Re: RFC fuse waitq latency

[Miklos Szeredi]

On Mon, 28 Mar 2022 at 15:21, Bernd Schubert <bschubert@xxxxxxx> wrote:
>
> I would like to discuss the user thread wake up latency in
> fuse_dev_do_read(). Profiling fuse shows there is room for improvement
> regarding memory copies and splice. The basic profiling with flame graphs
> didn't reveal, though, why fuse is so much
> slower (with an overlay file system) than just accessing the underlying
> file system directly and also didn't reveal why a single threaded fuse
> uses less than 100% cpu, with the application on top of use also using
> less than 100% cpu (simple bonnie++ runs with 1B files).
> So I started to suspect the wait queues and indeed, keeping the thread
> that reads the fuse device for work running for some time gives quite
> some improvements.

Might be related: I experimented with wake_up_sync() that didn't meet
my expectations.  See this thread:

https://lore.kernel.org/all/1638780405-38026-1-git-send-email-quic_pragalla@xxxxxxxxxxx/#r

Possibly fuse needs some wake up tweaks due to its special scheduling
requirements.

Thanks,
Miklos