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

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Message-ID: <AANLkTikvsEpQM4=fGj5sH7rS74-KfPL5nq0v18v59MOb@mail.gmail.com>
Date:	Wed, 24 Nov 2010 08:45:20 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Mel Gorman <mel@....ul.ie>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ben Gamari <bgamari@...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 Wed, Nov 24, 2010 at 7:10 AM, Mel Gorman <mel@....ul.ie> wrote:
> 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.

It can help very much. :)

Also, I need time since I am so busy.

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



-- 
Kind regards,
Minchan Kim
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