Interesting bits:
DF: Are you still excited by the potential of these consoles?
Oles Shishkovstov: I think what we achieved with the new consoles was a really good job given the time we had with development kits in the studio - just four months hands-on experience with Xbox One and six months with the PlayStation 4 (I guess the problems we had getting kits to the Kiev office are well-known now).
Out of context: And let's not forget that programming close to the metal will usually mean that we can get 2x performance gain over the equivalent PC spec. Practically achieving that performance takes some time, though!
Digital Foundry: Xbox One's lower compute unit count, memory bandwidth and ESRAM issues are well documented. Resolution differences in multi-platform games are commonplace and in some titles we're even looking at 720p vs 1080p. What's your take on the differences between Xbox One and PlayStation 4?
Oles Shishkovstov: Well, you kind of answered your own question - PS4 is just a bit more powerful. You forgot to mention the ROP count, it's important too - and let's not forget that both CPU and GPU share bandwidth to DRAM [on both consoles]. I've seen a lot of cases while profiling Xbox One when the GPU could perform fast enough but only when the CPU is basically idle. Unfortunately I've even seen the other way round, when the CPU does perform as expected but only under idle GPU, even if it (the CPU) is supposed to get prioritised memory access. That is why Microsoft's decision to boost the clocks just before the launch was a sensible thing to do with the design set in stone.
Digital Foundry: Is ESRAM really that much of a pain to work with?
Oles Shishkovstov: Actually, the real pain comes not from ESRAM but from the small amount of it. As for ESRAM performance - it is sufficient for the GPU we have in Xbox One. Yes it is true, that the maximum theoretical bandwidth - which is somewhat comparable to PS4 - can be rarely achieved (usually with simultaneous read and write, like FP16-blending) but in practice I've seen only a few cases where it becomes a limiting factor.
Digital Foundry: DirectX 11 vs GNMX vs GNM - what's your take on the strengths and weakness of the APIs available to developers with Xbox One and PlayStation 4? Closer to launch there were some complaints about XO driver performance and CPU overhead on GNMX.
Oles Shishkovstov: Let's put it that way - we have seen scenarios where a single CPU core was fully loaded just by issuing draw-calls on Xbox One (and that's surely on the 'mono' driver with several fast-path calls utilised). Then, the same scenario on PS4, it was actually difficult to find those draw-calls in the profile graphs, because they are using almost no time and are barely visible as a result.
In general - I don't really get why they choose DX11 as a starting point for the console. It's a console! Why care about some legacy stuff at all? On PS4, most GPU commands are just a few DWORDs written into the command buffer, let's say just a few CPU clock cycles. On Xbox One it easily could be one million times slower because of all the bookkeeping the API does.
But Microsoft is not sleeping, really. Each XDK that has been released both before and after the Xbox One launch has brought faster and faster draw-calls to the table. They added tons of features just to work around limitations of the DX11 API model. They even made a DX12/GNM style do-it-yourself API available - although we didn't ship with it on Redux due to time constraints.
