water-sieving on a 500 μm mesh sieve. To facilitate sieving, the sediment was sometimes dispersed by adding 35% hydrogen peroxide. The deposits were radiocarbon dated on charcoals contained in the samples. A few archaeological artefacts were present in some layers. These make it possible to place the succession of malacological assemblages in a cultural framework. A total of 79 shells of the new Vitrea species were collected in Manosque but only 2 in Aix-en-Provence. Many are immature or incomplete due to their fragility. The photographs were taken using a Euromex Stereoblue stereoscopic microscope and a ToupTek UCMOS USB2.0 5-MP camera. The measurements were made with the same microscope equipped with an ocular micrometre. I made 10 measurements (in mm) on each of the bestpreserved specimens with at least 3 whorls. I measured 6 V. pygmaea specimens (including 3 from reliable published figures), 20 V. etrusca, and 26 V. luberonensis n. sp. from the type locality. The discriminant analysis (linear, bivariate model, fitted with ordinary least squares) was performed using the software PAST (PAleontological STatistics) v. 3.2 (Hammer et al. 2001).

Materials used for comparison. Vitrea pygmaea: Ai Petri, Yalta, Crimea, in forest, 44.465° N, 034.064° E, May


Cai Chen
email: cai.chen@ulb.be

Pascale Bugnon

WeChat and the Chinese Queer Diaspora

Keywords: WeChat, Chinese diaspora, queer Chinese, virtual ethnography

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

1 WeChat, functioning both as a digital media platform and an infrastructure [START_REF] Plantin | WeChat as infrastructure: The techno-nationalist shaping of Chinese digital platforms[END_REF], transcends national borders and has gained widespread usage among Chinese diasporas (Sun & Yu, 2022), including the queer1 community. Chinese queer individuals, whether sojourners (e.g., international students) or settled residents in Western countries (e.g., immigrants or long-term residents), may have different migration motives and pathways, but they share a common intrinsic migration aspiration. That is, the yearning for sexual freedom (as discussed in [START_REF] Kam | Coming out and going abroad: The chuguo mobility of queer women in China[END_REF]Ponce & Chen, 2023 among others). Despite the relatively liberal and tolerant societal environment in Western countries, the Chinese queer diaspora in countries like France has been confronted not only with the marginalisation as a racial minority within predominantly white societies-like their heterosexual counterparts (see [START_REF] Chuang | Une minorité modèle ?: Chinois de France et racisme anti-Asiatiques[END_REF][START_REF] Wang | L'expérience du racisme et des discriminations des personnes originaires d'Asie de l'Est et du Sud-Est en France (REACTAsie)[END_REF])-but also with the multi-layered discrimination as queer migrants of colour (see C. [START_REF] Chen | Racisme sexuel: Quand les migrants gays asiatiques font face à la fétichisation. The Conversation (France)[END_REF]. As social norms and cultural values migrate with individuals, Chinese queer migrants find themselves in a dual predicament. On the one hand, they embrace the freedom to be "truly themselves" within the liberal French society, and on the other hand, many continue to conceal their homosexuality from their families in China or their immediate social networks within the Chinese diaspora community (C. Chen, 2023a). This dual minority status places Chinese queer migrants in a unique position, and WeChat plays a pivotal role in their "inbetween" social lives.

My prior research drew on an extensive "virtual ethnography" [START_REF] Hine | Virtual ethnography[END_REF]-with WeChat ethnography as an important component-to research the intricate interplay between migration and sexuality among Chinese gay (ex-)students in France (see C. [START_REF] Chen | Sexuality and Mobility: The Trajectories of Gay Students from China to France and beyond[END_REF]. Within this research context, WeChat, as an assemblage of group chats, subscription accounts, and Moments (pengyou quan), emerged as the primary site of observation. In conducting participant observations, I actively engaged with five group chats dedicated to Chinese queer migrants-encompassing more than just gay men-in France and across Europe. Additionally, I followed the subscription account of Collectif Sésame F (zhima she), an association based in Paris, and kept track of the Moments updates from key informants. Notably, Chinese diasporic LGBT+ associations in Europe2 favour WeChat's subscription account, despite the harsh censorship (see [START_REF] Ni | Outrage over shutdown of LGBTQ WeChat accounts in China[END_REF], as their primary communication channel with members and their target audience to facilitate the campaigning for the events that they organise (e.g., participating in the local Gay Pride and online events amid the Covid-19 pandemic).

These grassroots associations play a pivotal role in forming discussion groups for Chinese queer migrants, facilitating connections among individuals from the same homeland who share common cultural backgrounds and face similar social conditions as migrants and sexual minorities. Within these groups which constitute both an ethnic niche and a safe queer space, members can seek or provide both informational and emotional support, enhancing their sense of belonging and solidarity (as demonstrated in Figure 1). Unlike local associations, Chinese diasporic LGBT+ associations predominantly emphasize cultural activities over sexual health promotion. Nevertheless, WeChat serves as a transnational virtual space for fostering queer sociality and solidarity, nurturing cross-border digital queer activism [START_REF] Ayoub | Migration and queer mobilisations: How migration facilitates cross-border LGBTQ activism[END_REF], albeit on a limited scale, among (aspirant) migrants, returnees, and Chinese queer activists both within and beyond China.

Being part of the multiple sites of virtual ethnography, the use of WeChat as a research tool and an observation site presents both advantages and challenges. WeChat enables researchers to transcend the constraints of time and space, granting access to the inner social world of the Chinese queer diaspora and providing insights into their ways of being and doing queer in an ethnic virtual space. However, it's crucial to recognize that WeChat represents just one facet of the social lives of Chinese LGBT+ migrants. These individuals frequently utilize other digital platforms (see also [START_REF] Yu | Tongzhi on the move: Digital/social media and placemaking practices among young gay Chinese in Australia[END_REF], including Grindr and Romeo for online dating, X (formerly Twitter) for homoerotic content (see [START_REF] Song | Desiring Wanghuang: Live Streaming, Porn Consumption and Acts of Citizenship among Gay Men in Digital China[END_REF], and forums for sociality and information (particularly during the 2000s and 2010s). In light of this multifaceted digital landscape, adopting a "multi-sited" perspective [START_REF] Marcus | Ethnography in/of the world system: The emergence of multi-sited ethnography[END_REF] becomes essential to comprehensively understand the diverse identities and complex lived experiences of the Chinese queer diaspora.

Research participants, such as queer migrants, incorporate multiple identities and display various social roles within a single application, WeChat. This digital platform serves not only as a barrier for concealing one's "true" identity but also as a bridge connecting online and offline social spaces. Ethnographers utilizing WeChat must therefore possess a critical understanding of the offline context surrounding the online world, the nodes and connections that bridge online and offline realms, and constantly switch between these two interacting settings through self-adjustment (see [START_REF] Zani | Shall WeChat? Switching between online and offline ethnography[END_REF]. Furthermore, researchers must prioritize ethical considerations to protect the privacy and confidentiality of research participants. In my study, I employed covert observation to prevent any undue influence on the atmosphere within the discussion groups. However, I ensured that at least the group administrators were aware of the researcher's identity and the purpose of the project. Moreover, I emphasize the importance of exercising caution and compassion when collecting and presenting data from the social space of marginalized and vulnerable social groups. I chose to employ self-made mock-ups (as presented above) rather than screenshots of group chats to fully respect the anonymity of all participants. Any photographs used in my publications or presentations related to this research were authorized by their owners and subjected to complete anonymization prior to use. Lastly, I recommend that researchers maintain an appropriate social distance from their informants and continually negotiate their own identity, subjectivity, emotionality, and positionality, even within the context of online environments.

Figure 1 :

 1 Figure 1: WeChat discussion group for gay Chinese in France (screenshot mock-up by Cai Chen with anonymized data).

The term 'queer' is employed here as an academic shorthand rather than an identity label. It encompasses all non-normative gender identities and sexualities and can be used in this article interchangeably with LGBT+ (lesbian, gay, bisexual, transgender, and other).

For instance, aforementioned Collectif Sésame F (Paris), Happy Togayther (Paris), Queer China UK (London), Out&Abroad (Amsterdam), Queer Squad (Frankfurt am Main), and Shanfeng Dianhuo (Madrid).


With 𝑓 e sh, pred.

Corrected potential drop prediction

▪

  Wall immersed inside simulation domain ▪ No boundary condition @ plasma-wall transition ▪ Decorrelate sheath physics from numerical BC Yann Munschy -No known closure leads to correct prediction of heat flux ▪ Comparison of kinetic simulation of plasma-wall interaction with fluid theory ▪ Trajectories of particles in plasma highlight kinetic sheath physics at play : o Acceleration of ions, reflection of slow electrons o Collisions ▪ Reduced fluid model fails to predict plasma behavior : o Sound speed critically depends on closure : Bohm's criterion is unoperational o Vanishing heat flux closure not relevant o Validity of other closures can be questioned : isothermal, zero collisional flux, high order moments… o Sheath heat transmission factor in good agreement with predictions (6% difference)ConclusionKinetic plasma-sheath self organization[Munschy et al. 2023] Kinetic plasma-wall interaction using immersed boundary conditions[Munschy et al. 2023] Yann Munschy -EFTC 2023 Sheath heat transmission factor: good agreement with predictions 9

▪

  coefficients : conducting wall Charge conservation from Boltzmann eq. 2. Adaptive coefficients : insulating wall → currents in wall 𝜕 𝑡 𝜌 + 𝜕 𝑥 𝑗 tot = -{𝜈 iw 𝑛 i -𝑛 w } -𝜈 ew 𝑛 e -𝑛 w 𝑗 e + 𝑗 i 𝜈 iw = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 𝜈 ew 𝑥, 𝑡 = 𝜈 iw 𝑛 i -𝑛 w 𝑛 e -𝑛 w Rhs. vanishing → no net current in wall • Robust physics : almost identical potential drop in the sheath in both cases : focus on the conducting case in the following Phase space characteristics for electrons in the whole plasma ▪ Particle source fraction injected in passing region : 1% for a typical value of sheath potential drop Without collisions trapped particles stay trapped while passing are absorbed in the wall ▪ Collisions : necessary transfer mechanism between trapped and passing, and necessary condition to reach steady state (the particle source must be compensated by the sink in the wall

(•▪

  Filled by collisions in 1D, Electric field as well in higher dimensions) Electron reemission : finite steepness of penalization mask & absorption of electrons in wall not perfect Linear relation 𝚫𝝓 𝒔𝒉 ∝ 𝑻 𝒆 𝒔𝒉 recovered • The faster the electrons, the greater the potential drop • At a point of the simulation / plasma boundary • Count the number of ions going out of the plasma • Absorb an equal number of the fastest electrons • Reflect the other electrons back the plasma → Equality of ion and electron fluxes at plasma boundary → Equality of fluxes defines the electron threshold velocity ➢ Conducting sheath • define electron threshold velocity is defined with the electric potential • Absorb all outcoming ions, and electrons faster than 𝑣 𝑐 • No direct constraint on electron and ion fluxes • Idea : the potential at plasma boundary reacts to the current Backward semi Lagrangian scheme for Boltzmann eq. ▪ Cubic spline interpolation on non equidistant mesh o Test bench for C++ version of GYSELA (5D gyrokinetic code)

2023 Using Stangeby's prediction of the sheath potential drop to explain parametric dependencies

									𝑓 𝑒
	Δ𝜙 sh pred. =	𝑇 e sh 𝑒	log	2𝜋	𝑚 e 𝑚 i	𝑀 2 1 +	sh 𝑇 i 𝑇 e sh			Γ e +
	Correction term 1:					-Γ e
	• electron reemission from the wall due to finite
	penalization strength
							Δ𝜙 𝑠ℎ 𝑝𝑟𝑒𝑑. =	෩ 𝑻 𝒆 𝒔𝒉 𝑒	log	2𝜋	𝑚 𝑒 𝑚 𝑖	𝑀 2 1 +	𝑇 𝑖 𝑠ℎ ෩ 𝑻 𝒆 𝒔𝒉	1 +	-Γ 𝑒 Γ 𝑖

truncated maxwellian of temperature 𝑇 e sh 9/28/2023 Yann Munschy -EFTC 𝑠ℎ Correction term 2: • Effective temperature to match forward flux to predicted Maxwellian flux Potential drop prediction [Stangeby]

  Intra species: 𝐶 𝑎𝑎 𝑓 𝑎 = 𝜕 𝑣 𝑎 𝐷 𝑣 𝑓 𝑀𝑎 𝜕 𝑣 𝑎 Conservation of density:  𝑑𝑣 𝐶 𝑎𝑎 𝑓 𝑎 = 0 Conservation of momentum:  𝑑𝑣 𝑣 𝑎 𝐶 𝑎𝑎 𝑓 𝑎 = 0 Conservation of energy:  𝑑𝑣 𝑣 𝑎 𝐶 𝑎𝑎 𝑓 𝑎 = 0 Gives 𝑇 𝑀𝑎 𝑥, 𝑡 and 𝑢 𝑀𝑎 (𝑥, 𝑡) 𝑅 𝑎𝑏 𝑀 = -𝑛 𝑎 𝜈 𝑎𝑏 𝑢 𝑎 -

	Collisions
							𝑓 𝑎 𝑓 𝑀𝑎	; 𝑓 𝑀𝑎 =	𝑛 𝑎 2𝜋𝑇 𝑀𝑎	exp	1 2𝑇 𝑀𝑎	𝑎 -𝑢 𝑀𝑎	2
	[Dif-Pradalier2011]
	Inter species: 𝐶 𝑎𝑏 𝑓 𝑎 =	2𝑄 𝑎𝑏 𝑀 𝑛 𝑎 𝑇 𝑎	𝑚 𝑎 2𝑇 𝑎	𝑣 𝑎 -𝑉 𝑎	2 -	1 2	𝐹 𝑀𝑎 +	𝑀 𝑅 𝑎𝑏 𝑛 𝑎 𝑇 𝑎	𝑣 𝑎 -𝑉 𝑎 𝐹 𝑀𝑎
	[Esteve2015]
				𝐴 𝑏 𝐴 𝑎	𝑢 𝑏 friction force (momentum transfer)
	𝑄 𝑎𝑏 𝑀 = -3𝑛 𝑎	𝐴 𝑏 𝐴 𝑏 +𝐴 𝑎	𝜈 𝑎𝑏 𝑇 𝑎 -𝑇 𝑏 -𝑢 𝑎 𝑅 𝑎𝑏 𝑀 energy transfer
	Yann Munschy -EFTC 2023									9/28/2023