linux-kernel - Re: [PATCH 5/5] proc: export more page flags in /proc/kpageflags

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Message-Id: <1240955395.938.1031.camel@calx>
Date:	Tue, 28 Apr 2009 16:49:55 -0500
From:	Matt Mackall <mpm@...enic.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Tony Luck <tony.luck@...il.com>, fengguang.wu@...el.com,
	mingo@...e.hu, rostedt@...dmis.org, fweisbec@...il.com,
	lwoodman@...hat.com, a.p.zijlstra@...llo.nl,
	penberg@...helsinki.fi, eduard.munteanu@...ux360.ro,
	linux-kernel@...r.kernel.org, kosaki.motohiro@...fujitsu.com,
	andi@...stfloor.org, adobriyan@...il.com, linux-mm@...ck.org
Subject: Re: [PATCH 5/5] proc: export more page flags in /proc/kpageflags

On Tue, 2009-04-28 at 14:17 -0700, Andrew Morton wrote:
> On Tue, 28 Apr 2009 11:11:52 -0700
> Tony Luck <tony.luck@...il.com> wrote:
> 
> > On Tue, Apr 28, 2009 at 1:33 AM, Wu Fengguang <fengguang.wu@...el.com> wrote:
> > > 1) FAST
> > >
> > > It takes merely 0.2s to scan 4GB pages:
> > >
> > > __ __ __ __./page-types __0.02s user 0.20s system 99% cpu 0.216 total
> > 
> > OK on a tiny system ... but sounds painful on a big
> > server. 0.2s for 4G scales up to 3 minutes 25 seconds
> > on a 4TB system (4TB systems were being sold two
> > years ago ... so by now the high end will have moved
> > up to 8TB or perhaps 16TB).
> > 
> > Would the resulting output be anything but noise on
> > a big system (a *lot* of pages can change state in
> > 3 minutes)?
> > 
> 
> Reading the state of all of memory in this fashion would be a somewhat
> peculiar thing to do.

Not entirely. If you've got, say, a large NUMA box, it could be
incredibly illustrative to see that "oh, this node is entirely dominated
by SLAB allocations". Or on a smaller machine "oh, this is fragmented to
hell and there's no way I'm going to get a huge page". Things you're not
going to get from individual stats.

> Generally, I think that pagemap is another of those things where we've
> failed on the follow-through.  There's a nice and powerful interface
> for inspecting the state of a process's VM, but nobody knows about it
> and there are no tools for accessing it and nobody is using it.

People keep finding bugs in the thing exercising it in new ways, so I
presume people are writing their own tools. My hope was that my original
tools would inspire someone to take it and run with it - I really have
no stomach for writing GUI tools.

However, I've recent gone and written a pretty generically useful
command-line tool that hopefully will get more traction:

http://www.selenic.com/smem/

I'm expecting it to get written up on LWN shortly, so I haven't spent
much time doing my own advertising.

-- 
http://selenic.com : development and support for Mercurial and Linux


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