On Tue 30-06-15 15:04:55, Dave Hansen wrote: > > 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]
Looks good. You can add: Reviewed-by: Jan Kara <[email protected]> Honza > --- > > 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. > _ -- Jan Kara <[email protected]> SUSE Labs, CR -- 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/
