On Thu, Aug 15, 2013 at 04:01:49PM +1000, Dave Chinner 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. I don't think the in-memory updates of the data and the version have to be completely atomic, if that's what you mean. > 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'm not sure those two updates have to be a single atomic transaction on disk, either. (Though the reboot cases are more complicated, I may not have thought it through.) (By the way, I wonder what happens if we reuse a change attribute value after a crash? There's probably a (hard to hit) bug there.) --b. > > IOWs, fixing the "filesystems need a transaction on each page_mkwrite > call" problem isn't as simple as changing how timestamps are > updated. > > Cheers, > > Dave. > -- > Dave Chinner > david@xxxxxxxxxxxxx > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- 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
