On Thu 15-08-13 17:11:42, Dave Chinner wrote: > On Wed, Aug 14, 2013 at 11:14:37PM -0700, Andy Lutomirski wrote: > > On Wed, Aug 14, 2013 at 11:01 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote: > > > On Wed, Aug 14, 2013 at 09:32:13PM -0700, Andy Lutomirski wrote: > > >> On Wed, Aug 14, 2013 at 7:10 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote: > > >> > On Wed, Aug 14, 2013 at 09:11:01PM -0400, Theodore Ts'o wrote: > > >> >> On Wed, Aug 14, 2013 at 04:38:12PM -0700, Andy Lutomirski wrote: > > >> >> > > It would be better to write zeros to it, so we aren't measuring the > > >> >> > > cost of the unwritten->written conversion. > > >> >> > > > >> >> > At the risk of beating a dead horse, how hard would it be to defer > > >> >> > this part until writeback? > > >> >> > > >> >> Part of the work has to be done at write time because we need to > > >> >> update allocation statistics (i.e., so that we don't have ENOSPC > > >> >> problems). The unwritten->written conversion does happen at writeback > > >> >> (as does the actual block allocation if we are doing delayed > > >> >> allocation). > > >> >> > > >> >> The point is that if the goal is to measure page fault scalability, we > > >> >> shouldn't have this other stuff happening as the same time as the page > > >> >> fault workload. > > >> > > > >> > Sure, but the real problem is not the block mapping or allocation > > >> > path - even if the test is changed to take that out of the picture, > > >> > we still have timestamp updates being done on every single page > > >> > fault. ext4, XFS and btrfs all do transactional timestamp updates > > >> > and have nanosecond granularity, so every page fault is resulting in > > >> > a transaction to update the timestamp of the file being modified. > > >> > > >> I have (unmergeable) patches to fix this: > > >> > > >> http://comments.gmane.org/gmane.linux.kernel.mm/92476 > > > > > > The big problem with this approach is that not doing the > > > timestamp update on page faults is going to break the inode change > > > version counting because for ext4, btrfs and XFS it takes a > > > transaction to bump that counter. NFS needs to know the moment a > > > file is changed in memory, not when it is written to disk. Also, NFS > > > requires the change to the counter to be persistent over server > > > failures, so it needs to be changed as part of a transaction.... > > > > I've been running a kernel that has the file_update_time call > > commented out for over a year now, and the only problem I've seen is > > that the timestamp doesn't get updated :) > > > > I think I must be misunderstanding you (or vice versa). I'm currently > > Yup, you are. > > > redoing the patches, and this time I'll do it for just the mm core and > > ext4. The only change I'm proposing to ext4's page_mkwrite is to > > remove the file_update_time call. > > Right. Where does that end up? All the way down in > ext4_mark_iloc_dirty(), and that does: > > if (IS_I_VERSION(inode)) > inode_inc_iversion(inode); > > The XFS transaction code is the same - deep inside it where an inode > is marked as dirty in the transaction, it bumps the same counter and > adds it to the transaction. Yeah, I'd just add that ext4 maintains i_version only if it has been mounted with i_version mount option. But then NFS server would depend on c/mtime update so it won't help you much - you still should update at least one of i_version, ctime, mtime on page fault. OTOH if the filesystem isn't exported, you could avoid this relatively expensive dance and defer things as Andy suggests. Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
