linux-kernel - Re: [RFC 0/2] optimise local-tw task resheduling

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Message-ID: <9250606d-4998-96f6-aeaf-a5904d7027e3@kernel.dk>
Date:   Sat, 11 Mar 2023 10:24:27 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Pavel Begunkov <asml.silence@...il.com>, io-uring@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [RFC 0/2] optimise local-tw task resheduling

On 3/10/23 12:04?PM, Pavel Begunkov wrote:
> io_uring extensively uses task_work, but when a task is waiting
> for multiple CQEs it causes lots of rescheduling. This series
> is an attempt to optimise it and be a base for future improvements.
> 
> For some zc network tests eventually waiting for a portion of 
> buffers I've got 10x descrease in the number of context switches,
> which reduced the CPU consumption more than twice (17% -> 8%).
> It also helps storage cases, while running fio/t/io_uring against
> a low performant drive it got 2x descrease of the number of context
> switches for QD8 and ~4 times for QD32.
> 
> Not for inclusion yet, I want to add an optimisation for when
> waiting for 1 CQE.

Ran this on the usual peak benchmark, using IRQ. IOPS is around ~70M for
that, and I see context rates of around 8.1-8.3M/sec with the current
kernel.

Applied the two patches, but didn't see much of a change? Performance is
about the same, and cx rate ditto. Confused... As you probably know,
this test waits for 32 ios at the time.

Didn't take a closer look just yet, but I grok the concept. One
immediate thing I'd want to change is the FACILE part of it. Let's call
it something a bit more straightforward, perhaps LIGHT? Or LIGHTWEIGHT?
I can see this mostly being used for filling a CQE, so it could also be
named something like that. But could also be used for light work in the
same vein, so might not be a good idea to base the naming on that.

-- 
Jens Axboe