netdev - Re: [PATCH net 2/2] tcp: restore 1.5x per RTT limit to CUBIC cwnd growth in congestion avoidance

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Date:	Wed, 11 Mar 2015 16:52:14 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	ncardwell@...gle.com
Cc:	netdev@...r.kernel.org, ycheng@...gle.com, edumazet@...gle.com
Subject: Re: [PATCH net 2/2] tcp: restore 1.5x per RTT limit to CUBIC cwnd
 growth in congestion avoidance

From: Neal Cardwell <ncardwell@...gle.com>
Date: Tue, 10 Mar 2015 17:17:04 -0400

> Commit 814d488c6126 ("tcp: fix the timid additive increase on stretch
> ACKs") fixed a bug where tcp_cong_avoid_ai() would either credit a
> connection with an increase of snd_cwnd_cnt, or increase snd_cwnd, but
> not both, resulting in cwnd increasing by 1 packet on at most every
> alternate invocation of tcp_cong_avoid_ai().
> 
> Although the commit correctly implemented the CUBIC algorithm, which
> can increase cwnd by as much as 1 packet per 1 packet ACKed (2x per
> RTT), in practice that could be too aggressive: in tests on network
> paths with small buffers, YouTube server retransmission rates nearly
> doubled.
> 
> This commit restores CUBIC to a maximum cwnd growth rate of 1 packet
> per 2 packets ACKed (1.5x per RTT). In YouTube tests this restored
> retransmit rates to low levels.
> 
> Testing: This patch has been tested in datacenter netperf transfers
> and live youtube.com and google.com servers.
> 
> Fixes: 9cd981dcf174 ("tcp: fix stretch ACK bugs in CUBIC")
> Signed-off-by: Neal Cardwell <ncardwell@...gle.com>
> Signed-off-by: Yuchung Cheng <ycheng@...gle.com>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Applied.
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