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    SubjectRe: [RFC][PATCH 6/8] mm: handle_speculative_fault()

    On Tue, 5 Jan 2010, Andi Kleen wrote:
    > > Oh well. Somebody who is bored might look at trying to make the wrapper
    > > code in arch/x86/lib/semaphore_32.S work on x86-64 too. It should make the
    > > successful rwsem cases much faster.
    > Maybe, maybe not.

    If there is actual contention on the lock, but mainly just readers (which
    is what the profile indicates: since there is no scheduler footprint, the
    actual writer-vs-reader case is probably very rare), then the xadd is
    likely to be _much_ faster than the spinlock.

    Sure, the cacheline is going to bounce regardless (since it's a shared
    per-mm data structure), but the spinlock is going to bounce wildly
    back-and-forth between everybody who _tries_ to get it, while the regular
    xadd is going to bounce just once per actual successful xadd.

    So a spinlock is as cheap as an atomic when there is no contention (which
    is the common single-thread case - the real cost of both lock and atomic
    is simply the fact that CPU serialization is expensive), but when there is
    actual lock contention, I bet the atomic xadd is going to be shown to be

    Remember: we commonly claim that 'spin_unlock' is basically free on x86 -
    and that's true, but it is _only_ true for the uncontended state.


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      Last update: 2010-01-05 19:01    [W:0.021 / U:151.520 seconds]
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