netdev - Re: [PATCH net-next] tcp: ack immediately when a cwr packet arrives

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Date:   Tue, 24 Jul 2018 21:57:27 -0400
From:   Neal Cardwell <ncardwell@...gle.com>
To:     Lawrence Brakmo <brakmo@...com>
Cc:     Yuchung Cheng <ycheng@...gle.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Netdev <netdev@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>, ast@...com,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH net-next] tcp: ack immediately when a cwr packet arrives

On Tue, Jul 24, 2018 at 1:42 PM Lawrence Brakmo <brakmo@...com> wrote:
>
> Note that without this fix the 99% latencies when doing 10KB RPCs
> in a congested network using DCTCP are 40ms vs. 190us with the patch.
> Also note that these 40ms high tail latencies started after commit
> 3759824da87b30ce7a35b4873b62b0ba38905ef5 in Jul 2015,
> which triggered the bugs/features we are fixing/adding. I agree it is a
> debatable whether it is a bug fix or a feature improvement and I am
> fine either way.

Good point. The fact that this greatly mitigates a regression in DCTCP
performance resulting from 3759824da87b30ce7a35b4873b62b0ba38905ef5
("tcp: PRR uses CRB mode by default and SS mode conditionally") IMHO
seems to be a good argument for putting this patch ("tcp: ack
immediately when a cwr packet arrives") in the "net" branch and stable
releases.

thanks,
neal