linux-kernel - Re: [PATCH 06/11] mm, compaction: distinguish between full and partial COMPACT

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Date:	Mon, 11 Apr 2016 15:46:44 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Mel Gorman <mgorman@...e.de>,
	David Rientjes <rientjes@...gle.com>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	Joonsoo Kim <js1304@...il.com>,
	Hillf Danton <hillf.zj@...baba-inc.com>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 06/11] mm, compaction: distinguish between full and
 partial COMPACT_COMPLETE

On Mon 11-04-16 15:42:21, Vlastimil Babka wrote:
> On 04/11/2016 03:27 PM, Michal Hocko wrote:
> >On Mon 11-04-16 14:53:36, Vlastimil Babka wrote:
> >>On 04/11/2016 02:46 PM, Michal Hocko wrote:
> >>
> >>The racy part is negligible but I didn't realize the sync/async migrate
> >>scanner part until now. So yeah, free_pfn could have got to middle of zone
> >>when it was in the async mode. But that also means that the async mode
> >>recently used up all free pages in the second half of the zone. WRT free
> >>pages isolation, async mode is not trying less than sync, so it shouldn't be
> >>a considerable missed opportunity if we don't rescan the it, though.
> >
> >I am not really sure I understand. The primary intention of this patch
> >is to distinguish where we have scanned basically whole zones from cases
> >where a new scan started off previous mark and so it was just unlucky to
> >see only tiny bit of the zone where we would clearly give up too early.
> >FWIU this shouldn't be the case if we start scanning from the beginning
> >of the zone even if we raced on the other end of the zone because the
> >missed part would be negligible. Is that understanding correct?
> 
> Yes, it should be less unlucky than seeing a tiny bit of the zone. Just
> wanted to point out that you might still not see the whole zone in one
> compaction attempt. E.g. async compaction is first, advances the free
> scanner and caches its position when it bails out due to being contended.
> Then direct reclaim frees some pages behind the cached position. Sync
> compaction attempts starts migration scanner from start_pfn, but picks up
> the cached free scanner pfn. The result is missing some free pages and the
> scanners meeting somewhat earlier than they otherwise would. Probably not
> critical even for OOM decisions, as that's also racy anyway.

OK, I see now. I agree this shouldn't be critical and thanks for the
clarification.
-- 
Michal Hocko
SUSE Labs