Re: [patch 00/15] Generic Mutex Subsystem

[Steven Rostedt]

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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