I have seen some similar threads, but my setup is slightly different. I too am seeing that BOINC only appears to recognize and use a single card, even though I have 2 in SLI. I was debating buying a 3rd, but not if the apps can't use it. Here's what I have:

Asus P6T6 WS Revolution motherboard
Two GTX 260's in slots 1 and 5 as instructed by the manual for two way SLI.
I did verify in the nVidia control panel that SLI is enabled and non-BOINC apps do see it.

However, here is what BOINC reports:
1/30/2009 6:06:15 PM||Starting BOINC client version 6.6.3 for windows_x86_64
1/30/2009 6:06:15 PM||log flags: task, file_xfer, sched_ops
1/30/2009 6:06:15 PM||Libraries: libcurl/7.19.2 OpenSSL/0.9.8i zlib/1.2.3
1/30/2009 6:06:15 PM||Data directory: C:\ProgramData\BOINC
1/30/2009 6:06:15 PM||Running under account pharrg
1/30/2009 6:06:15 PM||Processor: 8 GenuineIntel Intel(R) Core(TM) i7 CPU 920 @ 2.67GHz [Intel64 Family 6 Model 26 Stepping 4]
1/30/2009 6:06:15 PM||Processor features: fpu tsc pae nx sse sse2 pni
1/30/2009 6:06:15 PM||OS: Microsoft Windows Vista: Home Premium x64 Editon, Service Pack 1, (06.00.6001.00)
1/30/2009 6:06:15 PM||Memory: 5.99 GB physical, 12.09 GB virtual
1/30/2009 6:06:15 PM||Disk: 233.81 GB total, 148.92 GB free
1/30/2009 6:06:15 PM||Local time is UTC -7 hours
1/30/2009 6:06:15 PM||Not using a proxy
1/30/2009 6:06:16 PM||CUDA device: GeForce GTX 260 (896MB, est. 96GFLOPS)

In a project forum, another user suggested disabling SLI. However, I don't want to have to manually enable and disable SLI everytime I start and stop BOINC to play a game or something. Maybe that could be an option that executes automatically when BOINC either starts, stops, or is suspended? Although, I think the better route would be to enable BOINC to make use of all boards with SLI enabled. Or, is there something I'm doing wrong? Thanks.

SLI basically combines 2 (or more) matched GPU devices into 1 logical GPU device. When in SLI mode, the system sees only 1 logical GPU and unfortunately for CUDA this means that it only has visibility to 1 physical device (not 2, 3 or 4). Disabling SLI mode for CUDA is best because it allows SETI to take advantage of each GPU as its own device.

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Thanks. That worked. I wonder how many people think they're using 2, 3, or even 4 CUDA cards and are really just using 1 since they have SLI enabled for game performance and such.

I think that'd be a great option to add to the BOINC core client, a check box that would tell BOINC to automatically issue commands to disable SLI when doing CUDA, though probably it should also only do that when in screensaver mode so it doesn't impact games and such if they set it to continue working when the computer is in use. Otherwise, on multi-card machines, you'll miss out on a lot of processing speed if the user doesn't take the time to go into the control panel everytime which, to an extent, defeats the whole purpose of using CUDA.

I don't understand why CUDA can't use all cores in SLI mode. Games are written to do that. They in fact are the driving factor for having SLI technology. Is nVidia just behind on enabling their own SLI technology in CUDA?

Anyway, thanks again. I'll disable SLI whenever I'm not playing games for now.

I think that'd be a great option to add to the BOINC core client, a check box that would tell BOINC to automatically issue commands to disable SLI when doing CUDA

That is going to be difficult, as on a lot of the cards, SLI mode is enabled with a bridge on top of the cards. How is BOINC going to physically remove this bridge? Nothing said about the motherboards with the integrated GPUs, where you set SLI mode in the BIOS. Should BOINC know about all these different mobo's and their BIOSes as well? Impossible!

But again it's not a BOINC problem, but an nVidia problem. If they can disable it in their drivers, the request should be that they do it, not BOINC.

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Actually, the bridge does make the physical connection required for SLI, but it stays in place all the time even if SLI isn't being used. I enable or disable the SLI mode with a simple checkbox in the nVidia Control Panel. It's a realtime change. Just check the 'Enable SLI' box and apply. No reboot or physical hardware changes required. And, BOINC does see this change as it shows in the messages about CUDA devices.

In fact, SLI is disabled by default, even if the bridge is there. Many gamers forget to do this step and don't even realize they aren't using SLI. Further, I've also seen where installing a driver update disables it and you have to manually re-enable it again. Again, something people miss a lot.

Anyway, that's what I was talking about, that perhaps BOINC could issue whatever commands that checkbox is doing. Basically, disable SLI when BOINC CUDA starts, then re-enable it when it stops. Or even let the user decide if and when it changes the SLI settings in their preferences for BOINC. Actually, if I had my way, that should be a CUDA feature. Of course, really, you'd think nVidia would simply make CUDA SLI compatible to begin with. But until then...

I don't know how hard nVidia makes things like that to do, but many games do this already. Anyway, just a thought to maybe eak out more processing power for BOINC. If not, that's cool.

Actually, the bridge does make the physical connection required for SLI, but it stays in place all the time even if SLI isn't being used. I enable or disable the SLI mode with a simple checkbox in the nVidia Control Panel. It's a realtime change. Just check the 'Enable SLI' box and apply. No reboot or physical hardware changes required. And, BOINC does see this change as it shows in the messages about CUDA devices.

OK, I see. I forwarded that question to the BOINC and nVidia developers.

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;-) . . .