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Multicore Chips As 'Mini-Internets' 132

An anonymous reader writes "Today, a typical chip might have six or eight cores, all communicating with each other over a single bundle of wires, called a bus. With a bus, only one pair of cores can talk at a time, which would be a serious limitation in chips with hundreds or even thousands of cores. Researchers at MIT say cores should instead communicate the same way computers hooked to the Internet do: by bundling the information they transmit into 'packets.' Each core would have its own router, which could send a packet down any of several paths, depending on the condition of the network as a whole."
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Multicore Chips As 'Mini-Internets'

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  • Buses are so '90s (Score:5, Informative)

    by rrohbeck ( 944847 ) on Tuesday April 10, 2012 @11:54PM (#39640447)

    AMD uses HT and Intel has its ring bus, both of which use point-to-point links. Buses have serious trouble with the impedance jumps at the taps and clock skew between the lines, that's why nobody is using them in high speed applications any more. Even the venerable SCSI and ATA buses went the way of the dodo. The only bus I can see in my system is DDR3 (and I think that will go away with DDR4 due do the same problems.)

  • by Electricity Likes Me ( 1098643 ) on Wednesday April 11, 2012 @12:31AM (#39640681)

    Also this is exactly what chip makers already do to a great extent: the binning of CPUs by speeds is not a targeted process. You make a bunch of a chips, test them, and then sell them as whatever clock speed they are robustly stable at.

  • by jd ( 1658 ) <> on Wednesday April 11, 2012 @12:46AM (#39640771) Homepage Journal

    The Transputer was a brilliant design. Intel came up with a next-gen variant, called the iWarp, but never did anything with it and eventually abandoned the concept.

    IIRC, each Transputer had four serial lines where each could be in transmit or receive mode. They each had their own memory management (16K on-board, extendable up to 4 gigs - it was a true 32-bit architecture) so there was never any memory contention. Arrays of thousands of Transputers, arranged in a Hypercube topology, were developed and could out-perform the Cray X-MP at a fraction of the cost.

    Having a similar communications system in modern CPUs would certainly be doable. It would have the major benefit over a bus in that it's a local communications channel so you always have maximum bandwidth. Having said that, a switched network would have fewer interconnects and be simpler to construct and scale since the switching logic is isolated and not part of the core. You can also multicast and anycast on a switched network - technically doable on the Transputer but not trivial. Multicasting is excellent for MISD-type problems (multi-instruction, single-data) since you can have the instructions in the L1 cache and then just deliver the data in a single burst to all applicable cores.

    (Interestingly, although PVM and MPI support collective operations of this kind, they're usually done as for loops, which - by definition - means your network latency goes up with the number of processes you send to. Since collective operations usually end in a barrier, even the process you first send to has this extra latency built into it.)

    It's also arguable that it would be better if the networking in the CPU was compatible with the networking on the main bus since this would mean core-to-core communications across SMP would not require any translation or any extra complexities in the support chips. It would also mean CPU-to-GPU communications would be greatly simplified.

  • by Sarusa ( 104047 ) on Wednesday April 11, 2012 @12:49AM (#39640797)

    As mentioned in other comments, this has been done before. The method of message passing isn't as fundamental as one key point - that it is all explicit message passing.

    Intel and AMD x86/x64 CPUs use coherent cache between cores to make sure that a thread running on CPU 1 sees the same RAM as a thread running on CPU 3. This leads to horrible bottlenecks and huge amounts of die tied up in trying to coordinate the writes, maintain coherency between N cores (N-1 ^2 connections!), and it all just goes to hell pretty fast. Intel has this super new transactional memory rollback thing, but it's turd polishing.

    The next step is pretty obvious (see Barrelfish) and easy: no shared coherency. Everything is done with message passing. If two threads or processes (it doesn't really matter at that point) want to communicate they need to do it with messages. It's much cleaner than dealing with shared memory synchronization, and makes program flow much more obvious (to me at least - I use message queues even on x86/x64). If you need to share BIG MEMORY between threads, which is reasonable for something like image processing, you at least use messages to explicitly coordinate access to shared memory and the cores don't have to worry about coherency.

    This scales extremely well for at least a couple thousand CPUs, which is where the 'local internet' becomes useful.

    Where it becomes not easy is that almost all programs written for x86/x64 assume threads can share memory at will. They'd need to be rewritten for this model or would suddenly run a whole lot slower since you'd have to lock them to one core or somehow do the coordination behind their back. It'd be worth it for me!

  • by Anonymous Coward on Wednesday April 11, 2012 @12:50AM (#39640805)

    You are misinterpreting it. The chips CAN work independently. It is only when one needs to talk to another or use a shared resource (hard drive, main memory, network) that this becomes a potential issue. It is like a family of three sharing a single bathroom - not such a big deal, Bump that up to 20 using the same bathroom, and you start having serious issues.

  • by mikkelm ( 1000451 ) on Wednesday April 11, 2012 @01:16AM (#39640937)

    Slashdot in 2012 is largely technical support people and Windows administrators who hold their MCSAs more dear than their first born. This is how it has to be explained.

"If you lived today as if it were your last, you'd buy up a box of rockets and fire them all off, wouldn't you?" -- Garrison Keillor