linux-kernel - Re: [PATCH v8 0/9] rwsem performance optimizations

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Message-ID: <20131003073212.GC5775@gmail.com>
Date:	Thu, 3 Oct 2013 09:32:12 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Tim Chen <tim.c.chen@...ux.intel.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Alex Shi <alex.shi@...aro.org>,
	Andi Kleen <andi@...stfloor.org>,
	Michel Lespinasse <walken@...gle.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Matthew R Wilcox <matthew.r.wilcox@...el.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Rik van Riel <riel@...hat.com>,
	Peter Hurley <peter@...leysoftware.com>,
	"Paul E.McKenney" <paulmck@...ux.vnet.ibm.com>,
	Jason Low <jason.low2@...com>,
	Waiman Long <Waiman.Long@...com>, linux-kernel@...r.kernel.org,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH v8 0/9] rwsem performance optimizations


* Tim Chen <tim.c.chen@...ux.intel.com> wrote:

> For version 8 of the patchset, we included the patch from Waiman to 
> streamline wakeup operations and also optimize the MCS lock used in 
> rwsem and mutex.

I'd be feeling a lot easier about this patch series if you also had 
performance figures that show how mmap_sem is affected.

These:

> Tim got the following improvement for exim mail server 
> workload on 40 core system:
> 
> Alex+Tim's patchset:    	   +4.8%
> Alex+Tim+Waiman's patchset:        +5.3%

appear to be mostly related to the anon_vma->rwsem. But once that lock is 
changed to an rwlock_t, this measurement falls away.

Peter Zijlstra suggested the following testcase:

===============================>
In fact, try something like this from userspace:

n-threads:

  pthread_mutex_lock(&mutex);
  foo = mmap();
  pthread_mutex_lock(&mutex);

  /* work */

  pthread_mutex_unlock(&mutex);
  munma(foo);
  pthread_mutex_unlock(&mutex);

vs

n-threads:

  foo = mmap();
  /* work */
  munmap(foo);

I've had reports that the former was significantly faster than the
latter.
<===============================

this could be put into a standalone testcase, or you could add it as a new 
subcommand of 'perf bench', which already has some pthread code, see for 
example in tools/perf/bench/sched-messaging.c. Adding:

   perf bench mm threads

or so would be a natural thing to have.

Thanks,

	Ingo
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