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

[Dave Chinner <david@fromorbit.com>]

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. 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....

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@fromorbit.com