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From: Jens Axboe <axboe@kernel.dk>
To: Pavel Begunkov <asml.silence@gmail.com>, io-uring@vger.kernel.org
Subject: Re: [RFC 0/2] 3 cacheline io_kiocb
Date: Sat, 25 Jul 2020 09:45:48 -0600
Message-ID: <467e93fb-876d-e2a5-7596-4b9e21317d67@kernel.dk> (raw)
In-Reply-To: <cover.1595664743.git.asml.silence@gmail.com>

On 7/25/20 2:31 AM, Pavel Begunkov wrote:
> That's not final for a several reasons, but good enough for discussion.
> That brings io_kiocb down to 192B. I didn't try to benchmark it
> properly, but quick nop test gave +5% throughput increase.
> 7531 vs 7910 KIOPS with fio/t/io_uring
> 
> The whole situation is obviously a bunch of tradeoffs. For instance,
> instead of shrinking it, we can inline apoll to speed apoll path.
> 
> [2/2] just for a reference, I'm thinking about other ways to shrink it.
> e.g. ->link_list can be a single-linked list with linked tiemouts
> storing a back-reference. This can turn out to be better, because
> that would move ->fixed_file_refs to the 2nd cacheline, so we won't
> ever touch 3rd cacheline in the submission path.
> Any other ideas?

Nothing noticeable for me, still about the same performance. But
generally speaking, I don't necessarily think we need to go all in on
making this as tiny as possible. It's much more important to chase the
items where we only use 2 cachelines for the hot path, and then we have
the extra space in there already for the semi hot paths like poll driven
retry. Yes, we're still allocating from a pool that has slightly larger
objects, but that doesn't really matter _that_ much. Avoiding an extra
kmalloc+kfree for the semi hot paths are a bigger deal than making
io_kiocb smaller and smaller.

That said, for no-brainer changes, we absolutely should make it smaller.
I just don't want to jump through convoluted hoops to get there.

-- 
Jens Axboe


  parent reply index

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-25  8:31 Pavel Begunkov
2020-07-25  8:31 ` [PATCH 1/2] io_uring: allocate req->work dynamically Pavel Begunkov
2020-07-25  8:31 ` [PATCH 2/2] io_uring: unionise ->apoll and ->work Pavel Begunkov
2020-07-25 15:45 ` Jens Axboe [this message]
2020-07-25 18:24   ` [RFC 0/2] 3 cacheline io_kiocb Pavel Begunkov
2020-07-25 19:40     ` Jens Axboe
2020-07-25 20:14       ` Pavel Begunkov
2020-07-25 20:25         ` Jens Axboe

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