linux-kernel - Re: [RFC] Tracking mlocked pages and moving them off the LRU

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Message-ID: <Pine.LNX.4.64.0702031744060.31986@schroedinger.engr.sgi.com>
Date:	Sat, 3 Feb 2007 17:49:13 -0800 (PST)
From:	Christoph Lameter <clameter@....com>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Arjan van de Ven <arjan@...radead.org>,
	linux-kernel@...r.kernel.org,
	Nick Piggin <nickpiggin@...oo.com.au>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [RFC] Tracking mlocked pages and moving them off the LRU

On Sat, 3 Feb 2007, Andrew Morton wrote:

> Do we actually need NR_MLOCK?  Page reclaim tends to care more about the
> size of the LRUs and doesn't have much dependency on ->present_pages,

Yes, we'd be fine with general reclaim I think. But the calculation of the 
dirty ratio based on ZVCs would need it if we take the mlocked pages off. 
Otherwise we may have dirty ratios > 100%.

> I guess we could use NR_MLOCK for writeback threshold calculations, to
> force writeback earlier if there's a lot of mlocked memory in the affected
> zones.  But that code isn't zone-aware anyway, and we don't know how to make
> it zone aware in any sane fashion and making it cpuset-aware isn't very
> interesting or useful..

Exclusion or inclusion of NR_MLOCK number is straightforward for the dirty 
ratio calcuations. global_page_state(NR_MLOCK) f.e. would get us totals on 
mlocked pages per zone. node_page_state(NR_MLOCK) gives a node specific 
number of mlocked pages. The nice thing about ZVCs is that it allows
easy access to counts on different levels.

> So..  Why do we want NR_MLOCK?

Rik also had some uses in mind for allocation?
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