VR SLI: Accelerating OpenGL Virtual Reality with Multi-GPU Rendering

High-performance stereo head-mounted display (HMD) rendering is a fundamental component of the virtual reality ecosystem. HMD rendering requires substantial graphics horsepower to deliver high-quality, high-resolution stereo rendering with a high frame rate.

Today, NVIDIA is releasing VR SLI for OpenGL via a new OpenGL extension called “GL_NVX_linked_gpu_multicast” that can be used to greatly improve the speed of HMD rendering. With this extension, it is possible to control multiple GPUs that are in an NVIDIA SLI group with a single OpenGL context to reduce overhead and improve frame rates.

Autodesk VRED has successfully integrated NVIDIA’s new multicast extension into its stereo rendering code used for HMDs like the Oculus Rift and the HTC Vive, achieving a 1.7x speedup (see Figure 1). The multicast extension also helps Autodesk VRED with rendering for stereo displays or projectors, with nearly a 2x speedup.

Figure 1: Autodesk VRED Professional 2016 SR1-SP4 rendering a scene for Oculus Rift.
Figure 1: Autodesk VRED Professional 2016 SR1-SP4 rendering a scene for Oculus Rift.

The GL_NVX_linked_gpu_multicast extension is available in the NVIDIA GeForce and Quadro driver version 361.43 and newer. With the driver in the new multicast mode, all OpenGL operations that are not related to the extension are broadcast to all GPUs in the SLI group. Allocations are mirrored, data uploads are broadcast, and rendering commands are executed on all GPUs. This means that without the use of the special functionality in the new multicast extension, all GPUs are executing the same work on the same data.

For stereo rendering of a frame in VR, the GPU must render the same scene from two different eye positions. A normal application using only one GPU must render these two images sequentially, which means twice the CPU and GPU workload.

With the OpenGL multicast extension, it’s possible to upload the same scene to two different GPUs and render it from two different viewpoints with a single OpenGL rendering stream. This distributes the rendering workload across two GPUs and eliminates the CPU overhead of sending the rendering commands twice, providing a simple way to achieve substantial speedup (see Figure 2).

Figure 2: Performance increase in an Oculus rendering sample, going from one GPU to two GPUs using VR SLI for OpenGL.
Figure 2: Performance increase in an Oculus rendering sample, going from one GPU to two GPUs using VR SLI for OpenGL.

Code Samples

The following samples show the differences between a normal application doing sequential stereo rendering and an application doing parallel rendering using the new multicast extension. Code changes are minimized because all “normal” OpenGL operations are mirrored in multicast mode, so the application code for preparing the scene for rendering is unchanged.

The first sample shows how a standard application typically renders a stereo image. First, data is uploaded to the GPU containing data for the left eye (1.), and the first image is rendered (2.). Then, data for the right eye is uploaded (3.), followed by rendering the second image (4.). Calling the rendering function twice means that the OpenGL stream for rendering the image needs to be generated twice, causing twice the CPU overhead compared to the second sample.

// render into left and right texture sequentially

// 1. set & upload left eye for the first image
sceneData.view = /* left eye data */;
glNamedBufferSubDataEXT( ..., &sceneData );

// use left texture as render target
glFramebufferTexture2D( ..., colorTexLeft, ... );
 
// 2. render into left texture
render( ... );
 
// 3. set & upload right eye for second image
sceneData.view = /* right eye data */;
glNamedBufferSubDataEXT( ..., &sceneData );
 
// use right texture as render target
glFramebufferTexture2D( ..., colorTexRight, ... );
 
// 4. render into right texture
render( ... );

The second sample shows how to generate the same set of textures using two GPUs. The data for the left and the right eyes are uploaded to GPU 0 and GPU 1, respectively (1.). Since the scene data is identical on both GPUs, the single render call (2.) that gets broadcast to both GPUs generates both the left and the right eye views, on GPUs 0 and 1, respectively, in the corresponding instance of the left texture. To work with both textures on one GPU afterwards, the right eye image must be copied from GPU 1 into GPU 0 (3.).

// render into left texture on both GPUs
// copy left tex on GPU1 to right tex on GPU0
 
// 1. set & upload colors for both images
sceneData.view = /* left eye data */;
glLGPUNamedBufferSubDataNVX( GPUMASK_0, ..., &sceneData );
sceneData.view = /* right eye data */;
glLGPUNamedBufferSubDataNVX( GPUMASK_1, ..., &sceneData );
 
// use left texture as render target
glFramebufferTexture2D( ..., colorTexLeft, ... );
 
// 2. render into left texture
render( ... );
 
// make sure colorTexRight is safe to write
glLGPUInterlockNVX();
 
// 3. copy the left texture on GPU 1 to the right texture on GPU 0
glLGPUCopyImageSubDataNVX(
  1,         /* from GPU 1 */ 
  GPUMASK_0, /*   to GPU 0 */ 
  colorTexLeft,  GL_TEXTURE_2D, 0, 0, 0, 0,
  colorTexRight, GL_TEXTURE_2D, 0, 0, 0, 0,
  texWidth, texHeight, 1);

// make sure colorTexRight is safe to read
glLGPUInterlockNVX();

At the end of both of these code samples, texture 1 contains the left eye rendering of the scene and texture 2 contains the right eye rendering of the scene. The code after generating the images remains unmodified because both textures reside on the GPU that is used in non-multicast mode. As mentioned before, the simplicity of the extension requires only minimal code changes to modify an existing stereo application for multicast mode.

Try VR SLI Today!

The extension “GL_NVX_linked_gpu_multicast” is available starting with the new 361.43 driver, downloadable for both NVIDIA GeForce and Quadro GPUs. The DesignWorks VR and GameWorks VR SDK packages released in parallel with the driver contain more in-depth documentation, the extension text and a basic sample application that showcases the use of the extension. The sample that ships with the VR SDK is also available on GitHub.

We will also be releasing more code samples showing different applications of multicast on the nvpro-samples GitHub repository.

30 Comments
  • vak mad

    cant download the driver, just says ”
    unable to download,try later”

  • Wookie Groomer

    Interesting how there is no VR and yet it seems to be talked about and hyped as much as something that actually exists at a consumer level… Dev kits in the hands of a small amount of people don’t count.

    • lslencrypted

      200.000 dev units sold total and Gear VR is sold to consumers right now. Besides, there’s no point in selling VR if there’s no graphic driver support and that’s what’s happening now :) Release of consumer HMDs are slated Q1 2015 so they’re all in a hurry to get things ready.

      • Brandon Kinsman

        Q1 2016*

    • mpeniak

      wait for next year, everything is gonna explode with vr…people see it therefore people prepare

  • vak mad

    so???????????????????????????????????????????????????????????

    • Luke Antra

      This is quite interresting as it shows the progress the NVidia driver engineers are making for VR support.

      • vak mad

        I fix the problem via regedit. THANKS for responsible

  • Ahmed Sadek

    why Company of Heroes 2 Ardennes Assault not working with nvidia drivers ?
    but it’s working with cpu ghraphics when unistall the driver

    failed to find a supported hardware rendering device.ensure that your system meets the minimum requirements……………

    my card is gigabyte gtx 670

  • Joe Albert

    im having trouble downloading a driver also. The version is the 361.43 which was released on 12/21/2015. Im downloading this the day of the release yet it is saying that i have no internet connection, yet i am at my house trying to download it. I have just gotten rainbow six siege and it is saying i have an outdated driver and to update for maximum speed. If you could please respond back to me that would be very helpful. Thank you.

  • José Roberto Garcia

    Thank you, the driver has download sucesfull :3

  • Heidi

    I cant download it either. Last week i downloaded Windows 10, and since then my Nvidia Geforce Experience hasn’t been able to download. It says; that this Nvidia-graficcard isn’t compatible with this version of windows, but on your webside it says that it work with Windows 10.

  • Headshot CSGO

    cant download the driver “GeForce Gaming Driver” 21.12.2015
    Computer MSI Leopard pro NVIDIA DTX 950M

    • Which operating system? Are you downloading through GeForce Experience or our website?

  • Yasin Kaplan

    cant download the driver, just says ”
    unable to download,try later” too. ı am attempting to download gt 730, help me please

    • Which operating system are you trying to download for?

      • Yasin Kaplan

        windows 8

    • Yasin Kaplan

      ? can any one help,please

  • Rizowan Sarker

    i cant play assassin’s creed syndicate with gt610. too much lagging, but u said gt 610 supports that game. mine is core i5+8GB of RAM..

  • Jesmar Baldacchino

    I can’t download my Driver which was released 12/21/2015, my operating system is windows 10 GTX 970 GPU.

  • Killer_Bee

    Lowered my Teps and getting Higher Benchmarks , why wasnt this done before?
    Good Job! 980 @ 1507MHz

  • Tanya Watkins

    trying to join arc server and it keeps telling me my driver needs to be updated.

  • James Harwell

    Cant download all it says is “unable to download, try again later and I’ve tried like 10 times already.

  • Febrian Bagus Saputra

    For Download problem.

    Avast Antivirus detect this driver as virus.
    http://www.nvidia.com/download/driverResults.aspx/96885/en-us

    I’ve already submit to avast. hope avast will fix that immediately.

  • Yasin Kaplan

    cant download the driver, just says ”
    unable to download,try later”ı am attempting to download gt 730, and my operating system are win 8, help me please

  • klepp0906

    Have quad Titans, will VRSLI work or will I have to remove two cards?…

  • kevpatts

    Now that the Rift is released, where or how can we find out which games have actually implemented this feature? This seriously effects the rig I’ll build.