Crossfire bridge or not? 2xHD6870

[Damme]

Hello!

Running on 6870, settings as follows:

intensity 9,9
vectors 2,2
worksize 128,128
cpu core 1000,1000
gpu memclock 1000,1000
gpu threads 2,2
queue 1

What I realized was that I get 613.5MH/s with the crossfire bridge connected but not enabled, and 585MH/s with crossfire bridge disabled removed. I find this weird! (I cannot enable crossfire bcos I dont have any VGA or dummyplug connected.
How can this be? I herd that enabled crossfire will actually slow down progress.
Both gfx runs att 100%

tips for tweking appreciated. does lower memclock help? Should I try to clock cpu more ? (hangs at 1010 with standard vcore)

//Damme

edit : sorry should have been posted under hardware!
edit: wrong word!

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[swordfish6975]

I haven't tried xfire bridges for my mining rigs, I thought without was better as well.

[Psyrick]

I have 2 x 7970s with bridge connected. Since it's behind a cooling fan I added, I never pulled it out to try running without the bridge. With xfire enabled, card 2 never exceeds 89% GPU load and the hash rate suffers. With it disabled, both run independently at 98% GPU load.

I tweak my GPU voltage from 1175mV stock to just 1180mV to keep my overclock stable. The GPU clock speed I step up from 950MHz to 1125MHz. This boosts my hash rate from 560MH/s to 660MH/s. It could do more, but given my cooling system, I am getting GPU temps between 81C and 86C. Furthermore, the gain in hash rate per MHz drops at higher clock speeds. Can get 685MH/s at 1200MHz, but the extra 25MH/s is not worth the heat, so I don't push it past that. Using Afterburner, I turn my mem clock and mem voltage to minimum. This does not affect hash rate or stability, but reduces the heat output of the card, which allows to push the core a little further.

Hope you found this information relevant.
- Psyrick

[deslok]

because of the way crossfire tries to share things between gpu's enabling it causes a decrease in mining performance(loss incurred switching between them) installing the bridge itself however is harmless unless you actually enable crossfire.

[Damme]

But in my case I gain speed with the hardware bridge connected (but not enabled) compared to not having the hardware bridge! enabling crossfire does indeed slow down to lower speeds than two singlecards without the bridge.

[TiborB]

First, let us know what you get with only 1 card in your MoBo using the very same clock & other parameters. Crossfire is known to decrease mining performance, this is pretty much in contrast with what you experience. Obviously you will not get more with 2 GPUs than 2 times the performance of a single GPU - but let us check how close you are to this.

With those clock rates and 2 6870s you could easily go above 650MH/s this is why I suggest to test what you get with 1 GPU only.

I assume you are running windows - on Linux, multiple GPUs seem to work well without a crossfire cable and without dummy plugs, the hashrate is the sum of the rates of the individual cards in my case, even for heterogeneous config (mixed version GPUs).

Cheers,
   T

[Damme]

one single card: 305Mh/s
two single cards: 610Mh/s
two cards with crossfire hardware bridge connected, but NOT enabled: 640Mh/s
two cards with crossfire enabled: 510Mh/s

I gain ~30Mh/s with the bridge connected - not enabled.

I have neved been able to pull more than 320Mh/s card even with tweaking, scrypt is 312Kh/s.
tips on worksize, threads, vectors, etc? always test the intensity up until cpu goes high or speel lowers or I get lots of rejected

[TiborB]

Ok, so you get the boost with crossfire cable, but crossfire disabled in the OS. Thank for clarifying this.

Take a look at this: https://en.bitcoin.it/wiki/Mining_hardware_comparison#AMD_.28ATI.29

There you can find os/driver/miner combinations along with worksize, vector, threads, clock and memclock settings and measured hashrate.
If you are optimizing for sha and not scrypt, then you can downclock the memory to decrease thermal dissipation which in fact will allow you to get better GPU performance.

Hope this helps,
   T

[laris2]

It's quite interesting that you get improved results with crossfire bridge, and having the crossfire itself disabled.
Something I will look into this evenining.