From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andrew Morton <akpm@osdl.org>
Date: Mon, 22 Nov 2004 23:16:28 +0000
Subject: Re: deferred rss update instead of sloppy rss
Message-Id: <20041122151628.77ab87ca.akpm@osdl.org>
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To: Christoph Lameter <clameter@sgi.com>
Cc: hugh@veritas.com, torvalds@osdl.org, benh@kernel.crashing.org, nickpiggin@yahoo.com.au, linux-mm@kvack.org, linux-ia64@vger.kernel.org, linux-kernel@vger.kernel.org

Christoph Lameter <clameter@sgi.com> wrote:
>
> On Mon, 22 Nov 2004, Andrew Morton wrote:
> 
> > > The page fault code only increments rss. For larger transactions that
> > > increase / decrease rss significantly the page_table_lock is taken and
> > > mm->rss is updated directly. So no
> > > gross inaccuracies can result.
> >
> > Sure.  Take a million successive pagefaults and mm->rss is grossly
> > inaccurate.  Hence my suggestion that it be spilled into mm->rss
> > periodically.
> 
> It is spilled into mm->rss periodically. That is the whole point of the
> patch.
> 
> The timer tick occurs every 1 ms.

That only works if the task happens to have the CPU when the timer tick
occurs.  There remains no theoretical upper bound to the error in mm->rss,
and that's very easy to fix.