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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