Re: [PATCH 0/8] Various io_uring micro-optimizations (reducing lock contention)

[Max Kellermann <max.kellermann@ionos.com>]

On Wed, Jan 29, 2025 at 6:19 PM Jens Axboe <axboe@kernel.dk> wrote:
> The other patches look pretty straight forward to me. Only thing that
> has me puzzled a bit is why you have so much io-wq activity with your
> application, in general I'd expect 0 activity there. But Then I saw the
> forced ASYNC flag, and it makes sense. In general, forcing that isn't a
> great idea, but for a benchmark for io-wq it certainly makes sense.

I was experimenting with io_uring and wanted to see how much
performance I can squeeze out of my web server running
single-threaded. The overhead of io_uring_submit() grew very large,
because the "send" operation would do a lot of synchronous work in the
kernel. I tried SQPOLL but it was actually a big performance
regression; this just shifted my CPU usage to epoll_wait(). Forcing
ASYNC gave me large throughput improvements (moving the submission
overhead to iowq), but then the iowq lock contention was the next
limit, thus this patch series.

I'm still experimenting, and I will certainly revisit SQPOLL to learn
more about why it didn't help and how to fix it.

> I'll apply 1-7 once 6.14-rc1 is out and I can kick off a
> for-6.15/io_uring branch. Thanks!

Thanks Jens, and please let me know when you're ready to discuss the
last patch. It's a big improvement for those who combine io_uring with
epoll, it's worth it.

Max