linux-kernel - Re: [PATCH] mm: vmscan: fix setting reclaim mode

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Message-ID: <CAJd=RBCGN6LGqTH7ubQ5FuW-wYyLB5ao+58ffhipNni1Tt_JUg@mail.gmail.com>
Date:	Wed, 11 Jan 2012 00:58:50 +0800
From:	Hillf Danton <dhillf@...il.com>
To:	Mel Gorman <mgorman@...e.de>
Cc:	linux-mm@...ck.org,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: vmscan: fix setting reclaim mode

On Wed, Jan 11, 2012 at 12:44 AM, Mel Gorman <mgorman@...e.de> wrote:
> On Tue, Jan 10, 2012 at 11:58:03PM +0800, Hillf Danton wrote:
>> From: Hillf Danton <dhillf@...il.com>
>> [PATCH] mm: vmscan: fix setting reclaim mode
>>
>> The comment says, initially assume we are entering either lumpy reclaim or
>> reclaim/compaction, and depending on the reclaim order, we will either set the
>> sync mode or just reclaim order-0 pages later.
>>
>> On other hand, order-0 reclaim, instead of sync reclaim, is expected when
>> under memory pressure, but the check for memory pressure is incorrect,
>> leading to sync reclaim at low reclaim priorities.
>>
>> And the result is sync reclaim is set for high priorities.
>>
>
> RECLAIM_MODE_SYNC is only set for RECLAIM_MODE_LUMPYRECLAIM. Even when
> using RECLAIM_MODE_LUMPYRECLAIM, it should only be set when reclaim
> is under memory pressure and failing to reclaim the necessry pages
> (priority < DEF_PRIORITY - 2). Once in symc reclaim, reclaim will call
> wait_on_page_writeback() on dirty pages which potentially leads to
> significant stalls (one of the reasons why RECLAIM_MODE_LUMPYRECLAIM
> sucks and why compaction is preferred). Your patch means sync reclaim
> is used even when priority == DEF_PRIORITY. This is unexpected.
>
> Your changelog really needs to explain what the problem is that you
> have encountered and why this patch fixes it. It's not like some of
> your other patches which were minor performance optimisations that
> were self-evident.
>
Hi Mel

To avoid another step in wrong direction, I'd goto bed now,
it is about 1:09 AM.
When sane again, I will try to understand your comment.

Best
Hillf
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