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. If a filesystem is providing an i_version value, then NFS uses it to determine whether client side caches are still consistent with the server state. If the filesystem does not provide an i_version, then NFS falls back to checking c/mtime for changes. If files on the server are being modified without either the tiemstamps or i_version changing, then it's likely that there will be problems with client side cache consistency.... > Instead, ext4 will call > file_update_time on munmap, exit, MS_ASYNC, and at the end of > writepages. Unless I'm missing something, there's no need to > unconditionally start a transaction on page_mkwrite (and there had > better not be, because file_update_time won't start a transaction if > the time doesn't change). Right, there's no unconditional need for a transaction except if the filesystem is providing the inode version change feature for NFS. ext4, btrfs and XFS all do this unconditionally, and so therefore those filesystem have a need for an inode change transaction on every page fault, just like they do for every write(2) call. Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
