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    SubjectRe: [PATCH 0/2] convert mmap_sem to a scalable rw_mutex
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    On Fri, 2007-05-11 at 18:52 +0200, Eric Dumazet wrote:
    > Ingo Molnar a écrit :
    > > * Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
    > >
    > >> I was toying with a scalable rw_mutex and found that it gives ~10%
    > >> reduction in system time on ebizzy runs (without the MADV_FREE patch).
    > >>
    > >> 2-way x86_64 pentium D box:
    > >>
    > >> 2.6.21
    > >>
    > >> /usr/bin/time ./ebizzy -m -P
    > >> 59.49user 137.74system 1:49.22elapsed 180%CPU (0avgtext+0avgdata 0maxresident)k
    > >> 0inputs+0outputs (0major+33555877minor)pagefaults 0swaps
    > >>
    > >> 2.6.21-rw_mutex
    > >>
    > >> /usr/bin/time ./ebizzy -m -P
    > >> 57.85user 124.30system 1:42.99elapsed 176%CPU (0avgtext+0avgdata 0maxresident)k
    > >> 0inputs+0outputs (0major+33555877minor)pagefaults 0swaps
    > >
    > > nice! This 6% runtime reduction on a 2-way box will i suspect get
    > > exponentially better on systems with more CPUs/cores.
    >
    > As long you only have readers, yes.
    >
    > But I personally find this new rw_mutex not scalable at all if you have some
    > writers around.
    >
    > percpu_counter_sum is just a L1 cache eater, and O(NR_CPUS)

    Yeah, that is true; there are two occurences, the one in
    rw_mutex_read_unlock() is not strictly needed for correctness.

    Write locks are indeed quite expensive. But given the ratio of
    reader:writer locks on mmap_sem (I'm not all that familiar with other
    rwsem users) this trade-off seems workable.

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