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Author Topic: Two 6970s not as efficient as one?  (Read 851 times)
BigBoote
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June 13, 2011, 05:56:58 PM
 #1

Hi,

I've set up a mining rig:

Intel Q6600
Asus P5K-E (a 16x PCI-E slot & a 4x PCI-E slot)
Antec HCG-900
Sapphire 6970 (x2, no crossfire cable connected)

GPU Clock: 900
Mem Clock: 500
Voltage +10%

phoenix -u http://XXXXXXX -k poclbm DEVICE=1 BFI_INT VECTORS AGGRESSION=11 WORKSIZE=128

When running with just device 0, I get about 410 MHashes/sec; when I run with just device 1, I get 375 MHashes/s.  But with both running, I get 365 Mhashes.  Is the x4 slot dragging me down?  I wouldn't think bus speed would be a limiting factor with all the computation taking place inside the card.

Is anyone else able to get scalable work with 2 6970s?
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Chris
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June 13, 2011, 05:58:38 PM
 #2

PCI-E speed shouldn't matter, but it looks like there could be a bottleneck somewhere.
BigBoote
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June 13, 2011, 08:02:32 PM
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After playing around for a while, I've managed to get both cards run at 392 Mhashes/s.  It seems that I don't entirely understand the interaction between Trixx and Catalyst Control Center.  I thought Trixx would control both cards when the "Synchronize cards in Multi-GPU config" was chosen.  However, this seems not to be the case.  The second card is not paying attention to Trixx controls, only CCC controls; yet the first card pays attention to both - they have to be the same in both apps or the performance of the first card suffers.

Also, it seems that mhash rate takes a hit whenever the fan speed changes; having the default "custom" setting in trixx (C=%) resulted in worse performance than just keeping a steady clock speed.
demoncrest
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June 13, 2011, 08:07:45 PM
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Hey just a quick question regarding your system I actually just finished the process of getting my 6970's (well, 6950's which I just finished flashing to 6970's... and I really hope u did the same), and anyways I havent been able to run both together since I believe I need a bigger power supply.

what PSU are you using? I was thinking of going 1000W so I could even add a third sometime down the line
BigBoote
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June 13, 2011, 09:21:17 PM
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Hey just a quick question regarding your system I actually just finished the process of getting my 6970's (well, 6950's which I just finished flashing to 6970's... and I really hope u did the same), and anyways I havent been able to run both together since I believe I need a bigger power supply.

what PSU are you using? I was thinking of going 1000W so I could even add a third sometime down the line

See my original post: Antec HCG-900.  Why do you think the power supply is stopping you?  Is it because you only see one adaptor when you use mining tools (or examine them in CCC)?  If so, have you made sure that there's a monitor plugged into both?  if nothing is plugged into your second adaptor, it will be disabled by windows & unavailable for mining.  If you don't have a second adaptor, google "Dummy Plug" for instructions on how to build your own or buy one.
BigBoote
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June 14, 2011, 09:28:20 PM
 #6

I figured out what was going on:  I had my two command shells open running phoenix, which was writing a line out to the shell every second or so & scrolling the rest up.  The act of scrolling the text (& therefore updating the screen pixels) was taxing the second card.  Minimizing all windows that had changing contents fixed the problem - both cards running at 100%.
benzz
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June 14, 2011, 09:31:18 PM
 #7

interesting, i seem to have a similar issue with my 5850 (im also using it for day to day tasks)
BigBoote
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June 14, 2011, 09:37:28 PM
 #8

interesting, i seem to have a similar issue with my 5850 (im also using it for day to day tasks)

I'm guessing that having a motherboard with onboard video & using that as your primary monitor solves the problem.  Not an option with me, but now I know.
xaxistech
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June 14, 2011, 09:38:26 PM
 #9

Video cards don't scale perfectly. It is usually up to the motherboard and CPU to make sure the work is evenly divided. But you will never see 100% performance from multiple cards.

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kickerconspiracy
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June 14, 2011, 09:57:54 PM
 #10

I had the same issue.  2x6970.  Looks like I need a dummy plug to fix it.  I assume running them in crossfire mode is not nearly as productive?
Fluff
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June 14, 2011, 10:11:06 PM
 #11

you need a dummy plug for any graphics card that is not in X-Fire mode, or the GPU on the card will just shut down, because there is not attached monitor to it, and gimp your Mhash/s

you can make dummy plug to fake a monitor being pluged into the graphics card with a DVI to VGA adapter and some resitors

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linenoise
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June 14, 2011, 10:39:01 PM
 #12

I've got a linux box running a few cards just fine without anything connected to their video ports. Is that a widows only issue, or specific card or MB problem?

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