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    SubjectRe: SMP/cc Cluster description
    At 06:36 PM 12/4/01 -0800, David S. Miller wrote:
    >What is the difference between your messages and spin locks?
    >Both seem to shuffle between cpus anytime anything interesting
    >happens.
    >
    >In the spinlock case, I can thread out the locks in the page cache
    >hash table so that the shuffling is reduced. In the message case, I
    >always have to talk to someone.

    While what I'm about to say has little bearing on the SMP/cc case: one
    significant advantage of messages over spinlocks is being able to assign
    priority with low overhead in the quick-response real-time multi-CPU
    arena. I worked with a cluster of up to 14 CPUs using something very much
    like NUMA in which task scheduling used a set of prioritized message
    queues. The system I worked on was designed to break transaction-oriented
    tasks into a string of "work units" each of which could be processed very
    quickly -- on the order of three milliseconds or less. (The limit of 14
    CPUs was set by the hardware used to implement the main system bus.)

    I bring this up only because I have never seen a spinlock system that dealt
    with priority issues very well when under heavy load.

    OK, I've said my piece, now I'll sit back and continue to watch your
    discussion.

    Satch

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