4x6950's vs 1x6950

[tinymouse]

My setup:

1. Daily use machine

1x6950 reference card with arctic cooling cooler stable @ 950/1375
Windows 7 x64
Consistent 400mhash rate - all the time, 24/7

2. Purpose built miner.
4xMSI 6950's @ 840/1250
1200Watt Antec - Huge Lian Li case etc
Varying Operating systems Vista/Linux

Consistent 1000mhash - all the time, 24/7 Operating System makes little difference if any

Problem: 400mhash from 1 card vs 1000mhash from 4 cards together!!

I've tried enabling crossfire, disabling it, removing the crossfire bridges etc. I've tried Linux, Windows etc!!

Here's the real issue - the numbers seem to go down as I enable the additional cards in the 4x machine. The first card will run at 300-350 out of the gate and settle just above 300mhashes. As I enable each additional card with a new instance of Diablominer or poclbm the overall numbers go down. I've tried everything but manipulating hardware settings on my motherboard which is an Asus Rampage Extreme III (love it). I just don't get it.. I should be getting 1200mhashes at the very least from the 4x miner after things even out. When I first kick off the cards in either crossfire or individually they hit a total of ~1450mhashes and then evenly drift off to ~1000 in crossfire or ~250 per card in non-crossfire mode.

Any ideas? 4 cards at 1000mhashes is pathetic vs 1 card at 400. These are all 6950's at the end of the day and yes, the single card is at 950mhz vs. 840mhz but that's not enough to cut the performance down that far. Remember  we're talking 4x6950's doing a total of ~1000 while one of them is doing ~400.

Please throw me some ideas. I'm using api.bitcoin.cz but I don't think my 5x 6950's are overwhelming his setup.

EDIT: I thought I would mention that the cards are not running at 100% when I have all the cards going at one time which is  a real concern - crossfire or not. This makes me thing one of two things. Motherboard configuration (maybe) or too much going to bitcoin.cz at one time.

[1715287624]

1715287624

[Opsamk]

Give them to me. Jk.

Your cards should never use crossfire when doing mining. Your video memory should also be set to something less than 500. There should be one miner per card with each set to aggressive.

[tinymouse]

Yep.. I turned off crossfire. I've adjusted the memory.. has no real effect other than heat etc. Yes, I'm using one miner per card. The best set up so far after days of testing is -w 128 -v2 -f2 or -f60(for desktop use).

[slippyrocks]

win7x64

if i don't set the affinity right in guiminer my output drops off a cliff

like two gpus per cpu core seems okay

i have crossfirex enabled but they still show up as two separate cards

360Mh/s ea @ 840

[WillMitchell]

Come up with a new method 
Mining isnt profitable...

[Keninishna]

I'm not sure how much data is being put through the video cards but you might want to check your motherboard pcie settings. It could so that the first port is set to 16x second to 8x and the last two are at 1x. You can try to split it up so they are all running at 8x.

[Wuked]

What's the load on your processor? Is it hitting 100% ?

[Seuros]

If they card get hot, they will throttle the speed automatically! 

[swop]

4x 6950.. That's cracy When do you think you'll mine all that money you've spent on the cards? :

[itxbc]

Its the speed. The difference in hash rate from 840 to 950 is substantial. I saw close to that variance when I was messing with my 6950s. I've got mine running at 900MHz and am seeing 355MH/s At 840, I was below 300. It is odd that you aren't at 100% though on your cores, regardless of the speed. Are your cards getting too hot? In multi card rigs, even with two cards, the PCIe slots are WAY too close together and the cards starve for airflow, causing overheating. That's why you see so many custom built rigs with cards 4+ inches apart or if the cards are next to each other, with dual fans sitting on top trying to force air into that slender airspace. The fact that you are talking about crossfire tells me the cards are right next to each other.

So my guess is the cards are too hot and are throttling down. Since it's clear you've unlocked, throttle your memory speed way down and see if that helps your temps. I'm having trouble unlocking the 6950s I have, so instead am looking to move them apart with PCIe ribbon cables so I can ramp up my interior cards, which right now run below max speed to keep the temps realistic.

[pbj sammich]

Have you tried adjusting the voltages of the 4 cards? perhaps it's a PS issue?

[tinymouse]

I'm guessing its the temps.. that makes a lot of sense.

Thanks for the massive replies. I'll write back with the results from my testing.

[tinymouse]

Its the heat.. and I did extensive testing to include top and bottom only vs two cards next to each other.

Thanks!

[mautobu]

Any luck watercooling those things?