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

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Date:	Fri, 2 Sep 2011 14:36:48 +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 Fri, Sep 02, 2011 at 02:09:55PM +0900, Minchan Kim wrote:
> On Thu, Sep 1, 2011 at 11:20 PM, Mel Gorman <mgorman@...e.de> wrote:
> > 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"
> 
> Thanks, I will fix it at next version. :)
> 
> >
> >>
> >> 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.
> 
> You seem to say "compact_pagemigrate_failed" not "compact_fail".
> 

I did. Thanks.

> >
> > I now recognise of course that "compact_blocks_moved" was an *awful*
> > choice of name for this stat.
> 
> I hope changing stat names as follows unless it's too late(ie, it
> doesn't break ABI with any tools)
> 

I'm not aware of any tools that depend on this except my own reporting
scripts and even those do not particularly care.

> compact_blocks_moved -> compact_blocks

compact_pageblock_scanned?

> compact_pages_moved -> compact_pgmigrated_success
> compact_pagemigrate_failed -> compact_pgmigrated_fail
> compact_stall -> compact_alloc_stall
> compact_fail -> compact_alloc_fail
> compact_success -> compact_alloc_success
> 

Seems reasonable to me.

-- 
Mel Gorman
SUSE Labs
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