posted by Federico Biancuzzi on Tue 27th Mar 2007 15:41 UTC

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What can you tell us about the CPU inside Microsoft's XBox 360?

Jon Stokes: The Xenon CPU is a three-core, multithreaded PowerPC processor designed by IBM. Each of the three cores is very similar to the general-purpose CPU in the Cell BE (the PS3's processor), but it has some additional vector processing resources.

Probably the most important thing to note about Xenon is that it handles caching in a very special way that makes it more effective for media-intensive workloads like video decoding and gaming.

Streaming media applications tend to "dirty" the cache, which means that instead of storing a single working set in the cache and using that data for a while, they're constantly moving data /through/ the cache. This kind of behavior makes very poor use of the cache, and in fact streaming data from one thread can result in non-streaming data from another thread being booted out of the cache needlessly.

Xenon's fix for this is to "wire down" certain sections of the cache and dedicate them to a single thread. That way, a thread that is only moving data through the cache, and not storing it, can just dirty a small, dedicated part of cache. In a way, this "cache locking" mechanism enables the Xenon's cache to function a little bit like the Cell's "local store" memory.

About PS3 and Xbox 360. From a pure computational point of view which system could be considered more powerful?

Jon Stokes: I think that PS3 has more raw computational horsepower on paper, but in practice the two consoles will probably equal out to about the same for most game developers. However, there are some problems in high-performance computing where the Cell Broadband Engine that powers PS3 is much more powerful than anything else out there. The problem is that programmers have to design their code from the algorithm level on up to fit Cell exactly in order to see benefits. And again, this doesn't seem to apply to games, but if some developer figures out that it does then eventually they could get more performance out of the PS3.

Do you think that the PS3 could be used to build cheaper computational clusters as happened with PS2? I was thinking at places like Google...

Jon Stokes: I think this is an interesting idea, but ultimately IBM's Cell-based products will be a better fit for clusters than a PS3 console. The advantage of the PS3 console is, of course, that it's cheap because Sony subsidizes it. So it's entirely possible that someone would want to use it for a cluster. It does have gigabit Ethernet, so I guess it could work.

How do their performance-per-watt compare to that of modern power-savvy CPUs such as Intel Core Duo 2?

Jon Stokes: Although I don't have any real numbers to back it up, I'd say that Core 2 Duo almost certainly has them both beat in performance/watt for ordinary workloads. But again, if you're solving one of these exotic HPC problems using Cell, and you have code that's custom-fitted to give you an outrageous performance delta vs. a traditional architecture, then those performance/watt numbers would skew pretty drastically in Cell's favor for those applications.

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