nvidia 7950gx2 multigpu performance

[GOD]Anck · 11-26-2007, 06:46 AM

My system has a nvidia 7950gx2 dual GPU graphics card in it. lspci output:

Code:

04:00.0 PCI bridge: nVidia Corporation Unknown device 01b3 (rev a3)
05:00.0 PCI bridge: nVidia Corporation Unknown device 01b3 (rev a3)
05:01.0 PCI bridge: nVidia Corporation Unknown device 01b3 (rev a3)
06:00.0 VGA compatible controller: nVidia Corporation G71 [GeForce 7950 GX2] (rev a1)
07:00.0 3D controller: nVidia Corporation G71 [GeForce 7950 GX2] (rev a1)

I am running Slackware 12 with X11 7.2.0, and I'm using the nvidia 100.14.19 driver. I've configured X to use the nvidia driver module, and set up SLI / MultiGPU. I only have one screen attached.

Code:

Section "Device"
...
  Identifier  "NVIDIA"
  Driver      "nvidia"
...
EndSection
...
Section "Screen"
  ...
  Option "SLI" "On"
  Option "MultiGPU" "Auto"
  ...
EndSection

I ran some doom3 benchmarks with MultiGPU set to Off, Auto, AFR and SFR, and don't see any performance difference at all. It's set for maximum detail, 1024x768, 4x AA, and I get 55-60fps on timedemo demo1. vsync is disabled, and my screen refresh rate is set to 85hz anyway, so I don't think that's what's limiting it. Benchmarks on various hardware sites show far higher performance should be possible. Besides this video card, the system has an Asus P5LD2 mobo, Pentium D 3.6GHz, and 2GB DDR2.

I also ran World of Warcraft in opengl mode through wine, and didn't get any performance difference with MultiGPU there either. This makes me suspect that it's still only using one GPU. I know both work; I have a WinXP partition with UT3 installed, and I was able to double my framerate there (from 30 to 60 @ 1024x768, full detail) by switching from single GPU mode to AFR2 in the nvidia driver panel.

My Xorg.0.log shows this:

Code:

(II) Setting vga for screen 0.
(**) NVIDIA(0): Depth 24, (--) framebuffer bpp 32
(==) NVIDIA(0): RGB weight 888
(==) NVIDIA(0): Default visual is TrueColor
(==) NVIDIA(0): Using gamma correction (1.0, 1.0, 1.0)
(**) NVIDIA(0): Option "SLI" "On"
(**) NVIDIA(0): Option "MultiGPU" "AFR"
(**) NVIDIA(0): Enabling RENDER acceleration
(**) NVIDIA(0): NVIDIA SLI auto-select rendering option.
(**) NVIDIA(0): NVIDIA Multi-GPU alternate frame rendering selected.
(WW) NVIDIA(0): DamageEvents are not currently compatible with SLI.  Disabling
(WW) NVIDIA(0):     DamageEvents.
(II) NVIDIA(0): NVIDIA Multi-GPU enabled.
(II) NVIDIA(0): NVIDIA GPU GeForce 7950 GX2 (G71) at PCI:6:0:0 (GPU-0)
(--) NVIDIA(0): Memory: 524288 kBytes
(--) NVIDIA(0): VideoBIOS: 05.71.22.24.06
(II) NVIDIA(0): Detected PCI Express Link width: 16X
(--) NVIDIA(0): Interlaced video modes are supported on this GPU
(--) NVIDIA(0): Connected display device(s) on GeForce 7950 GX2 at PCI:6:0:0:
(--) NVIDIA(0):     Iiyama HM903DADTA (CRT-0)
(--) NVIDIA(0): Iiyama HM903DADTA (CRT-0): 342.0 MHz maximum pixel clock
(II) NVIDIA(0): Assigned Display Device: CRT-0
(II) NVIDIA(0): Validated modes:
(II) NVIDIA(0):     "1400x1050"
(II) NVIDIA(0):     "1280x1024"
(II) NVIDIA(0):     "1024x768"
(II) NVIDIA(0):     "800x600"
(II) NVIDIA(0):     "640x480"
(II) NVIDIA(0): Virtual screen size determined to be 1400 x 1050
(--) NVIDIA(0): DPI set to (98, 91); computed from "UseEdidDpi" X config
(--) NVIDIA(0):     option
(==) NVIDIA(0): Disabling 32-bit ARGB GLX visuals.
(--) Depth 24 pixmap format is 32 bpp
(II) do I need RAC?  No, I don't.

My questions: how do I get maximum performance out of my hardware? Do I need additional X configuration? Is there a better benchmark than doom3 that will show a performance increase in dual gpu mode, so I can at least tell whether or not it's using both GPU's?

Thanks!

[GOD]Anck · 11-26-2007, 07:41 AM

Just to make sure, I installed Doom 3 on my WinXP partition. I changed the global settings and the application specific override in the nvidia panel to make sure it only used 1 GPU, and ran timedemo demo1: 80fps. Then I switched to multi GPU mode AFR1, as recommended by nvidia, and ran another benchmark: 104fps. It's not double, but at least it's a noticeable difference, so apparently doom3 does benefit from dual GPU in my system.

I also find it hard to believe that it would run that much slower on Linux (60fps compared to 104), so I feel it must be a configuration issue. What would be a typical X configuration for opengl games using an nvidia card?

EDIT: Google came up with this link describing the exact same problem, and a reference to a phoronix article recommending I use the same resolution in games that X is set to.. I tried that as well, made no difference.