linux-kernel - Re: [PATCH 0/2] task

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Message-ID: <f23434a2-dcc5-6f50-89a3-0a1a92740d73@gmail.com>
Date:   Mon, 20 Jul 2020 19:42:36 +0300
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Jens Axboe <axboe@...nel.dk>, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] task_put batching

On 20/07/2020 19:11, Jens Axboe wrote:
> On 7/20/20 10:06 AM, Pavel Begunkov wrote:
>> On 20/07/2020 18:49, Jens Axboe wrote:
>>> On 7/20/20 9:22 AM, Pavel Begunkov wrote:
>>>> On 18/07/2020 17:37, Jens Axboe wrote:
>>>>> On 7/18/20 2:32 AM, Pavel Begunkov wrote:
>>>>>> For my a bit exaggerated test case perf continues to show high CPU
>>>>>> cosumption by io_dismantle(), and so calling it io_iopoll_complete().
>>>>>> Even though the patch doesn't yield throughput increase for my setup,
>>>>>> probably because the effect is hidden behind polling, but it definitely
>>>>>> improves relative percentage. And the difference should only grow with
>>>>>> increasing number of CPUs. Another reason to have this is that atomics
>>>>>> may affect other parallel tasks (e.g. which doesn't use io_uring)
>>>>>>
>>>>>> before:
>>>>>> io_iopoll_complete: 5.29%
>>>>>> io_dismantle_req:   2.16%
>>>>>>
>>>>>> after:
>>>>>> io_iopoll_complete: 3.39%
>>>>>> io_dismantle_req:   0.465%
>>>>>
>>>>> Still not seeing a win here, but it's clean and it _should_ work. For
>>>>> some reason I end up getting the offset in task ref put growing the
>>>>> fput_many(). Which doesn't (on the surface) make a lot of sense, but
>>>>> may just mean that we have some weird side effects.
>>>>
>>>> It grows because the patch is garbage, the second condition is always false.
>>>> See the diff. Could you please drop both patches?
>>>
>>> Hah, indeed. With this on top, it looks like it should in terms of
>>> performance and profiles.
>>
>> It just shows, that it doesn't really matters for a single-threaded app,
>> as expected. Worth to throw some contention though. I'll think about
>> finding some time to get/borrow a multi-threaded one.
> 
> But it kind of did here, ended up being mostly a wash in terms of perf
> here as my testing reported. With the incremental applied, it's up a bit
> over before the task put batching.

Hmm, I need to get used to sensitivity of your box, that's a good one!

Do you mean, that the buggy version without atomics was on par comparing
to not having it at all, but the fixed/updated one is a bit faster? Sounds
like micro binary differences, like a bit altered jumps.

It'd also interesting to know, what degree of coalescing in
io_iopoll_complete() you manage to get with that.

>>> I can just fold this into the existing one, if you'd like.
>>
>> Would be nice. I'm going to double-check the counter and re-measure anyway.
>> BTW, how did you find it? A tool or a proc file would be awesome.
> 
> For this kind of testing, I just use t/io_uring out of fio. It's probably
> the lowest overhead kind of tool:
> 
> # sudo taskset -c 0 t/io_uring -b512 -p1 /dev/nvme2n1

I use io_uring-bench.c from time to time, but didn't know it continued living
under fio/t/. Thanks! I also put it under cshield for more consistency, but it
looks like io-wq ignores that.

-- 
Pavel Begunkov