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

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Message-ID: <CAKPOu+-0kT5PXt1WbEGJSC8=47pZDz311DHB7D920ZHuoXPvwQ@mail.gmail.com>
Date: Wed, 29 Jan 2025 20:43:44 +0100
From: Max Kellermann <max.kellermann@...os.com>
To: Pavel Begunkov <asml.silence@...il.com>
Cc: Jens Axboe <axboe@...nel.dk>, io-uring@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/8] Various io_uring micro-optimizations (reducing lock contention)

On Wed, Jan 29, 2025 at 8:30 PM Pavel Begunkov <asml.silence@...il.com> wrote:
> It's great to see iowq getting some optimisations, but note that
> it wouldn't be fair comparing it to single threaded peers when
> you have a lot of iowq activity as it might be occupying multiple
> CPUs.

True. Fully loaded with the benchmark, I see 400%-600% CPU usage on my
process (30-40% of that being spinlock contention).
I wanted to explore how far I can get with a single (userspace)
thread, and leave the dirty thread-sync work to the kernel.

> It's wasteful unless you saturate it close to 100%, and then you
> usually have SQPOLL on a separate CPU than the user task submitting
> requests, and so it'd take some cache bouncing. It's not a silver
> bullet.

Of course, memory latency always bites us in the end. But this isn't
the endgame just yet, we still have a lot of potential for
optimizations.