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From: Mel Gorman <>
To: Jan Kara <>
Cc: Amir Goldstein <>,,
Subject: Re: [PATCH] fsnotify: Rearrange fast path to minimise overhead when there is no watcher
Date: Mon, 8 Jun 2020 17:50:40 +0100	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

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 <>
> 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.

> The patch is simple enough so I have no problem merging it but I'm just
> surprised by the results... Hum, maybe the structure randomization is used
> in the builds and so e.g. sb->s_fsnotify_mask and sb->s_fsnotify_marks
> don't end up in the same cacheline? But I don't think we enable that in
> SUSE builds?

Correct, GCC_PLUGIN_RANDSTRUCT was not set.

Mel Gorman

  reply	other threads:[~2020-06-08 16:50 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 [this message]
2020-06-08 17:45     ` Amir Goldstein

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