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    SubjectRe: IO queueing and complete affinity w/ threads: Some results
    On Tue, Feb 19 2008, Mike Travis wrote:
    > Paul Jackson wrote:
    > > Jens wrote:
    > >> My main worry with the current code is the ->lock in the per-cpu
    > >> completion structure.
    > >
    > > Drive-by-comment here: Does the patch posted later this same day by Mike Travis:
    > >
    > > [PATCH 0/2] percpu: Optimize percpu accesses v3
    > >
    > > help with this lock issue any? (I have no real clue here -- just connecting
    > > up the pretty colored dots ;).
    > >
    >
    > I'm not sure of the context here but a big motivation for doing the
    > zero-based per_cpu variables was to optimize access to the local
    > per cpu variables to one instruction, reducing the need for locks.

    I'm afraid the two things aren't related, although faster access to
    per-cpu is of course a benefit for this as well. My expressed concern
    was the:

    spin_lock(&bc->lock);
    was_empty = list_empty(&bc->list);
    list_add_tail(&req->donelist, &bc->list);
    spin_unlock(&bc->lock);

    where 'bc' may be per-cpu data of another CPU

    --
    Jens Axboe



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