Re: [PATCH 00/19] fs: rework and optimize i_version handling in filesystems

[Dave Chinner <david@fromorbit.com>]

On Wed, Dec 13, 2017 at 03:14:28PM -0500, Jeff Layton wrote:
> On Wed, 2017-12-13 at 10:05 -0500, J. Bruce Fields wrote:
> > This is great, thanks.
> > 
> > On Wed, Dec 13, 2017 at 09:19:58AM -0500, Jeff Layton wrote:
> > > With this, we reduce inode metadata updates across all 3 filesystems
> > > down to roughly the frequency of the timestamp granularity, particularly
> > > when it's not being queried (the vastly common case).
> > > 
> > > The pessimal workload here is 1 byte writes, and it helps that
> > > significantly. Of course, that's not what we'd consider a real-world
> > > workload.
> > > 
> > > A tiobench-example.fio workload also shows some modest performance
> > > gains, and I've gotten mails from the kernel test robot that show some
> > > significant performance gains on some microbenchmarks (case-msync-mt in
> > > the vm-scalability testsuite to be specific), with an earlier version of
> > > this set.
> > > 
> > > With larger writes, the gains with this patchset mostly vaporize,
> > > but it does not seem to cause performance to regress anywhere, AFAICT.
> > > 
> > > I'm happy to run other workloads if anyone can suggest them.
> > > 
> > > At this point, the patchset works and does what it's expected to do in
> > > my own testing. It seems like it's at least a modest performance win
> > > across all 3 major disk-based filesystems. It may also encourage others
> > > to implement i_version as well since it reduces the cost.
> > 
> > Do you have an idea what the remaining cost is?
> > 
> > Especially in the ext4 case, are you still able to measure any
> > difference in performance between the cases where i_version is turned on
> > and off, after these patches?
> 
> Attached is a fio jobfile + the output from 3 different runs using it
> with ext4. This one is using 4k writes. There was no querying of
> i_version during the runs.  I've done several runs with each and these
> are pretty representative of the results:
> 
> old = 4.15-rc3, i_version enabled
> ivers = 4.15-rc3 + these patches, i_version enabled
> noivers = 4.15-rc3 + these patches, i_version disabled
> 
> To snip out the run status lines:
> 
> old:
> WRITE: bw=85.6MiB/s (89.8MB/s), 9994KiB/s-11.1MiB/s (10.2MB/s-11.7MB/s), io=50.2GiB (53.8GB), run=600001-600001msec
> 
> ivers:
> WRITE: bw=110MiB/s (115MB/s), 13.5MiB/s-14.2MiB/s (14.1MB/s-14.9MB/s), io=64.3GiB (69.0GB), run=600001-600001msec
> 
> noivers:
> WRITE: bw=117MiB/s (123MB/s), 14.2MiB/s-15.2MiB/s (14.9MB/s-15.9MB/s), io=68.7GiB (73.8GB), run=600001-600001msec
> 
> So, I see some performance degradation with -o iversion compared to not
> having it enabled (maybe due to the extra atomic fetches?), but this set
> erases most of the difference.

So what is the performance difference when something is actively
querying the i_version counter as fast as it can (e.g. file being
constantly stat()d via NFS whilst being modified)? How does the
performance compare to the old code in that case?

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com