linux-kernel - Re: [MM] Make mm counters per cpu instead of atomic

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Date:	Tue, 24 Nov 2009 16:02:33 +0800
From:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	"hugh.dickins@...cali.co.uk" <hugh.dickins@...cali.co.uk>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, Tejun Heo <tj@...nel.org>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: [MM] Make mm counters per cpu instead of atomic

On Mon, 2009-11-23 at 08:31 -0600, Christoph Lameter wrote:
> On Mon, 23 Nov 2009, Zhang, Yanmin wrote:
> 
> > Another theoretic issue is below scenario:
> > Process A get the read lock on cpu 0 and is scheduled to cpu 2 to unlock. Then
> > it's scheduled back to cpu 0 to repeat the step. eventually, the reader counter
> > will overflow. Considering multiple thread cases, it might be faster to
> > overflow than what we imagine. When it overflows, processes will hang there.
> 
> True.... We need to find some alternative to per cpu data to scale mmap
> sem then.
I ran lots of benchmarks such like specjbb2005/hackbench/tbench/dbench/iozone
/sysbench_oltp(mysql)/aim7 against percpu tree(based on 2.6.32-rc7) on a 4*8*2 logical
cpu machine, and didn't find big result difference between with your patch and without
your patch.



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