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

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Date:   Mon, 20 Jul 2020 17:18:56 +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 19/07/2020 21:49, Jens Axboe wrote:
> On 7/19/20 5:15 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
>>
>> Well, if this thing is useful, it'd be hard to quantify, because active
>> polling would hide it. I think, it'd need to apply a lot of isolated
> 
> It should be very visible in my setup, as we're CPU limited, not device
> limited. Hence it makes it very easy to show CPU gains, as they directly
> translate into improved performance.

IIRC, atomics for x64 in a single thread don't hurt too much. Disregarding
this patch, it would be good to have a many-threaded benchmark to look
after scalability.

>> pressure on cache synchronisation (e.g. spam with barriers), or try to
>> create and measure an atomic heavy task pinned to another core. Don't
>> worth the effort IMHO.
>> `
>> Just out of curiosity, let me ask how do you test it?
>> - is it a VM?
>> - how many cores and threads do you use?
>> - how many io_uring instances you have? Per thread?
>> - Is it all goes to a single NVMe SSD?
> 
> It's not a VM, it's a normal box. I'm using just one CPU, one thread,
> and just one NVMe device. That's my goto test for seeing if we reclaimed
> some CPU cycles.

Got it, thanks

-- 
Pavel Begunkov