Crossfiring a 7970 GHz with a 7950?

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timnswede

timnswede

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Apr 26, 2012
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I couldn't find any official information from AMD so all I found online were conflicting reports of people saying that crossfiring a 7970 with a 7950 would be the same as two 7950s, and other people saying it would be inbetween 2x 7970 and 2x 7950. Is there any official information out there, or better yet, do any of you guys have a crossfire setup with a 7970 and a 7950 and could you tell me your experience with it?
 
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Zooshooter

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Typically, when combining 2 like products where one operates at a slower speed, both products will operate at that slower speed. You see this in ram all the time. My guess would be that the 7970 would have to throttle itself down to match the 7950 in cases where data needs to be passed back and forth between the two cards. Unless you can OC the 7950 to match the 7970.
 
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ikaz

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Yes basically it the 7970 would act like a 7950 however there was some review out there (I wish I could find the link) that showed running 7970/7950 was actually faster than a pure 7950/7950. Though the test I think were ran pre 12.9 drivers so who know if that still true and I think it was more to do with with minium or ave frame rates being.
 
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bystander

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The idea of the faster card downclocking to match the slower one is false. This is easily confirmed by running MSI Afterburner or any other monitoring software (I've tested this on 5870 and 6950/6970). The problem with this, is that Crossfire doesn't do any frame metering. As a result the faster card will render faster than the slower card, and microstutter is possibly increased as a result.
 
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timnswede

timnswede

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This is the info I found by a guy named Ken1649 on overclock.net:

"AMD cards can work at their own specification (core, memory clocks, voltage and SP/Shaders) in Crossfire. They don't have to downclock or sync'ed, etc.



This example is exaggerated and over simplified with 100% perfect scaling;

6970 1536 shaders = Card A is faster clock per clock
6950 1408 shaders = Card B

In Crossfire;

Card A renders Frame #1 = 3ms
Card B renders Frame #2 = 4ms
Card A renders Frame #3 = 3ms
Card B .......

6970 + 6950 crossfire produces 2 frames/3.5ms
6970 + 6970 crossfire produces 2 frames/3.0ms
6950 + 6950 crossfire produces 2 frames/4.0ms


Edit:

This applies to:

1. 5850 + 5870
2. 6850 + 6870
3. 7950 + 7970
4. 7990 + 7970
5. Etc."


Which sounds like what ikaz and bystander is saying, but is against what everyone else is saying. Just wish I could find some benchmarks with a 7950/7970 setup. I could always get a second 7970 instead, but it's more than I need and $100 more expensive.

 
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vmem

vmem

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Feb 17, 2012
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clocks don't change, I agree with that completely. however in terms of actual performance (especially taking into account microstutter etc), you won't get better performance with 7970/7950 vs 7950/7950
 
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bystander

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You should get FPS in between 7970 CF and 7950 CF, but as I mentioned in my first post, microstutter is likely to be worse. This is because the 7970 will render its frame as fast as it can at its clocks with its shaders, while the 7950 is doing its thing with slower clocks. Since CF doesn't frame meter, it would result in something in between, but with microstutter.

The new prototype drivers may change things a bit, but we'll have to wait and see on those.
 
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timnswede

timnswede

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I won't be crossfiring until a couple weeks/months from now till i get a 2560x1440 monitor anyways, so I can wait for the drivers you are talking about. I might even wait till the 8970 comes out to see how well it plays 2560x1440, crossfire just kind of seems like a pain with the microstuttering, high temperatures, and power consumption.
 
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PCgamer81

PCgamer81

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In that case, could one manually downclock the 7970 to 7950 speeds in order to fix the microstuttering?
 
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bystander

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Nothing will fix microstuttering, even when they operate at the same speeds, as all the FCAT tests show, though FPS limiting seems to fix things the most often. That said, it might help make it less of a problem.
 
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