linux-kernel - Re: [PATCH 4/4] maps: /proc/<pid>/pmaps interface

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Message-ID: <20070818172225.GR30556@waste.org>
Date:	Sat, 18 Aug 2007 12:22:26 -0500
From:	Matt Mackall <mpm@...enic.com>
To:	Fengguang Wu <wfg@...l.ustc.edu.cn>
Cc:	Andrew Morton <akpm@...l.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	David Rientjes <rientjes@...gle.com>,
	John Berthels <jjberthels@...il.com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] maps: /proc/<pid>/pmaps interface - memory maps in granularity of pages

On Sat, Aug 18, 2007 at 04:45:31PM +0800, Fengguang Wu wrote:
> Matt,
> 
> On Sat, Aug 18, 2007 at 01:40:42AM -0500, Matt Mackall wrote:
> > > - On memory pressure,
> > >   - as VSZ goes up, RSS will be bounded by physical memory.
> > >     So VSZ:RSS ratio actually goes up with memory pressure.
> > 
> > And yes.
> > 
> > But that's not what I'm talking about. You're likely to have more
> > holes in your ranges with memory pressure as things that aren't active
> > get paged or swapped out and back in. And because we're walking the
> > LRU more rapidly, we'll flip over a lot of the active bits more often
> > which will mean more output.
> > 
> > >   - page range is a good unit of locality. They are more likely to be
> > >     reclaimed as a whole. So (RSS:page_ranges) wouldn't degrade as much.
> > 
> > There is that. The relative magnitude of the different effects is
> > unclear. But it is clear that the worst case for pmap is much worse
> 
> > than pagemap (two lines per page of RSS?). 
> It's one line per page. No sane app will make vmas proliferate.

Sane apps are few and far between.
 
> So let's talk about the worst case.
> 
> pagemap's data set size is determined by VSZ.
> 4GB VSZ means 1M PFNs, hence 8MB pagemap data.
> 
> pmaps's data set size is bounded by RSS hence physical memory.
> 4GB RSS means up to 1M page ranges, hence ~20M pmaps data.
> Not too bad :)

Hmmm, I've been misreading the output.

What does it do with nonlinear VMAs?

-- 
Mathematics is the supreme nostalgia of our time.
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