Date:   Wed, 13 Dec 2017 15:14:28 -0500
From:   Jeff Layton <jlayton@...nel.org>
To:     "J. Bruce Fields" <bfields@...ldses.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        hch@....de, neilb@...e.de, amir73il@...il.com, jack@...e.de,
        viro@...iv.linux.org.uk
Subject: Re: [PATCH 00/19] fs: rework and optimize i_version handling in
 filesystems

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.

> > 
> > [1]: On ext4 it must be turned on with the i_version mount option,
> >      mostly due to fears of incurring this impact, AFAICT.
> 
> So xfs and btrfs both have i_version updates on by default at this
> point?  (Assuming the filesystem's created with recent enough tools,
> etc.)
> 

Yes. With xfs and btrfs, I don't think you can disable it these days.

-- 
Jeff Layton <jlayton@...nel.org>
View attachment "ext4-ivers-4k" of type "text/plain" (12418 bytes)

View attachment "ext4-noivers-4k" of type "text/plain" (12221 bytes)

View attachment "ext4-old-4k" of type "text/plain" (12847 bytes)

View attachment "seq-write.fio" of type "text/plain" (192 bytes)

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