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

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Commit 814d488c61 ("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: 9cd981dcf1 ("tcp: fix stretch ACK bugs in CUBIC")
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Neal Cardwell 2015-03-10 17:17:04 -04:00 committed by David S. Miller
parent 9949afa42b
commit d578e18ce9
1 changed files with 4 additions and 2 deletions
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6
net/ipv4/tcp_cubic.c
View file

@ -306,8 +306,10 @@ static inline void bictcp_update(struct bictcp *ca, u32 cwnd, u32 acked)
}
}
if (ca->cnt == 0) /* cannot be zero */
ca->cnt = 1;
/* The maximum rate of cwnd increase CUBIC allows is 1 packet per
* 2 packets ACKed, meaning cwnd grows at 1.5x per RTT.
*/
ca->cnt = max(ca->cnt, 2U);
}
static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 acked)