3.19-stable review patch. If anyone has any objections, please let me know.
------------------ From: Neal Cardwell <[email protected]> [ Upstream commit d578e18ce93f5d33a7120fd57c453e22a4c0fc37 ] 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 <[email protected]> Signed-off-by: Yuchung Cheng <[email protected]> Signed-off-by: Eric Dumazet <[email protected]> Signed-off-by: David S. Miller <[email protected]> Signed-off-by: Greg Kroah-Hartman <[email protected]> --- net/ipv4/tcp_cubic.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) --- a/net/ipv4/tcp_cubic.c +++ b/net/ipv4/tcp_cubic.c @@ -306,8 +306,10 @@ tcp_friendliness: } } - 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) -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
