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Date: Mon, 09 Sep 2002 14:44:35 -0700
From: Andrew Morton <akpm@digeo.com>
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To: Alan Cox <alan@lxorguk.ukuu.org.uk>
CC: "Martin J. Bligh" <Martin.Bligh@us.ibm.com>,
       William Lee Irwin III <wli@holomorphy.com>,
       Paolo Ciarrocchi <ciarrocchi@linuxmail.org>,
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Subject: Re: LMbench2.0 results
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Alan Cox wrote:
> 
> On Sun, 2002-09-08 at 19:40, Andrew Morton wrote:
> > We need to find some way of making vm_enough_memory not call get_page_state
> > so often.  One way of doing that might be to make get_page_state dump
> > its latest result into a global copy, and make vm_enough_memory()
> > only get_page_state once per N invokations.  A speed/accuracy tradeoff there.
> 
> Unless the error always falls on the same side the accuracy tradeoff is
> fatal to the entire scheme of things. Sorting out the use of
> get_page_state is worth doing if that is the bottleneck, and
> snapshooting such that we only look at it if we might be close to the
> limit would work, but we'd need to know when the limit had shifted too
> much

It could be that the cost is only present on the IBM whackomatics,
so they can twiddle the /proc setting and we can all be happy.
Certainly I did not see any problems on the quad.

Does "heuristic" overcommit handling need so much accuracy?
Perhaps we can push some of the cost over into mode 2 somehow.

Or we could turn it the other way up and, in __add_to_page_cache(),
do:

	if (overcommit_mode == anal)
		atomic_inc(&nr_pagecache_pages);
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