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    SubjectRe: [PATCH -v5][RFC]: mutex: implement adaptive spinning


    On Wed, 7 Jan 2009, Gregory Haskins wrote:
    >
    > Can I ask a simple question in light of all this discussion?
    >
    > "Is get_task_struct() really that bad?"

    Yes. It's an atomic access (two, in fact, since you need to release it
    too), which is a huge deal if we're talking about a timing-critical
    section of code.

    And this is timing-critical, or we wouldn't even care - even in the
    contention case. Admittedly btrfs apparently makes it more so that it
    _should_ be, but Peter had some timings that happened with just regular
    create/unlink that showed a big difference.

    So the whole and only point of spinning mutexes is to get rid of the
    scheduler overhead, but to also not replace it with some other thing ;)

    Linus


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