linux-kernel - Re: [PATCH 0/8] Use memory compaction instead of lumpy reclaim during high-order allocations

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Date:	Thu, 18 Nov 2010 09:20:44 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/8] Use memory compaction instead of lumpy reclaim
	during high-order allocations

On Thu, Nov 18, 2010 at 09:38:28AM +0100, Johannes Weiner wrote:
> On Thu, Nov 18, 2010 at 05:26:27PM +0900, KAMEZAWA Hiroyuki wrote:
> > On Thu, 18 Nov 2010 08:12:54 +0000
> > Mel Gorman <mel@....ul.ie> wrote:
> > 
> > > > > I'm hoping that this series also removes the
> > > > > necessity for the "delete lumpy reclaim" patch from the THP tree.
> > > > 
> > > > Now I'm sad.  I read all that and was thinking "oh goody, we get to
> > > > delete something for once".  But no :(
> > > > 
> > > > If you can get this stuff to work nicely, why can't we remove lumpy
> > > > reclaim?
> > > 
> > > Ultimately we should be able to. Lumpy reclaim is still there for the
> > > !CONFIG_COMPACTION case and to have an option if we find that compaction
> > > behaves badly for some reason.
> > > 
> > 
> > Hmm. CONFIG_COMPACTION depends on CONFIG_MMU. lumpy reclaim will be for NOMMU,
> > finally ?
> 
> It's because migration depends on MMU.  But we should be able to make
> a NOMMU version of migration that just does page cache, which is all
> that is reclaimable on NOMMU anyway.
> 

Conceivably, but I see little problem leaving them with lumpy reclaim. As
page cache and anon pages are mixed together in MIGRATE_MOVABLE but only one
set of pages can be discarded, the success rates of either lumpy reclaim or
compaction is doubtful. It'd require a specific investigation.

> At this point, the MMU dependency can go away, and so can lumpy
> reclaim.
> 

The series never calls lumpy reclaim once CONFIG_COMPACTION is set. The code
could be shrunk with the below patch but the saving to vmlinux is minimal
(288 bytes for me on x86-64). My preference is still to have lumpy reclaim
available as a comparison point with compaction for a development cycle or two.

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 52a0f0c..7488983 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1048,7 +1048,7 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
 			BUG();
 		}
 
-		if (!order)
+		if (!order || COMPACTION_BUILD)
 			continue;
 
 		/*

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