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>

diff --git a/init/Kconfig b/init/Kconfig
index dcef8b5..f698a5a 100644 (file)
--- 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