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| author | Neal Cardwell <ncardwell@google.com> | 2015-03-10 17:17:04 -0400 |
|---|---|---|
| committer | David S. Miller <davem@davemloft.net> | 2015-03-11 16:51:51 -0400 |
| commit | d578e18ce93f5d33a7120fd57c453e22a4c0fc37 (patch) | |
| tree | c5a801604470af1670387f46c4d613ef11260ca0 | |
| parent | 9949afa42be0b76f5832db112ce51bb6b35b2abb (diff) | |
| download | linux-stable-d578e18ce93f5d33a7120fd57c453e22a4c0fc37.tar.gz linux-stable-d578e18ce93f5d33a7120fd57c453e22a4c0fc37.tar.bz2 linux-stable-d578e18ce93f5d33a7120fd57c453e22a4c0fc37.zip | |
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 <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>
| -rw-r--r-- | net/ipv4/tcp_cubic.c | 6 |
1 files changed, 4 insertions, 2 deletions
diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c index 4b276d1ed980..06d3d665a9fd 100644 --- 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) |