Chapter 20. Configuring SLI and Multi-GPU FrameRendering
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## Chapter 20. Configuring SLI and Multi-GPU FrameRendering

The NVIDIA FreeBSD driver contains support for NVIDIA SLI FrameRendering and NVIDIA Multi-GPU FrameRendering. Both of these technologies allow an OpenGL application to take advantage of multiple GPUs to improve visual performance.

The distinction between SLI and Multi-GPU is straightforward. SLI is used to leverage the processing power of GPUs across two or more graphics cards, while Multi-GPU is used to leverage the processing power of two GPUs colocated on the same graphics card. If you want to link together separate graphics cards, you should use the "SLI" X config option. Likewise, if you want to link together GPUs on the same graphics card, you should use the "MultiGPU" X config option. If you have two cards, each with two GPUs, and you wish to link them all together, you should use the "SLI" option.

In FreeBSD, with two GPUs SLI and Multi-GPU can both operate in one of three modes: Alternate Frame Rendering (AFR), Split Frame Rendering (SFR), and Antialiasing (AA). When AFR mode is active, one GPU draws the next frame while the other one works on the frame after that. In SFR mode, each frame is split horizontally into two pieces, with one GPU rendering each piece. The split line is adjusted to balance the load between the two GPUs. AA mode splits antialiasing work between the two GPUs. Both GPUs work on the same scene and the result is blended together to produce the final frame. This mode is useful for applications that spend most of their time processing with the CPU and cannot benefit from AFR.

With four GPUs, the same options are applicable. AFR mode cycles through all four GPUs, each GPU rendering a frame in turn. SFR mode splits the frame horizontally into four pieces. AA mode splits the work between the four GPUs, allowing antialiasing up to 64x. With four GPUs SLI can also operate in an additional mode, Alternate Frame Rendering of Antialiasing. (AFR of AA). With AFR of AA, pairs of GPUs render alternate frames, each GPU in a pair doing half of the antialiasing work. Note that these scenarios apply whether you have four separate cards or you have two cards, each with two GPUs.

Multi-GPU is enabled by setting the "MultiGPU" option in the X configuration file; see Appendix F, X Config Options for details about the "MultiGPU" option.

The nvidia-xconfig utility can be used to set the "MultiGPU" option, rather than modifying the X configuration file by hand. For example:

        % nvidia-xconfig --multigpu=on

SLI is enabled by setting the "SLI" option in the X configuration file; see Appendix F, X Config Options for details about the SLI option.

The nvidia-xconfig utility can be used to set the SLI option, rather than modifying the X configuration file by hand. For example:

        % nvidia-xconfig --sli=on

### Hardware requirements

SLI functionality requires:

- Identical PCI-Express graphics cards

- A supported motherboard

- In most cases, a video bridge connecting the two graphics cards

For the latest in supported SLI and Multi-GPU configurations, including SLI- and Multi-GPU capable GPUs and SLI-capable motherboards, see http://www.slizone.com.

### Other Notes and Requirements

The following other requirements apply to SLI and Multi-GPU:

- Mobile GPUs are NOT supported

- SLI on Quadro-based graphics cards always requires a video bridge

- TwinView is also not supported with SLI or Multi-GPU. Only one display can be used when SLI or Multi-GPU is enabled.

- If X is configured to use multiple screens and screen 0 has SLI or Multi-GPU enabled, the other screens will be disabled. Note that if SLI or Multi-GPU is enabled, the GPUs used by that configuration will be unavailable for single GPU rendering.

### 20.1. Frequently Asked SLI and Multi-GPU Questions
| Why is glxgears slower when SLI or Multi-GPU is enabled?
| When SLI or Multi-GPU is enabled, the NVIDIA driver must coordinate the operations of all GPUs when each new frame is swapped (made visible). For most applications, this GPU synchronization overhead is negligible. However, because glxgears renders so many frames per second, the GPU synchronization overhead consumes a significant portion of the total time, and the framerate is reduced.
| Why is Doom 3 slower when SLI or Multi-GPU is enabled?
| The NVIDIA Accelerated FreeBSD Graphics Driver does not automatically detect the optimal SLI or Multi-GPU settings for games such as Doom 3 and Quake 4. To work around this issue, the environment variable __GL_DOOM3 can be set to tell OpenGL that Doom 3's optimal settings should be used. In Bash, this can be done in the same command that launches Doom 3 so the environment variable does not remain set for other OpenGL applications started in the same session:
        % __GL_DOOM3=1 doom3
Doom 3's startup script can also be modified to set this environment variable:
        #!/bin/sh
        # Needed to make symlinks/shortcuts work.
        # the binaries must run with correct working directory
        cd "/usr/local/games/doom3/"
        export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:.
        export __GL_DOOM3=1
        exec ./doom.x86 "$@"
This environment variable is temporary and will be removed in the future.

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