netdev - Re: [PATCH net-next v3 0/2] tcp: fix high tail latencies in DCTCP

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Message-Id: <20180707.201500.1708121396951549394.davem@davemloft.net>
Date:   Sat, 07 Jul 2018 20:15:00 +0900 (KST)
From:   David Miller <davem@...emloft.net>
To:     brakmo@...com
Cc:     netdev@...r.kernel.org, kernel-team@...com, bmatheny@...com,
        ast@...com, ncardwell@...gle.com, ycheng@...gle.com,
        sibanez@...nford.edu, eric.dumazet@...il.com
Subject: Re: [PATCH net-next v3 0/2] tcp: fix high tail latencies in DCTCP

From: Lawrence Brakmo <brakmo@...com>
Date: Tue, 3 Jul 2018 09:26:13 -0700

> When have observed high tail latencies when using DCTCP for RPCs as
> compared to using Cubic. For example, in one setup there are 2 hosts
> sending to a 3rd one, with each sender having 3 flows (1 stream,
> 1 1MB back-to-back RPCs and 1 10KB back-to-back RPCs). The following
> table shows the 99% and 99.9% latencies for both Cubic and dctcp:
> 
>            Cubic 99%  Cubic 99.9%   dctcp 99%    dctcp 99.9%
> 1MB RPCs    2.6ms       5.5ms         43ms          208ms
> 10KB RPCs    1.1ms       1.3ms         53ms          212ms
 ...
> v2: Removed call to tcp_ca_event from tcp_send_ack since I added one in
>     tcp_event_ack_sent. Based on Neal Cardwell <ncardwell@...gle.com>
>     feedback.
>     Modified tcp_ecn_check_ce (and renamed it tcp_ecn_check) instead of modifying
>     tcp_ack_send_check to insure an ACK when cwr is received.
> v3: Handling cwr in tcp_ecn_accept_cwr instead of in tcp_ecn_check.
> 
> [PATCH net-next v3 1/2] tcp: notify when a delayed ack is sent
> [PATCH net-next v3 2/2] tcp: ack immediately when a cwr packet

Neal and co., what are your thoughts right now about this patch series?

Thank you.