This is a note to let you know that I've just added the patch titled
tcp: restore 1.5x per RTT limit to CUBIC cwnd growth in congestion avoidance
to the 3.19-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
tcp-restore-1.5x-per-rtt-limit-to-cubic-cwnd-growth-in-congestion-avoidance.patch
and it can be found in the queue-3.19 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From foo@baz Tue Mar 24 11:00:07 CET 2015
From: Neal Cardwell <ncardwell@xxxxxxxxxx>
Date: Tue, 10 Mar 2015 17:17:04 -0400
Subject: tcp: restore 1.5x per RTT limit to CUBIC cwnd growth in congestion avoidance
From: Neal Cardwell <ncardwell@xxxxxxxxxx>
[ 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 <ncardwell@xxxxxxxxxx>
Signed-off-by: Yuchung Cheng <ycheng@xxxxxxxxxx>
Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
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)
Patches currently in stable-queue which might be from ncardwell@xxxxxxxxxx are
queue-3.19/tcp-fix-tcp_cong_avoid_ai-credit-accumulation-bug-with-decreases-in-w.patch
queue-3.19/tcp-restore-1.5x-per-rtt-limit-to-cubic-cwnd-growth-in-congestion-avoidance.patch
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