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Old 08-18-11, 07:46 PM   #1
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Question Enabling SLI causes slow 2D and high CPU load

Today I received two identical GeForce 460 GTX. I'm especially interested in using the "basemosaic" feature introduced in the more recent proprietary NVIDIA drivers to drive my three screens without Xinerama.

I was pleasantly surprised that when I've set up the basemosaic and metamodes options as described in the NVIDIA readme, all three screens actually worked and I was able to move things around, without any Xinerama enabled. I was a bit puzzled why basemosaic insisted on the SLI bridge to be attached to the cards because in the readme, it sounds like it didn't need to be, but well, why not. Then I tried Second Life and indeed I could move around the OpenGL window beautifully between screens and even spread it across all of them.

So far, so good, but shortly thereafter I noticed that normal windows would sometimes render slowly... very slowly – for instance when minimizing them, or the bookmarks menu in Firefox would scroll extremely slow, or YouTube videos show a lot of tearing and slowness. At first I thought basemosaic might not be supported with those cards after all.

After researching and testing some more, I noticed that the exact same symptoms would show when I just say "SLI" "1" in the xorg.conf. No basemosaic. I just enable SLI. And then normal 2D operations become slow, even when only using a single monitor and without any 3D fanciness. Oh and the "X" process would show about 75% CPU load when, for instance, scrolling through the Firefox bookmarks which is much more than normal.

I didn't find many other threads that seem to describe this very problem, but especially this one sounds just like the problem I'm experiencing:


And maybe this is also related:

https://wiki.archlinux.org/index.php...a#Enabling_SLI (the remark about the Gnome 3 slowness)

I've found many threads saying that wrong MTRR settings might decrease performance a lot when using multiple cards, but these seem to be very old and outdated. Playing around with related settings (mtrr cleanup) didn't seem to help a bit either. Options like enabling SHMPixmaps didn't seem to make any difference. Trying to force both cards to always run on the highest PowerMizer setting didn't help neither.

As for the basic specs: Fully updated 64-bit Arch, Intel DX58SO board, two GeForce 460 GTX on the two PCI-E 16x slots, 6 GB RAM (3x2 GB), nvidia 280.13, xorg The usual suspects of the logs don't show much, however I noticed this new message early in init:

[    1.151295] pci_root PNP0A08:00: address space collision: host bridge window [mem 0x000c4000-0x000cbfff] conflicts with Video ROM [mem 0x000c0000-0x000c7fff]
although I heard that this could be ignored.

Anyone experiencing similar problems with SLI?
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Old 08-19-11, 09:02 AM   #2
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Default Re: Enabling SLI causes slow 2D and high CPU load

I too am experiencing similar issues with SLI. It sometimes works decent, although it introduces noticeable tearing in compiz with enhanced zoom. (I think it has something to do with SFR mode which I use because it seems to be the most stable.) But after a certain period of time which seems random, the screen will take forever to render, up to ten seconds or more and then becomes responsive for a second or two and becomes unresponsive again.

Another issue which happens fairly often is the bottom two thirds of the screen will become garbled after using vdpau or project-m music visualizer and other fullscreen applications. Using compiz will always cause the screen to update properly until another fullscreen application is exited. This does not always happen though, maybe 50% of the time it will. I use 3 GTX480 with SFR, I have never really tried with AFR as it seems much slower with compiz. I have experienced this issue without SLI enabled, so it may not be related.

When the computer does become slow or completely unresponsive, I Alt + PrntScrn + R, E, L, S, U, B to reboot and often times the computer will boot up and do the same thing again right from login. After another reboot the problem disappears (for another random period of time, at least a few hours usually.)

2D applications are the most noticeably hit by the performance loss of using SLI. 3D is slower as well, from what I find. Games are noticeably choppier with SLI. I believe the 2D and 3D slow acceleration has something to do with my unresponsiveness issue.

I am currently using driver version 275.21 because the 280.xx driver series seems to break SLI for me. When I boot using SLI and 280.xx drivers, the computer is completely unresponsive as soon as any compiz effect is used. I have not tried the latest 280 series driver, I will try it soon and report back.

Edit: just started on 285.03, seems really good so far, 3D performance seems ok, as well as SLI performance compared to older drivers, although in unigine benchmark at 1080p extreme tesselation I get 8 fps to start and it stays low. In windows it would be at least 60 Fps. Just tried 275.21 with and without SLI I get 20 FPS in unigine. Must be some sort of regression there.

Edit 2: Less than an hour, first hard freeze on 285.03. in SFR. I get the same FPS in unigine with SLI enable or disabled as well ,I have just found.

Last edited by cheechr1; 08-21-11 at 07:11 PM.
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Old 10-13-11, 05:45 AM   #3
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Thumbs up Re: Enabling SLI causes slow 2D and high CPU load

I just wanted to give you a little update. I just installed the 285.05.09 drivers (latest 3.0 kernel, xorg 1.11.1). I don't know what they did, but SLI+BaseMosaic now seems to work perfectly for me.

Performance is great, just as good as with a single card and screen (compared with the Heaven benchmark). Now I see how bad the performance was when I was using Xinerama to drive my three-screen setup (less than half the FPS).

I hope there won't be any surprises still, but as of now, it works great. I recommend everyone with similar issues to try the latest drivers. Now I'm looking forward to try compiz on all monitors. ^^
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