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From: Pavel Begunkov <>
To: Jens Axboe <>,,
Subject: Re: [PATCH 0/2] task_put batching
Date: Sun, 19 Jul 2020 14:15:21 +0300
Message-ID: <> (raw)
In-Reply-To: <>

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
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?

> 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.

I'll take a look whether I can reproduce.

Pavel Begunkov

  reply index

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-18  8:32 Pavel Begunkov
2020-07-18  8:32 ` [PATCH 1/2] tasks: add put_task_struct_many() Pavel Begunkov
2020-07-18  8:32 ` [PATCH 2/2] io_uring: batch put_task_struct() Pavel Begunkov
2020-07-18 14:37 ` [PATCH 0/2] task_put batching Jens Axboe
2020-07-19 11:15   ` Pavel Begunkov [this message]
2020-07-19 18:49     ` Jens Axboe
2020-07-20 14:18       ` Pavel Begunkov
2020-07-20 15:22   ` Pavel Begunkov
2020-07-20 15:49     ` Jens Axboe
2020-07-20 16:06       ` Pavel Begunkov
2020-07-20 16:11         ` Jens Axboe
2020-07-20 16:42           ` Pavel Begunkov

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