linux-kernel - Re: [PATCH 1/2] mm: page allocator: Adjust the per-cpu counter threshold when memory is low

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Message-ID: <20101109113337.GI32723@csn.ul.ie>
Date:	Tue, 9 Nov 2010 11:33:37 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Shaohua Li <shaohua.li@...el.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Christoph Lameter <cl@...ux.com>,
	David Rientjes <rientjes@...gle.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH 1/2] mm: page allocator: Adjust the per-cpu counter
	threshold when memory is low

On Fri, Oct 29, 2010 at 12:40:02PM -0700, Andrew Morton wrote:
> > ...
> >
> > Follow-on patch?
> 
> Sometime, please.
> 

How does this look?

==== CUT HERE ====
mm: vmscan: Comment on why kswapd reduces the per-cpu vmstat threshold

While kswapd is awake, the per-cpu vmstat threshold is reduced to
reduce per-cpu drift to acceptable levels. Add a comment explaining
why.

Signed-off-by: Mel Gorman <mel@....ul.ie>
---
 mm/vmscan.c |   13 +++++++++++++
 1 files changed, 13 insertions(+), 0 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 7966110..ba39948 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2378,6 +2378,19 @@ static int kswapd(void *p)
 				 */
 				if (!sleeping_prematurely(pgdat, order, remaining)) {
 					trace_mm_vmscan_kswapd_sleep(pgdat->node_id);
+
+					/*
+					 * vmstat counters are not perfectly
+					 * accurate and the estimated value
+					 * for counters such as NR_FREE_PAGES
+					 * can deviate from the true value by
+					 * nr_online_cpus * threshold. To
+					 * avoid the zone watermarks being
+					 * breached while under pressure, we
+					 * reduce the per-cpu vmstat threshold
+					 * while kswapd is awake and restore
+					 * them before going back to sleep.
+					 */
 					set_pgdat_percpu_threshold(pgdat,
 						calculate_normal_threshold);
 					schedule();
--
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