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    SubjectRe: [RFC] New locking primitive for 2.5

    On Thu, 7 Feb 2002, yodaiken wrote:

    > So what's the difference between combi_spin and combi_mutex?
    > combi_spin becomes
    > if not mutex locked, spin
    > else sleep
    > Bizzare

    no, the real optimization is that when spin meets spin, they will not
    mutex. If a mutex-user has it then spins turn into mutex, but that (is
    supposed to) happen rarely.

    i think one example *could* be to turn inode->i_sem into a combi-lock. Eg.
    generic_file_llseek() could use the spin variant.

    this is a real performance problem, i've seen scheduling storms in
    dbench-type runs due to llseek taking the inode semaphore.

    whether combi-locks truly bring performance benefits remains to be seen,
    but the patch definitely needs to provide some working example and some
    hard numbers for some real workload.

    Ingo

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