Emmanuel Podestá
email: emmanuel.podesta@posgrad.ufsc.br

Pedro Henrique Penna
email: pedro.penna@sga.pucminas.br

João Fellipe Uller

Márcio Castro
email: marcio.castro@ufsc.br

A Trace-Driven Methodology to Evaluate and Optimize Memory Management Services of Distributed Operating Systems for Lightweight Manycores

Keywords: Computer systems organization → System on a chip, Very long instruction word, Multiple instruction, multiple data, Parallel architectures, Trace-Driven Methodology, Distributed Operating System, Memory Management, Software-managed Cache, Lightweight Manycores

Lightweight manycores belong to a new class of emerging lowpower processors for the Exascale era. These processors present several challenges for the development of applications, such as distributed memory architecture, limited amount of on-chip memory and no cache coherence. Recently, distributed Operating Systems (OSs) have been proposed to address these challenges in a transparent way. In these systems, different OS services are deployed across the processor cores, being the memory management service one of the most important ones. However, the intrinsic characteristics and memory limitations of lightweight manycores bring several challenges to the design, implementation and future optimizations of memory management services. In this work, we propose a tracedriven methodology to evaluate and optimize features of a memory management service of distributed OSs for lightweight manycores. By using a compact representation of the page access pattern of the applications, our methodology is capable of mimicking the memory access pattern of the original applications on the target distributed OS running on a lightweight manycore. We integrated our methodology in a distributed OS (Nanvix) and validated it using three applications from a specific benchmark for lightweight manycores (CAP Bench). Then, we applied our methodology to carry out a case study using a software-managed cache implementation available in Nanvix. Our methodology enabled us to evaluate different page replacement policies on Kalray MPPA-256, even without the required support from the architecture to implement them.

INTRODUCTION

Historically, hardware architects were able to design faster and more advanced uniprocessor systems by increasing the number of transistors per chip and scaling the clock frequency. In addition, they implemented features such as speculative execution, instruction-level parallelism, out-of-order execution and larger caches, which contributed to improve single-threaded performance. Unfortunately, these aggressive strategies led processors to hit a new constraint: the power wall [START_REF] Villa | Scaling the Power Wall: A Path to Exascale[END_REF]. As a response to this challenge, the industry transitioned toward Chip Multiprocessor (CMP) designs, which feature more than one processor core to increase the aggregate performance of the chip.

More recently, the increase in performance and parallelism in a single chip has been taken to a next level with the emergence of a new class of highly-parallel processors named lightweight manycores [START_REF] Borkar | Thousand core chips[END_REF]. These processors integrate hundreds of low-power cores on a single chip, which allow to exploit both data and task parallelism, and have been proven to achieve better energy efficiency than CMPs [START_REF] Dupont De Dinechin | A Clustered Manycore Processor Architecture for Embedded and Accelerated Applications[END_REF]. In this design, cores are grouped into clusters, each one having its own limited local memory (a.k.a scratchpad memory) and address space, thus resulting in a distributed memory design. Clusters are interconnected by one or more Networks-on-Chip (NoCs) and communications between them take place through message exchanges. Examples of such processors are the Sunway SW26010 [START_REF] Fu | The Sunway TaihuLight Supercomputer: System and Applications[END_REF], which is the base processor of the Sunway Taihu-Light supercomputer (featuring 10.6 million low-power cores in total, Adapteva Epiphany [START_REF] Olofsson | Kickstarting High-performance Energy-efficient Manycore Architectures with Epiphany[END_REF] and Kalray MPPA-256 [START_REF] Dupont De Dinechin | A Distributed Run-Time Environment for the Kalray MPPA-256 Integrated Manycore Processor[END_REF].

One of the main problems that hinders the adoption of lightweight manycores by the industry is the lack of Operating System (OS) support. Currently, software engineers have to explicitly deal with data tiling, data prefetching and low-level communication abstractions to extract reasonable performance out of a lightweight manycore processor [START_REF] Varghese | Programming the Adapteva Epiphany 64-Core Network-on-Chip Coprocessor[END_REF]. Moreover, communications must take into account the NoC topology whenever possible to improve bandwidth and reduce latency.

Recently, distributed OSs were proposed to address these challenges in a transparent way [START_REF] Boyd-Wickizer | Corey: An Operating System for Many Cores[END_REF][START_REF] Simon | nOS: A Nano-Sized Distributed Operating System for Many-Core Embedded Systems[END_REF]. The multikernel is one of the distributed OS designs that has shown promising results. In this approach, multiple independent OS kernel instances are deployed on the lightweight manycore processor. Each kernel provides bareminimum abstractions and fully-featured system services are implemented in a distributed fashion. Furthermore, the distributed architecture brings: (i) better scalability; (ii) hardware-neutral characteristics, enabling portability between different architectures; and (iii) explicit inter-core communication, which offers more room for optimizations and efficient use of the processor network.

Among the services offered by distributed OSs for lightweight manycores, the memory management is one of the most important ones. This service overcomes most programming intricacies of a distributed memory architecture, and it must exploit the NoC in order to achieve decent performance. However, designing and implementing a memory management subsystem that takes into account the intrinsic characteristics and memory limitations of lightweight manycores is still an open problem, paving the way for different performance optimizations. For instance, part of the local memory of clusters can be used as a software-managed cache to store recently used pages. Moreover, a software prefetching mechanism can be adopted to bring pages from the main memory to the local memory of clusters in advance to hide NoC communication costs. Towards these optimizations, a possible approach would be to port several applications of different domains to the target distributed OS and carry out experiments with them. However, actually porting applications to distributed OSs that target lightweight manycores is a time-consuming and error-prone task.

Thus, to help on these analyses and relieve developers from the burden of porting several applications to the target distributed OS, in this paper we propose a trace-driven methodology that can be used to evaluate and optimize features of a memory management service of distributed OSs for lightweight manycores. In our methodology, applications are first executed in a standard Linux environment and their memory accesses are traced. Then, the traced information is transformed into compacted structures called heatmaps, which represent the memory access pattern of the applications. Finally, the heatmaps are used by a proxy application in the target distributed OS running on the lightweight manycore processor to mimic the memory access pattern of the original applications. In summary, this work delivers the following new contributions to the state of the art on the evaluation of memory management services of distributed OSs for lightweight manycores:

(i) a new trace-driven methodology that helps distributed OS developers to evaluate and optimize features of a memory management services without the need of porting applications to the target distributed OS;

(ii) an integration of the proposed methodology in Nanvix, an open-source distributed OS that targets lightweight manycores [START_REF] Penna | On the Performance and Isolation of Asymmetric Microkernel Design for Lightweight Manycores[END_REF]; and (iii) a case study of a software-managed cache evaluation in Nanvix using the proposed methodology.

All experiments with Nanvix were executed on Kalray MPPA-256 [START_REF] Dupont De Dinechin | A Distributed Run-Time Environment for the Kalray MPPA-256 Integrated Manycore Processor[END_REF], a NoC-based lightweight manycore processor that features a distributed memory architecture and integrates 288 cores in a single chip. We show that our methodology is able to correctly represent and mimic the memory access pattern of the original applications. Moreover, we show that the resolution of the heatmap can be fine-tuned to greatly reduce the number of memory accesses performed by the proxy application in the target distributed OS, while keeping the overall memory access pattern behavior of the original applications. This allows for much faster executions, enabling the evaluation of the memory management service in several scenarios in a feasible time. Finally, we show that the methodology can be applied to study the behavior of different page replacement policies of a software-managed cache implementation, even when the underlying lightweight manycore processor does not feature the necessary hardware support to implement them, helping hardware architects to decide whether or not is beneficial to include the necessary hardware support to allow the implementation of smarter page replacement policies in the software-managed cache.

The remainder of this work is organized as follows. Section 2 presents the background on lightweight manycores and distributed OSs. Section 3 describes our trace-driven methodology. Then, Section 4 discusses our evaluation methodology. Finally, Section 5 presents our results, Section 6 covers related works and Section 7 concludes this paper.

BACKGROUND

First, we discuss about lightweight manycores and their main characteristics (Section 2.1). Then, we present an overview of distributed OSs for these processors (Section 2.2).

Lightweight Manycore Processors

Lightweight manycore processors differ from other architectures that also have high core counts, such as Non-Uniform Memory Access (NUMA) platforms, Graphics Processing Units (GPUs) and other accelerators (e.g., Intel Xeon Phi). Their main distinctions are the following: Figure 1 shows an overview of a conceptual lightweight manycore. Some examples of lightweight manycores are the Sunway SW26010 [START_REF] Fu | The Sunway TaihuLight Supercomputer: System and Applications[END_REF] and the Kalray MPPA-256 [START_REF] Dupont De Dinechin | A Distributed Run-Time Environment for the Kalray MPPA-256 Integrated Manycore Processor[END_REF]. The former is a customdesigned processor used in the Sunway TaihuLight, currently the world's fourth most powerful system according to TOP500 1 . The latter, on the other hand, is a commercially available lightweight manycore processor designed by Kalray.

(i)

Distributed Operating Systems

Distributed OSs recently emerged as an alternative approach to address the challenges in software development and deployment in lightweight manycore processors [START_REF] Asmussen | M3: A Hardware/Operating-System Co-Design to Tame Heterogeneous Manycores[END_REF][START_REF] Penna | On the Performance and Isolation of Asymmetric Microkernel Design for Lightweight Manycores[END_REF]. These OSs are composed by a set of services that implement the main abstractions of a traditional OS. They are usually structured in three main layers. In the bottom layer, a Hardware Abstraction Layer (HAL) abstracts the underlying hardware, so as to provide portability across different architectures. In the middle layer, an OS kernel provides resource multiplexing as well as basic OS abstractions, such as processes and threads. Finally, the top layer features several OS services to provide a transparent programming environment for users.

In this paper, we are specially interested in distributed OSs designed specifically for lightweight manycores such as MOSSCA [START_REF] Kluge | An Operating System for Safety-Critical Applications on Manycore Processors[END_REF], M 3 [START_REF] Asmussen | M3: A Hardware/Operating-System Co-Design to Tame Heterogeneous Manycores[END_REF] and Nanvix [START_REF] Penna | On the Performance and Isolation of Asymmetric Microkernel Design for Lightweight Manycores[END_REF]. In these OSs, a microkernel design is employed in the middle layer so as to cope with the low amount of on-chip memory as shown in Figure 1. Usually, a single microkernel instance is deployed in each Compute Cluster (kernel instance) and all other cores of the Compute Cluster are used to run system services (service instance) or user-level applications. We chose Nanvix as our target distributed OS in this work because, to the best of our knowledge, it is currently the only open-source distributed OS that runs on commercially available baremetal lightweight manycores.

In Nanvix, the memory management service manages page allocation and sharing among Compute Clusters. The Distributed Paging System (DPS) is the main component of this service, which provides a shared memory abstraction and exposes a programming interface to manipulate these abstractions. In the next sections, we show how our trace-driven methodology can help distributed OS developers to evaluate and optimize features of this service without the need of porting applications to this distributed OS. 1 Available at: https://www.top500.org

TRACE-DRIVEN METHODOLOGY

Our trace-driven methodology is intended to aid developers to study, implement and optimize features of memory management services of distributed OSs. This methodology relieves developers from the burden of porting several applications to the target distributed OS, which is necessary to evaluate the memory management service under different scenarios.

Figure 2 presents an overview of this methodology. First, applications are compiled and executed in a standard Linux environment along with a memory profiling tool. The profiler collects memory accesses of the target application and dumps this information on trace files (Step 1 ). Then, each trace file is preprocessed (if necessary), so as to obtain a well-formatted output (Step 2 ).

Next, the well-formatted output is used to create a heatmap: a compact two-dimensional structure that describes how many times each memory page (or a set of pages) was accessed during some discrete time frames (Step 3 ). The resolution of the heatmap structure can be fine-tuned to significantly reduce the execution time of the proxy application (Step 4 ), at the cost of losing some details about the memory access pattern of the application. Then, a probabilistic approach is applied by the proxy application, which uses the heatmap structure to mimic the page access pattern of the original application in the target distributed OS running on the lightweight manycore processor (Step 4 ). Finally, the proxy application outputs statistics about the execution. In the next sections, we describe each of these steps in more details.

Trace Collection

This stage is composed of 2 main steps, which are described next.

3.1.1

Step 1: Profiling. This step consists in running the application of interest on a standard Linux OS and collecting information about its memory accesses (Step 1 ). An approach for obtaining such information without any application source code changes is to use a binary instrumentation tool such as Valgrind [START_REF] Nethercote | Valgrind: A framework for heavyweight dynamic binary instrumentation[END_REF] or Intel Pin [START_REF] Bach | Analyzing Parallel Programs with PIN[END_REF]. The output of this step is a raw trace file containing the timestamp and the address of every memory access made by the program.

3.1.2

Step 2: Trace Preprocessing. Some manipulations may be necessary on the raw trace file output in Step 1 to produce a wellformatted trace file that contains only the needed data (Step 2 ). The complexity of these manipulations are related to the output produced by the profiler. There are mainly three manipulations that may be considered in the preprocessing step: (i) segment selection; (ii) address/page translation; and (iii) page number normalization.

The segment selection concerns the selection of user space segments of interest in the raw trace file (stack, heap, BSS, data and/or text). Depending on how applications are implemented, most of the memory accesses of interest may occur in the heap segment (if data is dynamically allocated), in the BSS/data segments (if data is statically allocated) or both. The choice of selecting one or more segments is related to both the application itself and to the access pattern that should be mimicked later on.

Since we are interested in the page access pattern of the application, the address/page translation is necessary only if the events registered by the profiler refer to memory addresses (instead of memory pages). In this case, a trace file manipulation is needed to translate subsequent memory access events on addresses that fall into the same memory page to a single page access event in the output trace file. The size of the memory page should be considered to perform such translation.

Finally, a page number normalization is performed, so as to simplify the allocation and management of these pages by the proxy application (Step 4 ). The proxy application considers that all pages have consecutive numbers.

Page Access Pattern

This stage is composed of 2 main steps, which are described next.

3.2.1

Step 3: Heatmap. The preprocessed trace output in Step 2 is used to produce a heatmap structure in Step 3 . The heatmap is a compact two-dimensional graphical representation of measured values of numerical data using a chosen color scheme, with one end of the color scheme representing the high values and the other end representing the low values [START_REF] Suematsu | A Heatmap-Based Time-Varying Multi-variate Data Visualization Unifying Numeric and Categorical Variables[END_REF]. The variation in color may be by hue or intensity, giving visual insights to the reader about how a phenomenon is clustered or varies over space and time.

We use heatmaps to represent the page access pattern of the application, which show how many times each page (or a group of pages) is accessed during specific discrete time periods (the darker the color of the cell of the heatmap, the higher the number of page accesses). The 𝑥-axis represents a temporal behavior (timestamps of events), whereas the 𝑦-axis represents a space behavior (pages or groups of pages). The resolution of the heatmap can be fine-tuned by grouping several events into bins in 𝑥 and/or 𝑦 axes. Thus, a maximum resolution is achieved if the heatmap uses individual events. The higher is the number of events grouped into bins, the lower will be the heatmap resolution, resulting in a less accurate page access pattern of the application. By adjusting the resolution of the heatmap we can significantly reduce the time spent on mimicking the memory access pattern of the application on the target distributed OS running on the lightweight manycore processor as well as the amount of memory footprint required to store the page access pattern ś recall that this is an important constraint for lightweight manycores.

3.2.2

Step 4: Proxy Application. Finally, a proxy application mimics the page access pattern of the original profiled application on the distributed OS running on target lightweight manycore processor (Step 4 ). It takes as input a heatmap, which was obtained in Step 3 , and a trial factor (𝑡 𝑓 ). The heatmap is seen by the proxy application as a set of histograms (each time bin in the 𝑥-axis is actually a histogram of page accesses) and it uses a probabilistic approach to generate a variable number of random memory page accesses (trials) in each time bin based on its frequency distribution. The number of trials performed by the proxy application in each time bin 𝑥 corresponds to 𝑡𝑟𝑖𝑎𝑙𝑠 𝑥 = number of events in the original heatmap in 𝑥 𝑡 𝑓 .

Thus, the higher is the trial factor, the lower will be the number of page accesses (trials) generated by the proxy application. The value of 𝑡 𝑓 impacts on the overall behavior of the memory access pattern being reproduced: the more random values are generated, the closer will be the behavior produced by the proxy application compared to the original execution. However, it is possible to reduce considerably the number of random page accesses without losing the original access pattern, as we show in Section 5.1. This allows us to improve considerably the execution time of the proxy application as well as to deal with very large heatmaps.

EVALUATION METHODOLOGY

In this section, we discuss the evaluation methodology to validate our trace-driven methodology. First, we present the applications considered in this study. Then, we describe our experiment environments in more details. Finally, we discuss the experimental design and methods applied in this paper.

Applications

We selected three applications from CAP Bench [START_REF] Matheus | CAP Bench: A Benchmark Suite for Performance and Energy Evaluation of Low-power Many-core Processors[END_REF] that feature different memory access patterns. CAP Bench is an open-source benchmark suite that includes applications suitable to evaluate emerging lightweight manycore processors such as Kalray MPPA-256. These applications allow us to validate our trace-driven methodology as well as to show its potential to evaluate and optimize the memory management service of Nanvix.

In the following paragraphs we give a brief overview of these applications. In-depth descriptions of these applications can be found in [START_REF] Matheus | CAP Bench: A Benchmark Suite for Performance and Energy Evaluation of Low-power Many-core Processors[END_REF].

Friendly Numbers (FN) This application computes the amount of friendly numbers in a range [𝑚, 𝑛]. In number theory, two natural numbers are friendly if they share the same abundancy. In turn, the abundancy 𝐴 of a given number 𝑛 is defined as

𝐴(𝑛) = 𝜎 (𝑛)
𝑛 , where 𝜎 (𝑛) denotes the sum of divisors of 𝑛. Gaussian Filter (GF) This program implements a Gaussian blur, which is an image smoothing filter. GF consists in applying a specially computed two-dimensional Gaussian mask (𝑚) to an image (𝑖) using a matrix convolution operation. Thus, this application features a stencil pattern. K-Means (KM) This application implements the K-Means data clustering algorithm, which partitions 𝑛 points into 𝑘 partitions in a 2D space. Data points and centroids are evenly and randomly distributed in space. Then, data points are reclustered into 𝑘 partitions taking into account the minimum Euclidean distance between them and the centroids. Next, the centroid of each partition is recalculated taking the mean of all points within the partition. The whole procedure is repeated until all centroids remain unchanged.

Experimental Environment

We carried out our experiments on two platforms: We used the former platform to collect the traces from the CAP Bench applications. Then, we employed the latter platform to validate and evaluate our trace-driven methodology in Nanvix.

Intel
Figure 3 pictures an architectural overview of Kalray MPPA-256 (Bostan architecture), which was the processor adopted in this paper. It PEs and RMs are designed to target different goals. RMs are dedicated to manage communications, whereas PEs are generalpurpose cores, so they can run user programs. It is important to note that hardware cache coherence is not supported in Compute Clusters. Clusters have distinct address spaces, and they communicate with one another by explicitly exchanging messages through one of the available NoCs: a Data NoC (D-NoC), which should be used for system-and user-level large data transfers, and a Control NoC (C-NoC), which is reserved for small control messages. Kalray MPPA-256 has built-in Direct Memory Access (DMA) controllers in their NoC interfaces to enable asynchronous communications.

Experimental Design and Methods

We divided the experimental evaluation in two sets of experiments, each of which aiming at a different goal. In the following paragraphs we describe each set in more details.

In the first set (Section 5.1), we aim at validating our trace-driven methodology by comparing the heatmaps of the applications obtained from the trace files with the heatmaps generated by the proxy application. If both heatmaps are similar, we can conclude that our trace-driven methodology is able to correctly represent and mimic the memory access pattern of the original applications. Moreover, we analyze the heatmaps produced by the proxy application with different trial factors, so as to evaluate if the overall memory access pattern behavior of the original applications are preserved.

In the second set (Section 5.2), we intend to show one case study where our trace-driven methodology can be applied. In this case study, we evaluate the software-managed cache implementation of the memory management service in Nanvix. This cache keeps the pages being accessed by the application in the local memory of Compute Clusters, so as to avoid the high latency of fetching pages from remote memories. Originally, the software-managed cache implementation used a simple First-in First-out (FIFO) page replacement policy, since Kalray MPPA-256 has important hardware limitations2 that prevent OS developers to implement more sophisticated policies. However, since our proxy application mimics the memory access pattern of the original applications by making explicitly calls to the software-managed cache, we were able to implement other policies such as Not Frequently Used (NFU) and Aging. Based on the obtained results, hardware architects can decide whether or not is beneficial to include the necessary hardware support to allow the implementation of smarter page replacement policies at the OS-level.

In NFU and Aging policies, there is a reference counter associated to every page, which is initially set to zero. The main difference between these policies is on how the reference counters of pages that have been accessed are updated at fixed time intervals: in NFU, the reference counter is incremented by 1 whereas in Aging, it is first shifted right and then, its most significant bit is set to 1. Due to these operations, Aging ensures that pages referenced more recently will have higher counters. In both policies, the page with the lowest reference counter is chosen to be evicted from the cache.

We ran CAP Bench applications on Intel Xeon to collect their traces and to build their heatmaps (Steps 1 , 2 and 3 ). We implemented our proxy application (Step 4 ) as user-level application in Nanvix and ran it on the target lightweight manycore processor (Kalray MPPA-256). The size of the software-managed cache in all experiments was 128 KB, which allows us to store 32 pages of 4 KB each. We selected only the heap segments of CAP Bench applications during the trace preprocessing step (Step 2 ), since data processed by these applications are allocated dynamically.

Our proxy application has a deterministic behavior. Thus, a single execution is enough to obtain the desired results. For the softwaremanaged cache experiments, however, we carried out 10 trials of each experimental configuration to eliminate undesired cache warm-up effects. Then, we carried out a single execution to collect the results.

RESULTS

First, we discuss about the validation of our trace-drive methodology (Section 5.1). Then, we show the results obtained in our case study with a software-managed cache implementation (Section 5.2).

Methodology Validation

Figure 4 presents the heatmaps built from the traces collected from the execution of CAP Bench applications (original) as well as from our proxy application with two trial factors (𝑡 𝑓 = 1000 and 𝑡 𝑓 = 10000). The former value was chosen so as to have a scenario with a moderate yet significant reduction factor. The latter, on the other hand, represents the near maximum possible value for the GF application, which had no more than 160K memory page accesses per cell in the original heatmap.

The 𝑥-axis in Figure 4 represents the temporal behavior (timestamps of memory page accesses), whereas the 𝑦-axis represents the space behavior (memory pages). Noteworthy, cells in Figure 4c were highlighted with dashed boxes to improve visualization, since their values were very close to 0 (i.e., white color). Finally, the resolutions of heatmaps were fine-tuned so as to have 30 bins in the 𝑦-axis and 50 bins in the 𝑥-axis.

Overall, the proxy application was able to mimic the page access pattern of the original applications, even with a high trial factor. However, some information about less frequently accessed pages were lost when we increased the trial factor, as it can be seen in Figure 4c with 𝑡 𝑓 = 1000, and even more with 𝑡 𝑓 = 10000 (dashed boxes). This happens due the probabilistic approach of our methodology, which tends to keep information about memory pages that were accessed more frequently.

Table 1 shows the root-mean-square deviation (RMSD) of the results obtained from the proxy application with different trial factors (the lower the RMSD value, the higher the similarity between the access pattern of the original application and the one produced by the proxy application). The very low RMSD values indicate that our methodology is capable of producing a memory access pattern behavior very close to the original applications, even with a high trial factor. As it can be noticed, GF showed the highest RMSD values among all applications considered in this study, specially with 𝑡 𝑓 = 10000. The main reason for that is two-fold: (i) the number of accesses per page is much lower in this application compared to the other ones, which results in information losses when a high trial factor is employed; and (ii) this application has a set of pages that are accessed very few times throughout the execution, thus the probability of accessing a page in this set tends to be very low (near zero percent) with a high trial factor.

Another important aspect of our methodology that should be evaluated is the memory footprint to store the page access pattern of the applications. Table 2 presents the size of the original trace files and their respective representation with heatmaps. Overall, the size of the heatmaps is several orders of magnitude lower than the size of the original trace files. Since our proxy application uses heatmaps instead of trace files to mimic the memory access pattern of the original applications, it is able to cope with the limited amount

Case Study: Software-managed Cache

Figure 5 pictures the variation of the software-managed cache hit ratio obtained from our proxy application with three different replacement policies: FIFO, NFU and Aging. The hit ratio was computed periodically with a fixed period of 𝑝 = 600 memory access events. Thus, each point in Figure 5 represents the cache hit ratio computed as follows ℎ𝑖𝑡_𝑟𝑎𝑡𝑖𝑜 = number of cache hits in 𝑝 𝑝 .

(

) 2
We also fine-tuned the input heatmaps used by the proxy application as follows: maximum resolution in 𝑦-axis (i.e., 1 page per bin) and 200 bins 𝑥-axis. Our empirical tests with FN, KM and GF showed that this configuration was enough to obtain precise results.

Figure 5a shows the hit ratio obtained with FN. As shown in Figure 4a, this application performs memory accesses to every page in the beginning of the execution but its working set reduces as the execution proceeds. This is mainly due to the data access pattern of a nested loop that computes the abundancy of the numbers. This behavior is reflected in the hit ratio, which increases as the execution approaches its end because the working set starts fitting in the cache. The best results were achieved with FIFO, since the page access pattern of FN follows exactly the same behavior.

In Figure 5b, however, we observed that Aging was more beneficial for KM. This application has two well-defined page access behaviors: pages on top and bottom ranges in Figure 4b are always accessed throughout the execution whereas middle range pages are accessed in a FIFO ordering fashion. The FIFO page replacement policy is able to correctly keep middle range pages in cache, showing a stable cache hit ratio of 96% on average. NFU, on the other hand, presented a high hit ratio variation with several spikes. The main reason is that NFU constantly evicts middle range pages because they usually have lower reference counters. This problem is solved by Aging, since it is aware of the time span of page use. Finally, Figure 5c shows the results obtained with GF. As shown in Figure 4c, this application has few pages that are constantly accessed throughout the execution (top range) whereas the access pattern of all other pages follows the FIFO rule. We observed that all page replacement policies were able to keep a hit ratio very close to 100%, since the most accessed pages fit in the cache.

RELATED WORKS

Tracing is a well-known technique to collect information from the execution of applications. This information is used by researchers and developers to achieve different goals such as debugging, performance optimizations in hardware/software, as well as to allow for more realistic simulations of architectures and applications. When used in simulations, they provide means to evaluate several possible scenarios and configurations in a feasible time. In this section, we discuss some related works that make use of memory accesses traces to help to evaluate and/or optimize software and hardware solutions.

To the best of knowledge, the closest work to ours was proposed by M. M. Rahman and Dueck [START_REF] Kent | Trace Files for Automatic Memory Management Systems[END_REF], whose goal was to evaluate and test Automated Memory Management (MM) runtime systems, such as the Java Virtual Machine (JVM). The proposed approach is based on collecting MM operations in traces from benchmark suites at run-time. Since these traces are very large, the authors proposed a trace synthesizer that produces synthetic trace files with basic MM operations for given configuration parameters. These parameters can be adjusted based on the profiling results of real trace files. Their results show that the trace synthesizer can generate more test scenarios, helping developers to evaluate the MM system of the JVM. In contrast to this work, we were interested in distributed OS-level optimizations for lightweight manycores, which brings other challenges due to the intrinsic characteristics and limitations of these processors. Furthermore, our approach is able to mimic the page access pattern of the original applications using heatmaps along with a probabilistic approach. Because of that, we neither need to deal with large trace files when reproducing the behavior of the applications nor create new synthetic trace files.

Diener et al. [START_REF] Diener | Communication-aware process and thread mapping using online communication detection[END_REF] proposed CDSM, a mechanism that uses page faults to detect communications between threads and uses this information to map threads to cores. They used heatmaps to store information about the number of communication events between two processes or threads. Heatmaps are used to map processes and threads to close processing units according to their communication behavior at run-time. As it can be noticed, Diener et al. [START_REF] Diener | Communication-aware process and thread mapping using online communication detection[END_REF] used heatmaps to achieve a different goal, although sharing some aspects that are similar to our page access pattern representation.

Traces have also been used to build more realistic cache simulators [START_REF] Dumas | Trace-driven exploration of sharing set managementstrategies for cache coherence in manycores[END_REF][START_REF] Keramidas | Multicore cache simulations using heterogeneous computing on general purpose and graphics processors[END_REF][START_REF] Moeng | Scalable multi-cache simulation using GPUs[END_REF]. Moeng et al. [START_REF] Moeng | Scalable multi-cache simulation using GPUs[END_REF] proposed the use of GPUs to accelerate a trace-based cache simulator conceived to study the cache coherence in multithreaded workloads and multilevel cache implementations. The information is collected once using a functional simulator and a trace of events is generated. Then, the simulator replays the events registered in the traces. Similarly, Keramidas et al. [START_REF] Keramidas | Multicore cache simulations using heterogeneous computing on general purpose and graphics processors[END_REF] used both CPU and GPU processors to speedup cache simulation. Finally, Dumas et al. [START_REF] Dumas | Trace-driven exploration of sharing set managementstrategies for cache coherence in manycores[END_REF] proposed a trace-driven simulation method to accurately compare cache coherence protocols in NoC-based manycores. Their solution helps manycore architects to select and dimension the best cache coherence protocol for their application considering performance and hardware related costs. In contrast to these works, we were interested in evaluating a software-managed cache by carrying out experiments on a baremetal hardware without any cache-level simulation.

CONCLUSION

The development of applications for lightweight manycores is very challenging. Software engineers have to explicitly deal with the limited amount of on-chip memory, no cache coherence and multiple address spaces. To tackle these challenges, distributed OSs have been proposed to ease development and improve portability.

In this context, the memory management is one of the most important services offered by distributed OSs. However, the intrinsic characteristics and memory limitations of lightweight manycores bring several challenges to its design and implementation, opening new opportunities for optimizations. A possible approach to help the design and evaluation of the memory management service is to port several applications of different domains to the target distributed OS and carry out experiments with these applications.

Notwithstanding, porting software to distributed OSs for lightweight manycores is a time-consuming and error-prone task.

Aiming at this problem, in this work, we proposed a trace-driven methodology that can be used to evaluate and optimize features of a memory management service of distributed OSs for lightweight manycores. Thanks to a compact representation of memory access patterns, our methodology is capable of mimicking the memory access pattern of the original applications on the target distributed OS running on a lightweight manycore with low footprints. We integrated our methodology in Nanvix and validated it using three applications from CAP Bench. Furthermore, we carried out a case study using a software-managed cache implementation available in Nanvix. Our methodology enabled us to evaluate different page replacement policies on Kalray MPPA-256, even without the required support from the architecture to implement them.

As future work, we intend to apply our methodology to help the design and evaluation of new optimizations in Nanvix, such as the software prefetching module and a more sophisticated page sharing algorithm. We also intend to consider other applications from CAP Bench and other benchmarks. Finally, we intend to apply our methodology to other distributed OSs and/or lightweight manycore processors.

Figure 1 :

 1 Figure 1: A simplified overview of a conceptual lightweight manycore running a multikernel OS.

Figure 2 :

 2 Figure 2: Overview of the trace-driven methodology.

Figure 3 :

 3 Figure 3: MPPA-256 lightweight manycore processor.

Figure 4 :

 4 Figure 4: Heatmaps of FN, GF and KM.

Figure 5 :

 5 Figure 5: Software-managed cache hit ratio.

Table 1 :

 1 Root-mean-square deviation (RMSD) of the results obtained from the proxy application.

	Application RMSD (𝑡 𝑓 = 1000) RMSD (𝑡 𝑓 = 10000)
	FN	0.395	1.206
	KM	0.198	0.649
	GF	1.539	4.594

Table 2 :

 2 Sizes of trace files and heatmaps.

	Application Trace Heatmap
	FN	1.4 GB	8.0 KB
	KM	5.6 GB	7.2 KB
	GF	119 MB	4.4 KB
	of on-chip memory available in lightweight manycore processors.
	Moreover, it improves the execution of the proxy application, since
	heatmaps are allocated in memory and no I/O operation is needed.

The MMU hardware of this processor does not update page metadata when a page is accessed or modified. Therefore, the only information available for the page replacement policy is the time in which the page was admitted in the system.

ACKNOWLEDGMENTS

This work was partially supported by Conselho Nacional de Desenvolvimento Científico e Tecnológico ś Brasil (CNPq) and by Coordenação de Aperfeiçoamento de Pessoal de Nível Superior ś Brasil (CAPES) ś Finance Code 001 and under the CAPES-PrInt Program Grant No.: 88881.310783/2018-01.


Raynald Seveno

Thibault Dufay

Mohammed El Gibari

Benoit Guiffard

Hong Wu Li

Saber Morsli

Aude Pichon

Eric Tanguy

Teaching the basics of electricity using a flexible piezoelectric generator

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

Keywords-piezoelectricity; basics of electricity; electrokinetics laws

I. INTRODUCTION (HEADING 1)

Thanks to the two facets of their profession, teacherresearchers have the opportunity to present their research themes in their lectures, since they generally teach in their favoured disciplines. However, if the presentation of research work in a Master's degree remains relatively easy, this is not the case for undergraduate courses, and especially for the first-year public who do not often have the necessary scientific background to understand cutting-edge research. Nevertheless, it is often at this time that students choose their core discipline, and it is important to pay particular attention to the teachings they receive. At present, research pertaining to renewable energies to address the shortage of fossil fuels is a subject of common interest. Indeed, young people are deeply concerned about environmental issues. A lot of research revolves around the development of generators that are able to convert ambient energies into electricity, such as canonical solar cells [START_REF] Wang | Stability of perovskite solar cells[END_REF] and micro-power systems like electromagnetic [START_REF] Sari | An Electromagnetic Micro Power Generator for Low-Frequency Environmental Vibrations Based on the Frequency Upconversion Technique[END_REF] or electrostatic [START_REF] Peano | Design and optimization of a MEMS electretbased capacitive energy scavenger[END_REF] generators.

In this paper, we present a practical session based on the Nair-J research project, the objective of which is to study the possibility of recovering energy from air currents using a synthetic blade of grass able to convert mechanical energy into electricity (principle of piezoelectricity). This generator consists of a piezoelectric thin film of lead zirconate titanate (PZT) 3 μm thick, deposited by a Chemical Solution Deposition (CSD) process onto a 16 μm thick aluminum foil [START_REF] Seveno | Ultra light tunable capacitor based on PZT thin film deposited onto aluminium foil[END_REF]. An upper electrode of aluminum of 200 nm thick, evaporated by the Joule effect, makes it possible to obtain the piezoelectric capacitor. The last is then encapsulated by lamination into polyethylene terephthalate (PET, 150 µm) to make it flexible. During the lesson, the students will determine the piezoelectric coefficient d31 of the PZT film, which makes it possible to evaluate the piezoelectric potential of the used generator. The students will then characterize the flexible piezoelectric generator using the laws of electrokinetics. This will provide them with a concrete case to apply the theoretical knowledge they have learned in class.

Initially, we interest the students by the presentation of the generator (Fig. 1), which looks like a simple piece of plastic. By connecting it directly to an oscilloscope, the students can see that an electrical signal of random shape is produced by manual folding of the generator (Fig. 2). Then, the principle of piezoelectricity is explained in a few words, as is the research carried out on the development of piezoelectric materials used for the conception of the generator studied during the practical class. How can one compare the piezoelectric materials with one another? How can we quantify the amount of electricity produced by this type of generator? What is the electrical power produced? The experiments in this lesson should be able to answer these questions.

II. DETERMINATION OF THE PIEZOELECTRIC COEFFICIENT

The first experiment consists in the determination of the piezoelectric coefficient d31. Even if this coefficient is not directly related to the power delivered by the generator, it makes it possible to evaluate the piezoelectric ability of the material used to build the generator.

By applying an alternative electric voltage 𝑣 = 𝑉 0 cos (2𝜋𝑓) to an Al/PZT/Al cantilever beam, the latter oscillates at the same frequency f, due to the inverse piezoelectric effect. A laser vibrometer (Polytech OFV 2200) is then used to measure the tip deflection d at the free end of the beam (Fig. 3). The students perform these measurements at different frequencies and have to fit the corresponding plots to the canonical model of an oscillating system under sinusoidal excitation (eq. 1), as already used in the RLC circuit theory (Fig. 4). d0 is the quasi-static displacement, f0 is the resonance frequency of the oscillator in the free mode and Q is the quality factor [START_REF] Kneubühl | Oscillations and waves[END_REF].

𝑑 = 𝑑 0 / √ (1 -( 𝑓 𝑓 0 ) 2 ) 2 + ( 𝑓 𝑓 0×𝑄 ) ² (1)
The fit of the plots allows the determination of the d 0 value, which is used for the calculation of the piezoelectric coefficient d 31 (eq.2), where the  constant depends on the structure of the cantilever (dimensions and Young's moduli) [START_REF] Luginbuhl | [END_REF].

𝑑 31 = 𝛼 × 𝑑 0 /𝑉 0 (2)

III. ENERGY HARVESTING MEASUREMENTS

The piezoelectric generator is inserted into a mechanical excitation device (Fig. 5) able to generate a variation of quasisinusoidal deformation of the flexible generator, leading to a quasi-sinusoidal signal observed on the oscilloscope 𝑢 𝑅 = 𝑈 𝑅 cos(2𝜋𝑓𝑡)(frequency f of the signal is imposed by the motor speed of the mechanical excitation device), as shown in Fig. 6.

Students then realize that piezoelectric generators deliver alternative, not continuous, electricity. The measurement of the UR amplitude of the voltage observed at the terminals of the input impedance (1 M) of the oscilloscope makes it possible to calculate the power thus delivered by the generator with the help of the formula given in the equation 3. Important questions are: Is this power optimal? Can the device be improved to increase the level of power recovered? To answer these questions, the students must study the equivalent circuit corresponding to the assembly (Fig. 7). The piezoelectric generator is modeled by an ideal current generator i0 associated in parallel with a linear capacitance C. The input impedance of the oscilloscope is represented by a load resistor R connected in parallel to the generator. This resistance is modifiable by experimentally adding a resistance in parallel to the circuit.

𝑃 = 𝑈 𝑅 2 /2𝑅 (3)
At a fixed frequency f (𝜔 = 2𝜋𝑓), the students perform different measurements of the amplitude of the voltage UR by modifying the value of the load resistor R. To do this, they insert resistor associations in parallel with the circuit and need to use the equivalence resistance formula to compute the new values of R. Using the formula in equation 3, the Ohm's law (eq.4) and the formula of the current divider bridge (eq.5), Students can fit the experimental values of the (𝑈 𝑅 ; 𝐼 𝑅 ; 𝑃) = 𝑓(𝑅)curves and thus determine the value of the piezoelectric current's amplitude I0, also called the short-circuit current (Fig. 8). Error bar of the voltage has been determined at the oscilloscope, and the students calculate the corresponding error bars of current and power.

𝑈 𝑅 = 𝑅 × 𝐼 𝑅 (4) 𝐼 𝑅 = 𝐼 0 √ 1 + 𝑅²𝐶²𝜔² ⁄ (5)
The amplitude of the open-circuit voltage UOC, another intrinsic property of the generator, is obtained by causing the load resistor R to tighten to infinity, and is given by the equation 6:

𝑈 𝑂𝐶 = 𝐼 0 /𝐶𝜔 (6)
Figure 8. Amplitude of the current and voltage across the resistive load resistor R and the harvested power as a function of R.

In addition to the generator characteristics I0 and Uoc that are clearly legible on the curves, students may notice that the power passes through a maximum for a given resistance Ropt. They are then asked to recover the theoretical value 𝑅 𝑜𝑝𝑡 = 1/𝐶𝜔 of this resistance by solving the equation

𝑑𝑃 𝑑𝑅 = 0.
By injecting the Ropt value into the expression of 𝑃 = 𝑓(𝑅), we obtain the maximum value of the harvested power (eq.7).

Figure 9. Amplitude of the output power at the optimal load resistor as function of the frequency of the applied mechanical excitation.

𝑃 𝑚𝑎𝑥 = 𝐼 0 2 /4𝐶𝜔 (7)
To optimize the precision of the calculated values of the intrinsic properties of the generator, the students repeat the measurements at different frequencies, thus making it possible to plot the curve 𝑃 𝑚𝑎𝑥 = 𝑔(𝑓) as shown in Fig. 9. They notice that the maximum power Pmax value is proportional to the excitation frequency. A linear regression is then required, and the obtained slope coefficient  is used for the calculation of the energy density 𝐸 𝑆 = 𝛽/𝑉, where V is the volume of the piezoelectric capacitor.

IV. FEEDBACK OF STUDENTS

In the context of a dissemination campaign on scientific culture ("Passeport recherche"), 20 high school students in year one of a science and laboratory technology baccalaureate in France have taken this practical class (only the harvesting voltage part, not the determination of the piezoelectric coefficient). At the beginning of the school year, they had to choose between different subjects of scientific studies, and chose, along with their teachers, to work on the N-air-J project. Initially, two researchers from this project went to give an oral presentation to students in their high school. They introduced the context and objectives of the project, and explained the principle of piezoelectricity, which is the basis of the operation of the generators studied. Secondly, the students came to the science faculty of Nantes (France) to carry out a practical work on the piezoelectric generator, as described in Several weeks after doing the practical class, the students have answered to an anonymous survey. For each questions, they had to give a note ranging from 1 to 5 representing their agreement to the question (1: disagree; 5: totally agree). Results of the survey, including number of students who give the corresponding note and the average note per each question, are given in Table 1. Generally, the students did appreciate to experiment flexible piezoelectric generator and most of them understood the utility of the theoretical model to characterize the generator. However, they are not accustomed to fit experimental plots with formula, and the level of the practical session seems to be a little bit high for high school students.

V. CONCLUSION

Based on the research developed in the N-air-J project, a practical session has been build, in which a flexible piezoelectric generator is characterized. The production of a quasi-sinusoïdal voltage observable at an oscilloscope allows the students to apply theoretical knowledges, as resonance phenomenon of an oscillator, calculation of equivalent resistances, Ohm's law and current dividers. They fit the experimental plots to the electrokinetic's laws to determine the generator's properties. Through this experience, they access to actual researches and can directly apply the basics of electricity in a concrete case.

  Teach the basics of electricity by the use of a flexible piezoelectric generator. R. Seveno, T. Dufay, M. El Gibari, B. Guiffard, H. W. Li, S. Morsli, A. Pichon and E. Tanguy UFR des Sciences et des Techniques, Université de Nantes 2 rue de la houssinière, 44322 cedex 03, Nantes, France raynald.seveno@univ-nantes.fr Abstract-Lecturer-researchers, because of the duality of their profession, have the opportunity to introduce students directly to their research. Stimulating student interest through practical research topics enables students to see the relevance of the teaching/learning process and thereby enhances their motivation. As a major societal issue, research on renewable energies is held in great esteem, particularly among young students with a strong interest in environmental issues. The work presented here relates to the realization of a practical class on the testing of a flexible piezoelectric generator studied as part of the "N-air-J" regional research project. The particular characteristic of piezoelectric materials is that they produce electricity when compressed, which means that they can be used in energy recovery devices. The electrical model associated with this type of generator, consisting of an ideal current source in parallel with a linear capacitor and a load resistor, is sufficiently simple to be understood by first-year university students following a course on electricity. Using an appropriate mechanical excitation system (made out of Lego©), it is possible to generate variations in the quasi-sinusoidal compressions of a flexible piezoelectric film. By connecting an oscilloscope directly to the terminals of this experimental generator, students can visualize the produced current flowing through its input impedance, thus making it possible to use the laws of electrokinetics on an alternating sinusoidal current. As the objective of the practical class is to calculate the energy density produced by this type of generator, it uses the notions of Ohm's law, current dividers, equivalent resistances (serial/parallel associations), power and density of energy to provide concrete examples of some of the basics of electricity. High school students in year one of a science and laboratory technology baccalaureate in France have already taken this practical class as part of a dissemination campaign on scientific culture.

Figure 1 .

 1 Figure 1. Photograph of the piezoelectric generator.

Figure 2

 2 Figure 2 Photograph of the harvested random shape voltage.

Figure 3 .

 3 Figure 3. Photograph of the piezoelectric characterization device.

Figure 4 .

 4 Figure 4. Tip deflection of the cantilever beam as a function of the frequency of the applied voltage and the associated fits.

Figure 5 .

 5 Figure 5. Photograph of the bending device.

Figure 6 .

 6 Figure 6. Output voltage of the piezoelectric harvesting device.

Figure 7 .

 7 Figure 7. Scheme of the equivalent circuit of the piezoelectric harvesting device.

AKNOWLEDGMENT

The authors would like to thanks P. Derval, S. Hassini, B. Masse and P. Navaro for their contribution to the build of the practical session. They thank A. Cheillan and T. Morisseau, and all the students of their group from Alcide d'Orbigny high school (Bouaye, France), to their participation to the N-air-J project and its dedicated practical session. The N-air-J project is supported by the French region Pays de la Loire through the 2014 07965 contract.


Stephane Kouadio Koffi
email: koffi.ks@gmail.com

Syndou Meite

Abdoulaye Ouattara

Stephane Kan Kouassi

Sylla Aboubacar

Veronique Agbaya Akran

Pascale Bourhy

Mireille Dosso

Koffi Kouadio

Meite Syndou

Ouattara Abdoulaye

Kouassi Kan

Sylla

Akran Agbaya

Bourhy Pascale

Dosso Mireille

Kouadio Koffi

Stéphane

Geographic distribution of anti-Leptospira antibodies in humans in Côte d'Ivoire, West Africa

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

INTRODUCTION 15

Leptospirosis is a zoonotic bacterial disease with a worldwide distribution caused by pathogenic 16 bacteria of the genus Leptospira [START_REF] Haake | Leptospirosis in humans[END_REF]. It is an endemic disease especially in subtropical and tropical 17 regions and outbreaks can occur after heavy rain or flooding [START_REF] Haake | Leptospirosis in humans[END_REF]. Transmission occurs through direct or 18 indirect contact with urine of infected animals [START_REF] Haake | Leptospirosis in humans[END_REF][START_REF] Hartskeerl | Emergence, control and re-emerging 110 leptospirosis: dynamics of infection in the changing world[END_REF].

19

Leptospirosis may be unrecognized or misdiagnosed because of the lack of specific clinical symptoms, 20 thus human incidence is commonly underreported.

21

The World Health Organization (WHO) Leptospirosis Epidemiology Reference Group (LERG) 22 estimated the leptospirosis incidence at 1.03 million cases and 58,900 deaths each year worldwide [START_REF] Costa | Global Morbidity and Mortality of Leptospirosis: A Systematic Review[END_REF].

23

West African annual leptospirosis incidence was estimated to be 9.7 per 100,000 population [START_REF] Costa | Global Morbidity and Mortality of Leptospirosis: A Systematic Review[END_REF].

24

In West Africa, few studies on Leptospira prevalence have been published. And prevalence of acute

37

This study was carried out from August to November 2015 in the 82 health districts of Côte d'Ivoire.

38

The study was carry out on stored human specimens previously collected for the national surveillance 39 system for communicable diseases in 2014. Only negative specimens were included in the study.

47

Confirmation was carried out by MAT in IP with a threshold of 1:50. As antigens, 16 serogroups were 48 used (table 1).

49

Socio-demographic and clinical data from the National Reference Centre, which manage the samples 50 and results for analysis performed on studied samples, were used to build a database using Epi Info 51 software (CDC, Atlanta, Georgia). Data analyses were done using R software 3.2.0 [8].

53

RESULTS

54

Age of patients ranged from 1 month to 78 years old with a mean age of 20.8 years. Patients less than 55 25 years were more represented with 60.9% of all patients. We noted a male predominance with a sex 56 ratio of 1.55.

57

Of the samples tested, ELISA positively screened 90. Of these, 36 were confirmed by MAT

62

MAT anti-Leptospira antibodies titre ranged from 1:50 to 1:6400. Eight different serogroups of 63 Leptospira were found (table 2). Serogroup Panama was the most frequent (36.

40

  One to 13 serum samples were randomly selected per health district based on the number of notified 41 cases by health districts in the year to obtain a sample size of 384. 42 Blood samples were collected from each patient who matches the clinical WHO's case definition for 43 notifiable disease. Samples were sent, at +4°C, to the National Reference Centre at the Institut Pasteur 44 de Côte d'Ivoire for analysis. Anti-Leptospira antibody testing was carried out at Institut Pasteur de 45 Côte d'Ivoire (IPCI) and Institut Pasteur in Paris (IP). We used as screening methods the ELISA 46 against type IgG anti-Leptospira antibodies previously describe by Bourhy et al. (2013) in IPCI [7].

58 representing 9

 9 .4 % prevalence. People with anti-Leptospira antibodies had a mean age of 34.5 years 59 and a Sex ratio of 2. Of the 82 health districts of the country, anti-Leptospira antibodies were found in 60 samples from 22. Health districts with anti-Leptospira antibodies were mostly located in the western 61 and the southern parts of the country (fig 1).

  1 %) followed by 64 Louisiana (19.4 %) and Grippotyphosa and Sejroe (11.1% respectively).

Funding:

  65 66 and wet soils were Leptospira find proper conditions for his maintenance in the environment [10, 11, 79 12]. In these parts of the country, some irrigated farming activities like rice, banana and vegetable 80 farming are associated with risk behaviours like walking barefooted in wet farm soil, sludge or working 81 without wearing appropriated personal protective equipment. The maintenance of Leptospira in the 82 environment is better when bacteria are protected from direct UV sunlight [13]. This situation could be 83 achieved in forests or in large farms with trees like cocoa, coffee or rubber plantations mostly seen in 84 the western and southern part of the country. These plantations also offer habitats to small mammals 85 that are considered a reservoir of bacteria and create a Human/ Animal/ Environment interface, which 86 could improve the transmission of the disease. 87 Regarding people with anti-Leptospira antibodies, they were young adults (mean age 34.5 years) 88 mostly men (sex ratio 2) living in rural area; corresponding to active low-income farmers working into 89 agricultural fields. These people could represent an at-risk population and it could be suitable to 90 implement prevention activities against leptospirosis related to them. Indeed, farming is a major 91 activity in Côte d'Ivoire representing 43.5 % of jobs. 92 This study highlights the circulation of leptospirosis among people in Côte d'Ivoire. Unfortunately, no 93 routine laboratory-based diagnostic strategies are available for leptospirosis or other diseases such as 94 rickettsia or viral hepatitis. Numerous patients suffering from infectious diseases are not correctly 95 diagnosed, and the burden of these diseases is underestimated. Early diagnosis and treatment reduce 96 occurrence of complications and improve recovery rate significantly [1]. There is a need to improve 97 public health by Ivorian authorities and their international partners. No funding was received. 5. Allan KJ, Biggs HM, Halliday JEB, Kazwala RR, Maro VP, Cleaveland S, Crump JA. (2015) 118 Epidemiology of Leptospirosis in Africa: A Systematic Review of a Neglected Zoonosis and a 119 Paradigm for 'One Health' in Africa. Plos Neglected Tropical Diseases 9: 3899-3899.

120 6 .

 6 Roqueplo C, Marié JL, André-Fontaine G, Kodjo A, Davoust B (2015) Serological survey of 121 canine leptospirosis in three countries of tropical Africa: Sudan, Gabon and Ivory Coast. Comp 122 Immunol Microbiol Infect Dis 38:57-61. doi: 10.1016/j.cimid.2014.10.006. 123 7. Bourhy P, Vray M, Picardeau M (2013) Evaluation of an in-house ELISA using the intermediate 124 species Leptospira fainei for diagnosis of leptospirosis. J Med Microbiol 62(Pt 6):822-7.

125 8 .

 8 R Core Team (2015) R: A language and environment for statistical computing. R Foundation for 126 Statistical Computing, Vienna, Austria. URL http://www.R-project.org 127 9. Pappas G, Papadimitriou P, Siozopoulou V, Christou L, Akritidis N (2008) The globalization of 128 leptospirosis: worldwide incidence trends. Int J Infect Dis 12(4):351-7. Epub 2007 Dec 4.

129 10 .

 10 Adler B, De La Pena Moctezuma A (2010) Leptospira and leptospirosis. Veterinary Microbiology 130 140: 287-296. doi: 10.1016/j.vetmic.2009.03.012. 131 11. Faine S, Adler B, Bolin C, Pérolat P (1999) Leptospira and leptospirosis. Melbourne, Australia: 132 MediSci. 296 p.

133 12 .

 12 Levett PN (2001) Leptospirosis. Clinical Microbiology Reviews 14(2):296-326.

134 13 .Fig 1

 131 FIGURE CAPTIONS

PLoS Negl Trop Dis 9: e0003898. doi: 10.1371/journal.pntd.0003898.

De Vries SG, Visser BJ, Nagel IM, Goris MG, Hartskeerl RA, Grobusch MP (2014) Leptospirosis

in Sub-Saharan Africa: a systematic review. Int J Infect Dis 28:47-64.

Conflict of Interest:

The authors declare that they have no conflict of interest.


R Seveno

S Morsli
email: saber.morsli@univ-nantes.fr

A Pichon
email: aude.pichon@univ-nantes.fr

E Tanguy
email: eric.tanguy@univ-nantes.fr

Using renewable energies to teach the basics of electricity. Example of the "N-air-J" project

Keywords:

With a constantly evolving student population increasingly focused on new technologies, the teaching of science subjects also needs to evolve by incorporating as many applied examples as possible. Lecturer-researchers, because of the duality of their profession, have the opportunity to introduce students directly to their research. Stimulating student interest through practical research topics enables students to see the relevance of the teaching/learning process and thereby enhances their motivation. As a major societal issue, research on renewable energies is held in great esteem, particularly among young students with a strong interest in environmental issues. The work presented here relates to the realization of a practical class on the testing of a flexible piezoelectric generator studied as part of the "N-air-J" regional research project (http://www.n-air-j.univnantes.fr). The particular characteristic of piezoelectric materials is that they produce electricity when compressed, which means that they can be used in energy recovery devices. The electrical model associated with this type of generator, consisting of an ideal current source in parallel with a linear capacitor and a load resistor, is sufficiently simple to be understood by first-year university students following a course on electricity. Using an appropriate mechanical excitation system (made out of Lego©), it is possible to generate variations in the quasi-sinusoidal compressions of a flexible piezoelectric film. By connecting an oscilloscope directly to the terminals of this experimental generator, students can visualize the produced current flowing through its input impedance. As the recorded charges are proportional to the variations in compression of the piezoelectric generator, the current is also quasi-sinusoidal, thus making it possible to use the laws of electrokinetics on an alternating sinusoidal current. As the objective of the practical class is to calculate the energy density produced by this type of generator, it uses the notions of Ohm's law, current dividers, equivalent resistances (serial/parallel associations), power and density of energy and rectification of an alternating signal to provide concrete examples of some of the basics of electricity. It also uses the model of the single degree of freedom oscillator in forced mode and steady state, since this makes it possible to work back to the calculation of the piezoelectric coefficient of the material used to build the generator. In addition to applying basic knowledge on electricity, students are required to perform curvefitting with the experimental points measured on the oscilloscope in order to correlate them with the theoretical model used. This gets students thinking about one of the fundamental aspects of physics, that is, the use of mathematical theories to explain and predict the phenomena we see every day. High school students in year one of a science and laboratory technology baccalaureate in France have already taken this practical class as part of a dissemination campaign on scientific culture, and they very much appreciated the opportunity to experiment with the basics of a high-tech research project.

I. INTRODUCTION (HEADING 1)

Thanks to the two facets of their profession, teacherresearchers have the opportunity to present their research themes in their lectures, since they generally teach in their favoured disciplines. However, if the presentation of research work in a Master's degree remains relatively easy, this is not the case for undergraduate courses, and especially for the firstyear public who do not often have the necessary scientific background to understand cutting-edge research. Nevertheless, it is often at this time that students choose their core discipline, and it is important to pay particular attention to the teachings they receive. At present, research pertaining to renewable energies to address the shortage of fossil fuels is a subject of common interest. Indeed, young people are deeply concerned about environmental issues. A lot of research revolves around the development of generators that are able to convert ambient energies into electricity, such as canonical solar cells [START_REF] Wang | Stability of perovskite solar cells[END_REF] and micro-power systems like electromagnetic [START_REF] Sari | An Electromagnetic Micro Power Generator for Low-Frequency Environmental Vibrations Based on the Frequency Upconversion Technique[END_REF] or electrostatic [START_REF] Peano | Design and optimization of a MEMS electret-based capacitive energy scavenger[END_REF] generators.

In this paper, we present a practical session based on the N-air-J research project, the objective of which is to study the possibility of recovering energy from air currents using a synthetic blade of grass able to convert mechanical energy into electricity (principle of piezoelectricity). This generator consists of a piezoelectric thin film of lead zirconate titanate (PZT) 3 µm thick, deposited by a Chemical Solution Deposition (CSD) process onto a 16 µm thick aluminum foil [START_REF] Seveno | Ultra light tunable capacitor based on PZT thin film deposited onto aluminium foil[END_REF]. An upper electrode of aluminum of 200 nm thick, evaporated by the Joule effect, makes it possible to obtain the piezoelectric capacitor. The last is then encapsulated by lamination into polyethylene terephthalate (PET, 150 µm) to make it flexible. During the lesson, the students will determine the piezoelectric coefficient d 31 of the PZT film, which makes it possible to evaluate the piezoelectric potential of the used generator. The students will then characterize the flexible piezoelectric generator using the laws of electrokinetics. This will provide them with a concrete case to apply the theoretical knowledge they have learned in class. Initially, we interest the students by the presentation of the generator (Fig. 1), which looks like a simple piece of plastic. By connecting it directly to an oscilloscope, the students can see that an electrical signal of random shape is produced by manual folding of the generator (Fig. 2). Then, the principle of piezoelectricity is explained in a few words, as is the research carried out on the development of piezoelectric materials used for the conception of the generator studied during the practical class. How can one compare the piezoelectric materials with one another? How can we quantify the amount of electricity produced by this type of generator? What is the electrical power produced? The experiments in this lesson should be able to answer these questions.

II. DETERMINATION OF THE PIEZOELECTRIC COEFFICIENT

The first experiment consists in the determination of the piezoelectric coefficient d 31 . Even if this coefficient is not directly related to the power delivered by the generator, it makes it possible to evaluate the piezoelectric ability of the material used to build the generator.

By applying an alternative electric voltage = cos (2 ) to an Al/PZT/Al cantilever beam, the latter oscillates at the same frequency f, due to the inverse piezoelectric effect. A laser vibrometer (Polytech OFV 2200) is then used to measure the tip deflection d at the free end of the beam (Fig. 3). The students perform these measurements at different frequencies and have to fit the corresponding plots to the canonical model of an oscillating system under sinusoidal excitation (eq. 1), as already used in the RLC circuit theory (Fig. 4). d 0 is the quasi-static displacement, f 0 is the resonance frequency of the oscillator in the free mode and Q is the quality factor [START_REF] Kneubühl | Oscillations and waves[END_REF].

= / 1 - + × ² (1)
The fit of the plots allows the determination of the d 0 value, which is used for the calculation of the piezoelectric coefficient d 31 (eq.2), where the α constant depends on the structure of the cantilever (dimensions and Young's moduli) [START_REF] Luginbuhl | [END_REF]. The piezoelectric generator is inserted into a mechanical excitation device (Fig. 5) able to generate a variation of quasisinusoidal deformation of the flexible generator, leading to a quasi-sinusoidal signal observed on the oscilloscope ! " = # " cos 2 $ (frequency f of the signal is imposed by the motor speed of the mechanical excitation device), as shown in Fig. 6 [ 7 ]. Students then realize that piezoelectric generators deliver alternative, not continuous, electricity. The measurement of the U R amplitude of the voltage observed at the terminals of the input impedance (1 MΩ) of the oscilloscope makes it possible to calculate the power thus delivered by the generator with the help of the formula given in the equation 3.

= × / (2)
% # " /2&

Important questions are: Is this power optimal? Can the device be improved to increase the level of power recovered? To answer these questions, the students must study the equivalent circuit corresponding to the assembly (Fig. 7). The piezoelectric generator is modeled by an ideal current generator i 0 associated in parallel with a linear capacitance C. The input impedance of the oscilloscope is represented by a load resistor R connected in parallel to the generator. This resistance is modifiable by experimentally adding a resistance in parallel to the circuit. At a fixed frequency f (' 2 , the students perform different measurements of the amplitude of the voltage U R by modifying the value of the load resistor R. To do this, they insert resistor associations in parallel with the circuit and need to use the equivalence resistance formula to compute the new values of R. Using the formula in equation 3, the Ohm's law (eq.4) and the formula of the current divider bridge (eq.5), Students can fit the experimental values of the # " ; ) " ; % & curves and thus determine the value of the piezoelectric current's amplitude I 0 , also called the short-circuit current (Fig. 8). Error bar of the voltage has been determined at the oscilloscope, and the students calculate the corresponding error bars of current and power.

# " & ) " (4) The amplitude of the open-circuit voltage U OC , another intrinsic property of the generator, is obtained by causing the load resistor R to tighten to infinity, and is given by the equation 4:

) " ) * 1 &²+²'² , (5)
# -. = ) /+' (6)
In addition to the generator characteristics I 0 and U oc that are clearly legible on the curves, students may notice that the power passes through a maximum for a given resistance R opt . they are then asked to recover the theoretical value & /01 = 1/+' of this resistance by solving the equation By injecting the R opt value into the expression of % = (&), we obtain the maximum value of the harvested power (eq.7).

% 567 = ) /4+' (7)
To optimize the precision of the calculated values of the intrinsic properties of the generator, the students repeat the measurements at different frequencies, thus making it possible to plot the curve % 567 = 9( ) as shown in Fig. 9. They notice that the maximum power P max value is proportional to the excitation frequency. A linear regression is then required, and the obtained slope coefficient β is used for the calculation of the energy density : ; = </ , where V is the volume of the piezoelectric capacitor.

Figure 9. Amplitude of the output power at the optimal load resistor as function of the frequency of the applied mechanical excitation.

IV. FEEDBACK OF STUDENTS

In the context of a dissemination campaign on scientific culture ("Passeport recherche"), 20 high school students in year one of a science and laboratory technology baccalaureate in France have taken this practical class (only the harvesting voltage part, not the determination of the piezoelectric coefficient). At the beginning of the school year, they had to choose between different subjects of scientific studies, and chose, along with their teachers, to work on the N-air-J project. Initially, two researchers from this project went to give an oral presentation to students in their high school. They introduced the context and objectives of the project, and explained the principle of piezoelectricity, which is the basis of the operation of the generators studied. Secondly, the students came to the science faculty of Nantes (France) to carry out a practical work on the piezoelectric generator, as described in paragraph III. Several weeks after doing the practical class, the students have answered to an anonymous survey. For each questions, they had to give a note ranging from 1 to 5 representing their agreement to the question (1: disagree; 5: totally agree). Results of the survey, including number of students who give the corresponding note and the average note per each question, are given in Table 1. Generally, the students did appreciate to experiment flexible piezoelectric generator and most of them understood the utility of the theoretical model to characterize the generator. However, they are not accustomed to fit experimental plots with formula, and the level of the practical session seems to be a little bit high for high school students.

V. CONCLUSION

Based on the research developed in the N-air-J project, a practical session has been build, in which a flexible piezoelectric generator is characterized. The production of a quasi-sinusoïdal voltage observable at an oscilloscope allows the students to apply theoretical knowledges, as resonance phenomenon of an oscillator, calculation of equivalent resistances, Ohm's law and current dividers. They fit the experimental plots to the electrokinetic's laws to determine the generator's properties. Through this experience, they access to actual researches and can directly apply the basics of electricity in a concrete case.

Figure 1 .

 1 Figure 1. Photograph of the piezoelectric generator.

Figure 2

 2 Figure 2 Photograph of the harvested random shape voltage.

Figure 3 .

 3 Figure 3. Photograph of the piezoelectric characterization device.

Figure 4 .Figure 5 .

 45 Figure 4. Tip deflection of the cantilever beam as a function of the frequency of the applied voltage and the associated fits.

Figure 6 .

 6 Figure 6. Output voltage of the piezoelectric harvesting device.

Figure 7 .

 7 Figure 7. Scheme of the equivalent circuit of the piezoelectric harvesting device.

Figure 8 .

 8 Figure 8. Amplitude of the current and voltage across the resistive load resistor R and the harvested power as a function of R.

Table 1 .

 1 Results of the anonymous survey

	1	2	3	4	5	Note /5

Does the renewable energies' topic influence your choice of the study's project?

		2	6	6	5	1	2.9
	Does the research's project motivate you to learn electricity?	2	7	4	7	0	2.8
	Does the research's project allow to identify a real case of applied electricity?	1	2	7	7	3	3.5
	Does the experimental course illustrate the research's project?	1	7	5	3	4	3.1
	Is the experimental course helpful to understand the interest of a theoretical model to study electricity?	0	4	10	4	2	3.2
	Is the experimental course helpful to understand the interest the basics of electricity?	3	4	4	7	2	3.1

AKNOWLEDGMENT

The authors would like to thanks P. Derval, T. Dufay and S. Hassini for their contribution to the build of the practical session. They thank A. Cheillan and T. Morisseau, and all the students of their group from Alcide d'Orbigny high school (Bouaye, France), to their participation to the N-air-J project and its dedicated practical session. The N-air-J project is supported by the French region Pays de la Loire through the 2014 07965 contract.