On the opposite side of the ability to achieve such low idle power usage is the need to manage load power usage, which was also overhauled for the Cypress. As a reminder, TDP is not an absolute maximum, rather its a maximum based on whats believed to be the highest
reasonable load the card will ever experience. As a result its possible in extreme circumstances for the card to need power beyond what its TDP is rated for, which is a problem.
That problem reared its head a lot for the RV770 in particular, with the rise in popularity of stress testing programs like FurMark and OCCT. Although stress testers on the CPU side are nothing new, FurMark and OCCT heralded a new generation of GPU stress testers that were extremely effective in generating a maximum load. Unfortunately for RV770, the maximum possible load and the TDP are pretty far apart, which becomes a problem since the VRMs used in a card only need to be specd to meet the TDP of a card plus some safety room. They dont need to be able to meet whatever the true maximum load of a card can be, as it should never happen.
Why is this? AMD believes that the instruction streams generated by OCCT and FurMark are entirely unrealistic. They try to hit everything at once, and this is something that they dont believe a game or even a GPGPU application would ever do. For this reason these programs are held in low regard by AMD, and in our discussions with them they referred to them as power viruses, a term thats normally associated with malware. We dont agree with the terminology, but in our testing we cant disagree with AMD about the realism of their load we cant find anything that generates the same kind of loads as OCCT and FurMark.
Regardless of what AMD wants to call these stress testers, there was a real problem when they were run on RV770. The overcurrent situation they created was too much for the VRMs on many cards, and as a failsafe these cards would shut down to protect the VRMs. At a user level shutting down like this isnt a very helpful failsafe mode. At a hardware level shutting down like this isnt enough to protect the VRMs in all situations. Ultimately these programs were capable of permanently damaging RV770 cards, and AMD needed to do something about it. For RV770 they could use the drivers to throttle these programs; until Catalyst 9.8 they detected the program by name, and since 9.8 they detect the ratio of texture to ALU instructions (
Ed: Were told NVIDIA throttles similarly, but we dont have a good control for testing this statement). This keeps RV770 safe, but it wasnt good enough. Its a hardware problem, the solution needs to be in hardware, particularly if anyone really did write a power virus in the future that the drivers couldnt stop, in an attempt to break cards on a wide scale.
This brings us to Cypress. For Cypress, AMD has implemented a hardware solution to the VRM problem, by dedicating a very small portion of Cypresss die to a monitoring chip. In this case the job of the monitor is to continually monitor the VRMs for dangerous conditions. Should the VRMs end up in a critical state, the monitor will immediately throttle back the card by one PowerPlay level. The card will continue operating at this level until the VRMs are back to safe levels, at which point the monitor will allow the card to go back to the requested performance level. In the case of a stressful program, this can continue to go back and forth as the VRMs permit.
By implementing this at the hardware level, Cypress cards are fully protected against all possible overcurrent situations, so that its not possible for any program (OCCT, FurMark, or otherwise) to damage the hardware by generating too high of a load. This also means that the protections at the driver level are not needed, and weve confirmed with AMD that the 5870 is allowed to run to the point where it maxes out or where overcurrent protection kicks in.
On that note, because card manufacturers can use different VRMs, its very likely that were going to see some separation in performance on FurMark and OCCT based on the quality of the VRMs. The cheapest cards with the cheapest VRMs will need to throttle the most, while luxury cards with better VRMs would need to throttle little, if at all. This should make little difference in stock performance on real games and applications (since as we covered earlier, we cant find anything that pushes a card to excess) but it will likely make itself apparent in overclocking. Overclocked cards - particularly those with voltage modifications may hit throttle situations in normal applications, which means the VRMs will make a difference here. It also means that overclockers need to keep an eye on clock speeds, as the card shutting down is no longer a tell-tale sign that youre pushing it too hard.
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