linux-kernel - Re: thread stacks and strict vm overcommit accounting

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Message-Id: <20070316132903.e86c275d.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Fri, 16 Mar 2007 13:29:03 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	da-x@...atomic.org, linux-kernel@...r.kernel.org
Subject: Re: thread stacks and strict vm overcommit accounting

On Thu, 15 Mar 2007 11:06:21 -0800
Andrew Morton <akpm@...ux-foundation.org> wrote:

> > On Tue, 13 Mar 2007 18:33:20 +0200 Dan Aloni <da-x@...atomic.org> wrote:
> > Hello,
> > 
> > This question is relevent to 2.6.20.
> > 
> > I noticed that if the RSS for the stack size is say, 8MB, running
> > a single-threaded process doesn't incur an increase of 8MB to
> > Committed_AS (/proc/meminfo).
> > 
> > However, on multi-threaded apps linked with pthread (on Debian
> > Etch with 2.6.20 vanilla x86_64), every thread will incur the
> > the specified maximum stack size RSS (assuming that you use
> > the default attr). In other words, it appears that vm accounting
> > works differently in that case.
> > 
> > Is this the intended behaviour?
> 
> That sounds like a bug to me.

AFAIK, "main" thread's stack is marked as VM_GROWS?? and its size can be
changed dynamically. "other" threads' stack are alloced by mmap (or malloc maybe)
and it never grows. This is difference between multi-thread and single thread.

So, you should be carefull to the size of stack when you use multi-threaded apps
and vm_overcommit_ratio at the same time. Because MAP_NORESERVE is accounted
if sysctl_overcommit_memory == OVERCOMMIT_NEVER, a program like java will fail
to create a new thread sometimes.

I have no good idea to fix this difference, sorry.

-Kame

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