HD 5970 vs HD 5800 CrossFireGiven the strangely low performance levels of the Radeon HD 5970 in comparison with the less powerful CrossFire solution of 2 Radeon HD 5850s, we wanted to compare Radeon HD 5970 with a CrossFire system with the same processing power. To do this we clocked two Radeon HD 5870s at 725/1000 MHz. We also compared this solution to the Radeon HD 5850s in CrossFire which have 10% less processing units.
Keep the mouse over the graph to display actual performance.
You get a good deal more out of the equivalent CrossFire system than the Radeon HD 5970. The newcomer is between 1 and 13% down, depending on the games. We can only suppose that connecting the two GPUs to the system via a switch and a single PCI Express connector is a limitation on performance. It is possible of course that AMD might reduce this loss in efficiency with future drivers.
You’ll note that the Radeon HD 5870’s additional processing units don’t give much of a gain. The advantage comes rather from their higher clock.Update 23/11/2009:
Because the results for the Radeon HD 5970 were down on those for a CrossFire system based on the Radeon HD 5870 set at the same clock, and even on a CrossFire system based on the Radeon HD 5850, which is nevertheless less powerful on paper, we began to wonder if this would also have been the case on a P55 platform on which the PCI Express ports function at 8x, namely a bandwidth towards the GPUs reduced by half compared to our test system.
Given that the two GPUs on the Radeon HD 5970 have to share a single PCI Express 16x port to communicate with the rest of the system, we were justified in asking if that might not be the limitation. Although we thought it improbable that such a difference might result from this, we wanted to make sure by physically reducing the width of the PCI Express bus from 16 to 8 lanes per card.
As you can see in the graph that has been updated, this had nothing to do with it and the bandwidth towards the system isn’t the problem because the impact on performance is minimal, as we thought. This will reassure P55 platform users who may have thought that a CrossFire with the Radeon HD 5800s would be less worthwhile for them.
It remains to be seen what lies behind the Radeon HD 5970’s lower performances. It could still be that the PLX switch is a limitation (latency, efficiency etc.), that the drivers are not yet perfect or that the card puts a break on performance so as to avoid overloading/overheating, though we didn’t note any reduction in clock when we were monitoring it.