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tcp_cubic: better follow cubic curve after idle period

Jana Iyengar found an interesting issue on CUBIC :

The epoch is only updated/reset initially and when experiencing losses.
The delta "t" of now - epoch_start can be arbitrary large after app idle
as well as the bic_target. Consequentially the slope (inverse of
ca->cnt) would be really large, and eventually ca->cnt would be
lower-bounded in the end to 2 to have delayed-ACK slow-start behavior.

This particularly shows up when slow_start_after_idle is disabled
as a dangerous cwnd inflation (1.5 x RTT) after few seconds of idle

Jana initial fix was to reset epoch_start if app limited,
but Neal pointed out it would ask the CUBIC algorithm to recalculate the
curve so that we again start growing steeply upward from where cwnd is
now (as CUBIC does just after a loss). Ideally we'd want the cwnd growth
curve to be the same shape, just shifted later in time by the amount of
the idle period.

Reported-by: Jana Iyengar <>
Signed-off-by: Eric Dumazet <>
Signed-off-by: Yuchung Cheng <>
Signed-off-by: Neal Cardwell <>
Cc: Stephen Hemminger <>
Cc: Sangtae Ha <>
Cc: Lawrence Brakmo <>
Signed-off-by: David S. Miller <>
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1 parent 05c5a46 commit 30927520dbae297182990bb21d08762bcc35ce1d Eric Dumazet committed with davem330 Sep 10, 2015
Showing with 16 additions and 0 deletions.
  1. +16 −0 net/ipv4/tcp_cubic.c
@@ -151,6 +151,21 @@ static void bictcp_init(struct sock *sk)
tcp_sk(sk)->snd_ssthresh = initial_ssthresh;
+static void bictcp_cwnd_event(struct sock *sk, enum tcp_ca_event event)
+ if (event == CA_EVENT_TX_START) {
+ s32 delta = tcp_time_stamp - tcp_sk(sk)->lsndtime;
+ struct bictcp *ca = inet_csk_ca(sk);
+ /* We were application limited (idle) for a while.
+ * Shift epoch_start to keep cwnd growth to cubic curve.
+ */
+ if (ca->epoch_start && delta > 0)
+ ca->epoch_start += delta;
+ return;
+ }
/* calculate the cubic root of x using a table lookup followed by one
* Newton-Raphson iteration.
* Avg err ~= 0.195%
@@ -450,6 +465,7 @@ static struct tcp_congestion_ops cubictcp __read_mostly = {
.cong_avoid = bictcp_cong_avoid,
.set_state = bictcp_state,
.undo_cwnd = bictcp_undo_cwnd,
+ .cwnd_event = bictcp_cwnd_event,
.pkts_acked = bictcp_acked,
.owner = THIS_MODULE,
.name = "cubic",

41 comments on commit 3092752




what mean ?


what mean ?


what mean ?


what mean?


what mean?


what mean?


what mean?


mean ?


what man?

vingc commented on 3092752 Sep 28, 2015



It's Cool, thanks




So good!

Honghe commented on 3092752 Sep 28, 2015

对 cubic algorithm 不知,因此看不懂此重要修复,可惜了


good job


Signed-off-by: Yuchung Cheng

I guess, this is a Chinese name.
Good job.

lebiru commented on 3092752 Sep 28, 2015

awesome 👍


Is the return inside the if statement really needed?


good job


Yuchung Cheng is from Taiwan



kenmux commented on 3092752 Sep 29, 2015

Why mention about this man specially? Why?
They're all from Google. Good jobs, guys!

dhdone commented on 3092752 Sep 29, 2015



How much performance improvement could we have by applying this fix? Could anyone give us a hint?

guhb commented on 3092752 Sep 29, 2015





Good Job!



gbjtv commented on 3092752 Oct 6, 2015

Will this make it into other lts kernels?




I'm running linux mint 17.1, how do I apply this patch?
Thanks in advance


marc, there is no need to be so rude! I'm not that naive.




Well , we just need wait for redhat to backport this patch and roll up the new kernel updates...



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