Re: Performance regression from switching lock to rw-sem foranon-vma tree

[Ingo Molnar]

* Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx> wrote:

> On Wed, 2013-06-19 at 09:53 -0700, Tim Chen wrote: 
> > On Wed, 2013-06-19 at 15:16 +0200, Ingo Molnar wrote:
> > 
> > > > vmstat for mutex implementation: 
> > > > procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu-----
> > > >  r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
> > > > 38  0      0 130957920  47860 199956    0    0     0    56 236342 476975 14 72 14  0  0
> > > > 41  0      0 130938560  47860 219900    0    0     0     0 236816 479676 14 72 14  0  0
> > > > 
> > > > vmstat for rw-sem implementation (3.10-rc4)
> > > > procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu-----
> > > >  r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
> > > > 40  0      0 130933984  43232 202584    0    0     0     0 321817 690741 13 71 16  0  0
> > > > 39  0      0 130913904  43232 224812    0    0     0     0 322193 692949 13 71 16  0  0
> > > 
> > > It appears the main difference is that the rwsem variant context-switches 
> > > about 36% more than the mutex version, right?
> > > 
> > > I'm wondering how that's possible - the lock is mostly write-locked, 
> > > correct? So the lock-stealing from Davidlohr Bueso and Michel Lespinasse 
> > > ought to have brought roughly the same lock-stealing behavior as mutexes 
> > > do, right?
> > > 
> > > So the next analytical step would be to figure out why rwsem lock-stealing 
> > > is not behaving in an equivalent fashion on this workload. Do readers come 
> > > in frequently enough to disrupt write-lock-stealing perhaps?
> 
> Ingo, 
> 
> I did some instrumentation on the write lock failure path.  I found that
> for the exim workload, there are no readers blocking for the rwsem when
> write locking failed.  The lock stealing is successful for 9.1% of the
> time and the rest of the write lock failure caused the writer to go to
> sleep.  About 1.4% of the writers sleep more than once. Majority of the
> writers sleep once.
> 
> It is weird that lock stealing is not successful more often.

For this to be comparable to the mutex scalability numbers you'd have to 
compare wlock-stealing _and_ adaptive spinning for failed-wlock rwsems.

Are both techniques applied in the kernel you are running your tests on?

Thanks,

	Ingo
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