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Commit 918fc718 authored by Mel Gorman's avatar Mel Gorman Committed by Linus Torvalds
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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 rises 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: default avatarMel Gorman <mgorman@suse.de>
Cc: Rik van Riel <riel@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 5a1c9cbc

mm/vmscan.c

+1 −15
Original line number Diff line number Diff line
@@ -1443,25 +1443,11 @@ 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);



	/*