Re: page fault scalability (ext3, ext4, xfs)

Andy Lutomirski
2013-08-19T22:29:44Z

On Mon, Aug 19, 2013 at 3:17 PM, J. Bruce Fields <bfields@fieldses.org> wrote:
> 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@fromorbit.com> 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.
>

I hope not, because they aren't currently in the same transaction, and
putting them in the same transaction require starting a transaction on
page fault and doing the equivalent of writepages when the same
transaction is committed.

With my changes [1], they still aren't, but putting them in the same
transaction would probably be only a couple lines of code, and it
would actually improve performance. (I won't write those couple lines
of code because I don't know anything at all about jbd2.)

[1] https://lkml.org/lkml/2013/8/16/510

--Andy