From 0835ab53eab5bbeebe1c135e92fb0c4d483dde17 Mon Sep 17 00:00:00 2001
From: Andi Kleen <andi@firstfloor.org>
Date: Sat, 23 Feb 2008 15:24:02 -0800
Subject: [PATCH] cgroup memory controller: document huge memory/cache overhead
 in Kconfig

Document huge memory/cache overhead of memory controller in Kconfig

I was a little surprised that 2.6.25-rc* increased struct page for the
memory controller.  At least on many x86-64 machines it will not fit into a
single cache line now anymore and also costs considerable amounts of RAM.
At earlier review I remembered asking for a external data structure for
this.

It's also quite unobvious that a innocent looking Kconfig option with a
single line Kconfig description has such a negative effect.

This patch attempts to document these disadvantages at least so that users
configuring their kernel can make a informed decision.

Signed-off-by: Andi Kleen <ak@suse.de>
Cc: Balbir Singh <balbir@in.ibm.com>
Acked-by: Paul Menage <menage@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---
 init/Kconfig | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/init/Kconfig b/init/Kconfig
index dcef8b55011a0..f698a5af50079 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -394,6 +394,14 @@ config CGROUP_MEM_CONT
 	  Provides a memory controller that manages both page cache and
 	  RSS memory.
 
+	  Note that setting this option increases fixed memory overhead
+	  associated with each page of memory in the system by 4/8 bytes
+	  and also increases cache misses because struct page on many 64bit
+	  systems will not fit into a single cache line anymore.
+
+	  Only enable when you're ok with these trade offs and really
+	  sure you need the memory controller.
+
 config PROC_PID_CPUSET
 	bool "Include legacy /proc/<pid>/cpuset file"
 	depends on CPUSETS
-- 
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