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    SubjectRe: -rt dbench scalabiltiy issue

    On Wed, 18 Nov 2009, Nick Piggin wrote:
    > > So yes, on -rt, the overhead from lock contention is way way worse then
    > > any extra atomic ops. :)
    > How about overhead for an uncontended lock? Ie. is the problem caused
    > because lock *contention* issues are magnified on -rt, or is it
    > because uncontended lock overheads are higher? Detailed callgraph
    > profiles and lockstat of +/-atomic case would be very interesting.

    In the uncontended case we have the overhead of calling might_sleep()
    before we acquire the lock with cmpxchg(). The uncontended unlock is a
    cmpxchg() as well.

    I don't think that this is significant overhead and we see real lock
    contention issues magnified by at least an order of magnitude.



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