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

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Message-ID: <CADVnQynGT7neCE5d6WaGQYs2WtBLjVhVXrOoJu3PYQDx-hTmjg@mail.gmail.com>
Date:   Mon, 23 Jul 2018 22:15:23 -0400
From:   Neal Cardwell <ncardwell@...gle.com>
To:     Lawrence Brakmo <brakmo@...com>
Cc:     Netdev <netdev@...r.kernel.org>, Kernel Team <kernel-team@...com>,
        ast@...com, Yuchung Cheng <ycheng@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH net-next] tcp: ack immediately when a cwr packet arrives

On Mon, Jul 23, 2018 at 8:49 PM Lawrence Brakmo <brakmo@...com> wrote:
>
> We observed high 99 and 99.9% latencies when doing RPCs with DCTCP. The
> problem is triggered when the last packet of a request arrives CE
> marked. The reply will carry the ECE mark causing TCP to shrink its cwnd
> to 1 (because there are no packets in flight). When the 1st packet of
> the next request arrives, the ACK was sometimes delayed even though it
> is CWR marked, adding up to 40ms to the RPC latency.
>
> This patch insures that CWR marked data packets arriving will be acked
> immediately.
...
> Modified based on comments by Neal Cardwell <ncardwell@...gle.com>
>
> Signed-off-by: Lawrence Brakmo <brakmo@...com>
> ---
>  net/ipv4/tcp_input.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)

Seems like a nice mechanism to have, IMHO.

Acked-by: Neal Cardwell <ncardwell@...gle.com>

Thanks!
neal