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authorNeal Cardwell <ncardwell@google.com>2015-03-10 17:17:04 -0400
committerDavid S. Miller <davem@davemloft.net>2015-03-11 16:51:51 -0400
commitd578e18ce93f5d33a7120fd57c453e22a4c0fc37 (patch)
treec5a801604470af1670387f46c4d613ef11260ca0 /net
parent9949afa42be0b76f5832db112ce51bb6b35b2abb (diff)
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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>
Diffstat (limited to 'net')
-rw-r--r--net/ipv4/tcp_cubic.c6
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)