On Tue, Jan 29, 2013 at 10:12:45AM +0100, Ingo Molnar wrote:
>
> * Yuanhan Liu <yuanhan....@linux.intel.com> wrote:
>
> > On Tue, Jan 29, 2013 at 09:44:00AM +0100, Ingo Molnar wrote:
> > >
> > > * Yuanhan Liu <yuanhan....@linux.intel.com> wrote:
> > >
> > > > [...]
> > >
> > > Very nice measurements and analysis, thanks!
> > >
> > > > As stated above, anybody can have a chance to own the lock in
> > > > mutex once somebody release the lock. Well, there is only one
> > > > to own the lock in rwsem write lock, and the one is known
> > > > already: the one in the head of wait list. That would result
> > > > to more contention in rwsem write lock case, especially if the
> > > > one _will_ own the lock is not running now.
> > >
> > > I think we should allow lock-steal between rwsem writers - that
> > > will not hurt fairness as most rwsem fairness concerns relate to
> > > reader vs. writer fairness.
> >
> > Agreed, and I'm sure this will improve performance and may
> > make this performance regression go away.
> >
> > David, is that Ok to you? If so, I may have a try.
>
> I'm not David but please try it :-)
>
> Making rwsem behavior and scalability similar to mutexes would
> have numerous advantages.
>
> > > Am I correct to assume that all relevant users in this
> > > workload are down_write() users?
> >
> > Yes, as commit 5a50508 just convert all mutex to down_write.
>
> A second track of inquiry would be to see whether any of the key
> usage sites could be converted to down_read()
I tried before, and seems I didn't find one.
Well, I did find an irrelevant one: vma_lock_anon_vma at validate_mm()
in mm/mmap.c. That function is _real_ only if CONFIG_DEBUG_VM_RB is
set, and there is no vma_lock_anon_vma_read() or something similar,
thus I guess it's may not worthy to turn it.
> or whether the
> lock hold times could be reduced drastically
I also found one, but it doesn't sound like the one will reduce lock
hole times drastically:
vma_lock_anon_vma() seems covered too much code at
expand_up/downwards.
Well, again, it's quite a tiny optimization for reducing the coverage.
Thanks.
--yliu
> - but I doubt
> that's really possible on such heavily forking workloads.
>
> Thanks,
>
> Ingo
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