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    SubjectRe: CPU affinity & IPI latency
    On Thu, Jul 12, 2001 at 05:36:41PM -0700, Larry McVoy wrote:
    > Be careful tuning for LMbench (says the author :-)
    >
    > Especially this benchmark. It's certainly possible to get dramatically better
    > SMP numbers by pinning all the lat_ctx processes to a single CPU, because
    > the benchmark is single threaded. In other words, if we have 5 processes,
    > call them A, B, C, D, and E, then the benchmark is passing a token from
    > A to B to C to D to E and around again.
    >
    > If the amount of data/instructions needed by all 5 processes fits in the
    > cache and you pin all the processes to the same CPU you'll get much
    > better performance than simply letting them float.
    >
    > But making the system do that naively is a bad idea.

    I agree, and can't imagine the system ever attempting to take this
    into account and leave these 5 tasks on the same CPU.

    At the other extreme is my observation that 2 tasks on an 8 CPU
    system are 'round robined' among all 8 CPUs. I think having the
    2 tasks stay on 2 of the 8 CPUs would be an improvement with respect
    to CPU affinity. Actually, the scheduler does 'try' to do this.

    It is clear that the behavior of lat_ctx bypasses almost all of
    the scheduler's attempts at CPU affinity. The real question is,
    "How often in running 'real workloads' are the schduler's attempts
    at CPU affinity bypassed?".

    --
    Mike Kravetz mkravetz@sequent.com
    IBM Linux Technology Center
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