Re: [PATCH 0/2] convert mmap_sem to a scalable rw_mutex

[Eric Dumazet <dada1@cosmosbay.com>]

Ingo Molnar a écrit :
> * Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
>
> > I was toying with a scalable rw_mutex and found that it gives ~10% 
> > reduction in system time on ebizzy runs (without the MADV_FREE patch).
> >
> > 2-way x86_64 pentium D box:
> >
> > 2.6.21
> >
> > /usr/bin/time ./ebizzy -m -P
> > 59.49user 137.74system 1:49.22elapsed 180%CPU (0avgtext+0avgdata 0maxresident)k
> > 0inputs+0outputs (0major+33555877minor)pagefaults 0swaps
> >
> > 2.6.21-rw_mutex
> >
> > /usr/bin/time ./ebizzy -m -P
> > 57.85user 124.30system 1:42.99elapsed 176%CPU (0avgtext+0avgdata 0maxresident)k
> > 0inputs+0outputs (0major+33555877minor)pagefaults 0swaps
>
> nice! This 6% runtime reduction on a 2-way box will i suspect get 
> exponentially better on systems with more CPUs/cores.

As long you only have readers, yes.

But I personally find this new rw_mutex not scalable at all if you have some 
writers around.

percpu_counter_sum is just a L1 cache eater, and O(NR_CPUS)



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