The NVIDIA Linux 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 Linux, 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.
With a Quadro Plex Visual Computing System (VCS), the same options as above are available, if your Quadro Plex VCS has two or four GPUs. In addition, there is a special SLI Mosaic Mode to extend a single X screen transparently across all of the available display outputs on the Quadro Plex VCS.
Multi-GPU is enabled by setting the "MultiGPU" option in the X configuration file; see Appendix B, 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 B, 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
The simplest way to configure Mosaic Mode using a grid of monitors is to use nvidia-settings (see Chapter 24, Using the nvidia-settings Utility). The steps to perform this configuration are as follows:
Install your Quadro Plex VCS into the system, following the steps provided in the Quadro Plex Installation Guide.
Connect all of the monitors you would like to use to the Quadro Plex VCS. If you are going to use fewer monitors than there are connectors, connect one monitor to each GPU before adding a second monitor to any GPUs.
Install the NVIDIA display driver set.
Configure an X screen to use the "nvidia" driver on at least one of the GPUs in your Quadro Plex VCS (see Chapter 6, Configuring X for the NVIDIA Driver for more information).
Run nvidia-settings. You should see a tab in the left pane of nvidia-settings labeled "SLI Mosaic Mode Settings". Note that you may need to expand the entry for the X screen you configured earlier.
Check the "Use SLI Mosaic Mode" check box.
Select the monitor grid configuration you'd like to use from the "display configuration" dropdown.
Choose the resolution and refresh rate at which you would like to drive each individual monitor.
Set any overlap you would like between the displays.
Click the "Save to X Configuration File" button. NOTE: If you
don't have permissions to write to your system's X configuration
file, you will be prompted to choose a location to save the file.
After doing so, you must
copy the X configuration file into a location the X server will
consider upon startup (usually
/etc/X11/xorg.conf for X.Org servers or
/etc/X11/XF86Config for XFree86
Exit nvidia-settings and restart your X server.
SLI functionality requires:
Identical PCI-Express graphics cards
A supported motherboard
In most cases, a video bridge connecting the two graphics cards
A Quadro Plex VCS to use Mosaic Mode
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.
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, with the exception of Mosaic.
If X is configured to use multiple screens and screen 0 has SLI or Multi-GPU enabled, the other screens configured to use the nvidia driver 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.
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 Linux 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.
Why does SLI or MultiGPU fail to initialize?
There are several reasons why SLI or MultiGPU may fail to initialize. Most of these should be clear from the warning message in the X log file; e.g.:
The warning message
Below are some specific troubleshooting steps to help deal with SLI and MultiGPU initialization failures.
How can I determine if my kernel correctly detects my PCI Bridge?
As discussed above, the NVIDIA driver must have access to the PCI Bridge that each NVIDIA GPU is connected to in order to configure SLI or MultiGPU correctly. The following steps will identify whether the kernel correctly recognizes the PCI Bridge: