linux-kernel - [PATCH] doc: describe memcg swappiness more precisely memory.swappiness==0

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Message-ID: <20121015220725.GB11682@dhcp22.suse.cz>
Date:	Tue, 16 Oct 2012 00:07:25 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-mm@...ck.org, David Rientjes <rientjes@...gle.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: [PATCH] doc: describe memcg swappiness more precisely
 memory.swappiness==0

And a follow up for memcg.swappiness documentation which is more
specific about spwappiness==0 meaning.
---
>From 1bc3a94fea728107ed108edd42df464b908cd067 Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@...e.cz>
Date: Mon, 15 Oct 2012 11:43:56 +0200
Subject: [PATCH] doc: describe memcg swappiness more precisely

since fe35004f (mm: avoid swapping out with swappiness==0) memcg reclaim
stopped swapping out anon pages completely when 0 value is used.
Although this is somehow expected it hasn't been done for a really long
time this way and so it is probably better to be explicit about the
effect. Moreover global reclaim swapps out even when swappiness is 0
to prevent from OOM killer.

Signed-off-by: Michal Hocko <mhocko@...e.cz>
---
 Documentation/cgroups/memory.txt |    4 ++++
 1 file changed, 4 insertions(+)

diff --git a/Documentation/cgroups/memory.txt b/Documentation/cgroups/memory.txt
index c07f7b4..71c4da4 100644
--- a/Documentation/cgroups/memory.txt
+++ b/Documentation/cgroups/memory.txt
@@ -466,6 +466,10 @@ Note:
 5.3 swappiness
 
 Similar to /proc/sys/vm/swappiness, but affecting a hierarchy of groups only.
+Please note that unlike the global swappiness, memcg knob set to 0
+really prevents from any swapping even if there is a swap storage
+available. This might lead to memcg OOM killer if there are no file
+pages to reclaim.
 
 Following cgroups' swappiness can't be changed.
 - root cgroup (uses /proc/sys/vm/swappiness).
-- 
1.7.10.4

-- 
Michal Hocko
SUSE Labs
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