From: Eric Wong <normalperson@yhbt.net>
Date: Wed, 20 Mar 2013 04:07:46 +0000 (+1100)
Subject: epoll: trim epitem by one cache line
X-Git-Tag: next-20130322~1^2~252
X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=464e547739450151f5699ce5214416631e0bf691;p=karo-tx-linux.git

epoll: trim epitem by one cache line

It is common for epoll users to have thousands of epitems, so saving a
cache line on every allocation leads to large memory savings.

Since epitem allocations are cache-aligned, reducing sizeof(struct epitem)
from 136 bytes to 128 bytes will allow it to squeeze under a cache line
boundary on x86_64.

Via /sys/kernel/slab/eventpoll_epi, I see the following changes on my
x86_64 Core2 Duo (which has 64-byte cache alignment):

	object_size  :  192 => 128
	objs_per_slab:   21 =>  32

Also, add a BUILD_BUG_ON() to check for future accidental breakage.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Cc: Davide Libenzi <davidel@xmailserver.org>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 495d15558f42..0edf94dd578e 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -105,7 +105,7 @@
 struct epoll_filefd {
 	struct file *file;
 	int fd;
-};
+} EPOLL_PACKED;
 
 /*
  * Structure used to track possible nested calls, for too deep recursions