The Scientific Computing Service is preparing a gradual migration of selected remote desktop applications on the Correfoc cluster from the current TurboVNC/noVNC environment to a new architecture based on Selkies, WebRTC and H.264 video streaming. The new approach will initially be introduced as a pilot in a limited number of applications, allowing us to assess its performance, identify possible issues and gather user feedback before considering a wider deployment across the cluster.

Remote graphical applications on Correfoc will continue to be launched through Open OnDemand and executed as Slurm jobs inside Apptainer containers based on Rocky Linux 9. For users, the way sessions are requested and accessed will remain familiar, while the underlying technology used to display and interact with the remote desktop will progressively change.

Why are we testing an alternative to TurboVNC/noVNC?

TurboVNC/noVNC has provided a reliable way of accessing graphical applications through a web browser. However, the interactive and visualisation workloads running on Correfoc increasingly include three-dimensional rendering, GPU-accelerated applications and interfaces with frequent visual updates.

These workloads can benefit from technologies designed specifically for low-latency multimedia streaming.

Selkies uses WebRTC to transport the desktop image, audio, and keyboard and mouse events between the remote session running on Correfoc and the user’s browser. This makes use of capabilities already available in modern browsers and may provide a more responsive experience, particularly for scientific visualisation, interactive 3D content and GPU-accelerated rendering.

The migration will be progressive. TurboVNC/noVNC will not be replaced in every Correfoc application immediately, and the outcome of the pilot will determine how and where Selkies is adopted.

What changes for users?

Users will continue to start interactive sessions from the Correfoc Open OnDemand portal, select the required Slurm resources and connect to the application from their browser. No additional software, browser extension or local client will be required.

Applications will continue to be launched from Open OnDemand in the usual way. The main change will be the technology used to display and interact with the remote desktop.

Authentication will also remain transparent. Open OnDemand will manage access to each session, while Selkies will use short-lived authentication tokens associated with the individual Slurm job. Users will not be asked to enter their username and password again when opening the remote desktop.

Each session will create its own X11 desktop and Xvfb display inside the Correfoc compute job. It will therefore not depend on a persistent X11 or TurboVNC server running on the compute node. This allows different remote desktop jobs to use independent display environments.

Two or more desktop sessions may share the same Correfoc compute node while using different X11 displays and GPU resources, provided that the application configuration and the resources assigned by Slurm allow it.

The main expected advantages are:

• lower interaction and display latency;
• smoother visualisation and navigation in graphical applications;
• more efficient use of Correfoc GPU resources;
• reduced bandwidth and encoder usage when a session is not being viewed;
• improved compatibility with modern web browsers;
• independent desktop environments for each Slurm job.

How do WebRTC, H.264 and NVENC work?

WebRTC is the technology used to establish the interactive multimedia connection between the session running on Correfoc and the user’s browser. Web traffic and session signalling will continue to pass through Open OnDemand, while the multimedia stream will be negotiated through WebRTC.

The exact network path used by WebRTC may depend on the user’s browser and network environment. This negotiation is handled automatically and does not require any additional configuration from the user.

The desktop image is encoded as an H.264 video stream before being sent to the browser. On Correfoc GPU jobs, this encoding is performed by NVIDIA NVENC, a dedicated hardware video encoder available on NVIDIA GPUs. Using NVENC avoids placing the main video-encoding workload on the CPU.

The same GPU assigned by Slurm can therefore serve several related purposes within the job. VirtualGL uses it to render OpenGL graphics, while NVENC encodes the resulting desktop image. CUDA applications can continue to access all GPUs assigned to the job.

On the Correfoc high-gpu partition, users can request between one and four GPUs. The std-gpu partition provides one GPU per node. Jobs running on high-cpu, which does not provide GPU resources, will use CPU-based video encoding instead.

Selkies can also react when the browser tab containing the desktop is hidden or moved into the background. The browser notifies the session, allowing Selkies to pause desktop capture and temporarily release the NVENC encoder. Streaming resumes automatically when the user returns to the tab.

This behaviour reduces unnecessary GPU usage and network traffic while keeping the Slurm job and application session active on Correfoc.

No permanent or system-wide Selkies installation is required on the Correfoc compute nodes. The necessary components are provided through the Apptainer application environment and launched within each Slurm job.

Gradual deployment and pilot phase

The new architecture will first be enabled in selected applications on Correfoc where interactive performance and GPU-accelerated visualisation are particularly relevant. It will not be applied to all applications at the same time.

During the pilot period, the Scientific Computing Service will monitor stability, browser compatibility, latency, resource usage and application-specific behaviour. The configuration may be adjusted as experience is gained and different usage patterns on Correfoc are evaluated.

Users participating in the pilot may notice differences in image quality, responsiveness or connection behaviour compared with TurboVNC/noVNC. These observations will be important in deciding the next stages of the migration.

We encourage Correfoc users to report any incidents, unexpected behaviour or usability issues through the usual support channels. Comments on responsiveness, visual quality and the general user experience will also help us refine the service before considering a broader deployment.