Groucho said:
http://blogs.msdn.com/shawnhar/archive/2007/11/21/rendertarget-changes-in-xna-game-studio-2-0.aspx It's actually the eDRAM of the GPU that's the issue. It gets clobbered every time the rendertarget is switched. Multiple targets is possible on the X360, but it doesn't look like its decently fast, due to this issue. Lots of memory copying to support the feature, it seems. Shawn Hargreaves, the MS engineer whose MSDN blog this is says it clearly enough: "Deferred rendering is actually quite an awkward fit for the 360, because of the limited size but very fast EDRAM hardware design. The technique as a whole may work ok, but I think that light volumes idea is probably more work than it is worth." Deferred Rendering is great for fancy lighting, and some fancy lighting/shading techniques, when it comes to speed. The X360's flexible GPU allows it to largely get around this issue, if the game is engineered well, I imagine, but it may not look quite as good as an engine using deferred rendering -- depends on how much of a graphics fanatic you are, I suppose. Game reviewers go crazy for it, but the general public may only be able to tell the difference in a subtle "I dunno, it just looks better" kinda way.
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Hmm...interesting. That is only in regards to one method(deffered rendering) of calculating lots and lots of light sources more simply than other methods. It looks like the depth information needed for the process takes up a good bit more than the 10mb of EDRAM which in turn causes predicate tiling(but the 360 was built to handle tiling very very efficiently). It doesn't look like rendering to two different targets is a problem though, just when used in conjunction with this method.
"For that technique you would need to enable the preserve contents mode, yes.
I'm not sure that would actually give you a perf gain on Xbox, though. I'm guessing restoring the depth contents is going to be slower than however much you gain through the stencil tests.
Deferred rendering is actually quite an awkward fit for the 360, because of the limited size but very fast EDRAM hardware design. The technique as a whole may work ok, but I think that light volumes idea is probably more work than it is worth."
He is saying that the benefits of using this method do not make up for the memory reads and writes, not that the memory read and write speeds are bad.
And this is a very interesting read:
http://download.microsoft.com/download/d/3/0/d30d58cd-87a2-41d5-bb53-baf560aa2373/next_generation_graphics_programming_on_xbox_360.ppt