Sun's 'Rock' chip: Lots of threads, but how many?

Sun Chief Executive dangled a tantalizingly large number in front of financial analysts on conference call Tuesday: the number of threads that its forthcoming 16-core Rock processor can handle.

That number was 256, which is 64 times that of Intel's current Itanium and Xeon or of IBM's Power.

But Sun fans should be careful before they trumpet the figure. John Fowler, Sun's executive vice president in charge of servers, said later that day that's not the number of threads available to software running on the machine. (A thread is an independent sequence of software instructions; servers, which often juggle multiple tasks, such as database queries, at once, can benefit from using multiple processors, processors with multiple cores, and cores that can run multiple threads.)

Fowler didn't elaborate on how exactly Schwartz's number was at variance with the true design, but one possibility is that the CEO was counting internally used "scout" threads that blaze the trail for the "real" threads that software uses. Scout threads handle tasks such as fetching data the real thread will need, and Sun likes scout threads so much that each real thread gets its own scout thread.

So one possibility for Schwartz's 256-thread tally, is that each Rock core has eight real threads and eight scout threads. Sun wouldn't elaborate on Schwartz's view.

Update: Another possibility is that Schwartz was talking about the number of threads in an entire system. An eight-processor Rock system with 256 threads would have 32 threads per chip, or 2 threads per core.

Rock has "taped out," meaning that the design has been sent to Texas Instruments for manufacturing of the first real prototypes. The chip is due to ship in servers in the second half of 2008.

Sun is more gung-ho than any of its competitors when it comes to cramming lots of threads into its processors. Niagara, formally called the UltraSparc T1, has eight processing cores with four threads apiece, giving a single chip a 32-thread capacity, for example.

A given Niagara thread likely won't execute as fast as on, say, IBM's forthcoming Power6 processor, which has dual two-thread cores or an x86 chip from AMD or Intel with four single-thread cores. But Sun likes its approach so much it's building eight eight-thread cores into its upcoming Niagara 2 processor, due in the second half of 2007.

Rock, though, has a focus of high thread speed as well as lots of threads, making it suitable for high-end server work, Sun says.