linux-kernel - Re: [PATCH 7/7] mm: compaction: Introduce sync-light migration for use by compaction

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Date:	Tue, 22 Nov 2011 14:56:51 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Linux-MM <linux-mm@...ck.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Minchan Kim <minchan.kim@...il.com>, Jan Kara <jack@...e.cz>,
	Andy Isaacson <adi@...apodia.org>,
	Johannes Weiner <jweiner@...hat.com>,
	Rik van Riel <riel@...hat.com>, Nai Xia <nai.xia@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 7/7] mm: compaction: Introduce sync-light migration for
 use by compaction

On Tue, 2011-11-22 at 02:36 +0800, Mel Gorman wrote:
> This patch adds a lightweight sync migrate operation MIGRATE_SYNC_LIGHT
> mode that avoids writing back pages to backing storage. Async
> compaction maps to MIGRATE_ASYNC while sync compaction maps to
> MIGRATE_SYNC_LIGHT. For other migrate_pages users such as memory
> hotplug, MIGRATE_SYNC is used.
> 
> This avoids sync compaction stalling for an excessive length of time,
> particularly when copying files to a USB stick where there might be
> a large number of dirty pages backed by a filesystem that does not
> support ->writepages.
Hi,
from my understanding, with this, even writes
to /proc/sys/vm/compact_memory doesn't wait for pageout, is this
intended?
on the other hand, MIGRATE_SYNC_LIGHT now waits for pagelock and buffer
lock, so could wait on page read. page read and page out have the same
latency, why takes them different?

Thanks,
Shaohua

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