linux-kernel - [PATCH 7/7] sched,numa: change scan period code to match intent

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Date:	Mon, 23 Jun 2014 11:41:35 -0400
From:	riel@...hat.com
To:	linux-kernel@...r.kernel.org
Cc:	chegu_vinod@...com, peterz@...radead.com, mgorman@...e.de,
	mingo@...nel.org
Subject: [PATCH 7/7] sched,numa: change scan period code to match intent

From: Rik van Riel <riel@...hat.com>

Reading through the scan period code and comment, it appears the
intent was to slow down NUMA scanning when a majority of accesses
are on the local node, specifically a local:remote ratio of 3:1.

However, the code actually tests local / (local + remote), and
the actual cut-off point was around 30% local accesses, well before
a task has actually converged on a node.

Changing the threshold to 7 means scanning slows down when a task
has around 70% of its accesses local, which appears to match the
intent of the code more closely.

Cc: Mel Gorman <mgorman@...e.de>
Signed-off-by: Rik van Riel <riel@...hat.com>
---
 kernel/sched/fair.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index ee35576..1aaa3b4 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1452,12 +1452,12 @@ static void update_numa_active_node_mask(struct numa_group *numa_group)
 /*
  * When adapting the scan rate, the period is divided into NUMA_PERIOD_SLOTS
  * increments. The more local the fault statistics are, the higher the scan
- * period will be for the next scan window. If local/remote ratio is below
- * NUMA_PERIOD_THRESHOLD (where range of ratio is 1..NUMA_PERIOD_SLOTS) the
- * scan period will decrease
+ * period will be for the next scan window. If local/(local+remote) ratio is
+ * below NUMA_PERIOD_THRESHOLD (where range of ratio is 1..NUMA_PERIOD_SLOTS)
+ * the scan period will decrease. Aim for 70% local accesses.
  */
 #define NUMA_PERIOD_SLOTS 10
-#define NUMA_PERIOD_THRESHOLD 3
+#define NUMA_PERIOD_THRESHOLD 7
 
 /*
  * Increase the scan period (slow down scanning) if the majority of
-- 
1.8.5.3

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