[RFC PATCH] getting misc stats/attributes via xattr API

[Miklos Szeredi]

On Tue, 10 May 2022 at 06:27, Ian Kent <raven at 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