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Old 01-02-08, 10:38 AM   #1
gentooruwest
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Default If SLI is correctly enabled, why do my FPS drop by almost half

Let me begin by saying that I've been scouring the internet for the past two weeks for instructions on getting SLI to work under Linux. Seems all I should need to do is run "nvidia-xconfig --sli=auto" or manually edit /etc/X11/xorg.conf and add

option "SLI" "auto"

In either the Screen section or the Device section. When I do this /var/log/Xorg.0.log states that SLI is enabled and the link is 16X. yay, that's what I want. However, when I run glxgears, I get about 8,000 fps @1680x1050. When I disable SLI in xorg.conf and restart X I get 14,000 fps @1680x1050. What's the deal?

Side questions I have

1. Do I need a device section for each of my cards in order for SLI to work? I'm not planning on using twinview or multiple monitors in any form/fashion.
2. If both device sections are needed, do I add both device identifiers to the screen section, even though I've only got the monitor attached to one of them?
3. If I add a device section for both and specify the BusID for each card, then why is it that Xorg.0.log spews out the message that I need to add a device section for the second card (always the card that doesn't have the monitor attached).

Overall, there's just a lack of documentation on getting this to run at peak performance.

Thanks!
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Old 01-02-08, 10:44 AM   #2
Thunderbird
 
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Default Re: If SLI is correctly enabled, why do my FPS drop by almost half

In case of SLI there is synchronization needed between the two cards and the cpu also has some more work. The speed gain of SLI depends a lot on the situation. In some situations you might get a 70% speedup but in other cases it can be relatively low (10-20%) and it all depends on how the program is written.

Glxgears is a very bad benchmark it depends a lot on the cpu and second when it is using sli the gpu syncing is the bottleneck which kills the performance. Just try it in a game like quake wars, ut2004 or whatever reasonable game.
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Old 01-02-08, 10:51 AM   #3
gentooruwest
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Default Re: If SLI is correctly enabled, why do my FPS drop by almost half

Sounds reasonable. However I don't game much under Linux. The system also dual boots Windows which I use for the games. I was only hoping to utilize the SLI to improve compiz performance, which by the way if I enable it brings my system to a crawl. I guess my answer is shaping up to be, "don't use SLI under Linux for everyday applications that utilizes GLX" Which is fine I guess. I bought all this for the windows games anyway. I'm just disappointed that SLI performs so rotten for standard GLX applications.

BTW, the processor is an AMD 64 X2 5400+, so I shouldn't be running into too much CPU bottleneck.

--jwest
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Old 01-02-08, 11:19 AM   #4
coopstah13
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Default Re: If SLI is correctly enabled, why do my FPS drop by almost half

http://wiki.cchtml.com/index.php/Glx...ot_a_Benchmark
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Old 01-02-08, 12:03 PM   #5
gentooruwest
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Default Re: If SLI is correctly enabled, why do my FPS drop by almost half

*sigh* ... thanks for the pointless commentary. Now getting back to the question at hand. Why are GLX applications such as "COMPIZ" running so slowly with SLI enabled. Forget I ever said anything about glxgears or it's performance, because inevitably people always construe that to mean you were trying to "benchmark".
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Old 01-05-08, 05:55 PM   #6
gentooruwest
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Default Re: If SLI is correctly enabled, why do my FPS drop by almost half

I got the performance I was looking for by editing my xorg.conf file as follows. I also found out that you only need one device section even though you have two cards present. You only need to add a second device section if you're planning on connecting a display to it.

Section "Device"
Identifier "Device0"
Driver "nvidia"
VendorName "NVIDIA Corporation"
BusID "PCI:1:00:0"
Option "AddARGBGLXVisuals" "true"
Option "UseEvents" "false"
Option "RenderAccel" "true"
Option "XAANoOffscreenPixmaps" "true"
Option "backingstore" "true"
Option "NoLogo"
Option "AllowGLXWithComposite" "true"
Option "Triplebuffer" "true"
Option "NoFlip" "True"
Option "SLI" "SFR"
EndSection
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Old 01-05-08, 07:59 PM   #7
AaronP
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Default Re: If SLI is correctly enabled, why do my FPS drop by almost half

GeForce cards generally only get SLI scaling when running fullscreen OpenGL applications. Compiz doesn't count because most of the time, it doesn't call glXSwapBuffers. As for glxgears, see the question entitled "Why is glxgears slower when SLI or Multi-GPU is enabled?" in Chapter 25 of the README.
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