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    SubjectRe: [PATCH 2/2] smp_call_function: use rwlocks on queues rather than rcu
    On Fri, Aug 22, 2008 at 11:35:46AM -0700, Jeremy Fitzhardinge wrote:
    > Andi Kleen wrote:
    > > Right now my impression is that it is not well understood why
    > > the kmalloc makes the IPI that much slower. In theory a kmalloc
    > > shouldn't be all that slow, it's essentially just a
    > > "disable interrupts; unlink object from cpu cache; enable interrupts"
    > > with some window dressing. kfree() is similar.
    > >
    > > Does it bounce a cache line on freeing perhaps?
    > I think it's just an assumption that it would be slower. Has anyone
    > measured it?

    It's likely slower than no kmalloc because
    there will be more instructions executed, the question is just how much.

    > (Note: The measurements I posted do not cover this path, because it was
    > on a two cpu system, and it was always using the call-single path.)

    Ah so it was already 25% slower even without kmalloc? I thought
    that was with already. That doesn't sound good. Any idea where that slowdown
    comes from?


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