Re: [PATCH] rwsem: reduce spinlock contention in wakeup code path

[Tim Chen]

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.Long@xxxxxx> 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%

Tim

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