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Date:   Mon, 26 Sep 2022 21:08:27 +0100
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Subject: Re: [PATCH net-next v4 00/27] io_uring zerocopy send
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To:     David Ahern <dsahern@kernel.org>, io-uring@vger.kernel.org,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org
Cc:     "David S . Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Jonathan Lemon <jonathan.lemon@gmail.com>,
        Willem de Bruijn <willemb@google.com>,
        Jens Axboe <axboe@kernel.dk>, kernel-team@fb.com
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From:   Pavel Begunkov <asml.silence@gmail.com>
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On 7/24/22 19:28, David Ahern wrote:
> On 7/17/22 8:19 PM, David Ahern wrote:
>>>
>>> Haven't seen it back then. In general io_uring doesn't stop submitting
>>> requests if one request fails, at least because we're trying to execute
>>> requests asynchronously. And in general, requests can get executed
>>> out of order, so most probably submitting a bunch of requests to a single
>>> TCP sock without any ordering on io_uring side is likely a bug.
>>
>> TCP socket buffer fills resulting in a partial send (i.e, for a given
>> sqe submission only part of the write/send succeeded). io_uring was not
>> handling that case.
>>
>> I'll try to find some time to resurrect the iperf3 patch and try top of
>> tree kernel.
> 
> With your zc_v5 branch (plus the init fix on using msg->sg_from_iter),
> iperf3 with io_uring support (non-ZC case) no longer shows completions
> with incomplete sends. So that is good improvement over the last time I
> tried it.
> 
> However, adding in the ZC support and that problem resurfaces - a lot of
> completions are for an incomplete size.
> 
> liburing comes from your tree, zc_v4 branch. Upstream does not have
> support for notifications yet, so I can not move to it.
> 
> Changes to iperf3 are here:
>     https://github.com/dsahern/iperf mods-3.10-io_uring

Tried it out, the branch below fixes a small problem, adds a couple
of extra optimisations and now it actually uses registered buffers.

     https://github.com/isilence/iperf iou-sendzc

Still, the submission loop looked a bit weird, i.e. it submits I/O
to io_uring only when it exhausts sqes instead of sending right
away with some notion of QD and/or sending in batches. The approach
is good for batching (SQ size =16 here), but not so for latency.

I also see some CPU cycles being burnt in select(2). io_uring wait
would be more natural and perhaps more performant, but I didn't
spend enough time with iperf to say for sure.

-- 
Pavel Begunkov