Digital Foundry: How would you characterise the combination of Xenos and Xenon compared to the traditional x86/GPU combo on PC? Surely on the face of it, Xbox 360 is lacking a lot of power compared to today's entry-level "enthusiast" PC hardware?
Oles Shishkovstov: You can calculate it like this: each 360 CPU core is approximately a quarter of the same-frequency Nehalem (i7) core. Add in approximately 1.5 times better performance because of the second, shared thread for 360 and around 1.3 times for Nehalem, multiply by three cores and you get around 70 to 85 per cent of a single modern CPU core on generic (but multi-threaded) code.
Bear in mind though that the above calculation will not work in the case where the code is properly vectorised. In that case 360 can actually exceed PC on a per-thread per-clock basis. So, is it enough? Nope, there is no CPU in the world that is enough for games!
The 360 GPU is a different beast. Compared to today's high-end hardware it is 5-10 times slower depending on what you do. But performance of hardware is only one side of equation. Because we as programmers can optimise for the specific GPU we can reach nearly 100 per cent utilisation of all the sub-units. That's just not possible on a PC.
In addition to this we can do dirty MSAA tricks, like treating some surfaces as multi-sampled (for example hi-stencil masking the light-influence does that), or rendering multi-sampled shadow maps, and then sampling correct sub-pixel values because we know exactly what pattern and what positions sub-samples have, etc. So, it's not directly comparable.
