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    SubjectRe: [patch 00/15] Generic Mutex Subsystem

    On Mon, 19 Dec 2005, Andi Kleen wrote:

    > > $ ./test-mutex V 16 10 $ ./test-mutex V 16 10
    > > 8 CPUs, running 16 tasks. 8 CPUs, running 16 tasks.
    > > checking VFS performance. checking VFS performance.
    > > avg loops/sec: 34713 avg loops/sec: 84153
    > > CPU utilization: 63% CPU utilization: 22%
    > >
    > > i.e. in this workload, the mutex based kernel was 2.4 times faster
    > > than the semaphore based kernel, _and_ it also had 2.8 times less CPU
    > > utilization. (In terms of 'ops per CPU cycle', the semaphore kernel
    > > performed 551 ops/sec per 1% of CPU time used, while the mutex kernel
    > > performed 3825 ops/sec per 1% of CPU time used - it was 6.9 times
    > > more efficient.)
    >
    > Do you have an idea where this big difference comes from? It doesn't look
    > it's from the fast path which is essentially the same. Do the mutexes have
    > that much better scheduling behaviour than semaphores? It is a bit hard to
    > believe.
    >
    > I would perhaps understand your numbers if you used adaptive mutexes
    > or similar that spin for a bit, but just for pure sleeping locks it seems
    > weird. After all the scheduler should work in the same way for both.
    >

    Perhaps it's the smaller structures, as Ingo said, which would allow for
    better cache handling.

    -- Steve

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