The patch titled
Subject: epoll: trim epitem by one cache line
has been removed from the -mm tree. Its filename was
epoll-trim-epitem-by-one-cache-line-on-x86_64.patch
This patch was dropped because it was merged into mainline or a subsystem tree
------------------------------------------------------
From: Eric Wong <normalperson@xxxxxxxx>
Subject: 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.
[akpm@xxxxxxxxxxxxxxxxxxxx: use __packed, for all architectures]
Signed-off-by: Eric Wong <normalperson@xxxxxxxx>
Cc: Davide Libenzi <davidel@xxxxxxxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
fs/eventpoll.c | 10 +++++++++-
1 file changed, 9 insertions(+), 1 deletion(-)
diff -puN fs/eventpoll.c~epoll-trim-epitem-by-one-cache-line-on-x86_64 fs/eventpoll.c
--- a/fs/eventpoll.c~epoll-trim-epitem-by-one-cache-line-on-x86_64
+++ a/fs/eventpoll.c
@@ -104,7 +104,7 @@
struct epoll_filefd {
struct file *file;
int fd;
-};
+} __packed;
/*
* Structure used to track possible nested calls, for too deep recursions
@@ -128,6 +128,8 @@ struct nested_calls {
/*
* Each file descriptor added to the eventpoll interface will
* have an entry of this type linked to the "rbr" RB tree.
+ * Avoid increasing the size of this struct, there can be many thousands
+ * of these on a server and we do not want this to take another cache line.
*/
struct epitem {
/* RB tree node used to link this structure to the eventpoll RB tree */
@@ -1964,6 +1966,12 @@ static int __init eventpoll_init(void)
/* Initialize the structure used to perform file's f_op->poll() calls */
ep_nested_calls_init(&poll_readywalk_ncalls);
+ /*
+ * We can have many thousands of epitems, so prevent this from
+ * using an extra cache line on 64-bit (and smaller) CPUs
+ */
+ BUILD_BUG_ON(sizeof(void *) <= 8 && sizeof(struct epitem) > 128);
+
/* Allocates slab cache used to allocate "struct epitem" items */
epi_cache = kmem_cache_create("eventpoll_epi", sizeof(struct epitem),
0, SLAB_HWCACHE_ALIGN | SLAB_PANIC, NULL);
_
Patches currently in -mm which might be from normalperson@xxxxxxxx are
origin.patch
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