From: Mel Gorman <mgorman@techsingularity.net>
To: Jan Kara <jack@suse.cz>
Cc: Amir Goldstein <amir73il@gmail.com>,
linux-fsdevel@vger.kernel.org, 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 17:50:40 +0100 [thread overview]
Message-ID: <20200608165040.GI3127@techsingularity.net> (raw)
In-Reply-To: <20200608151943.GA861@quack2.suse.cz>
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.
> 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
SUSE Labs
next prev parent 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 [PATCH] fsnotify: Rearrange fast path to minimise overhead when there is no watcher 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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