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Old 08-18-11, 07:46 PM   #1
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Join Date: Mar 2004
Posts: 5
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: (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that wrong MTRR settings might decrease performance a lot when using multiple cards, but these seem to be very old and outdated.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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