[PATCH] Document huge memory/cache overhead of memory controller in Kconfig

[Andi Kleen <andi@firstfloor.org>]

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.

Cc: balbir@linux.vnet.ibm.com

Signed-off-by: Andi Kleen <ak@suse.de>

Index: linux/init/Kconfig
===================================================================
--- linux.orig/init/Kconfig
+++ linux/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