| Message ID | 1426022224-28793-2-git-send-email-ncardwell@google.com |
|---|---|
| State | Accepted, archived |
| Delegated to: | David Miller |
| Headers | show |
From: Neal Cardwell <ncardwell@google.com> Date: Tue, 10 Mar 2015 17:17:04 -0400 > 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> Applied. -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Wed, Mar 11, 2015 at 4:52 PM, David Miller <davem@davemloft.net> wrote: > From: Neal Cardwell <ncardwell@google.com> > Date: Tue, 10 Mar 2015 17:17:04 -0400 > >> 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> > > Applied. Thanks! My sense is that both of these fixes are appropriate for the stable queue, since they fix serious packet loss issues that folks running 3.19.x will run into: d578e18 tcp: restore 1.5x per RTT limit to CUBIC cwnd growth in congestion avoidance 9949afa tcp: fix tcp_cong_avoid_ai() credit accumulation bug with decreases in w neal -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
From: Neal Cardwell <ncardwell@google.com> Date: Wed, 11 Mar 2015 18:10:42 -0400 > My sense is that both of these fixes are appropriate for the stable > queue, since they fix serious packet loss issues that folks running > 3.19.x will run into: Ok, queued up, thanks. -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c index 4b276d1..06d3d66 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)
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> --- net/ipv4/tcp_cubic.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-)