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    SubjectRe: 2.4.23aa2 (bugfixes and important VM improvements for the high end)
    Andrea Arcangeli wrote:
    > > Can you use a read-write lock, so that userspace copies only need to
    > > take the lock for reading? That doesn't eliminate cacheline bouncing
    > > but does eliminate the serialisation.
    >
    > normally the bouncing would be the only overhead, but here I also think
    > the serialization is a significant factor of the contention because the
    > critical section is taking lots of time. So I would expect some
    > improvement by using a read/write lock.

    For something as significant as user<->kernel data transfers, it might
    be worth eliminating the bouncing as well - by using per-CPU * per-mm
    spinlocks.

    User<->kernel data transfers would take the appropriate per-CPU lock
    for the current mm, and not take page_table_lock. Everything that
    normally takes page_table_lock would, and also take all of the per-CPU locks.

    That does require a set of per-CPU spinlocks to be allocated whenever
    a new mm is allocated (although the sets could be cached so it needn't
    be slow).

    -- Jamie
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