How does 7950+7970 cfx work?

[psycoblaster]

My XFX 7970ghz is being RMA'd currently, and I won't have it for a maximum of 2 weeks. I really don't think I can wait that long, so I'm wondering if it is fine if I buy a 7950 for now and CFX it with the 7970 once it comes back.

How does CFX with the two different cards work, and is there any benchmarks for it as of today? I haven't seen a single review after researching and that's why I'm asking here.

Some say the 7970 will underclock to perform at the same clock, and some say both works separately, rendering 1 frame per card without the cores/clocks having any affect on one another.

 

[lxgoldsmith]

it doesn't work, the cards must have identical specifications, including chipsets, most often being the two of the same card

 

[vmem]

^this applies only to Nvidia cards

for AMD cards, only the first too numbers: 79xx, need to be the same for this to work. this is all based on the basic architecture of the GPU die. in fact, the new 7870LE, which is based on the Tahiti design, will also crossfire with a 7970. You will have control of the two cards' clocks separately and both will run at full potential

Edit: what you described about the two cards each rendering alternating frames is more or less correct, however you can see that if all they do is alternate frames, they will only perform at 7950 potential, which is actually not the case. Unfortunately I am not too familiar with the details, but I can tell you that 7970 cfx 7950 will be faster than a pair of 7950s

 

[psycoblaster]

it doesn't work, the cards must have identical specifications, including chipsets, most often being the two of the same card

Well, amd cards don't work like nvidia that they do crossfire as long as they are in the same series:
http://sites.amd.com/PublishingImages/Public/Graphic_Illustrations/WebBannerJPEG/AMD_CrossfireX_Chart_1618W.jpg
but I just wanted to see benchmarks to see how well they actually perform though.

 

[psycoblaster]

^this applies only to Nvidia cards

for AMD cards, only the first too numbers: 79xx, need to be the same for this to work. this is all based on the basic architecture of the GPU die. in fact, the new 7870LE, which is based on the Tahiti design, will also crossfire with a 7970. You will have control of the two cards' clocks separately and both will run at full potential

Edit: what you described about the two cards each rendering alternating frames is more or less correct, however you can see that if all they do is alternate frames, they will only perform at 7950 potential, which is actually not the case. Unfortunately I am not too familiar with the details, but I can tell you that 7970 cfx 7950 will be faster than a pair of 7950s

ah, as long as 7970+7950 works better than 7950x2, I guess that means the scaling isn't too bad, huh?

 

[vmem]

here you can see Toms cfx a 7870LE with a 7950:
http://www.tomshardware.com/reviews/tahiti-le-7870-7930-benchmark,3401-5.html

they note that they're getting horrible microstuttering until they Overclocked the 7870LE to a higher clock than the 7950... perhaps this is a driver issue with the new 7870LE. There will be some minor issues with your set up, but I don't think it's anything you can't fix as the 7950 and 7970 are very similar in architecture and design (7950 just have 2 disabled shaders vs the 7970). worse case I guess you can just OC the 7950 really hard or something

 

[vmem]

hopefully this will explain how it works:
http://www.overclock.net/t/1220193/crossfiring-between-different-card-of-the-same-series/10#post_16551322

 

[lxgoldsmith]

^this applies only to Nvidia cards

sorry, my bad