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

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Message-ID: <20101123093859.GE19571@csn.ul.ie>
Date:	Tue, 23 Nov 2010 09:38:59 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	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 Mon, Nov 22, 2010 at 09:01:32PM -0800, Andrew Morton wrote:
> On Tue, 23 Nov 2010 13:52:05 +0900 Minchan Kim <minchan.kim@...il.com> wrote:
> 
> > >> +/*
> > >> + * Function used to forecefully demote a page to the head of the inactive
> > >> + * list.
> > >> + */
> > >
> > > This comment is wrong? __The page gets moved to the _tail_ of the
> > > inactive list?
> > 
> > No. I add it in _head_ of the inactive list intentionally.
> > Why I don't add it to _tail_ is that I don't want to be aggressive.
> > The page might be real working set. So I want to give a chance to
> > activate it again.
> 
> Well..  why?  The user just tried to toss the page away altogether.  If
> the kernel wasn't able to do that immediately, the best it can do is to
> toss the page away asap?
> 

I'm just guessing here on the motivation but maybe it is in case FADV_DONENEED
was called on a page in use by another process (via read/write more do than
being mapped). Process A says "I don't need this" but by moving it to the
head of the list we give Process B a chance to reference it and reactivate
without incurring a major fault?

> > If it's not working set, it can be reclaimed easily and it can prevent
> > active page demotion since inactive list size would be big enough for
> > not calling shrink_active_list.
> 
> What is "working set"?  Mapped and unmapped pagecache, or are you
> referring solely to mapped pagecache?
> 
> 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.

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