linux-kernel - [PATCH 2/2] mm: vmscan: Do not scale writeback pages when deciding whether to set ZONE

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Message-Id: <1372250364-20640-3-git-send-email-mgorman@suse.de>
Date:	Wed, 26 Jun 2013 13:39:24 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Jiri Slaby <jslaby@...e.cz>,
	Valdis Kletnieks <Valdis.Kletnieks@...edu>,
	Rik van Riel <riel@...hat.com>,
	Zlatko Calusic <zcalusic@...sync.net>,
	Johannes Weiner <hannes@...xchg.org>,
	dormando <dormando@...ia.net>, Michal Hocko <mhocko@...e.cz>,
	Jan Kara <jack@...e.cz>, Dave Chinner <david@...morbit.com>,
	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Linux-FSDevel <linux-fsdevel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>, Mel Gorman <mgorman@...e.de>
Subject: [PATCH 2/2] mm: vmscan: Do not scale writeback pages when deciding whether to set ZONE_WRITEBACK

After the patch "mm: vmscan: Flatten kswapd priority loop" was merged the
scanning priority of kswapd changed. The priority now raises until it is
scanning enough pages to meet the high watermark.  shrink_inactive_list sets
ZONE_WRITEBACK if a number of pages were encountered under writeback but
this value is scaled based on the priority. As kswapd frequently scans with
a higher priority now it is relatively easy to set ZONE_WRITEBACK. This
patch removes the scaling and treates writeback pages similar to how it
treats unqueued dirty pages and congested pages.  The user-visible effect
should be that kswapd will writeback fewer pages from reclaim context.

Signed-off-by: Mel Gorman <mgorman@...e.de>
---
 mm/vmscan.c | 16 +---------------
 1 file changed, 1 insertion(+), 15 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 65f2fbea..f677780 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1477,25 +1477,11 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
 	 * as there is no guarantee the dirtying process is throttled in the
 	 * same way balance_dirty_pages() manages.
 	 *
-	 * This scales the number of dirty pages that must be under writeback
-	 * before a zone gets flagged ZONE_WRITEBACK. It is a simple backoff
-	 * function that has the most effect in the range DEF_PRIORITY to
-	 * DEF_PRIORITY-2 which is the priority reclaim is considered to be
-	 * in trouble and reclaim is considered to be in trouble.
-	 *
-	 * DEF_PRIORITY   100% isolated pages must be PageWriteback to throttle
-	 * DEF_PRIORITY-1  50% must be PageWriteback
-	 * DEF_PRIORITY-2  25% must be PageWriteback, kswapd in trouble
-	 * ...
-	 * DEF_PRIORITY-6 For SWAP_CLUSTER_MAX isolated pages, throttle if any
-	 *                     isolated page is PageWriteback
-	 *
 	 * Once a zone is flagged ZONE_WRITEBACK, kswapd will count the number
 	 * of pages under pages flagged for immediate reclaim and stall if any
 	 * are encountered in the nr_immediate check below.
 	 */
-	if (nr_writeback && nr_writeback >=
-			(nr_taken >> (DEF_PRIORITY - sc->priority)))
+	if (nr_writeback && nr_writeback == nr_taken)
 		zone_set_flag(zone, ZONE_WRITEBACK);
 
 	/*
-- 
1.8.1.4

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