From: Dave Hansen <[email protected]> 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. This means that there will both be nothing to notify for *and* implies that there is no need for clearing the ignore mask. This patch gave a 13.1% speedup in writes/second on my test, which is an improvement from the 10.8% that I saw with the last version. Signed-off-by: Dave Hansen <[email protected]> Cc: Andrew Morton <[email protected]> Cc: Jan Kara <[email protected]> Cc: Al Viro <[email protected]> Cc: Eric Paris <[email protected]> Cc: John McCutchan <[email protected]> Cc: Robert Love <[email protected]> Cc: Andi Kleen <[email protected]> Cc: [email protected] --- 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 2015-06-30 15:03:56.819399401 -0700 +++ b/fs/notify/fsnotify.c 2015-06-30 15:03:56.822399536 -0700 @@ -205,6 +205,16 @@ int fsnotify(struct inode *to_tell, __u3 mnt = NULL; /* + * Optimization: srcu_read_lock() has a memory barrier which can + * be expensive. It protects walking the *_fsnotify_marks lists. + * However, if we do not walk the lists, we do not have to do + * SRCU because we have no references to any objects and do not + * need SRCU to keep them "alive". + */ + if (hlist_empty(&to_tell->i_fsnotify_marks) && + (!mnt || hlist_empty(&mnt->mnt_fsnotify_marks))) + return 0; + /* * if this is a modify event we may need to clear the ignored masks * otherwise return if neither the inode nor the vfsmount care about * this type of event. _ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
