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Old 02-10-12, 03:35 AM   #1
jimduchek
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Join Date: Feb 2012
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Default Dual GPU's, 4 monitors, extra load on 2nd GPU?

Hi all. I've got two identical GTX460's, connected to four monitors. I noticed that the 2nd GPU is regularly running about 10 degrees C hotter than the 1st at more-or-less idle. In investigating, I see in the nvidia-settings panel that the 2nd GPU never drops below 'performance level 2', while the first GPU regularly drops down to 1 or 0 when I'm not doing much. Turning the 'preferred mode' to 'maximum performance' on both GPU's causes the temperatures to even out (at the higher level, around 45 degrees), so I doubt I have any sort of physical cooling problem with the 2nd card.

My setup is using Xinerama, so no composite. Two of the monitors are rotated (one on each card).

Am I right in assuming that the load a card sees should be related to which screen stuff is running on? I can run fullscreen video on either of the two GPU1 screens and still have the 'performance level' only scale up to 1 and occasionally 2. But GPU2 just sits at 2 even with totally blank desktops.

If I fire up glxgears, the clock speed goes up on the 'correct' card if I drag it across screens. I notice the FPS is halved on the two monitors that are rotated, but as there's one of these on _each_ card, I don't think it's relevant.

Anyway, does anybody know why the 2nd card won't idle properly? Is it a bug or is there perhaps something I'm doing?
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Old 02-10-12, 06:22 PM   #2
jimduchek
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Default Re: Dual GPU's, 4 monitors, extra load on 2nd GPU?

Figured it out for anybody who might search upon this topic in the future.

If the two monitors connected to a card have even slightly different mode timings, the card will _always_ run at a high clock rate. Obviously if the monitors are different resolutions, this will always be the case. In my case, three of my four 1920x1200 monitors used the same mode timings, while one used a slightly different one. Oddly (and luckily) enough, the monitor with different mode timings in the EDID was the same make and model as one of the others (strange, right?), so I forced it to use the EDID from the other. Everything is cool now.

Jim
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Old 02-12-12, 11:46 AM   #3
JaXXoN
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Default Re: Dual GPU's, 4 monitors, extra load on 2nd GPU?

So that's a typical case of "tiny cause with huge impact" :-)

Thanks for reporting!

regards

Bernhard
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