Mark Cerny said:
Another issue involving the GPU involves size and frequency. How big do we make the GPU, and what frequency do we run it at. This is a balancing act. The chip has a cost, and there is a cost for whatever we use to supply that chip with power and to cool it. In general I like running the GPU at a higher frequency. Let me show you why.
*shows an example of a hypothetical ps4 pro level with 36 or 48 cus at the same TF, faster chip have all sections of the chip faster, etc...*
It's easier to fully use 36CU in parallel than 48CU, when triangles are small it's much harder to fill all those CUs with useful work. So there's a lot to be said for faster assuming you can handle the resulting power and heat issues, which frankly we haven't always done the best job at. Part of the reason for that is, historically, our process for setting cpu and gpu frequencies have relied on heavy duty guesswork with regards to how much electrical power it will consume and how much heat will be produced as a result inside the console.
Power consumption varies A LOT from games to games. When I play GoW on ps4pro I kmow the power consumption is high just be the fan noise but power isn't simply about engine quality, it's about the minutiae of what's being displayed and how. It's counter-intuitive but processing dense geometry typically consumes less power than processing simple geometry which is, I suspect, why Horizon's map screen makes my ps4pro heat up so much.
Our process on previous consoles has been to try to guess the maximum power during the entire console lifetime might be. Which is to say, the worst case scene in the worst case game, and prepare a cooling solution which we think will be quiet at that power level. If we get it right, fan noise is minimal. If we get it wrong, the console will be quite loud for the highest power games and a change it might overheat or shutdown if we misestimated power too badly.
PS5 is especially challenging because the CPU supports 256bit native instructions that consume a lot of power. These are great here and there but presumably only minimally used... Or are they? If we plan for major 256bit instruction usage, we need to set the cpu clock substantially lower or noticeably increase the size of the power supply and fan. So after long discussions we decided to go with a very different direction for PS5.
*blah blah about gcn vs rdna cu sizes*
We went with a variable clock strategy for PS5, which is to say we continuously run the gpu and cpu in boost mode. We supply a generous supply of electrical power, and increase the frequency until it reaches the capability of the power and cooling solution. It's a completely different paradigm. Rather than running at constant frequency and letting power vary based on workload, we run essentially at constant power and let the frequency vary based on workload.
We then tackle the engineering challenge of a cost effective and high performance of a cooling solution designed for that specific power level. It's a simpler problem because there are no more unknowns. No need to guess what power consumption the worst case game might have. As for the details of our cooling solution, we're saving them for the teardown. I think you'll be quite happy with what the engineering team came up with.
So how fast can we run the GPU and CPU with this strategy?
The simplest approach would be to look at the temperature of the silicon die and throttle frequencies on thst basis. But that won't work, it fails to create a consistent PS5 experience. It wouldn't do to run a console slower simply because it was in a hot room.
So rather than look at the temperature we look at the activities the CPU and GPU are performing, and set the frequencies on that basis which makes everything determimistic and repeateable. While we're at it we also used AMD smartshift tech and send any unused power from the cpu to the gpu so we can squeeze out a few more pixels.
The benefits of this strategy are quite large. Running a gpu at 2ghz was looking like an unreachable target with the old fixed frequency strategy. With this new paradigm we're able to run way over that, in fact we have to cap the gpu at 2.23 so we can guarantee that the on chip logic operates properly. 36CU at 2.23 is 10.3TF and we expect the gpu to spend most of it's time at or close to that frequency and performance. Similarly, running the CPU at 3ghz was causing headaches with the old strategy. But now we can run it as high as 3.5ghz. In fact, it spends most of it's time at that frequency.
That doesn't mean ALL games will be running at 2.23 and 3.5. When that worst case game arrives, it will run at a lower clock speed, but not too much lower. To reduce power by 10% it only takes a couple percent in frequency. So I would expect any downclock to be pretty minor.
All things considered, the change to a variable frequency approach will show significant gains for playstation gamers.
