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What frequency will production Power6 chips use?
McCredie: That's the thing we haven't got down yet. Our actual shipping frequency is going to be set by the system environment we choose to put the chip into. Chip development has to be well ahead of system development. Those system environments will have various power constraints and thermal constraints. As we settle those down we'll be able to pin the ultimate frequency. We're telling people now the ultimate frequency will be between 4GHz and 5GHz.
How do you get up to that speed?
McCredie: That's what Brian Curran (the lead author on IBM's 4GHz paper for ISSCC) showed. If you're holding the pipe depth constant, you have to put half as much logic between each pipe stage. We had to get to the point where our circuits were doing double and triple duty, where one set of transistors were doing multiple functions. We had half as much gate delay between latches but had to get more work out of them.
The chip will come out in 2007. Isn't that later than what IBM had said earlier?
McCredie: We're trying to stay on a three-year cadence.
I imagine you're trying to balance the Power6 systems so that a 4GHz chip won't just idle away more processing cycles waiting for data.
McCredie: We did scale the system with the chip. While we didn't show it at ISSCC, one of the key things is our next-generation I/O (input-output). We also have our third-generation elastic interface, so we are scaling our memory and processor-to-processor communication. We're focusing on scaling our system structure as well so we don't just spin our wheels faster.
Multicore processors are all the rage, and IBM led the market with the dual-core Power4. What does the future hold for Power6?
McCredie: The key thing is the balance between maintaining our single-core performance as well as maintaining our system throughput. There are many applications that still count on that single-core, single-thread, uniprocessor performance. Not all the applications have been migrated over to exploit multiple cores. We're trying to strike the balance between single-thread performance on uniprocessor apps and SMP (symmetrical multiprocessing with many threads). That's one of the reasons we did pursue the frequency.
How many cores does Power6 have?
McCredie: It's still a dual-core chip (like Power5). But we will, as we did on Power5, exploit our chip to get more than two cores per socket, just like we did on Power5.
Your multichip module packaging approach?
McCredie: Exactly. Power5 was the first four-core-per-socket design out there.
Will it still have two threads per core?
McCredie: We haven't talked about that yet.
Sun's UltraSparc T1 "Niagara" processor takes a different balance, emphasizing throughput much more than single-thread performance. What do you think of it?
McCredie: The problem you find is this: When you go over and specialize too much on one aspect of performance, you generally get in trouble. Life is never so kind to us architects. If I do this one thing, like great throughput SMP performance, and ignore all these other things, the world is not kind to you. We usually end up making trade-offs. Every single customer has at least one key app that's never been threaded.
Sun isn't arguing that Niagara is good for everything. They're aiming for the front-end server jobs like Web site hosting or Java programs.
McCredie: I don't believe there's a big role in this world for too-specialized hardware. We have to stay on general-purpose hardware that we can do a lot with. Cray, Thinking Machines--the roadside is littered with people building specialized scientific hardware.
What do you think of asynchronous clocking, where different parts of the chip run on independent schedules?
McCredie: For me, personally, for this particular architect, I'm not a big fan. It breaks too many of our tools and verification suites to build large processors.
Power is a big issue these days. What will power consumption look like going from Power5 to Power6?
McCredie: We're targeting the same classes and categories for Power5 and Power6. We're telling people we're hitting the same power envelope as with the Power5. We are holding the power aligned for the most part. Holding the power is a concern for everybody these days.
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