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From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Tue, 10 May 2022 10:06:17 +0200
Message-ID: <CAJfpegv3MCHMzur9R+K+yZC3Z_Wmbq3=pQwuQ=+kQSrihg0c9g@mail.gmail.com>
Subject: Re: [RFC PATCH] getting misc stats/attributes via xattr API
To:     Ian Kent <raven@themaw.net>
Cc:     Christian Brauner <brauner@kernel.org>,
        linux-fsdevel@vger.kernel.org, Dave Chinner <david@fromorbit.com>,
        "Theodore Ts'o" <tytso@mit.edu>, Karel Zak <kzak@redhat.com>,
        Greg KH <gregkh@linuxfoundation.org>,
        linux-kernel@vger.kernel.org,
        Linux API <linux-api@vger.kernel.org>,
        linux-man <linux-man@vger.kernel.org>,
        LSM <linux-security-module@vger.kernel.org>,
        David Howells <dhowells@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <christian@brauner.io>,
        Amir Goldstein <amir73il@gmail.com>,
        James Bottomley <James.Bottomley@hansenpartnership.com>
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On Tue, 10 May 2022 at 06:27, Ian Kent <raven@themaw.net> wrote:

> > Was there ever a test patch for systemd using fsinfo(2)?  I think
> > not.
>
> Mmm ... I'm hurt you didn't pay any attention to what I did on this
> during the original fsinfo() discussions.

I can't find anything related to this in my mailbox.  Maybe you
mentioned it at some point, but I have not been involved with the
actual systemd changes.  So not meant to belittle your work at all.

> > Until systemd people start to reengineer the mount handing to allow
> > for retrieving a single mount instead of the complete mount table we
> > will never know where the performance bottleneck lies.
>
> We didn't need the systemd people to do this only review and contribute
> to the pr for the change and eventually merge it.
>
> What I did on this showed that using fsinfo() allone about halved the
> CPU overhead (from around 4 processes using about 80%) and once the
> mount notifications was added too it went down to well under 10% per
> process. The problem here was systemd is quite good at servicing events
> and reducing event processing overhead meant more events would then be
> processed. Utilizing the mount notifications queueing was the key to
> improving this and that was what I was about to work on at the end.
>
> But everything stopped before the work was complete.
>
> As I said above it's been a long time since I looked at the systemd
> work and it definitely was a WIP so "what you see is what you get"
> at https://github.com/raven-au/systemd/commits/. It looks like the
> place to look to get some idea of what was being done is branch
> notifications-devel or notifications-rfc-pr. Also note that this
> uses the libmount fsinfo() infrastrucure that was done by Karal Zak
> (and a tiny bit by me) at the time.

Looks great as a first step.

What do you mean by "Utilizing the mount notifications queueing"?

Do you mean batching of notifications?   I think that's a very
important issue: processing each individual notifcation may not make
sense when there are lots of them.  For example, doing ceate
mount+remote mount in a loop a million times will result in two
million notification messages (with high likelyhood of queue
overflow), but in the end the mount table will end up being the
same...

Thanks,
Miklos