Problems when mining with 4x 6870

[nikkuchan]

Hi,

I'm currently having problems trying to get a computer up with 4x 6870's mining rig going. This is what I have:

Specs:
Asus P8P67 WS Revolution
4x XFX 6870's (16x, 8x, 8x, 8x)
Corsair AX1200 PSU

Drivers/Software/OS:
AMD lastest drivers
ATI APP SDK 2.4
GUIMiner-20110614
Win7 SP1 32-Bit Pro.

I was able to get 3x 6870 to start mining which each of them going at 255+Mhash/s (some higher due to OC'ing. However, once I started the 4th one, all the other GPU's started to drop down between 50-150Mash/s. It would keep on doing that and all GPU's would not max out at 255+Mhash/s.

Anybody have any ideas why this is happening?

I wasn't able to find anything on the forums (based on my searching skills) regarding motherboard only having a max number of GPU's. For what I know, you can have as many GPU's you want on the motherboard no matter how fast the PCI-E lane is whether its 16x 8x 4x or 1x. Correct me if I'm wrong. 

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[scar]

the manual seems to indicate all 4 PCI slots should operate at 8x.  it could be that the 32-bit OS can't handle all that bandwidth.  you may want to try asking ASUS as well....  good luck.

[nikkuchan]

the manual seems to indicate all 4 PCI slots should operate at 8x.  it could be that the 32-bit OS can't handle all that bandwidth.  you may want to try asking ASUS as well....  good luck.

Hmmm... interesting. I'll probably check out the OS first and see if it's because of that. I might actually just try installing Ubuntu on it instead and test if the problem still exists.

[nikkuchan]

the manual seems to indicate all 4 PCI slots should operate at 8x.  it could be that the 32-bit OS can't handle all that bandwidth.  you may want to try asking ASUS as well....  good luck.

Hmmm... interesting. I'll probably check out the OS first and see if it's because of that. I might actually just try installing Ubuntu on it instead and test if the problem still exists.

Looks like it's the OS + software that's causing the issue according to this thread http://forum.bitcoin.org/index.php?topic=19038.0

I guess I really have to use Ubuntu to get all 4 going.

[DirtySanchez77]

Not sure but maybe you misread the post?  It states that Windows is clearly able to accept more than 4 GPU's, but that it's currently unable to mine with more than 4 GPU's.  I know of many Windows based rigs out there with 4 GPU's and all 4 are mining at optimum speeds.

It's possible that it might be as OS issue, specifically a 32-bit OS issue.  Your 4 GPU's alone have 4GB of memory, which is the addressable memory limit of a 32-bit OS.