Digital Foundry: There's a cluster of CPU cores [in PS4]. Their purpose in the PC market - I think there are dual and quad configurations - is for tablets. There are two of those [in PS4]. Then you have what I can only describe as a massive GPU...
Mark Cerny: I call it a super-charged PC architecture. And that's because we have gone in and altered it in a number of ways to make it better for gaming. So we have unified memory which certainly makes creating a game easier - that was the number one feature requested by the games companies. Because of that you don't have to worry about splitting your programmatical assets from your graphical assets because they are never in the ratios that the hardware designers chose for the memory. And then for the GPU we went in and made sure if would work better for asynchronous fine-grain compute because I believe that in the GPU, we'll get in a couple of years into the hardware cycle and it'll be used for a lot more than graphics.
Now when I say that many people say, "but we want the best possible graphics". It turns out that they're not incompatible. If you look at how the GPU and its various sub-components are utilised throughout the frame, there are many portions throughout the frame - for example during the rendering of opaque shadowmaps - that the bulk of the GPU is unused. And so if you're doing compute for collision detection, physics or ray-casting for audio during those times you're not really affecting the graphics. You're utilising portions of the GPU that at that instant are otherwise under-utilised. And if you look through the frame you can see that depending on what phase it is, what portion is really available to use for compute.
Digital Foundry: I think the thing that excites me most about the PS4 is that it's a fixed hardware design, developers can get the most visually from it but they don't need to just concentrate GPU power on, say, slightly higher quality shadowmaps. They've got GPU compute sitting there that can be doing new things, exciting things. Is this something we'll be seeing further on down the line? Is this something you're pathfinding with Knack?
Mark Cerny: Knack is a small title. Knack will not be leading the way in terms of utilisation. We'll be looking much more towards teams that have done that in the past like Ubisoft with Assassin's Creed and Watch_Dogs, Naughty Dog with their next titles or Sony Santa Monica with theirs. I think Killzone is going to be doing that. All we are doing ultimately is setting it up so that developers can take it any way that they want go. So if increased visual fidelity is what is most interesting to that team, that is what they can focus on. We just wanted to be sure that they could also dig in on GPGPU [general purpose computing on GPU] as well.
Digital Foundry: Going back to GPU compute for a moment, I wouldn't call it a rumour - it was more than that. There was a recommendation - a suggestion? - for 14 cores [GPU compute units] allocated to visuals and four to GPU compute...
Mark Cerny: That is bad leaks and not any sort form of formal evangelisation. The point is the hardware is intentionally not 100 per cent round. It has a little bit more ALU in it than if you were thinking strictly about graphics. As a result of that you have an opportunity, almost like an incentivisation, to use that ALU for GPGPU.
Digital Foundry: I seem to recall you might have talked about a toolchain where code could be compiled either for CPU or GPU. Is that right or have I got that completely wrong?
Mark Cerny: That's completely wrong! Such a toolchain does exist. It's AMD's HSA [Heterogeneous System Architecture]. That's very exciting but our current strategies are about exposing the low-level aspects of the GPU to a higher-level language. We think that's where the greatest benefit is in year one.
