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January 27, 2013, 12:22:44 AM |
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I faced similar issue (same symptoms) with 6990 and a 750W power supply using windows/guiminer/phoenix. I could start one instance of phonenix loading one GPU, but not two as the computer would shut down in about 10 secs unless I dropped the gpu clock to about 700 MHz.
Surely enough hash rate suffered as best I could hope for was 650 MH/s using both GPUs.
Switching to linux(ubuntu) and bfg/cgminer using the same hardware proved to be a solution once I was able to properly install Ati drivers with appropriate xorg.conf file that initialized both GPUs.
I was therefore able to increase hash GPU frequency to 900 MHz and consequently hash rate to about 760 MH/s give or take a few percent.
+ the whole thing can boot from 4GB usb key and needs no disk + reboot time is quite fast + linux provides for such niceties like "byobu". if you don't know what that is you are indeed missing out. + no need for vnc/remote desktop
I've stayed with 900 MHz as I feel comfortable running these cards in the 80°C range, not going over 85°C.
That's all fine and well, however recently I began to suspect that the real reason for instant poweroffs was related to sudden increase in temperature which apparently phoenix could not manage properly. so I began to experiment with cgminer on windows. sure enough I was able to get the card running at say 850 MHz range. Going to 900 was still a problem as when I did that I would see in Afterburner GPU frequency jumping between 800 and 900 MHz and not staying stable at some predefined frequency.
as I was not happy with that and because I was worried about the temperature spikes, well not really spikes, but rather slow 10 minute temperature cycles where temp would rise to almost 90°C, so I returned to ubuntu. I did get slightly better hash rate in windows though (777 MH/s)
one more thing: when you say 750W PSU, that does not say a lot. for example 750W PSU from say Corsair is able to absorb quite a load and system will be stable. 750W noname PSU can have a theoretical peak power rate of 750W with 4x 12V lines that should not be loaded with more than 16A, and even approaching 16A can cause problems.
I had one cheap noname 500W PSU and I tried it with 5850 which with the whole system should not use more than 380W (CPU was not loaded and I measured power usage at the wall). So plenty of headroom right? Wrong: PSU was dead in about 1h.
for what its worth... BR
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