linux-kernel - Re: [Bugme-new] [Bug 15618] New: 2.6.18->2.6.32->2.6.33 huge regression in performance

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Date:	Tue, 23 Mar 2010 18:34:09 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	bugzilla-daemon@...zilla.kernel.org,
	bugme-daemon@...zilla.kernel.org, ant.starikov@...il.com,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [Bugme-new] [Bug 15618] New: 2.6.18->2.6.32->2.6.33 huge
 regression in performance


* Andrew Morton <akpm@...ux-foundation.org> wrote:

> lolz.  Catastrophic meltdown.  Thanks for doing all that work - at a guess 
> I'd say it's mmap_sem. [...]

Looks like we dont need to guess, just look at the call graph profile (a'ka 
the smoking gun):

> > I perf'ed on 2.6.32.9-70.fc12.x86_64 kernel
> >
> > [...]
> >
> > callgraph(top part only):
> > 
> > 53.09%      dve22lts-mc  [kernel]                                         [k]
> > _spin_lock_irqsave
> >                |          
> >                |--49.90%-- __down_read_trylock
> >                |          down_read_trylock
> >                |          do_page_fault
> >                |          page_fault
> >                |          |          
> >                |          |--99.99%-- __GI_memcpy
> >                |          |          |          
> >                |          |          |--84.28%-- (nil)
> >                |          |          |          
> >                |          |          |--9.78%-- 0x100000000
> >                |          |          |          
> >                |          |           --5.94%-- 0x1
> >                |           --0.01%-- 
> > [...]
> > 
> >                |          
> >                |--49.39%-- __up_read
> >                |          up_read
> >                |          |          
> >                |          |--100.00%-- do_page_fault
> >                |          |          page_fault
> >                |          |          |          
> >                |          |          |--99.99%-- __GI_memcpy
> >                |          |          |          |          
> >                |          |          |          |--84.18%-- (nil)
> >                |          |          |          |          
> >                |          |          |          |--10.13%-- 0x100000000
> >                |          |          |          |          
> >                |          |          |           --5.69%-- 0x1
> >                |          |           --0.01%-- 
> > [...]

It shows a very brutal amount of page fault invoked mmap_sem spinning 
overhead.

> Perhaps with some assist from the CPU scheduler.

Doesnt look like it, the perf stat numbers show that the scheduler is only 
very lightly involved:

  > > 129875.554435 task-clock-msecs # 10.210 CPUs 
  > >          1883 context-switches # 0.000 M/sec 
 
a context switch only every ~68 milliseconds.

	Ingo
	Ingo
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