Arif Ahmed

Guillaume Pierre

Docker Container Deployment in Fog Computing Infrastructures

Keywords: Docker, Container, Edge Cloud, Fog Computing

The transition from virtual machine-based infrastructures to container-based ones brings the promise of swift and efficient software deployment in large-scale computing infrastructures. However, in fog computing environments which are often made of very small computers such as Raspberry PIs, deploying even a very simple Docker container may take multiple minutes. We demonstrate that Docker makes inefficient usage of the available hardware resources, essentially using different hardware subsystems (network bandwidth, CPU, disk I/O) sequentially rather than simultaneously. We therefore propose three optimizations which, once combined, reduce container deployment times by a factor up to 4. These optimizations also speed up deployment time by about 30% in datacenter-grade servers.

I. INTRODUCTION

Fog computing extends datacenter-based cloud platforms with additional resources located in the immediate vicinity of the end users. By bringing computation where the input data was produced and the resulting output data will be consumed, fog computing is expected to support new types of applications which either require very low network latency to their end users (e.g., augmented reality applications) or produce large volumes of data which are relevant only locally (e.g., IoT-based data analytics).

Fog computing architectures are fundamentally different from classical cloud platforms: to provide computing resources in the physical vicinity of any end user, fog computing platforms must necessarily rely on very large numbers of small Points-of-Presence connected to each other with commodity networks, whereas clouds are typically organized with a handful of extremely powerful data centers connected by dedicated ultra-high-speed networks. This geographical spread also implies that the machines used in any Pointof-Presence may not be datacenter-grade servers but much weaker commodity machines. As a matter of fact, one option which is being explored is to use single-board computers such as Raspberry PIs for this purpose. Despite their obvious hardware limitations, Raspberry PIs offer excellent performance/cost/energy ratios and are well-suited to scenarios where the device's physical size and energy consumption are important enablers for actual deployment [START_REF] Van Kempen | MEC-ConPaaS: An experimental single-board based mobile edge cloud[END_REF], [START_REF] Hajji | Understanding the performance of low power Raspberry Pi cloud for big data[END_REF].

However, building a high-performance fog platform based on tiny single-board computers is a difficult challenge: in particular these machines have very limited I/O performance.

In this paper, we focus on the issue of downloading and deploying Docker containers in single-board computers. We assume that server machines have limited storage capacity and therefore cannot be expected to keep in cache the container images of many applications that may be used simultaneously in a public fog computing infrastructure.

Deploying container images can be painfully slow, in the order of multiple minutes depending on the container's image size and network condition. However, such delays are unacceptable in scenarios such as a fog-assisted augmented reality application where the end users are mobile and new containers must be dynamically created when a user enters a new geographical area. Reducing deployment times as much as possible is therefore instrumental in providing a satisfactory user experience.

We show that this poor performance is not only due to hardware limitations. In fact it results from the way Docker implements the container's image download operation: Docker exploits different hardware subsystems (network bandwidth, CPU, disk I/O) sequentially rather than simultaneously. We therefore propose three optimization techniques which aim to improve the level of parallelism of the deployment process. Each technique reduces deployment times by 10-50% depending on the content and structure of the container's image and the available network bandwidth. When combined together, the resulting "Docker-pi" implementation makes container deployment up to 4 times faster than the vanilla Docker implementation, while remaining totally compatible with unmodified Docker images.

Interestingly, although we designed Docker-pi in the context of single-board computers, it also provides 23-36% performance improvements on high-end servers as well, depending on the image size and organization.

This paper is organized as follows. Section II presents the background and related work. Section III analyzes the deployment process and points out its inefficiencies. Section IV proposes and evaluates three optimizations. Finally, Section V discusses practicalities, and Section VI concludes.

II. BACKGROUND A. Docker background

Docker is a popular framework to build, package, and run applications inside containers [START_REF] Inc | Docker: Build, ship, and run any app, anywhere[END_REF]. Applications are packaged in the form of images which contain a part of a file system with the required libraries, executables, configuration files, etc. Images are stored in centralized repositories where they are accessible from any compute server. To deploy a container, Docker therefore first downloads the image from the repository and locally installs it, unless the image is already cached in the compute node. Starting a container from a locally-installed image is as quick as starting the processes which constitute the container's application. The deployment time of any container is therefore dictated by the time it takes to download, decompress, verify, and locally install the image before starting the container itself.

1) Image structure: Docker images are composed of multiple layers stacked upon one another: every layer may add, remove, or overwrite files present in the layers below itself. This enables developers to build new images very easily by simply specializing pre-existing images.

The same layering strategy is also used to store file system updates performed by the applications after a container has started: upon every container deployment, Docker creates an additional writable top-level layer which stores all updates following a Copy-on-Write (CoW) policy. The container's image layers themselves remain read-only. Table I shows the structures of the three images used in this paper.

2) Container deployment process: Docker images are identified with a name and a tag representing a specific version of the image. Docker users can start any container by simply giving its name and tag using the command: docker run IMAGE:TAG [parameters] Docker keeps a copy of the latest deployed images in a local cache. When a user starts a container, Docker checks its cache and pulls the missing layers from the docker registry before starting the container. This work aims to better understand the hardware resource usage of the Docker container deployment process, and to propose alternative techniques to speed up the download and installation of the required image layers. We assume that the image cache is empty at the time of the deployment request: fog computing servers will most likely have very limited storage capacity so in this context we expect that cache misses will be the norm rather than the exception.

B. Related work

Many research efforts have recognized the potential of single-board devices for building fog computing infrastructures and have evaluated their suitability for handling cloud-like types of workloads. For instance, Bellavista et al demonstrated that even extremely constrained devices such as Raspberry PIs may be successfully used to build IoT cloud gateways. [START_REF] Bellavista | Feasibility of fog computing deployment based on Docker containerization over RaspberryPi[END_REF]. With proper configuration, these devices can achieve scalable performance with minimal overhead. However, the study assumes that the Docker container images are already cached in the local nodes. In contrast, we focus on the download-and-install part of the container deployment, and show that simple modifications can significantly improve the performance of this operation.

A number of approaches propose to improve the design of Docker registries [START_REF] Anwar | Improving Docker registry design based on production workload analysis[END_REF]. CoMICon is a distributed Docker registry which distributes layers of an image among multiple nodes to increase availability and reduce the container provisioning time [START_REF] Nathan | CoMICon: A co-operative management system for docker container images[END_REF]. Distribution allows one to pull an image from multiple registries simultaneously, which reduces the average layer's download times. Similar approaches rely on peer-to-peer protocols instead [START_REF] Babrou | Docker registry bay[END_REF], [START_REF] Ma | Dockyard -container and artifact repository[END_REF]. However, distributed downloading relies on the assumption that multiple powerful servers are interconnected with a high-speed local-area network, and therefore that the main performance bottleneck is the long-distance network to a remote centralized repository. In the case of fog computing platforms, servers will be geographically distributed to maximize proximity to the end users, and they will rarely be connected to one another using high-capacity networks. As we discuss in the next section, the main bottleneck in fog computing nodes is created by hardware limitations of every individual node.

Another way to improve the container deployment time is to propose a new Docker storage driver. Slacker proposes to rely on a centralized NFS file system to share the images between all the nodes and registries [START_REF] Harter | Slacker: Fast distribution with lazy Docker containers[END_REF]. The lazy pulling of the container image in the proposed model significantly improves the overall container deployment time. However, Slacker expects that the container image is already present in the local multi-server cluster environment; in contrast, a fog computing environment is made of large numbers of nodes located far from each other, and the limited storage capacity of each node implies that few images can be stored locally for future use. Besides, Slacker requires flattening the Docker images in a single layer. This makes it easier to support snapshot and clone operations, but it deviates from the standard Docker philosophy which promotes the layering system as a way to simplify image creation and updates. Slacker therefore requires the use of a modified Docker storage driver (with de-duplication features) while our work keeps the image structure unmodified and does not constraint the choice of a storage driver. We discuss the topic of flattening Docker images in Section V-A.

III. UNDERSTANDING THE DOCKER CONTAINER DEPLOYMENT PROCESS

To understand the Docker container deployment process in full details we analyzed the hardware resource usage during the download, installation and deployment of a number of Docker images on a Raspberry PI-based infrastructure.

A. Experimental setup

We monitored the Docker deployment process on a testbed which consists of three Raspberry Pi 3 machines connected to each other and to the rest of the Internet with 10 Gbps Ethernet [START_REF]The mobile edge cloud testbed at IRISA Myriads team[END_REF]. The testbed was installed with Table I depicts the images we used for this study. The first one simply conveys a standard Ubuntu operating system: it is composed of one layer containing most of the content, and four small additional layers which contain various updates of the base layer. We created a so-called Mubuntu image by adding an extra 51 MB layer which represents a typical application's code and data which rely on the base Ubuntu image, following the incremental approach promoted by the Docker system. Finally, the BigLayers image is composed of four big layers which allow us to highlight the effect of the layering system on container deployment performance. We stored these images in the public Docker repository [START_REF] Inc | Docker hub[END_REF] so every experiment includes realistic download performance from a highly-utilized public repository.

We instrumented the testbed nodes to monitor the overall deployment time as well as the utilization of important resources during the container deployment process:

• Deployment time: We measured deployment times from the moment the deployment command is issued, to the time when Docker reports that the container is started. • Network activities: We monitored the traffic from/to the Docker daemon (excluding other unrelated processes) on the Ethernet interface using NetHogs tool at a 1second granularity. • Disk throughput: We monitored the disk activity with the iostat Linux command which monitors the number of bytes written to or read from disk at a 1-second granularity.

• CPU usage: We monitored CPU utilization by watching the /proc/stat file at a 1-second granularity. Every container deployment experiment was issued on an otherwise idle node, and with an empty image cache.

B. Monitoring the Docker container deployment process

Figure 1 depicts the results when deploying the three images using regular Docker. Figure 1(a) shows the deployment time of our three images in different network conditions: deploying the Ubuntu, Mubuntu and Biglayers images with unlimited network bandwidth respectively takes 240, 333 and 615 seconds. Clearly, the overall container deployment time is roughly proportional to the size of the image. When throttling the network capacity, deployment times grow steadily as the network capacity is reduced to 1 Mbps, 512 kbps, and 256 kbps. For instance, deploying the Ubuntu container takes 6 minutes when the network capacity is reduced to 512 kbps. This is considerable with regards to the deployment efficiency one would expect from a container-based infrastructure. However, the interesting information for us is the reason why deployment takes so long, as we discuss next.

Figure 1(b) depicts the utilization of different hardware resources from the host machine during the deployment of the standard Ubuntu image. The red line shows incoming network bandwidth utilization, while the blue curve represents the number of bytes written to the disk and the black line shows the CPU utilization. The first phase after the container creation command is issued involves intensive network activities, which indicates that Docker is downloading the image layers from the remote image registry. By default Docker downloads up to three image layers in parallel. The duration of downloads clearly depend on the image size and the available network capacity: between 55 s and 110 s for the Ubuntu and Mubuntu images. During this phase, we observe no significant disk activity in the host machine, which indicates that the downloaded file is kept in main memory.

After the download phase, Docker extracts the downloaded image layers to the disk before building the final image of the application. The extraction of a layer involves two operations: decompression (which is CPU-intensive) and writing to the disk (which is disk-intensive). We observe that the resource utilization alternates between periods during which the CPU is busy (∼40% utilization) while few disk activities are performed, and periods during which disk writes are the only notable activity of the system. We conclude that, after the image layers have been downloaded, Docker sequentially decompresses the image and writes the decompressed data to disk. When the image data is big, Docker alternates between partial decompressions and disk writes, while maintaining the same sequential behavior.

We see exactly the same phenomenon in Figure 1(c). However, here, the downloading of the first layer terminates before the other layers have finished downloading. The extraction of the first layer can therefore start before the end of the download phase, creating a small overlap between the downloading and extraction phases. the other two take a negligible amount of time. In this paper, we therefore focus on optimizing the pull operation.

2) Pulling image layers in parallel: By default, Docker downloads image layers in parallel with a maximum parallelism level of three. These layers are then decompressed and extracted to disk sequentially starting from the first layer. However, when the available network bandwidth is limited, downloading multiple layers in parallel will delay the download completion of the first layer, and therefore will postpone the moment when the decompression and extraction process can start. Therefore, delaying the downloading of the first layer ultimately leads to slowing down the extraction phase.

3) Single-threaded decompression: Docker always ships the image layers in compressed form, usually implemented as a gzipped tar file. This reduces the transmission cost of the image layers but it increases the CPU demand on the client node to decompress the images before extracting the image to disk. Docker decompresses the images via a call to the standard gunzip.go function, which happens to be singlethreaded. However, even very limited machines usually have several CPU cores available (4 cores in the case of a Raspberry Pi 3). The whole process is therefore bottlenecked by the single-threaded decompression. As a result the CPU utilization never grows beyond ∼40% of the four cores of the machine, wasting precious computation resources which may be exploited to speed up image decompression.

4) Resource under-utilization: The standard Docker container deployment process under-utilizes the available hardware resources. Essentially, deploying a container begins with a network-intensive phase during which the CPU and disk are mostly idle. It then alternates between CPUintensive decompression operations (during which the network and disk are mostly idle) and I/O-intensive image extraction operations (during which the network and CPU are mostly idle). The only case where these operations slightly overlap are images such as Mubuntu and BigLayers when the decompress and extraction process of the first layer can start while the last images are still being downloaded.

This resource under-utilization is one of the main reason for the poor performance of the overall container deployment process. The main contribution of this paper is to show how one may reorganize the Docker deployment process to maximize resource utilization during deployment.

IV. OPTIMIZING THE CONTAINER DEPLOYMENT

PROCESS

To address the inefficiencies presented in the previous section we propose and evaluate three optimizations which deal with different issues in the deployment process. We can therefore combine them all together, which brings significant performance improvement.

A. Sequential image layer downloading

Simultaneous downloading of multiple images obviously aims to maximize the overall network throughput. However, it has negative effects because the decompress and extraction phases of each image layer must take place sequentially to preserve the Copy-on-Write policy of Docker storage drivers. The decompress & extract phase can start only after the first layer has been downloaded. Downloading multiple image layers in parallel will therefore delay the download completion of the first layer because this download must share network resources with other image layer downloads, and will therefore also delay the moment when the first layer can start its decompress & extract phase. We therefore propose to download image layers sequentially rather than concurrently.

Figure 2 illustrates the effect of downloading layers sequentially rather than in parallel. In both cases, three threads are created to handle the three image layers. However, in the first option the downloads take place in parallel whereas the only required inter-thread synchronization requires that the decompression and extraction of layer n can start only after the decompression and extraction of layer n -1 has completed. In sequential downloading, the second layer starts downloading only when the first download has completed, which means that it takes place while the first layer is being decompressed and extracted to disk. This allows the first-layer extraction to start sooner and it also increases resource utilization because the download and Implementing sequential downloading can be done easily by setting the "max-concurrent-downloads" parameter to 1 in the /etc/docker/daemon.json configuration file.

Figure 3(a) depicts the resource usage of the host machines when deploying the Mubuntu image with sequential downloading and a 1 Mbps network capacity. We observe that after the downloading of layer 1 has completed, the utilization of hardware resources is much greater, with in particular a clear overlap between periods of intensive network, CPU and I/O resources. Also we can observe that the decompression of the first layer (visible as the first spike of CPU utilization) takes place sooner than in Figure 1(c).

Figure 3(b) compares the overall container deployment times with parallel and sequential downloads in various network conditions. When the network capacity is unlimited the performance gains in the deployment of the Ubuntu, Mubuntu and BigLayers images are 3%, 4.2% and 6% respectively.

However, the performance gains grow steadily as the available network bandwidth gets reduced. With a bandwidth cap of 256 kbps, sequential downloading brings improvements of 6% for the Ubuntu image, 10% for Mubuntu and 12% for BigLayers. This is due to the fact that slower network capacities exacerbate the duration of the download phases and increases the delaying effect of parallel layer downloading.

B. Multi-threaded layer decompression

By default, Docker uses the gunzip.go library to decompress the downloaded image layers before extracting to the disk. However, this function is single-threaded, which implies that the CPU utilization during decompression never exceeds 40% of the four available cores in the Raspberry Pi machine. We therefore propose to replace the single-threaded gunzip.go library with a multi-threaded implementation so that all the available CPU resources may be used to speed up this part of the container deployment process.

We use pgzip, which is a multi-threaded implementation of the standard gzip/gunzip functions [START_REF] Post | klauspost/pgzip: Go parallel gzip (de)compression[END_REF]. Its functionalities are exactly the same as those of the standard gzip, however it splits the work between multiple independent threads. When applied to large files of at least 1 MB, this can significantly speed up decompression.

Figure 4(a) depicts the deployment time of a singlelayered image while using various of threads for decompression. When pgzip uses a single thread, the performance and CPU utilization during decompression are very similar to the standard gunzip implementation. However, when we increase the number of threads, the overall container deployment time decreases from 154 s to 136 s. At the same time, the CPU utilization during decompression steadily increases from 40% to 71% of the four available CPU cores. If we push beyond 12 threads, no additional gains are observed. We clearly see that the parallel decompression does not scale linearly, as it is not able to exploit the full capacity of the overall CPU: this is due to the fact that gzip decompression must process data blocks of variable size so the decompression operation itself is inherently singlethreaded [START_REF] Sitaridi | Massively-parallel lossless data decompression[END_REF]. The benefit of multi-threading decompression is that other necessary operations during decompression such as data buffering and CRC verification can be delegated to other threads and moved out of the critical path.

Figure 4(b) shows the effect of using parallel decompression when deploying Mubuntu images with 12 threads. We observe that the CPU utilization is greater during the decompression phases than with standard Docker, in the order of 70% utilization instead of 40%. Also, the decompression phase is notably shorter.

Figure 4(c) compares the overall container deployment times with parallel decompression against that of the standard Docker. The network performance does not influence the time of the decompression phase so we conducted the evaluation only with an unlimited network capacity. The performance gain from multi-threaded decompression is similar for all three images, in the order of 17% of the overall deployment time.

C. I/O pipelining

Despite the sequential downloading and the multithreaded decompression techniques, the container deployment process still under-utilizes the hardware resources. This is due to the sequential nature of the workflow which is applied to each individual layer: each layer is first downloaded in its entirety, then it is decompressed entirely, then it is extracted to disk. This requires Docker to keep the entire decompressed layer in memory, which can be significant considering that a Raspberry Pi 3 has only 1 GB of main memory. Also, it means that the first significant disk activity can start only after the first layer has been fully downloaded and decompressed. Similarly, Docker necessarily decompresses and extracts the last layer to disk while the networking device is mostly inactive. However, there is no strict requirement for the download, decompress and extraction of a single layer to take place sequentially. For example, decompression may start right after the first bytes of the compressed layer have been downloaded. Similarly, extracting the layer may start immediately after the beginning of the layer image has been decompressed.

We therefore propose to reorganize the download, decompression and extraction of a single layer in three separate threads where each thread pipelines data to the next as soon as some data is available. In Unix shell syntax this essentially replaces the sequential "download; decompress; crc-check; extract" command with the concurrent "download | decompress | crc-check | extract" command. Since we stream the incoming downloaded data without buffering the entire layer, the extraction can start writing content to disk long before the download process has completed.

We implemented pipelining using the io.pipe() GO API [START_REF]The Go Authors[END_REF], which creates a synchronized in-memory pipe between an io.reader(pgzip/decompress) and an io.writer(network/download) without internal buffering. However, we must be careful about synchronizing this process between multiple image layers: for example, if we created an independent pipeline for each layer separately, the result would violate the Docker policy that layers must be extracted to disk sequentially, as one layer may overwrite a file which is present in a lower layer. If we extracted multiple layers simultaneously we could end up with the wrong version of the file being given to the container. Rather than building complex synchronization mechanisms, we instead decided to rely on Docker's sequential downloading feature already discussed in Section IV-A. When a multi-layer image is deployed, this imposes that layers are downloaded and extracted one after the other, while using the I/O pipelining technique within each layer.

Figure 5 evaluates the I/O pipelining technique using a single-layer image. We can see that the pipelined version is roughly 50% faster than its standard counterpart: in the standard deployment, resources are used one after the other: first network-intensive download, then CPU-intensive decompression, then finally disk-intensive image creation. In the pipelined version all operations take place simultaneously, which better utilizes the available hardware and significantly reduces the container deployment time.

D. Docker-pi

The three techniques presented previously address different issues. Sequential downloading of the image layers speeds up the downloading of the first layer in slow network We therefore propose Docker-pi, an optimized version of Docker which combines the three techniques to optimize container deployment on single-board machines. Figure 6(a) shows the resource usage while deploying the Mubuntu image using Docker-pi. We can clearly see that the networking, CPU and disk resources are used simultaneously and have a much greater utilization than with the standard Docker implementation. In particular, the CPU and disk activities start very early after the first few blocks of data have been downloaded.

Figure 6(b) highlights significant speedups compared to vanilla Docker: with no network cap, Docker-pi is 73% faster than Docker for the Ubuntu image, 65% faster for Mubuntu and 58% faster for BigLayer. When we impose bandwidth caps the overall deployment time becomes constraint by the download times, while the decompression and extraction operations take place while the download is taking place. In such bandwidth-limited environments the deployment time therefore cannot be reduced any further other than by pre-fetching images before the container deployment command is issued.

The reason why the gains are slightly lower for the Mubuntu and BigLayers images is that the default Docker download concurrency degree of 3 already makes them benefit from some of the improvements we proposed in Docker-pi. If we increase the concurrency degree of Docker to 4, the BigLayers image deploys in 644 s whereas Dockerpi needs only 207 s, which represents 68% improvement.

V. DISCUSSION

A. Should we flatten all Docker images? Flattening Docker images may arguably provide performance improvement in the deployment process. Indeed, multiple image layers may contain successive versions of the same file whereas a flattened image contains only a single version of every file. A flattened image is therefore slightly smaller than its multi-layered counterpart. Systems such as Slacker actually rely on the fact that images have been flattened [START_REF] Harter | Slacker: Fast distribution with lazy Docker containers[END_REF]. On the other hand, Docker-pi supports both flattened images and unmodified multi-layer images. We however do not believe that flattening all images would bring significant benefits.

Docker does not provide any standard tool to flatten images. This operation must be done manually by exporting an image with all its layers, and re-importing the result as a single layer while re-introducing the startup commands from all the initial layers. The operation must be redone every time any update is made in any of the layers. Although this process could be integrated in an image build workflow, it contradicts the Docker philosophy which promotes incremental development based on image layer reusability.

In a system where many applications execute concurrently, one may reasonably expect many images to share at least the same base layers (e.g., Ubuntu) which produce a standard execution environment. If all images were flattened this would result in large amounts of redundancy between different images, creating the need for sophisticated deduplication techniques [START_REF] Harter | Slacker: Fast distribution with lazy Docker containers[END_REF]. On the other hand, we believe that the layering system can be seen as a domain-specific form of de-duplication which naturally integrates in a developer's devops workflow. We therefore prefer keeping docker images unmodified, and demonstrated that container deployment can be made extremely efficient without the need for flattening images.

B. Does Docker-pi work also for powerful server machines?

Although we designed Docker-pi for single-board machines, the inefficiencies of vanilla Docker also exist in powerful server environments. We therefore evaluated the respective performance of Docker and Docker-pi in the Grid'5000 testbed which is commonly used for research on parallel and distributed computing including Cloud, HPC and Big Data [START_REF] Balouek | Adding virtualization capabilities to the Grid'5000 testbed[END_REF]. We specifically used a Dell PowerEdge C6220 server equipped with two 10-core Intel Xeon E5- 2660v2 processors running at 2.2GHz, 128 GB of main memory and two 10 Gbps network connections. Figure 6(c) compares the deployment times of Docker and Docker-pi with our three standard images. Obviously container deployment is much faster in this environment than in Raspberry PIs. However, here as well Docker-pi provides respectable performance improvement in the order of 23-36%. In this powerful server the network and CPU resources cannot be considered as bottlenecks so the sequential layer downloading and multi-threaded decompression techniques bring little improvement compared to the standard Docker. On the other hand, the sequential nature of the download/decompress/extract process is still present regardless of the hardware architecture, so the I/O pipelining technique brings similar performance gains as with the Raspberry PI.

VI. CONCLUSION

The transition from virtual machine-based infrastructures to container-based ones brings the promise of swift and efficient software deployment in large-scale computing infrastructures. However, this promise is not being held in fog computing platforms which are often made of very small computers such as Raspberry PIs, and where deploying even a very simple Docker container may take multiple minutes.

We identified three sources of inefficiency in the Docker deployment process and proposed three optimization techniques which, once combined together, speed up container deployment roughly by a factor 4. Last but not least, we demonstrated that these optimizations also bring significant benefits in regular server environments.

This work eliminates the unnecessary delays that take place during container deployment. Depending on the hardware, deployment time is now dictated only by the slowest of the three main resources: network bandwidth, CPU, or disk I/O. As hardware will evolve in the future the bottleneck may shift from one to the other. But, regardless of the specificities of any particular machine, Docker-pi will exploit the available hardware to its fullest extent.

C. Critical observations 1 )Figure 1 .

 11 Figure 1. Deployment times and resource usage using standard Docker

Figure 2 .

 2 Figure 2. Standard and sequential layer pull operations

Figure 3 .Figure 4 .

 34 Figure 3. Resource usage and deployment time with sequential layers downloading

Figure 5 .

 5 Figure 5. Evaluation of I/O pipelining

Figure 6 .

 6 Figure 6. Evaluation of Docker-pi in RPI and powerful server machines

Table I STRUCTURE

 I OF THE DOCKER IMAGES This setup also allowed us to emulate slower network connections -which are arguably representative of real fog computing scenarios -by throttling network traffic at the network interface level. We used the tc command to run experiments either with unlimited bandwidth, or with limits of 1 Mbps, 512 kbps or 256 kbps.

		Ubuntu Mubuntu BigLayers
	6th layer	-	51 MB	-
	5th layer	<1 MB	<1 MB	-
	4th layer	<1 MB	<1 MB	62 MB
	3rd layer	<1 MB	<1 MB	54 MB
	2nd layer <1 MB	<1 MB	64 MB
	1st layer	46 MB	46 MB	52 MB
	Total size	50 MB	101 MB	232 MB
	Docker version 17.06.


Anaïk Purenne

«TOWARDS CITIES OF INFORMERS? COMMUNITY-BASED SURVEILLANCE IN FRANCE AND CANADA"

What are the effects of citizen-based surveillance? Examining contrasted programs in France and Canada, this article shows that citizen involvement in surveillance actions can have ambivalent, multifaceted effects. Participatory surveillance can help to strengthen the community's sense of belonging, while paradoxically contributing to instil fear. However, these initiatives do not inevitably lead to a culture of generalized suspicion. Depending on the ability of residents to open up controversial subjects for debate, such programs can also leave the way open to a democratization of public action.

Introduction

Since the end of the 1980s, the sociological reflection on the new political uses of "surveillance" (i.e., data collection and analysis) has emerged as an important theme in the academic literature under the label of Surveillance Studies. In both North America and Europe, research attention has focused on surveillance technologies as an instrument of subtle and diffuse social control, and on their effects in terms of privacy and social inequalities. This research agenda, however, has minimized forms of data collection that are not (or not necessarily) based on new technologies-in particular, citizen-based surveillance.

"Participatory surveillance" (Hier and Greenberg 2009) exercised by the average citizen has long been negatively perceived by the institutions in charge of social control. Indeed, citizenbased surveillance was often dedicated to identifying inadequate practices and abuses of force, or to compensating for the police's failure to act in deprived areas [START_REF] Marx | Commentary: Some Trends and Issues in Citizen Involvement in the Law Enforcement Process[END_REF][START_REF] Vindevogel | La mobilisation des énergies privées pour l'amélioration de la sécurité publique à New York[END_REF]). However, this kind of community-based surveillance is now perceived and used by public institutions as a means to extend their own capacity for surveillance, as is illustrated by the spread of public vigilance campaigns against terrorism [START_REF] Chan | The New Lateral Surveillance and a Culture of Suspicion, In: "Surveillance and Governance: Crime Control and Beyond[END_REF][START_REF] Larsen | Public Vigilance Campaigns and Participatory Surveillance after 11 September 2001[END_REF]. For the police, citizen involvement also offers the opportunity to improve their image and to give the impression of being close to local communities, the members of which are encouraged to take an active part in crime prevention (Garland 2001: 123s) or in national security programs. As Mark Andrejevic suggests (2005), such citizen involvement pushed by top-down programs can be analyzed as an extension of the government surveillance toolbox. In this sense, it deserves the same attention as CCTV, computerized databases, electronic monitoring, and other technological devices [START_REF] Larsen | Public Vigilance Campaigns and Participatory Surveillance after 11 September 2001[END_REF][START_REF] Parnaby | Natural Surveillance, Crime Prevention, and the Effects of Being Seen[END_REF].

The present article focuses on the formalization of community-based surveillance in crimeprevention citizens' groups. Two prominent approaches characterize the literature on community-based initiatives such as Neighborhood Watch: on the one hand, quantitative evaluation research that addresses, for instance, the overall "effectiveness" of these programs to deter crime (see for example [START_REF] Bennett | The Effectiveness of Neighborhood Watch[END_REF][START_REF] Rosenbaum | The Theory and Research behind Neighborhood Watch: is it a sound Fear and Crime Reduction Strategy?[END_REF], and, on the other hand, in-depth ethnographic observation of anti-crime citizens' initiatives in a given city [START_REF] Bénit | Nous avons dû prendre la loi entre nos mains'. Pouvoirs publics, politique sécuritaire et mythes de la communauté à Johannesburg[END_REF][START_REF] Raoulx | Un centre ville 'safe and clean' (sûr et propre)? Marginalité, interventions urbanistiques et contrôle social en Amérique du Nord. Les exemples de Vancouver (Colombie-Britannique) et de Richmond (Virginie)[END_REF][START_REF] Schneider | Refocusing Crime Prevention. Collective Action and the Quest for Community[END_REF][START_REF] Vindevogel | La mobilisation des énergies privées pour l'amélioration de la sécurité publique à New York[END_REF]. While these studies have generated empirical knowledge on a wide range of forms and meanings associated with communitybased surveillance [START_REF] Dupont | Police communautaire et de résolution des problèmes[END_REF], the results are hardly cumulative given the absence of any explicit theoretical framework and/or comparative dimension. For his part, Gary Marx has proposed a research program for considering citizen involvement in anti-crime groups across time and place. His underlying hypothesis is that the coproduction of security has ambiguous and contradictory effects. On the one hand, these initiatives can facilitate cooperation among inhabitants of various backgrounds, as the perception of shared concerns cements local communities and prompts them to take responsibility for local issues. On the other hand, they can generate unanticipated effects that require careful examination. First, insofar as poor neighborhoods generally lack the social resources needed to mobilize around local problems, community-based surveillance programs can increase social and spatial inequalities by giving rise to a two-tier system (on this topic, see [START_REF] Schneider | Refocusing Crime Prevention. Collective Action and the Quest for Community[END_REF]. Second, and most importantly, there is a risk that, as [START_REF] Marx | Commentary: Some Trends and Issues in Citizen Involvement in the Law Enforcement Process[END_REF] observes, fear of crime and suspicion of others will be exacerbated, prompting the emergence of a "nation of informers." This last idea echoes the related literature on "lateral surveillance." While civilian participation is often assumed to give a democratic face to surveillance, Andrejevic emphasizes that "the result has not so much been a democratization of politics […], but the injunction to embrace strategies of law enforcement. […] In an era in which everyone is to be considered potentially suspect, we are invited to become spies" (2005: 494). The culture of suspicion may indeed result in reinforcing racial stereotyping and racism [START_REF] Chan | The New Lateral Surveillance and a Culture of Suspicion, In: "Surveillance and Governance: Crime Control and Beyond[END_REF].

The purpose of this article is to discuss this hypothesis through empirical investigation in Canada and France. The marked contrasts between these two contexts (see below) allow us to appreciate the degree of convergence or divergence regarding the effects of civilian crime prevention initiatives. Indeed, comparing contrasted rather than similar cases better reflects the plurality of social reality [START_REF] Giraud | Les défis de la comparaison à l'âge de la globalisation : pour une approche centrée sur les cas les plus différents inspirée de Clifford Geertz[END_REF]. This attention to plurality is especially important because "often, surveillance technologies and practices are seen as being undesirable, antithetical to democracy and individual autonomy" (Albrechtslund and Glud 2010: 235).

Covering very different contexts may provide a more nuanced picture of surveillance.

In Anglo-American countries, citizen involvement in self-defense and crime-prevention groups has a long and complex history [START_REF] Brown | The American Vigilante Tradition[END_REF][START_REF] Wilson | Broken Windows. The Police and Neighborhood Safety[END_REF]. As in other countries, "the informal watching of communities by their members [preceded] the institution of public police" (Chan 2008: 224). Yet the tradition of vigilantism remained vivid even after the creation of regular, professional police forces, and vigilante groups have sometimes taken the law into their own hands in an attempt to substitute for the justice system. Unlike these earlier forms of civilian participation, most contemporary initiatives are devoted to protecting local communities by reporting crime to the authorities [START_REF] Marx | Citizen Involvement in the Law Enforcement Process[END_REF].

Significantly, the formalization of citizen-based surveillance into neighborhood patrols and Neighborhood Watch groups in the 1970s and 1980s has often been supported by governmental agencies or by voluntary police associations such as the National Sheriffs' Association. While these surveillance programs have received most media and academic attention, they are only the tip of the iceberg. Community mobilization is not confined to police-run programs in which residents are trained to become the eyes and ears of the police and help them through information sharing. As Michele Elizabeth Cairns outlines, "for a number of reasons, citizens have become involved in crime prevention. Some of them include a desire to increase the livability in their neighborhoods, to educate themselves on ways to protect against crime and to avoid victimizations, and to address underlying reasons for criminality" (Cairns 1998: 17). In cities like New York and Vancouver, such bottom-up, grassroots initiatives were implemented by residents who had become aware of the police's inability to solve local problems on their own. The Community Policing Centers that we studied in Vancouver offer a good example of these grassroots initiatives.

Community-based surveillance developed more recently in some European countries. This late development coincides with the decline of Neighborhood Watch programs in the Anglo-American world as a result of limited effectiveness and lack of participation [START_REF] Chan | The New Lateral Surveillance and a Culture of Suspicion, In: "Surveillance and Governance: Crime Control and Beyond[END_REF]. In France, Claude Guéant, Home Minister in the former right-wing government of Nicolas Sarkozy, promoted the so-called "Voisins vigilants" in the mid-2000s. This government policy mainly focuses on information sharing between the police and citizens. Explicit reference is made to the Anglo-Saxon model of Neighborhood Watch, which is seen as an appropriate model of action, especially for suburban areas. It is stated that, in these neighborhoods, "part of the local population is present throughout the day, and there is preexisting social cohesion." 1 Without being officially supported by the left-wing government elected in 2012, these initiatives have not been abolished despite strong media criticism. Instead, the Voisins vigilants programs have been renamed under the more neutral label of "civic participation," and continue to develop to this day. According to the latest official report published by the Gendarmerie Nationale, 2 the figures rose from eight programs in mid-2011 to 123 in mid-2012 and to 484 in mid-2013. The Home Ministry indicates that these numbers-which do not even give the full picture since they only concern rural and suburban areas 3 -are likely to double in the next few years.

In addition to the different origins of the programs studied, another strong contrast lies in the diametrically opposed political cultures of France and Canada. In France, a strong emphasis is laid on the nation-state, which plays a central role in maintaining social order. In this country, the mere mention of strengthening the role of communities and intermediary bodies leads to 1 http://asset.rue89.com/files/cir_33332.pdf [Accessed: January 20, 2013]. 2 Direction Générale de la Gendarmerie Nationale, Participation citoyenne: bilan de la mise en oeuvre, 19 septembre 2013. The Gendarmerie Nationale is a military institution under the Home Ministry that is in charge of public safety and has jurisdiction over rural areas and small towns. 3 To our knowledge, no official estimate is publicly available regarding urban areas and larger towns placed under the Police Nationale jurisdiction, which is the main civil law enforcement agency in France. heated political debate [START_REF] Donzelot | Faire société. La politique de la ville aux Etats-Unis et en France[END_REF]. Hence the vehement criticism voiced against the idea that local communities might develop a capacity for social control, and the widely held perception that public safety should remain a matter of professional state actors [START_REF] Robert | Les territoires du contrôle social, quels changements ?[END_REF]. By contrast, Anglo-American countries such as the United States and Canada are more open to community and lateral surveillance. For instance, the US has "more formal public and private programs for involving citizens in information gathering and less ambivalence toward (and suspicion of) such efforts. This attitude reflects Anglo-American traditions of government and police in principle being a part of the community. […] The English language has no equivalent for the French la délation, the activity of informers (called les corbeaux, for crows)" (Marx 2013: 59).

A further difference that needs to be highlighted concerns the degree of urbanization of the sites under study. In France, the sites we selected are located in a suburban zone of the Essonne department, which was one of the pilot areas used under the governmental Voisins vigilants policy. The city of Breuillet (approximately 8,000 inhabitants) and the adjacent village of Saint-Yon (approximately 1,000 inhabitants) signed an official agreement with representatives of state agencies in October 2012. While these two municipalities are fairly representative of the French context, their population density and social composition (with an overwhelming majority of white residents) are very different from those of the site selected in Canada. Vancouver, which is one of the most multicultural cities in Canada, is populated by approximately 600,000 inhabitants distributed across 24 districts. Almost half of these districts have developed a Community Policing Center since the mid-1990s. These differences in the social composition of the neighborhoods and urban/suburban features appear to play an important role in terms of perceptions of insecurity and suspicion of others. Indeed, it is generally believed that inner cities with a high population density and mixed communities foster the acceptance of otherness. On the contrary, low density suburban areas marked by greater social homogeneity are said to encourage rejection of other people and to foster identitarian closure. The contrasts between these two contexts allow us to consider the potential diversity of "lateral surveillance" and of its implications for citizens.

Both case studies were based on document analysis (official speeches, policy guidelines, agency reports, newspapers) and a series of interviews (n=circa 30). Respondents were recruited through a snowball method. Half of them were volunteers or community organizers; the other half were police officers or elected officials in communication with the community-based crime-prevention groups we studied. Some of them were top officials (mayor, chief of police), and others were lower grades (front-line or neighborhood police officers). The questions asked concerned the following: organization of the group, means for recruiting participants, motivation to participate and issues at stake, type of operations initiated, relationship between the group and local authorities, theory of crime prevention, and potential added value of these initiatives.

It must be outlined that most interviews in Vancouver were conducted in 2012 and 2013, at a time when the police were questioning the Community Policing Centers, whereas the study in France took place in 2013, that is, when the Voisins vigilants programs were becoming popular. These contextual differences, as well as the small number of participants actually interviewed, constitute obvious limitations of the present analysis. This article, however, is not intended to reach any firm conclusions or broad generalizations; instead, it is exploratory in nature. The purpose of this comparative analysis is to identify the conditions under which surveillance initiatives can result in positive or negative effects. Indeed, "questions concerning the potential of surveillance for contributing to individual autonomy and dignity, fairness and due process, community cooperation […] have been rare in the field" (Monahan et al. 2010: 106).

These contrasting experiences confirm that anti-crime citizens' groups can strengthen ties within local communities, echoing the personal desire of a majority of participants to get involved in their own neighborhoods. This is especially true when the groups' objectives are not limited to safety concerns (1). While these efforts are intended to promote inclusion and civic participation among local populations, they can also lead to reinforcing the fear of others. Indeed, public authorities often wish to stay at the frontline to fight crime, and are prone to dramatize safety concerns in order to ensure sustained involvement in these programs (2). Can these trends be resisted and result in a democratization of politics (3)?

"It is not about Safety, it is about Quality of Life" 4

As pointed out a few decades ago by Gary Marx and Dane Archer (1971), citizen involvement in crime prevention initiatives and self-defense groups is often a controversial issue among local communities. The specter of vigilantism is still vivid, and often gives a negative aura to groups whose participants are depicted as obsessed by law and order. The advocates of these programs, for their part, lay emphasis on the fact that participatory surveillance is like any other form of civilian participation in collective action. Our case study suggests that the reality is not so binary. These two dimensions are often combined owing to the heterogeneous nature of these groups and the diversity of members' expectations. In addition, while certain participants clearly value law and order, most activities are dedicated to preventing the degradation of the neighborhood.

Suspicious or Civic-Minded Participants?

In fact, whether they are instigated by public authorities or not, participatory surveillance initiatives bring together people from diverse backgrounds. Both the Voisins vigilants programs in the Essonne department and the Community Policing Centers in Vancouver appear to have a relatively mixed membership. 5 One can therefore observe a variety of expectations regarding surveillance.

In Breuillet and Saint-Yon, as in many French small towns, the projects were initiated and designed by the mayor in partnership with the local police. After publishing a formal call for participation, the elected officials coopted a number of citizens. The latter can be distinguished in two categories. First, military staff of the nearby French Air Force base are well represented among the residents involved in the experience, with the mayor of Breuillet being himself a former Air Force colonel. These residents explicitly value an ordermaintenance perspective and a close partnership with local public safety officers, and some even advocate for a conservative "get tough" philosophy of crime control. Second, there is a majority of retired people who seek involvement in civic life. In this case, participants' expectations focus on building a connection with their community in order to feel useful. Some of these pensioners are already involved in the activities of other associations, while others volunteer for local services such as the public library. For these people, participation in surveillance programs is all the more attractive since it does not involve a huge investment in time. Except for attending two or three meetings per year, no specific effort is required because domestic burglary is limited in the area. All that participants have to do is report, if necessary, evidence of wrongdoing to the police, which is evidently a duty for all citizens. In other words, these programs are often seen as a way of demonstrating one's goodwill and civic spirit without having to commit too strongly. Such flexible or "plug-in volunteering" [START_REF] Lichterman | Elusive Togetherness. Church groups trying to bridge America's Divisions[END_REF]) is particularly convenient for pensioners who frequently travel to other countries. This is illustrated by the case of a volunteer we met: a retired woman who decided to abandon her term as a municipal councilor in order to spend more time with her grandchildren living abroad. These observations are not confined to crime prevention programs. On the contrary, they follow widespread trends that have been well documented by Nina [START_REF] Eliasoph | Making Volunteers. Civic Life after Welfare's End[END_REF] and other scholars concerning collective participation at large. This literature emphasizes the fact that involvement in community-based activities is especially attractive because it is brief, irregular, and uncomplicated (on this topic, see for instance [START_REF] Talpin | Participating Is Not Enough. Civic Engagement and Personal Change[END_REF]).

The polarization of expectations and trends is also observable in Vancouver, even though the ten Community Policing Centers (CPCs) spread throughout the city engage in more substantial day-to-day activities than the French Voisins vigilants groups. Most of them are run by residents and operated by volunteers, with the support of professional organizers. 6These volunteers are coopted by CPC members, without any interference from elected members of the City Council. Among other activities, volunteers are in charge of keeping a permanent office open during weekdays, with the aim of supporting victims and encouraging people to report crime. Those most involved are thus seniors, who have more time to participate on a regular basis. These citizens often put forward the idea that "it is important to take responsibility for your neighborhood." 7 Indeed, the main focus of the CPCs is on "community building." By contrast, participants in the Voisins vigilants programs in France are not expected to know each other since the mayor is at the core of the network. These contrasts between the two models of action reflect the differences between the French and the Anglo-American traditions of government. Echoing the belief that local communities are legitimate actors of social control, a major concern in Vancouver is to build social relationships in neighborhoods through the organization of community events like barbecues, parties, and fairs that promote multiculturalism or intergenerational understanding. These community events also give people the opportunity to meet each other, and to reinforce social interaction both between neighbors and between neighbors and the police.

Yet, students are also well represented among participants of the CPCs. The majority of young volunteers are applicants to the police force, insofar as volunteering is a mandatory requirement to enter the police academy. These students do not necessarily live in the area. In some centers, most young volunteers come from other neighborhoods or other cities. As a result, they do not necessarily care about community development or about building social relationships with the community. This creates a tension between the practices promoted by professional organizers and the expectations of these "police wannabees," who prefer patrolling to detect stolen cars or suspicious behaviors.

Fear of Crime or Neighborhood Preservation?

Despite the differences between the two countries and the contrasted expectations of participants, the daily issues at stake seem fairly convergent. For both in the Essonne department and in Vancouver, attention is focused not so much on crime as on disorder and quality of life.

In the French case, the specific traits and makeup of the neighborhood as well as the residential trajectories of the volunteers are important factors to consider for understanding this focus on disorder. Indeed, residential mobility often equates to social mobility. In Breuillet and Saint-Yon, many inhabitants belong to middle-income groups who have reached the end of an upward residential trajectory. Most of the volunteers we interviewed previously lived in an apartment in Paris or in a neighborhood near Paris. They then decided to move to a more remote location, where they bought an individual house for their family. As is the case with suburban populations in general, their primary concern is to maintain their current living conditions and a pleasant environment surrounded by fields and forests [START_REF] Charmes | La vie périurbaine face à la menace des gated communities[END_REF].

Their fear is to be caught up by lower income groups and to experience a loss of social status [START_REF] Palierse | Voisins Vigilants ou le 'Neighborhood Watch' à la française : une nouvelle forme de reproduction des pouvoirs locaux[END_REF][START_REF] Humez | La construction sociale de la nuisance. Un exemple dans un quartier industriel de la banlieue lyonnaise[END_REF]. Getting involved in collective projects of crime prevention is a way to address this concern, even though security issues are not necessarily central. The area is not directly affected by crime and most of the concerns raised by the volunteers relate to troubles such as abandoned waste or noisy behavior, which are deemed as important as protecting one's home from burglary. As noted by a volunteer who had been living in Breuillet for ten years, the implementation of a Voisins vigilants program also offered the opportunity to develop social relationships in a type of neighborhood where these are generally weak-even though this was not a stated objective of the mayor:

It allowed me to meet neighbors. I organized a cocktail reception. I invited everyone. Some have been here for ten years or more and have never met. They have been talking to each other ever since. This is also true in Vancouver where participants describe themselves as "people caring more about their neighborhood than about safety."8 In this case, however, the concern is not to maintain one's social status (since the social and ethnic makeup of the neighborhoods is more mixed), but to develop a voluntary approach to crime prevention. In this perspective, most CPCs organize citizen patrols to inspect the neighborhood as well as regular community cleanup campaigns to remove litter or graffiti. The aim is to reduce visible signs of disorder, which are believed to attract criminals and criminal activities. This approach, which focuses on environmental improvements, seems consistent with the well-known "broken window" theory developed by James Wilson and George Kelling (1982). According to the latter, the physical image of the neighborhood must be enhanced to prevent further disorder, rising fear, and criminal victimization. As a CPC website indicates, "removing debris and unwanted graffiti not only makes the neighborhood cleaner, it also makes it safer. Research has shown that systematic removal of garbage and graffiti can greatly reduce crime, vandalism and mischief." 9 As a volunteer summarized it a few years ago in a local newspaper, the aim is to address "broader quality of life issues such as garbage-strewn alleys and streets, unsafe traffic conditions and just plain neighborliness."10 These observations confirm other studies' findings that the fear of urban decay, which can be synonymous with a loss of social status for individuals, often takes precedence over the fear of victimization.11 Thus, participatory surveillance resembles at first glance other forms of civic engagement, and can be viewed as one of many forms of community action to strengthen ties between neighbors and prevent urban decay [START_REF] Raoulx | Un centre ville 'safe and clean' (sûr et propre)? Marginalité, interventions urbanistiques et contrôle social en Amérique du Nord. Les exemples de Vancouver (Colombie-Britannique) et de Richmond (Virginie)[END_REF][START_REF] Vindevogel | La mobilisation des énergies privées pour l'amélioration de la sécurité publique à New York[END_REF]).

Yet findings concerning the genuine motivation of participants do not capture the whole picture, as these actions may have unanticipated effects.

Institutional Re-Framing of Social Expectations and Rise of Suspicion

Whether these community actions are initiated by the mayor or by residents' associations, public institutions often try to use them to their advantage. These initiatives are perceived as an opportunity to improve police-community relations in a context of trust deficit. Yet the police agenda is less focused on using citizen-based surveillance as a source of knowledge and information about local issues than it is on rallying support for their "crime fighting" approach.12 Their main concern is to increase the one-way flow of information from the police to citizens, and not vice versa-this being especially true when the police use sophisticated information systems. This asymmetric partnership appears as a double-edged sword insofar as it may lead to increased suspicion and discrimination among citizens.

The Police Agenda on Participatory Surveillance

In both Canada and France, widespread distrust of police has become a major political concern in the past few decades. In British Columbia, for instance, a Commission of Inquiry into Policing was established in 1992 by the Attorney General of the province after a series of police killings. This independent commission, conducted by local lawyer Wallace Oppal, published a report entitled Closing the Gap: Policing and the Community that advocated a new governance of public safety and a reform of police departments in order to "respond to society's changing social conditions." 13 The report further emphasized that the police needed to recognize the role of citizens and civil society-based initiatives. In the wake of this report, the then Chief Constable of Vancouver Police examined different ways of aligning the police department with local needs and concerns. The proposal was made to the Vancouver city council to support the creation of community-based associations run by residents and operated by volunteers, instead of expensive local police stations. This cost-effective means to improve relationships between police and the community was supported by the mayor, who encouraged the creation of "community policing centers" through public funding and training programs [START_REF] Cairns | Community-Police Partnerships: coproducing Crime Prevention Services[END_REF]. Nevertheless, most interviews with senior police officers indicate that the Vancouver police continued to envision their role as one of fighting crime, and that the philosophy of community policing is nothing but a slogan. Even though the police department officially stresses that it is important to "empower citizens to participate in community affairs," 14 success is measured in terms of arrest statistics and crime breakdowns. 15 As a result, the main focus remains on securing community support for police work.

Question: In your eyes, what is the most valuable function of the Community Policing Centers?

Answer: The crime analyst identifies a trend and can send information and crime alerts to people in the neighborhood. We also talk to the Community Policing Centers, giving them information about crime hotspots to do foot or bike patrol. It is in this sense that the Community Policing Centers are valuable for us, not so much as information sources.

(Interview with the Chief Constable of Vancouver Police, December 20th, 2011) Thus, week, the CPCs receive fresh information and crime alerts about hotspots, which encourage them to target a certain time and place. As an interviewed sergeant clearly put it, citizen participation in safety issues amounts to "targeted patrols where the police needs are."

In France, relations between citizens and the police also appear to be largely negative.

Tensions initially emerged in the banlieues, and more specifically among youth of immigrant origin, who are targeted by law and order policies [START_REF] Boucher | Casquettes contre képis. Enquête sur la police de rue et l'usage de la force dans les quartiers populaires[END_REF][START_REF] Marlière | La police et les "jeunes de cité[END_REF][START_REF] Marlière | La France nous a lâchés! Le sentiment d'injustice chez les jeunes des cités[END_REF][START_REF] Mohammed | La police dans les "quartiers sensibles[END_REF][START_REF] Mucchielli | Le rap de la jeunesse des quartiers relégués. Un univers de représentations structuré par des sentiments d'injustice et de victimation collective[END_REF]. In cities where riots broke out in response to police involvement in deaths, these tensions are so extreme that inhabitants are demanding "the right to life" before public institutions that are perceived as aggressive and even deadly [START_REF] Kokoreff | Refaire la cité[END_REF]. This negative perception was initially limited to groups subjected to disproportionate police surveillance. In the past few years, however, it has spread to the broader society, as law and order policies have come to affect the average man [START_REF] Purenne | L'introduction des technologies de surveillance dans le travail policier[END_REF]. And yet, this dissatisfaction appears to be an ambivalent one, insofar as criticism of improper policies coexists with strong expectations regarding the state's role as a supplier of resources and facilities [START_REF] Merklen | La politique dans les cités ou les quartiers comme cadre de la mobilisation[END_REF].

In this context, buzzwords such as community-based surveillance and citizen participation have been given more attention as a way of minimizing these tensions and rallying support for the police. Residents are encouraged to help the police solve crime. For instance, in Breuillet and Saint-Yon, participants in the Voisins vigilants program are given a cell phone number where they can reach constables at any time should they witness a burglary or hear an alarm sound in the neighborhood. Meetings are organized to keep them updated. The dramatization 15 Just as in France, the so-called "traditional approach" to policing was renewed by the importation of Compstat (for "COMPared STATistics"), a computerized information tool developed in New York in the 1990s. The Vancouver Police department (VPD) was the first Canadian police force to adopt the Compstat program, which is intended to improve police effectiveness through crime analysis and targeted enforcement. of safety concerns, which is aimed at motivating participants, is also fuelled by the local media, with burglaries and violent thefts regularly hitting the headlines. These convergent discourses sustain the impression that crime is commonplace, even though the neighborhood is safe compared to other areas. Thus, while insecurity is not the number one preoccupation of inhabitants, efforts on the part of public authorities and the media to raise awareness contribute to a kind of fatalism and rampant fear. 16 Detecting suspicious behaviors then becomes a way of serving society and the community. This, in turn, raises the question of the targets of participatory surveillance.

Detecting Suspicious Behavior or Suspicious Individuals?

Suspicious behavior is difficult to grasp and define. As a result, even well intended citizens are driven to guard against "suspicious individuals," the perception of risk being itself based on the appearance of the person (age, clothing, ethnicity, etc.) rather than on his/her effective behavior. 17 Most volunteers explain that they generally focus their attention on people who are not from the neighborhood, especially if they look "different." One of the participants in a Voisins vigilants program in France regretted that public transport allows "youths and thugs [meaning: from working-class districts]" to reach the residential area where he lives. Another volunteer, an old lady, felt suspicious of and even angry with nomads and Roma who took the regional train to collect items from the clothing containers in the neighborhood. According to another interviewee, "they come from everywhere: Roma, Gypsies and nomads." This "us and them" perception is reinforced whenever certain kinds of activities, generally attributed to teenagers, take place in the neighborhood: drinking alcohol, listening to music, etc. These activities seem suspicious in that young people who do not reside in the neighborhood are presumed, rightly or wrongly, to be disrespectful and to leave trash behind. Participants in the Voisins vigilants programs are thus prone to engage in the proactive surveillance of every act and gesture of these "strangers." As a volunteer puts it: So when I walk along the banks [of the lake], I have to be careful, I look to see if there is anything […] And when you do this on a regular basis, it is often the same faces you see, people who live here. So when you see a new face, something a bit suspicious, you become more vigilant. I generally come around a second time to see if the person is still there.

Thus, everything happens as if nobody was allowed to disturb the peace of the community. Suspicion of others and racial discrimination, which are commonplace among the interviewees in France,18 are probably reinforced by the fact that the suburban areas in which they live are fairly homogenous in terms of their ethnic and social make up. As explained earlier, they are mostly populated by white middle-class people whose first concern is to preserve their living environment and their social status. In general, the specific structure of these neighborhoods is not considered favorable to developing an openness to otherness. It makes it more likely that participants will align themselves with police positions and professional "expertise," according to a French political culture that emphasizes the role of professional state actors in ensuring law and order. These probably mutually reinforcing factors can play an important role in explaining why the capacity to distance oneself from prejudices and negative stereotyping that we observed in some Vancouver CPCs does not characterize the French situation (see excerpts below), which is marked rather by conservatism and defensive attitudes.

Excerpts of Interviews with a Couple of Volunteers in Breuillet and Saint-Yon

Participants are typically French [meaning: white], I am not racist in saying that. There are a lot of seniors like me, there are a few young ones, but these are all people who are aware that thugs are going around. […] A lot of people bury their heads in the sand. We say: No there are offenders and we must protect ourselves. […] We don't want to become a ghetto or a poor suburb. We must try to perfect the appearance of the neighborhood, maintain the look of the place.

Our dream was a house with a large garden for the kids, so we had to move further away from our workplace, since I work in Paris. But for this quality of life... It is hard to imagine leaving this place now, even though we have a problem with burglaries. So all possible efforts must be made to stop them.

I wish there were more young people. They can bring a fresh perspective, you know, as new residents can see the situation through a different lens. I am willing to try any new idea, any change in the right direction. But we cannot ask too much of young people. The condo is becoming more democratic: Some of the houses aren't well kept; the population structure is changing. I'm not particularly racist, no more than anyone else. But you now see people with dark skin among the population. This "us and them" attitude also exists to a lesser extent in Vancouver, as liaison police officers in charge of community-based programs emphasize the "image of a society where criminal events come from outside and where the neighborhood is called for a united front against it" (Dupont 2007: 110). In particular, residents are encouraged to focus their attention on a given group that is supposed to pose a threat to citizens: drug addicts. Indeed, addiction is viewed by the police as a "criminogenic" factor and as the main reason behind high levels of property crime in Vancouver. 19 You have to pay particular attention to individuals with hoodies, caps, sunglasses, backpacks. I set up the typical profile of the offender who deserves surveillance: If they look like drug addicts, not healthy, a little dirty, badly dressed. […] Of course it could be an eccentric billionaire, but there is a 70 per cent chance that this is a person with bad intentions. We'd rather be safe than sorry. 20 Thus, restoring the trust between the police and local residents may inhibit confidence building between citizens at large, as the police are tempted to exploit participatory surveillance for their own purpose. Indeed, whether intentionally or not, the way in which the police use surveillance programs can reinforce prejudices regarding certain social groups, which proves to be counterproductive and fear instilling. This, in turn, may give rise to targeted surveillance and result in social/racial discrimination. Suspicion can even translate into a quasi-privatization of public spaces, as inhabitants become tempted to guard against "different" people. These results are convergent with the analysis of surveillance scholars who regard surveillance at large as a means of social sorting and categorical suspicion [START_REF] Ericson | Policing the Risk Society[END_REF][START_REF] Lyon | Surveillance as Social Sorting. Privacy, Risk and Digital Discrimination[END_REF]. Whether technology-dependent or citizen-based, surveillance can thus contribute to reinforcing social divisions.

How Collective Reflection and Alternative Approaches can address the Fear of Crime

In this context, "strangers" and non-residents may be potentially dangerous in the eyes of local residents, especially if they are young, from an ethnic background, etc. Should these exclusionary trends, which are similar to the social and racial profiling often used in police work, be seen as inevitable? As Elizabeth Comack and Jim Silver have argued, far from being intrinsically reactionary or tainted with populism, citizens can also mobilize to promote "a sophisticated understanding of the underlying causes of (2008: 818).

Interviews that were conducted in three Vancouver CPCs placed under the supervision of senior organizers confirm this argument. Even though critical reflexivity is more the exception than the rule, local observations indicate that it can lead inhabitants to promote a comprehensive approach to deal with marginalized people present in the neighborhood. These CPCs are located in working-class residential areas with a large immigrant population from Asia. One of them is situated close to the Downtown Eastside, a neighborhood marked by persistent poverty, drug trafficking, mental illness, homelessness, etc. The presence of homeless, drug users and poor people appears to be a major concern for residents. The CPC volunteers repeatedly record complaints related to drug sale or aggressive panhandling. CPC business plans outline that many residents "fear that the street disorder which appears to characterize the Downtown Eastside will move east into [the] neighborhood. Addressing community disintegration and visible signs of disorder is of utmost importance for community members."

At the same time, organizers and volunteers do not share the "us and them" vision that is increasingly popular among police forces and may lead "to demonize the criminal, to excite popular fears and hostilities, and to promote support for state punishment" (Garland 1996: 461). As a community organizer puts it: "We are much aware that sometimes the remedy can be more dangerous than the illness. One may aggravate safety problems when fear of crime spreads." Rather than viewing "outsiders" as potential offenders and "career criminals" that ought to be neutralized, he defends the idea that "there are no people who are criminal by nature. Some people steal because they are hungry. […] There is no such thing as the good guys on the one side and the bad guys on the other." 21 This vision appears to be convergent with what David Garland and others call a "criminology of the self." This "welfarist criminology that depicted the offender as disadvantaged or poorly socialized and made it the state's responsibility-in social as well as penal policy-to take positive steps of a remedial kind" (Garland 1996: 462). Thus, even though CPC members face pressure from the city and the police to focus surveillance on crime hotspots and "at risk" groups, they stress the need for alternative, "creative" responses to crime issues and organize themselves to develop their own agenda. Different programs have therefore been implemented to reduce fear of crime and help vulnerable persons.

First, CPC efforts aim to reduce underreporting. Volunteers provide a communication channel that is accessible to everyone, especially the most vulnerable or new residents. As a 2009 report indicates, "low rates of reporting crime, often accompanied by reports of fear, complacency or futility, have been major issues." Indeed, many residents do not have English as a first language, or else do not feel comfortable reporting to the police because of negative experiences in their country of origin. In view of this, organizers and volunteers have extended office hours to try and reduce fear of crime among vulnerable people who have been victimized.

Second, volunteers also meet marginalized people in order to create platforms for dialogue about their needs and concerns. In their view, excluding people deemed socially undesirable because of their disturbing behavior (for instance, the homeless or the mentally ill, who have often been the target of zero tolerance policies) and pushing the problem outside the neighborhood are not appropriate solutions. They would rather listen to marginalized people and provide referral to dedicated state or community resources.The aim is to help them "negotiate the system," as they often "have a lack of knowledge of systems and resources of both the government and the police."22 Some CPCs promote this inclusive approach and view themselves as "the safety net for everything" and as "the middle man"23 between institutions and citizens, particularly the most vulnerable. One of the CPC coordinators even tries to enroll people with drug addiction or disabilities as part-time volunteers in order to help them develop personal and social skills. In her view, these actions also contribute to reducing fear of crime and street disorder in the neighborhood by providing resources and support to those who are deprived.

Third, CPC organizers also promote peer mediation to reduce fear of crime. To that end, they sometimes use humorous and ludic methods that are somewhat similar to those endorsed by US radical movements. 24 Starting from the assumption that legal and traditional policing responses are unhelpful and even counterproductive to solve certain recurring problems, the objective is to develop a capacity for informal social control without dramatizing the issues at stake. The example of a "pyjama patrol" initiated by a CPC is emblematic. Residents living near a park were complaining about young people who drank alcohol at night. They reported the trouble over and over again to the police. Patrol cars were sent occasionally, with no effect on the problem. Alerted by neighbors, the CPC members decided to take on the problem.They organized citizen teams and trained them to patrol in the park while wearing a nightcap. After several weeks of this "pyjama patrol," the disturbance ceased.

Conclusion: Favorable and Unfavorable Conditions for a Democratization of Public Action

Based on the examination of two contrasting programs in France and Canada, this comparative study supports the hypothesis that anticrime citizens' initiatives have ambivalent and multifaceted effects. Participatory surveillance can help to strengthen social relationships and the sense of belonging in a community, while paradoxically contributing to instilling fear instead of reducing the sense of insecurity felt by citizens. Such widespread fear currently reinforces popular prejudices against "strangers." Although streets are public spaces, the mere presence of certain groups (typically youth, nomads, drug addicts, homeless people, etc.) is disapproved by residents and regarded as an intrusion into their private lives. This is not a new trend, but instead a long-term phenomenon. According to the French historian Gérard Noiriel, suspicion towards impoverished, marginalized and migrant populations dates back to the advent of modern society. This mistrust, coupled with the increased mobility of individuals and goods, prompted the development of remote identification protocols such as logbooks for workers and anthropometric identity booklets for nomads [START_REF] Noiriel | Profiling Minorities. A Study of Stop-and-Search Practices in Paris[END_REF]). These unanticipated impacts deserve attention. Indeed, "citizen involvement in law enforcement […] is unlike other forms of citizen participation. The stakes are higher; the risk of miscarriage is greater, and the consequences of abuse or error appear more serious" (Marx and Archer 1971: 71). In this regard, our comparative analysis suggests that these prejudices and risks are: 1) more prevalent in areas marked by greater social and ethnic homogeneity; and 2) fuelled by police forces who want to exploit civilian mobilization to serve their own organizational interests. These two conditions are not conducive to the empowerment of citizens.

Be that as it may, is participatory surveillance necessarily associated with exclusion, stigmatization and racism? Or, on the contrary, is it empowering in the sense that it "allows a community to better understand itself and its environment" (Monahan et al. 2010: 109)?

Citizens involved in surveillance are often portrayed as "denunciators," obsessed with watching one another and prone to reporting any suspicious behavior to law enforcement agencies [START_REF] Dupont | Police communautaire et de résolution des problèmes[END_REF]. This negative representation is especially present in France, where the specter of collaboration with Nazi Germany is still vivid. Yet our data suggest that community-based surveillance initiatives do not necessarily constitute a "reactionary form of political mobilization" (Comack and Silver 2008: 817), nor do they inevitably lead to a culture of generalized suspicion. General assertions about participants who uncritically buy into the law and order discourse fail to encapsulate the contrasting reality of participatory surveillance. Our comparative study offers a more balanced view, one that stresses an important mitigating factor, namely the ability of residents to open up controversial issues for debate and to reflect collectively on the complex causes of crime. The latter is the condition for lateral surveillance to open the way for a democratization of public action. Such critical ability is by no way innate: It can be reinforced or, conversely, weakened by two complementary factors that are present in the Vancouver context and absent in the French one. Firstly, the presence of professional organizers with a background in community development or criminology can foster the capacity for reflection. The role of professional organizers is crucial to broadening the debate and to resisting basic slogans and beliefs about crime and urban disorder. Secondly, the socialization of citizens in a multicultural context likely plays an important role insofar as the acceptance of otherness can create a more fertile ground for a tolerant approach to risk.

The capacity for collective reflection is especially important because basic beliefs that provide legitimacy to "tough on crime" strategies are now proliferating and becoming a kind of new common sense in many advanced countries. According to Loïc Wacquant:

These punitive policies are conveyed everywhere by an alarmist, even catastrophist discourse on "insecurity" animated with martial images and broadcast to saturation by the commercial media, the major political parties, and professionals of order. […] This discourse heedlessly revalorizes repression and stigmatizes the youths […], the jobless, homeless, beggars, drug addicts and prostitutes, and immigrants […], designated as the natural vectors of a pandemic of minor offenses that poison daily life. (2009: 2-3) As a counterpoint to this dominant discourse, citizen mobilization can shed additional light on urban disorder and crime, for instance by combining knowledge derived from usage and field expertise gained from being present every day in the neighborhood. This type of mobilization can even help public institutions "think outside the box," and broadens the scope of remedies to regulate social problems, as the experience of the vigies (watchdogs) of Villiers-le-Bel illustrates 25 [START_REF] Besnier | Approche genrée de la participation coproductive des habitants en matière de sécurité publique. L'expérience du collectif du 29 juin à Villiers-le-Bel[END_REF][START_REF] Evita | Les habitants, co-producteurs de sécurité. Une expérience de coproduction participative en matière de sécurité ou les ingrédients d'un système collaboratif[END_REF]. This informal group brings together committed women of different backgrounds and operates with the support of a community organizer. It has been set up to tackle often interlinked juvenile violence and institutional violence, its motto being "No to all forms of violence." Very active in the media, this group of residents has contributed to shifting the terms of the debate by emphasizing that deficiencies in basic public services (such as poor transport links) fuel the feeling of social and territorial exclusion and cause many young people to occupy public space. As a result, the promotion of youth mobility and improved provision of public transport have become some of their favorite themes. 26 As a founding member of the group explains: "We do not intend to take the place of the institutions. But when we feel that something can help to strengthen social cohesion, we put the emphasis on it" (quoted in Besnier 2012: 47-48).

  [safety and security] issues [...] [and can] have a very clear vision of what they believe the role of the police in the inner city should be: one in which the police are part of a wider effort of community mobilization"

Interview with a community organizer, July 17th, 2013, Vancouver.

In the case of France, population mix is defined according to gender and age. It is important to outline that, in contrast to the Vancouver groups whose members often come from different ethnic and social backgrounds, French participants in the Voisins vigilants programs are more homogeneous. The social and cultural makeup of the Voisins vigilants groups reflects the composition of the neighborhood, which is populated by a majority of white, middle income citizens.

A lot of offices were created in 1995 by residents' associations as part of their crime prevention programs. Their number rose from five to 17 at the time. Since then, their number has decreased, and there are now eight community-run offices, plus two police-run offices. Each center receives a core funding of one hundred thousand dollars from the City of Vancouver. The centers also develop fundraising activities to obtain money from shopkeepers, insurance companies, etc.

Interview with a volunteer, December 19th, 2012.

Interview with a community organizer, April 10th, 2012.

http://www.hastingssunrisecpc.com/page/community-cleanup [Accessed: May 2013].

The Vancouver Sun, March 16th, 1996 (quoted in Cairns 1998).

For similar results regarding gated communities, see for instance[START_REF] Charmes | Gated Communities: Ghettos for the Rich? La Vie des Idées[END_REF].

In the case of a criminal event, information is welcomed to assist in solving investigations.

http://www.pssg.gov.bc.ca/policeservices/shareddocs/specialreport-opal-closingthegap.pdf [Accessed: May 2013].

VPD, Evaluation Report back of CPCs, Administrative report toVancouver City Council, Oct. 24, 2008, p. 10.

In addition, one cannot exclude the possibility that the way in which the police use citizen participation for their own ends may result in unequal treatment between districts. Indeed, a two-tier system may develop between neighborhoods that demonstrate goodwill in helping the police (and hence deserve special police attention through intensive patrols and prompt care) and ones that are less organized and more disadvantaged (and hence do not deserve the same degree of attention and reaction). Nevertheless, the data we collected do not appear to document this trend.

This kind of prejudice is also commonplace in police operations, as many researchers have shown. It is viewed as "the result of a habitual, and often subconscious, use of widely accepted negative stereotypes in making decisions about who appears suspicious or who is more prone to commit certain types of crimes. […] Ethnic profiling targets certain persons because of what they look like and not what they have done" (Open Society Justice Initiative 2009: 19-20).

These trends need to be confirmed or refuted by further quantitative or qualitative research, as they are based on a small number of interviews and observations.

http://vancouver.ca/police/assets/pdf/reports-policies/vpd-chronic-offenders-sentencing.pdf [Accessed: January

20, 2013] 20 Observation, December 2011, course of training given to volunteers by a police officer in charge of Neighborhood Watch.

Interview with a community organizer, July 17th 2013, Vancouver.

CPC Business Plan 2009, April 17th, 2009, p.13.

Interview with a community organizer, April 10th, 2012.

See for instance[START_REF] Alinsky | Rules for Radicals. A Pragmatic Primer for Realistic Radicals[END_REF].

Acknowledgments

Our sincerest thanks to all the people who contributed to this empirical study by answering our questions, and especially to Clair MacGougan, Urvashi Singh, Eric Charmes and Emmanuel Martinais for their comments on earlier drafts. Thanks as well to Ariane Dorval and to the anonymous reviewers of Surveillance & Society for their helpful recommendations to improve the paper.