Linux-Fsdevel Archive on lore.kernel.org
help / color / mirror / Atom feed
From: Amir Goldstein <amir73il@gmail.com>
To: Mel Gorman <mgorman@techsingularity.net>
Cc: Jan Kara <jack@suse.cz>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] fsnotify: Rearrange fast path to minimise overhead when there is no watcher
Date: Mon, 8 Jun 2020 20:45:56 +0300	[thread overview]
Message-ID: <CAOQ4uxg2BKuRCfn7BH4FYnvrp_2rsgfgJ05pjjGOarx4C3E9Mw@mail.gmail.com> (raw)
In-Reply-To: <20200608165040.GI3127@techsingularity.net>

On Mon, Jun 8, 2020 at 7:50 PM Mel Gorman <mgorman@techsingularity.net> wrote:
>
> On Mon, Jun 08, 2020 at 05:19:43PM +0200, Jan Kara wrote:
> > > This is showing that the latencies are improved by roughly 2-9%. The
> > > variability is not shown but some of these results are within the noise
> > > as this workload heavily overloads the machine. That said, the system CPU
> > > usage is reduced by quite a bit so it makes sense to avoid the overhead
> > > even if it is a bit tricky to detect at times. A perf profile of just 1
> > > group of tasks showed that 5.14% of samples taken were in either fsnotify()
> > > or fsnotify_parent(). With the patch, 2.8% of samples were in fsnotify,
> > > mostly function entry and the initial check for watchers.  The check for
> > > watchers is complicated enough that inlining it may be controversial.
> > >
> > > Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
> >
> > Thanks for the patch! I have to tell I'm surprised this small reordering
> > helps so much. For pipe inode we will bail on:
> >
> >        if (!to_tell->i_fsnotify_marks && !sb->s_fsnotify_marks &&
> >            (!mnt || !mnt->mnt_fsnotify_marks))
> >                return 0;
> >
> > So what we save with the reordering is sb->s_fsnotify_mask and
> > mnt->mnt_fsnotify_mask fetch but that should be the same cacheline as
> > sb->s_fsnotify_marks and mnt->mnt_fsnotify_marks, respectively.
>
> It is likely that the contribution of that change is marginal relative
> to the fsnotify_parent() call. I'll know by tomorrow morning at the latest.
>
> > We also
> > save a function call of fsnotify_parent() but I would think that is very
> > cheap (compared to the whole write path) as well.
> >
>
> To be fair, it is cheap but with this particular workload, we call
> vfs_write() a *lot* and the path is not that long so it builds up to 5%
> of samples overall. Given that these were anonymous pipes, it surprised
> me to see fsnotify at all which is why I took a closer look.
>

I should note that after:
7c49b8616460 fs/notify: optimize inotify/fsnotify code for unwatched files
Which speaks of a similar workload,
the code looked quite similar to your optimization.

It was:
60f7ed8c7c4d fsnotify: send path type events to group with super block marks

That started accessing ->x_fsnotify_mask before ->x_fsnotify_marks,
although I still find it hard to believe that this makes a real difference.

Thanks,
Amir.

      reply	other threads:[~2020-06-08 17:46 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-06-08 14:05 Mel Gorman
2020-06-08 15:03 ` Amir Goldstein
2020-06-08 16:06   ` Mel Gorman
2020-06-08 16:26     ` Amir Goldstein
2020-06-08 18:01       ` Mel Gorman
2020-06-08 18:12         ` Amir Goldstein
2020-06-08 20:39           ` Amir Goldstein
2020-06-10 12:59             ` Mel Gorman
2020-06-10 13:24               ` Amir Goldstein
2020-06-12  9:38                 ` Amir Goldstein
2020-06-12 12:42                   ` Mel Gorman
2020-06-08 15:19 ` Jan Kara
2020-06-08 16:50   ` Mel Gorman
2020-06-08 17:45     ` Amir Goldstein [this message]

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=CAOQ4uxg2BKuRCfn7BH4FYnvrp_2rsgfgJ05pjjGOarx4C3E9Mw@mail.gmail.com \
    --to=amir73il@gmail.com \
    --cc=jack@suse.cz \
    --cc=linux-fsdevel@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mgorman@techsingularity.net \
    --subject='Re: [PATCH] fsnotify: Rearrange fast path to minimise overhead when there is no watcher' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).