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From:   Pavel Begunkov <asml.silence@gmail.com>
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Subject: Re: [RFC PATCH 0/2] Optimise io_uring completion waiting
Message-ID: <cb105638-2394-5eef-216f-9c6ff918ee59@gmail.com>
Date:   Sat, 14 Sep 2019 13:11:08 +0300
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From: Pavel Begunkov <asml.silence@gmail.com>
To: Jens Axboe <axboe@kernel.dk>, Ingo Molnar <mingo@redhat.com>,
 Peter Zijlstra <peterz@infradead.org>, linux-block@vger.kernel.org,
 linux-kernel@vger.kernel.org
Message-ID: <cb105638-2394-5eef-216f-9c6ff918ee59@gmail.com>
Subject: Re: [RFC PATCH 0/2] Optimise io_uring completion waiting
References: <cover.1568413210.git.asml.silence@gmail.com>
 <af5ba045-2ea8-b213-3625-354c10334540@kernel.dk>
In-Reply-To: <af5ba045-2ea8-b213-3625-354c10334540@kernel.dk>

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It solves much of the problem, though still have overhead on traversing
a wait queue + indirect calls for checking.

I've been thinking to either
1. create n wait queues and bucketing waiter. E.g. log2(min_events)
bucketing  would remove at least half of such calls for arbitary
min_events and all if min_events is pow2.

2. or dig deeper and add custom wake_up with perhaps sorted wait_queue.
As I see it, it's pretty bulky and over-engineered, but maybe somebody
knows an easier way?

Anyway, I don't have performance numbers for that, so don't know if this
would be justified.


On 14/09/2019 03:31, Jens Axboe wrote:
> On 9/13/19 4:28 PM, Pavel Begunkov (Silence) wrote:
>> From: Pavel Begunkov <asml.silence@gmail.com>
>>
>> There could be a lot of overhead within generic wait_event_*() used fo=
r
>> waiting for large number of completions. The patchset removes much of
>> it by using custom wait event (wait_threshold).
>>
>> Synthetic test showed ~40% performance boost. (see patch 2)
>=20
> Nifty, from an io_uring perspective, I like this a lot.
>=20
> The core changes needed to support it look fine as well. I'll await
> Peter/Ingo's comments on it.
>=20

--=20
Yours sincerely,
Pavel Begunkov


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