Xfire with 2x 69xx and impact on shader use

[jsharpley]

Hello folks,

I'm curious to hear from anybody with appropriate technical expertise on the topic of varying numbers of shader processors within a crossfire arrangement between two or more cards.

For instance, the stock 6950 has 1408 shader processors, while the stock 6970 features 1536. It is possible to crossfire these two cards as they are part of the same 'generation', and I believe it's widely accepted that the faster card will downclock to match the speed of the 6950.

However.... the big question for me right now is how does crossfire handle different numbers of shader processors? Are the extra shaders on the 6970 artificially crippled for the sake of compatibility? Is core clock and memory speed the only thing that gets adjusted when two different cards are paired?

We hear a lot about clock speed and crossfire, but not so much about shaders...

[TheQuadFather]

well i dont think the 6970 and the 6950 can be cross-fired at all, they have to be the same core architecture, maybe just maybe if the 6950 was flashed to unlock the 6970s shaders it has there might be a way to run it but it would be better to buy another 6950, flash and overclock them.

[fall]

It is possible, but then your Dependant on the weaker card. So if the 6950 has 1408 shader's the 6970 will only use 1408 shader's... Same goes with Core Clock, Memory Speed/Size etc.

[TheQuadFather]

It is possible, but then your Dependant on the weaker card. So if the 6950 has 1408 shader's the 6970 will only use 1408 shader's... Same goes with Core Clock, Memory Speed/Size etc.

after a bit of reading i think your right, aslong as they are the same 69** series its okay, well everybody learns :)

[fall]

after a bit of reading i think your right, aslong as they are the same 69** series its okay, well everybody learns :)

Yes :) 6950s and 70s are essentially the same card with minor differences.

They have the same GPU's just one is down clocked and has locked shader's. (Kinda like that Athlon x3 or was it phenom... idk ha)
50s have Cheaper Memory (i think)
and one has an 8x6 pin and the other has 6x6 pin.

What i want to do is find a 8 pin GPU connection and solder it in place of the first 6 pin and see if you can make a LEGIT 6970 :D

[jsharpley]

well i dont think the 6970 and the 6950 can be cross-fired at all

This link:

http://sites.amd.com/PublishingImages/Public/Graphic_Illustrations/WebBannerJPEG/AMD_CrossfireX_Chart_1618W.jpg

gives a good overview of crossfire compatibility.


It is possible, but then your Dependant on the weaker card. So if the 6950 has 1408 shader's the 6970 will only use 1408 shader's... Same goes with Core Clock, Memory Speed/Size etc.

So it does drop shaders from the faster card eh... good to know.

[fall]

So it does drop shaders from the faster card eh... good to know.

Yes this goes for SLI or CF, and it does not matter which you put on top your still only as fast as the weakest link.

[SeeThruHead]

It is possible, but then your Dependant on the weaker card. So if the 6950 has 1408 shader's the 6970 will only use 1408 shader's... Same goes with Core Clock, Memory Speed/Size etc.
Im not sure about shaders, but I believe this is mostly wrong info.
A 6970 + 6950 is faster than 2 6950's etc. However I dont believe the scaling is as good because different algorithms or whatever are used when crossfiring different cards (something about 1 card rendering more of the frame than the other i believe)
as for sli, witht he new drivers the cores no longer downclock to match the lowest core, the cards can have different clocks as well, although i think you may still need the same memory size.

[jsharpley]

I'd be interested in some benches of a 6970 + 6950 vs 2x6950, if you have some handy.

[Bobnova]

It's not that it drops the shaders, it's that it's just not as fast.

Both SLI and Crossfire the cards alternate between which one renders the next frame.
Frame1 renders on card1.
Frame2 renders on card2.
Frame3 renders on card1.
And so on and so forth.

Now if a 6950 can render a frame in 3ms and a 6970 can render the same frame in 2ms, 6950x2 crossfire gives you 6ms per two frames, while 6950/6970 crossfire gives you 5ms per two frames.
The numbers are made up, but they illustrate the point nicely.

You can OC one card and not the other in some cases too. I did it on SLI 580s and gained a little bit. Not much, but a little.

[SeeThruHead]

i cant find anything with 69xx series cards but it's been this way for some time
Crossfire Myth (http://www.overclock.net/amd-ati/688472-popular-crossfire-myth-busted.html)
http://img693.imageshack.us/img693/84/24792389.jpg

EDIT: as to the above post that is not always true. there are many ways to render frames. There is alternate frame rendering and scissor rendering, supertiling and so on. When i was running SLI i remember a setting to change the frame rendering style which isn't available in CCC but i think that if 5770+5750 is faster than 5750x2 than they might be using something other than alternate frame rendering when you put two different cards in crossfire.

[jsharpley]

so you guys both are saying that the clocks aren't artificially balanced on two different cards? (let's use amd specifically for now).

edit: i assume this means extra shaders are still in use as well, for instance when 6970+6950 is used, following the same line of logic as above. going by seethruhead's chart, it certainly seems this way.

[SeeThruHead]

I would assume that since the clocks are no longer throttled to the lowest card that the shaders would work independently as well. Of course I'm not 100% sure on that since I dont personally have two different cards to test but it makes sense to me.

[jsharpley]

yea, me too. it is a leap to assume that cards with asynchronous shader counts run at their full shader capacity, even with crossfired, but if the clocks can be out of synch... well... without further information, it seems reasonable to believe that shaders can work that way as well. in any event, glad i posted this thread :thup: thanks guys.