multi GPU crunching guide

[WhiteFireDragon]

dam i feel like recently i've been spamming this subforum with all my questions haha. and yet, here's another thread. there's no guide on this, so i'm going to ask all the questions here.

1) crunching two of the same GPU separately will give about the same output as two of those GPU's in SLI since SLI can get a single WU done twice as fast correct?

2) does the GPU's have to have the same drivers to be used? for example, the gtx200 series uses different nvidia drivers than the gtx400 series. i'm assuming you can't install different drivers and make it work?

3) does the pci-e bandwidth matter? my evga mobo has 4 x16 slots, but they only run at x16/x8/x4/x8. would i get lower output by putting it in the 3rd slot running at x4?

4) is it possible if i filled all 4 slots with GPU's and it crunches 4 WU separately? i know some mobos won't support more than two WU, and i'm not sure if 2 is a limitation or if it's only certain mobos.

[Otis11]

1) no matter how you configure them they will run 2 seperate WUs... I think you might have to disable sli for them to run though. depends on your setup iirc.

2) You can use different gens of cards, but if that's not your question, i'm not sure.

3) x4 is plenty for gpugrid. I don't think you'll notice a point penalty.

4) Yes, some have 4 295s for 8 total units at the same time. I am not sure however on how to get these setups to work as some have trouble with 295s... but i don't recall any unresolved sli and tri sli problems.

Hope this helps

[WhiteFireDragon]

yup, that helps a bunch. what about pci-e slot running at only x1? my H55 board has two x16 pci-e slot, but i believe the second only runs at x1. i still have questions about the drivers part. when i plug in another card, i notice win7 automatically installs its own drivers again. do i need to reinstall the nvidia CUDA driver again after the second card is plugged in?

[Snow Crash]

Why don't you test the 1x slot and let us know
If Windows is overwriting the drivers you want then I would say you should do the whole driversweeper routine to make sure all is in tip top shape. You can sometimes define in your mobo not to allow the OS to install drivers ... helps to keep M$ hands' off

[pwolfe]

In Linux the only way I can run 2 WU's is to have SLI enabled. Just throwing that out there.

[Otis11]

idk about the 1x... I think that might be pushing it, but if no one ever tries we won't know...

And I'm no SLI wizard... But the one time i ran two cards on the grid in the same computer, I never put it in sli... So you'll just have to tinker with it.

[WhiteFireDragon]

ah i think i know where my problem is. no matter if i enable SLI or not, i still need to connect the SLI bridge for the mobo to use both cards at the same time? i plugged in two cards, but could never get the second to work because i didn't have the SLI bridge connected.

i also found out the pci-e slot is x2 and not x1 haha. it's a x16 PCI-e 1.0 slot running at x4 only, which really means it a x2 in pci-e 2.0. i'm running one of my cards on there now, but i think it'll be hard to determine the point output compared to a ful x16 2.0 slot.

[Otis11]

I never used my sli bridge...

But if it works

[WhiteFireDragon]

was yours just plug and play? or did you have to reinstall drivers and/or BOINC? trying to get more than one GPU to crunch on the same mobo is giving me headaches

[Snow Crash]

There should be no need to do anything to BOINC at all unless you had previously added a <no_gpus>1</no_gpus> option in your cc_config.
I have a 295 and a 285 crunching on one rig. I did reinstall the driver version I just because I wanted to :-)
After that I didn't do anything else and they work just fine. The whole SLI thing can be a PITA ... I usually try it one way, if it works, great, if not I switch it back around and then I reboot, again, just because I like to :-0
I have them installed in an EVGA SLI LE using slots 1 and 4 on so I can have a bit of extra ventilation space for a 120mm fan resting across the top of the cards blowing down between them.

[lkiller123]

Are dummy plugs needed to run multiple cards?

[WhiteFireDragon]

what are dummy plugs? the SLI bridge connector?

[Snow Crash]

IIRC dummy plugs were only needed for Vista pre SP1.
A dummy plug fools the OS into thinking there is actaully a second monitor and then you can "extend" the desktop to the second monitor. This way it will actually see the card and let it be used properly. I have only done multi-gpu on XP which does not require a dummy plug. There are lots of guides around for making one, really pretty easy, you take a DVI to VGA adapter that came with the card and stuff 3 resistors into it. THe guides say what ohm and which pins they need to be put in.

[Otis11]

Unfortunately I can't be too helpful here. I put one card in, installed the drivers and BOINC, turned it off. Put the second card in, turned it on, and called it a day. No plugs, bridge, tweeks, nothing.

I'm on Win7 if that makes a difference, I'd try to mess with mine and see what makes it work, but I'm no longer running SLI...

[WhiteFireDragon]

i give up on this multi GPU thing. i wasted all my free time today trying to make it work. i'm just going to say the problem is either the two cards don't work well with each other, or the mobos i tried it on is at fault. i'll give it a try later when i get different hardware, but for now, this is no pause for me because i'm just wasting crunch cycles trying to get this thing to work.