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    SubjectRe: [PATCH] Replace completions with semaphores
    On Sun, Apr 13 2008, Matthew Wilcox wrote:
    > On Sun, Apr 13, 2008 at 09:05:13AM +0200, Ingo Molnar wrote:
    > > there's also another aspect: completions are faster a bit
    > > in theory, because they know that they will schedule most of the time -
    > > while semaphores assume that they will _not_ schedule. (And that's
    > > exactly because the intent of the developer when using a completion is
    > > crystal clear.)
    >
    > In practice though, the current implementation is slower. Of course,
    > that's fixable, and I strongly suspect that the current users of
    > completions simply don't care about speed -- the normal use of
    > completions is in startup and shutdown paths where a millisecond extra
    > isn't going to be noticable.

    I'd be surprised if it was in that range, milisecond range would
    definetely mean that something was completely bused in that area :-).
    IOW, I'd be surprised if you can measure much of a difference, even in
    microbencmarks.

    And performance does matter somewhat, it's not ONLY used in
    startup/shutdown scenarios. The block layer uses it for sync requests,
    for instance.

    --
    Jens Axboe



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