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How The 360 Works
The always informative How Stuff Works site has an article today entitled How the Xbox 360 Works. From the piece: "The other interesting thing to note about the Xbox 360 CPU is that each core is capable of processing two threads simultaneously. Think of a thread as a set of instructions for a program's job. The core processes these instructions and does the heavy lifting to get the job done. A conventional processor is traditionally capable of running a single execution thread. Because the Xbox 360 cores can each handle two threads at a time, the 360 CPU is the equivalent of having six conventional processors in one machine."
I thought something like 15% of Xbox's *didn't* work. Source? Slashdot. Surprise, surprise. It's funny. Laugh.
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"Because the Xbox 360 cores can each handle two threads at a time, the 360 CPU is the equivalent of having six conventional processors in one machine."
No it isn't.
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All the CPUs have to share access to memory, especially if one CPU is locking part of the cache for a big signal processing job. The bottleneck at the memory controller keeps SMP (symmetric multiprocessing) from reaching its theoretical throughput.
In order to get true performance out of a multicore system, you have to use NUMA, which is not just part of the hook from a Romanian pop song. It stands for non-uniform memory access, and it refers to associating a physical memory chip to each CPU. It's "non-uniform" because it takes longer for a CPU to read or write another CPU's memory than to read or write its own. But running threads that don't need to communicate too much and putting their memory on separate cores does ease up on the memory controller bottleneck.
Wonder if How Things Work got paid for that?
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That's how it works? And here I thought it was just the latest console using Hype technology.
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After reading the how the XBox 360 works, I checked out a couple other systems.
.999 teraflops?
None of them had any details about how any of the systems actually worked; it was just a list of marketing bullet-points and features published by the manufacturers.
How does knowing the system's launch lineup help me know how it works, anyway?
As for things that were flat out wrong about the 360...
9 billion dot products per second? Are they claiming that each core can compute a dot product at nearly every cycle? And if so, how is that number helpful? You still have a ton of other stuff to do in a game engine besides just computing dot products.
1 teraflop? Each thread on each core can calculate 166 billion FLOPS? Oh wait, you mean that you're also counting GPU performance in that number, which accounts for probably
500 million triangles per second... With how many textures applied? How many light sources? Oh, zero textures, using flat shading, with no light sources? And all 500 million triangles are part of a single triangle strip and are each 1 pixel in size? And that's just the theoretical maximum anyway?
What a worthless site. I feel dumber for having read it.
--Jeremy
Jesus was a liberal
It's like a reflex action to bash Microsoft here. First off, quality is always shaky on the first batch of any gaming console. It's a business where the hardware launches are historically tight. The only company that releases solid, well built, well tested consoles at launch is Nintendo, and look where their share of the market has gone. Second, based on everything I've read about the XBOX 360 it has a solid, almost textbook design, for the best technology of the next 5 years (multi core processor, unified pipeline shader/vertex on the graphics card, etc). I see alot more to question about the PS3's cell (limitations of SPE's etc) & the late addition of NVidia, since Sony only pulled them into the PS3 when they realized the Cell wouldn't be able to do the GPU work. Need I remind everyone that you are bashing a 3-GHZ G5 w/ 3 Core's paired with ATI's finest and all for $400? Yes, the marketing BS is just that, but if this was anyone else's product it would get treated alot better.
> Because the two threads in the chips share arithmetic and floating point units and whatnot,
> they get best case throughput of 1.3x a single threaded chip. This is according to Sony who has the same PPU on their PS3.
I don't mean to be too pedantic here, but you are not correct.
The Cell PPU unit and the XBOX360 PPC unit are not the same. They are related by the fact that they are both PPC designs, but that is as far as it goes. The XBOX360 PPC has two fixed-point, two floating-point and two VMX units per core - thread switching is done on fetch stalls. The Cell PPU has two register files but only single fixed-point, floating-point and VMX units - threading is accomplished by switching between the register files. The branch prediction units are also different, and the caches, and the memory mapping. As a matter of fact, the only thing the two processors share is an instruction set and an IBM invoice.
The number you (mis-)quote originally came from a lecture in an SCEA conference. You apparently don't understand the context under which it was said, and thus why it makes no sense to discuss here - nor do you appear to understand the NDA which, if you heard this directly from SCEA, are under. Although much of the Cell design and tools are public knowledge, it is necessary to keep confidential that non-public information which you have access to if you wish to continue to have access to it.
The Onion has a summary that is just as informative and much shorter.
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