linux-kernel - Re: [PATCH 3/3] compaction accouting fix

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Message-ID: <20110901142027.GI14369@suse.de>
Date:	Thu, 1 Sep 2011 15:20:27 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Johannes Weiner <jweiner@...hat.com>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 3/3] compaction accouting fix

On Sun, Nov 13, 2011 at 01:37:43AM +0900, Minchan Kim wrote:
> I saw the following accouting of compaction during test of the series.

s/accouting/accounting/ both here and in the subject. A nicer name the
patch would have been

"mm: compaction: Only update compact_blocks_moved if compaction was successful"

> 
> compact_blocks_moved 251
> compact_pages_moved 44
> 
> It's very awkward to me although it's possbile because it means we try to compact 251 blocks
> but it just migrated 44 pages. As further investigation, I found isolate_migratepages doesn't
> isolate any pages but it returns ISOLATE_SUCCESS and then, it just increases compact_blocks_moved
> but doesn't increased compact_pages_moved.
> 
> This patch makes accouting of compaction works only in case of success of isolation.
> 

compact_blocks_moved exists to indicate the rate compaction is
scanning pageblocks. If compact_blocks_moved and compact_pages_moved
are increasing at a similar rate for example, it could imply that
compaction is doing a lot of scanning but is not necessarily useful
work. It's not necessarily reflected by compact_fail because that
counter is only updated for pages that were isolated from the LRU.

I now recognise of course that "compact_blocks_moved" was an *awful*
choice of name for this stat.

-- 
Mel Gorman
SUSE Labs
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