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Strange SLI problem, GTX 460, 3D Vision, low FPS on one GPU in 50Hz


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Hi,

I have problem I have been investigating now for several days regarding SLI.
I am running two Gigabyte GTX 460 1GB in SLI. Using the latest Nvidia driver that includes 3D vision and SLI profile for Dirt Rally.
I'm running the game at 1280x720 with Nvidia 3D vision and 3D glasses on a 3D projector in 50hz. Had to run 50Hz instead of 60Hz to get stable FPS.
Have to set resolution in windows desktop to 1280x720 3D and in game to 1280x768 then alt tab twice to switch to window mode and back to fullscreen to enable 1280x720 with 3D vision in game.

I've been experimenting with graphics settings from lowest on all to Ultra/High/Media and everything inbetween, all kinds of Multisampling, Vsync on/off etc. But no matter what I try I get perfect FPS on one GPU (left eye in 3D vision) and random serious cutdown on FPS on the other eye as well as stuttering. The problem gets slightly worse if a crank up the graphics settings, but even on lowest, I still have problem that one GPU is lagging while the other is not lagging at all. This causes serious eye strain and makes the game unplayable with 3D vision. Disabling SLI fixes the issue, but the game only runs at around 25-30 fps.

I have also tried running Physx on CPU instead of GPU and at first it seamed to have improved things a lot, but after a while I noticed that there were still problems.

Things I've tried so far that did not resolve things

- Checked PCI-Express power cables to GPU's.
- Checked clock speed of both GPU, they are identical, factory default.
- Checked temperature of both CPU, running between 50-60 C.
- Tried vsync on/off, every form of multisampling, and all graphics options in just about every combination
- Tried Nvidia control panel to use alternate frame rendering for SLI, FXAA, Tripple buffering etc

My conclusion is that either one of my GPU's are defect (but I do not think so as other games run fine) or the SLI support in either the latest Nvidia driver or in game itself have issues with Nvidia GTX 460 in SLI.

Could someone help me out? Anyone else experienced this? Anyone else running with SLI and 3D vision?

This game is so awesome, I've been waiting for a real rally game for so many years now and Dirt Rally really is a dream come true, so I really really want to make it work on my hardware.

Regards,

/Linus




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Hi again,

Just made a new interesting discovery. If I switch to 60hz instead of 50hz, I get no problems! GPU1 and GPU2 now produce equal FPS and 3D vision is in perfect sync all the time.

My problem is, that even on shader set to ultra low, I do not get a constant 60 fps. Sometimes it drops down to 57 fps and then 3D vision cuts down to 30 fps because of vsync is forced. 

If I run with shader set to high, I get around 52-55 fps so 3D vision with vsync starts running at constant 30 fps. That is why is originally switched to 50 hz because at 50 hz I can run the game at quite high graphics settings and still maintain a steady 50 fps. 

Too bad it's not working properly with SLI in 50hz. Why could that be? Something must be optimized/hard coded somewhere to only run correct in 60 hz. 

/Linus
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Instead of using vsync can you use something else to cap framerate ? problem with vsync is as soon as it goes under then it cuts framerate in half. I have posted asking devs to look at monitors and resolutions as I get issues with only 49Hz available on a 60Hz monitor.
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Done some more testing.

Overclocked the GPU a bit, now it runs on 60fps almost stable, drops down to 56-57 at the lowest. But then I have to put the shader down to the lowest setting in order to get that high FPS which affects the visual look of the game quite a bit.

Tried with vSync off, but since I use stereoscopic 3D using Nvidia 3D vision (3DTV play on Epson eh-tw6000 projector at 720P 60Hz), as soon as I drop below 60 fps, I get the framerate cut in half, or at least that is what I think I am experiencing visually since it feels like very low FPS.

I wish I could find a way to run it at 1280x720 50Hz instead without the strange SLI artifacts because I can for some reason, crank up the graphics quite a bit without dropping under a stable 50 fps, and that would be perfect for me as I would lever get any cuts down to half framerate then and with these graphics settings the visuals are beautiful.

/Linus
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I honestly don't think a 460 is going to cut it to get 60fps stable unless you do dial the graphics down, I am on a 770 at 1680x1050 and in the benchmark mine can dip below 60 on the min frames although max can be over 100 but as you know with 3d vision if the fps goes below 60 on sli or 120 on single it will default to 30fps, it is one reason why I got rid of my 3d setup because to maintain over 120fps on mine I really had to reduce graphics settings and it looked a bit poop.

You need 60fps min on each card to maintain a good 3d experience.which means your average will be around 80 and max around 100fps.
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I know I cannot get 60 fps with this hardware on anytyhing but low settings, but I have noticed I can get 50 fps stable even with quite high settings. That is why it's frustrating that as soon as I switch from 60 to 50 hz I get serious artifacts with just one GPU (right eye in 3d vision) gets seriously low FPS while the other eye gets super smooth stable 50 fps. 50 fps is enough for me, the flow is good enough, but 25/30 fps is way too low.

I would rather play at stable 50 fps with high graphics settings than stable 60 fps with low graphics settings. But for some SLI bug reason, I cannot at the moment.

/Linus
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I swapped the 460's today, but there was no change in the behavior. Also, I installed and in game on screen display of GPu temps, CPU temps as well as GPU load and CPU load. My CPU is loaded at about 40-50% while running Physx (got an Intel i5 2,8Ghz). I suspect Physx is a very low load compared to the graphics, and I know the GPU of the 460 is well capable of doing 50 fps. 

Somwhere is there is a glitch that causes problems in 50hz. I will do some more testing and run other games in 50hz as that is something i do not usually do. I only did it in Dirt Rally since I discovered my hardware was capable of stable 50-60 fps and thus would handle 50 hz, but not 60.

/Linus
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