On Thu, Aug 15, 2013 at 02:31:14PM -0700, Andy Lutomirski wrote: > On Thu, Aug 15, 2013 at 2:28 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote: > > On Thu, Aug 15, 2013 at 09:45:31AM +0200, Jan Kara wrote: > >> 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. > > > > The problem with "not exported, don't update" is that files can be > > modified on server startup (e.g. after a crash) or in short > > maintenance periods when the NFS service is down. When the server is > > started back up, the change number needs to indicate the file has > > been modified so that clients reconnecting to the server see the > > change. > > > > IOWs, even if the NFS server is not up or the filesystem not > > exported we still need to update change counts whenever a file > > changes if we are going to tell the NFS server that we keep them... > > This will keep working as long as the clients are willing to wait for > writeback (or msync, munmap, or exit) on the server. I don't follow you - what will keep working? If we don't record changes while the filesystem is not exported, then NFS clients can't determine if files have changed while the server was down for a period.... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- 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
