linux-kernel - Re: aim7 scalability issue on 4 socket machine

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Date:	Thu, 17 Sep 2009 12:35:02 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: aim7 scalability issue on 4 socket machine


* Zhang, Yanmin <yanmin_zhang@...ux.intel.com> wrote:

> ???Aim7 result is bad on my new Nehalem machines (4*8*2 logical cpu). 
> Perf counter shows spinlock consumes 70% cpu time on the machine. 
> Lock_stat shows anon_vma->lock causes most of the spinlock contention. 
> Function tracer shows below call chain creates the spinlock.
> 
> do_brk => vma_merge =>vma_adjust
> 
> Aim7 consists of lots of subtests. One test is to fork lots of 
> processes and every process calls sbrk for 1000 times to grow/shrink 
> the heap. All the vma of the heap of all sub-processes point to the 
> same anon_vma and use the same anon_vma->lock. When sbrk is called, 
> kernel calls do_brk => vma_merge =>vma_adjust and lock anon_vma->lock 
> to create spinlock contentions.
> 
> There is a comment section in front of spin_lock(&anon_vma->lock. It 
> says anon_vma lock can be optimized when just changing vma->vm_end. As 
> a matter of fact, anon_vma->lock is used to protect anon_vma->list 
> when an entry is deleted/inserted or the list is accessed. There is no 
> such deletion/insertion if only vma->end is changed in function 
> vma_adjust.
> 
> Below patch fixes it.
> 
> Test results with kernel 2.6.31-rc8. The improvement on the machine is 
> about 150%.

Impressive speedup!

[ Also, the array of tools you used to debug this is impressive as well
  ;-) ]

	Ingo
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