[PATCH] epoll: trim epitem by one cache line on x86_64

[Eric Wong <normalperson@yhbt.net>]

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.

>From /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

I have no access to other 64-bit machines, so I am limiting this to
x86_64-only with EPOLL_PACKED instead of __attribute__((packed))

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Cc: Davide Libenzi <davidel@xmailserver.org>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
 fs/eventpoll.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index cfc4b16..06f3d0e 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -107,7 +107,7 @@
 struct epoll_filefd {
 	struct file *file;
 	int fd;
-};
+} EPOLL_PACKED;
 
 /*
  * Structure used to track possible nested calls, for too deep recursions
-- 
Eric Wong