linux-kernel - Re: [PATCH 4/4] capture pages freed during direct reclaim for allocation by the reclaimer

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Message-Id: <20081002162414.03470f46.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Thu, 2 Oct 2008 16:24:14 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Andy Whitcroft <apw@...dowen.org>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mel@....ul.ie>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 4/4] capture pages freed during direct reclaim for
 allocation by the reclaimer

On Wed,  1 Oct 2008 13:31:01 +0100
Andy Whitcroft <apw@...dowen.org> wrote:

> When a process enters direct reclaim it will expend effort identifying
> and releasing pages in the hope of obtaining a page.  However as these
> pages are released asynchronously there is every possibility that the
> pages will have been consumed by other allocators before the reclaimer
> gets a look in.  This is particularly problematic where the reclaimer is
> attempting to allocate a higher order page.  It is highly likely that
> a parallel allocation will consume lower order constituent pages as we
> release them preventing them coelescing into the higher order page the
> reclaimer desires.
> 
> This patch set attempts to address this for allocations above
> ALLOC_COSTLY_ORDER by temporarily collecting the pages we are releasing
> onto a local free list.  Instead of freeing them to the main buddy lists,
> pages are collected and coelesced on this per direct reclaimer free list.
> Pages which are freed by other processes are also considered, where they
> coelesce with a page already under capture they will be moved to the
> capture list.  When pressure has been applied to a zone we then consult
> the capture list and if there is an appropriatly sized page available
> it is taken immediatly and the remainder returned to the free pool.
> Capture is only enabled when the reclaimer's allocation order exceeds
> ALLOC_COSTLY_ORDER as free pages below this order should naturally occur
> in large numbers following regular reclaim.
> 
> Thanks go to Mel Gorman for numerous discussions during the development
> of this patch and for his repeated reviews.
> 

Hmm.. is this routine better than
  mm/memory_hotplug.c::do_migrate_range(start_pfn, end_pfn) ?

Thanks,
-Kame

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