linux-kernel - Re: [RFC 1/2] deactive invalidated pages

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Message-ID: <20101123221049.GR19571@csn.ul.ie>
Date:	Tue, 23 Nov 2010 22:10:49 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Ben Gamari <bgamari@...il.com>,
	Minchan Kim <minchan.kim@...il.com>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Nick Piggin <npiggin@...nel.dk>
Subject: Re: [RFC 1/2] deactive invalidated pages

On Tue, Nov 23, 2010 at 12:35:35PM -0800, Andrew Morton wrote:
> On Tue, 23 Nov 2010 14:58:56 +0000
> Mel Gorman <mel@....ul.ie> wrote:
> 
> > On Tue, Nov 23, 2010 at 09:55:49AM -0500, Ben Gamari wrote:
> > > On Tue, 23 Nov 2010 09:38:59 +0000, Mel Gorman <mel@....ul.ie> wrote:
> > > > > If it's mapped pagecache then the user was being a bit silly (or didn't
> > > > > know that some other process had mapped the file).  In which case we
> > > > > need to decide what to do - leave the page alone, deactivate it, or
> > > > > half-deactivate it as this patch does.
> > > > > 
> > > > 
> > > > What are the odds of an fadvise() user having used mincore() in advance
> > > > to determine if the page was in use by another process? I would guess
> > > > "low" so this half-deactivate gives a chance for the page to be promoted
> > > > again as well as a chance for the flusher threads to clean the page if
> > > > it really is to be reclaimed.
> > > > 
> > > Do we really want to make the user jump through such hoops as using
> > > mincore() just to get the kernel to handle use-once pages properly?
> > 
> > I would think "no" which is why I support half-deactivating pages so they won't
> > have to.
> 
> If the page is page_mapped() then we can assume that some other process
> is using it and we leave it alone *altogether*.
> 

Agreed, that makes perfect sense.

> If the page is dirty or under writeback (and !page_mapped()) then we
> should assume that we should free it asap.  The PageReclaim() trick
> might help with that.
> 

Again agreed.

> I just don't see any argument for moving the page to the head of the
> inactive LRU as a matter of policy.  We can park it there because we
> can't think of anythnig else to do with it, but it's the wrong place
> for it.
> 

Is there a better alternative? One thing that springs to mind is that we are
not exactly tracking very well what effect these policy changes have. The
analysis scripts I have do a reasonable job on tracking reclaim activity
(although only as part of the mmtests tarball, I should split them out as
a standalone tool) but not the impact - namely minor and major faults. I
should sort that out so we can put better reclaim analysis in place.

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