* Tim Chen <tim.c.c...@linux.intel.com> wrote:
> On Fri, 2013-09-27 at 12:39 -0700, Davidlohr Bueso wrote:
> > On Fri, 2013-09-27 at 12:28 -0700, Linus Torvalds wrote:
> > > On Fri, Sep 27, 2013 at 12:00 PM, Waiman Long <waiman.l...@hp.com> wrote:
> > > >
> > > > On a large NUMA machine, it is entirely possible that a fairly large
> > > > number of threads are queuing up in the ticket spinlock queue to do
> > > > the wakeup operation. In fact, only one will be needed. This patch
> > > > tries to reduce spinlock contention by doing just that.
> > > >
> > > > A new wakeup field is added to the rwsem structure. This field is
> > > > set on entry to rwsem_wake() and __rwsem_do_wake() to mark that a
> > > > thread is pending to do the wakeup call. It is cleared on exit from
> > > > those functions.
> > >
> > > Ok, this is *much* simpler than adding the new MCS spinlock, so I'm
> > > wondering what the performance difference between the two are.
> >
> > Both approaches should be complementary. The idea of optimistic spinning
> > in rwsems is to avoid putting putting the writer on the wait queue -
> > reducing contention and giving a greater chance for the rwsem
> > to get acquired. Waiman's approach is once the blocking actually occurs,
> > and at this point I'm not sure how this will affect writer stealing
> > logic.
> >
>
> I agree with the view that the two approaches are complementary to each
> other. They address different bottleneck areas in the rwsem. Here're
> the performance numbers for exim workload compared to a vanilla kernel.
>
> Waimain's patch: +2.0%
> Alex+Tim's patchset: +4.8%
> Waiman+Alex+Tim: +5.3%
I think I'd like to see a combo series submitted :-)
Thanks,
Ingo
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