Messages in this thread Patch in this message | | | Subject | [RFC][PATCH] fs: optimize inotify/fsnotify code for unwatched files | From | Dave Hansen <> | Date | Fri, 19 Jun 2015 14:50:25 -0700 |
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From: Dave Hansen <dave.hansen@linux.intel.com>
I have a _tiny_ microbenchmark that sits in a loop and writes single bytes to a file. Writing one byte to a tmpfs file is around 2x slower than reading one byte from a file, which is a _bit_ more than I expecte. This is a dumb benchmark, but I think it's hard to deny that write() is a hot path and we should avoid unnecessary overhead there.
I did a 'perf record' of 30-second samples of read and write. The top item in a diffprofile is srcu_read_lock() from fsnotify(). There are active inotify fd's from systemd, but nothing is actually listening to the file or its part of the filesystem.
I *think* we can avoid taking the srcu_read_lock() for the common case where there are no actual marks on the file being modified *or* the vfsmount.
The *_fsnotify_mask is an aggregate of each of the masks from each mark. If we have nothing set in the masks at all then there are no marks and no need to do anything with 'ignored masks' since none exist. This keeps us from having to do the costly srcu_read_lock() for a check which is very cheap.
This patch gave a 10.8% speedup in writes/second on my test.
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Jan Kara <jack@suse.cz> Cc: Al Viro <viro@zeniv.linux.org.uk> Cc: Eric Paris <eparis@redhat.com> Cc: John McCutchan <john@johnmccutchan.com> Cc: Robert Love <rlove@rlove.org> Cc: Tim Chen <tim.c.chen@linux.intel.com> Cc: Andi Kleen <ak@linux.intel.com> Cc: linux-kernel@vger.kernel.org ---
b/fs/notify/fsnotify.c | 10 ++++++++++ 1 file changed, 10 insertions(+)
diff -puN fs/notify/fsnotify.c~optimize-fsnotify fs/notify/fsnotify.c --- a/fs/notify/fsnotify.c~optimize-fsnotify 2015-06-19 13:29:53.117283581 -0700 +++ b/fs/notify/fsnotify.c 2015-06-19 13:29:53.123283853 -0700 @@ -213,6 +213,16 @@ int fsnotify(struct inode *to_tell, __u3 !(test_mask & to_tell->i_fsnotify_mask) && !(mnt && test_mask & mnt->mnt_fsnotify_mask)) return 0; + /* + * Optimization: The *_fsnotify_mask is an aggregate of each of the + * masks from each mark. If we have nothing set in the masks at + * all then there are no marks and no need to do anything with + * 'ignored masks' since none exist. This keeps us from having to + * do the costly srcu_read_lock() for a check which is very cheap. + */ + if (!to_tell->i_fsnotify_mask && + (!mnt || !mnt->mnt_fsnotify_mask)) + return 0; idx = srcu_read_lock(&fsnotify_mark_srcu); _ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in Please read the FAQ at http://www.tux.org/lkml/
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