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

[Peter Zijlstra]

On Mon, 2007-05-14 at 13:58 +0200, Nick Piggin wrote:
> On Fri, May 11, 2007 at 05:56:21PM +0200, Ingo Molnar wrote:
> > 
> > * Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> 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.
> 
> Is this with the MADV_DONTNEED kernel and glibc work?

No, just the old stuff.

> 
> > i also like the design, alot: instead of doing a full new lock type 
> > (with per-arch changes, extra lockdep support, etc. etc) you layered the 
> > new abstraction ontop of mutexes. This makes this hard locking 
> > abstraction look really, really simple, while the percpu_counter trick 
> > makes it scale _perfectly_ for the reader case. Congratulations!
> > 
> > given how nice this looks already, have you considered completely 
> > replacing rwsems with this? I suspect you could test the correctness of 
> 
> Not to take anything away from this lock type (because it can have its
> uses), but have you considered the size of this lock and its write side
> performance? 

Yeah, I did, I was initially just toying about with the concept, to see
if it was doable at all. I only posted because I got these fairly good
numbers.

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