tcp: restore 1.5x per RTT limit to CUBIC cwnd growth in congestion avoidance
tcp: restore 1.5x per RTT limit to CUBIC cwnd growth in congestion avoidance

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 <58d301c70203188c35d379474c26c9cc0636a189@google.com>
Signed-off-by: Yuchung Cheng <5b5a0a360fa35167ac197fe6359c229fde284cf2@google.com>
Signed-off-by: Eric Dumazet <312c7d941f03386814be517ffdb3e0eb6624e275@google.com>
Signed-off-by: David S. Miller <fe08d3c717adf2ae63592e4c9aec6e3e404d8e3e@davemloft.net>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs