linux-kernel - Re: [PATCH 1/1] pagemap clear_refs: modify to specify anon or mapped vma clearing

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Message-ID: <20090729145839.GB15102@csn.ul.ie>
Date:	Wed, 29 Jul 2009 15:58:39 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Moussa Ba <moussa.a.ba@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, rientjes@...gle.com,
	xiyou.wangcong@...il.com, adobriyan@...il.com, mpm@...enic.com,
	yinghan@...gle.com, npiggin@...e.de, jaredeh@...il.com
Subject: Re: [PATCH 1/1] pagemap clear_refs: modify to specify anon or
	mapped vma clearing

On Tue, Jul 28, 2009 at 05:00:54PM -0700, Moussa Ba wrote:
> The patch makes the clear_refs more versatile in adding the option to
> select anonymous pages or file backed pages for clearing.  This
> addition has a measurable impact on user space application performance
> as it decreases the number of pagewalks in scenarios where one is only
> interested in a specific type of page (anonymous or file mapped).
> 

I think what Andrew might be looking for is a repeat of the use-case for
using clear_refs at all and what the additional usecase is that makes this
patch beneficial. To be honest, I had to go digging for a bit to find out
why clear_refs is used at all and the potential benefits of this patch were
initially unclear to me although they were obvious to others. I confess I
haven't read the patch closely to determine if it's behaving as advertised
or not.

Bonus points for a wee illustration of a script that measures the apparent
working set of a process using clear_refs, showing the working set for anon
vs filebacked and the difference of measuring just anon versus the full
process for example. Such a script could be added to Documentation/vm as
I didn't spot any example in there already.

Total aside, is there potential to really mess with LRU aging by abusing
clear_refs a lot, partcularly as clear_refs is writable by the process
owner? Does it matter?

> 
> On Tue, Jul 28, 2009 at 4:52 PM, Andrew Morton<akpm@...ux-foundation.org> wrote:
> > On Tue, 28 Jul 2009 15:52:05 -0700
> > "Moussa A. Ba" <moussa.a.ba@...il.com> wrote:
> >
> >> This patch adds anonymous and file backed filters to the clear_refs interface.
> >> echo 1 > /proc/PID/clear_refs resets the bits on all pages
> >> echo 2 > /proc/PID/clear_refs resets the bits on anonymous pages only
> >> echo 3 > /proc/PID/clear_refs resets the bits on file backed pages only
> >> Any other value is ignored
> >
> > The changelog is missing any rationale for making this change.
> > Originally we were told that it "makes the clear_refs proc interface a
> > bit more versatile", but that's a bit thin.
> >
> > How do we justify making this change to Linux?  What value does it
> > have?  Can you describe a usage scenario which would help people
> > understand the usefulness of the change?
> >
> > Thanks.
> >
> 

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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