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

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Date:   Mon, 13 Mar 2023 03:45:43 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Jens Axboe <axboe@...nel.dk>, io-uring@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [RFC 0/2] optimise local-tw task resheduling

On 3/12/23 15:30, Jens Axboe wrote:
> On 3/11/23 1:45?PM, Pavel Begunkov wrote:
>> On 3/11/23 17:24, Jens Axboe wrote:
>>> 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.
>>
>> If I'd to guess it already has perfect batching, for which case
>> the patch does nothing. Maybe it's due to SSD coalescing +
>> small ro I/O + consistency and small latencies of Optanes,
>> or might be on the scheduling and the kernel side to be slow
>> to react.
>>
>> I was looking at trace_io_uring_local_work_run() while testing,
>> It's always should be @loop=QD (i.e. 32) for the patch, but
>> the guess is it's also 32 with that setup but without patches.
> 
> It very well could be that it's just loaded enough that we get perfect
> batching anyway. I'd need to reuse some of your tracing to know for
> sure.

I used existing trace points. If you see a pattern

trace_io_uring_local_work_run()
trace_io_uring_cqring_wait(@count=32)

trace_io_uring_local_work_run()
trace_io_uring_cqring_wait(@count=32)

...

that would mean a perfect batching. Even more so
if @loops=1


>>> 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 don't really care, will change, but let me also ask why?
>> They're more or less synonyms, though facile is much less
>> popular. Is that your reasoning?
> 
> Yep, it's not very common and the name should be self-explanatory
> immediately for most people.

That's exactly the problem. Someone will think that it's
like normal tw but "better" and blindly apply it. Same happened
before with priority tw lists.

-- 
Pavel Begunkov