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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
time.

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 <jri@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Cc: Sangtae Ha <sangtae.ha@gmail.com>
Cc: Lawrence Brakmo <lawrence@brakmo.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
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Eric Dumazet authored and davem330 committed Sep 10, 2015
1 parent 05c5a46 commit 30927520dbae297182990bb21d08762bcc35ce1d
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

@zhouyongtao

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@zhouyongtao zhouyongtao replied Sep 28, 2015

不明觉厉的样子

@TapXWorld

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@TapXWorld TapXWorld replied Sep 28, 2015

what mean ?

@TapXWorld

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@TapXWorld TapXWorld replied Sep 28, 2015

what mean ?

@TapXWorld

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@TapXWorld TapXWorld replied Sep 28, 2015

what mean ?

@TapXWorld

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@TapXWorld TapXWorld replied Sep 28, 2015

what mean?

@TapXWorld

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@TapXWorld TapXWorld replied Sep 28, 2015

what mean?

@TapXWorld

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@TapXWorld TapXWorld replied Sep 28, 2015

what mean?

@TapXWorld

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@TapXWorld TapXWorld replied Sep 28, 2015

what mean?

@TapXWorld

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@TapXWorld TapXWorld replied Sep 28, 2015

mean ?

@TapXWorld

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@TapXWorld TapXWorld replied Sep 28, 2015

what man?

@vingc

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@vingc vingc replied Sep 28, 2015

膜拜,感谢贡献者

@douglarek

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@douglarek douglarek replied Sep 28, 2015

It's Cool, thanks

@anythink-wx

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@anythink-wx anythink-wx replied Sep 28, 2015

顶~

@openproject

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@openproject openproject replied Sep 28, 2015

So good!

@Honghe

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@Honghe Honghe replied Sep 28, 2015

对 cubic algorithm 不知,因此看不懂此重要修复,可惜了

@xormplus

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@xormplus xormplus replied Sep 28, 2015

good job

@teddysun

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@teddysun teddysun replied Sep 28, 2015

Signed-off-by: Yuchung Cheng ycheng@google.com

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

@lebiru

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@lebiru lebiru replied Sep 28, 2015

awesome 👍

@swaechter

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@swaechter swaechter replied Sep 28, 2015

Is the return inside the if statement really needed?

@liuchangdong

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@liuchangdong liuchangdong replied Sep 29, 2015

good job

@porscheyin

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@porscheyin porscheyin replied Sep 29, 2015

Yuchung Cheng is from Taiwan

@name327

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@name327 name327 replied Sep 29, 2015

我赵日天并不服

@kenmux

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@kenmux kenmux replied Sep 29, 2015

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

@dhdone

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@dhdone dhdone replied Sep 29, 2015

感谢修复者

@zhenhua

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@zhenhua zhenhua replied Sep 29, 2015

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

@marctmiller

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@marctmiller marctmiller replied Sep 29, 2015

@hephaex

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@hephaex hephaex replied Sep 29, 2015

@guhb

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@guhb guhb replied Sep 29, 2015

看不懂~先顶一个

@kbengine

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@kbengine kbengine replied Oct 3, 2015

wow!

@rafavg77

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@rafavg77 rafavg77 replied Oct 4, 2015

Good Job!

@LINGQ1991

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@LINGQ1991 LINGQ1991 replied Oct 4, 2015

不明觉厉!!!

@gbjtv

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@gbjtv gbjtv replied Oct 6, 2015

Will this make it into other lts kernels?

@theharveyz

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@theharveyz theharveyz replied Oct 13, 2015

awesome!

@dale76uk

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@dale76uk dale76uk replied Oct 25, 2015

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

@marctmiller

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@marctmiller marctmiller replied Oct 25, 2015

@nkeck720

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@nkeck720 nkeck720 replied Oct 25, 2015

@dale76uk

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@dale76uk dale76uk replied Oct 26, 2015

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

@marctmiller

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@marctmiller marctmiller replied Oct 26, 2015

@arjun024

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Contributor

@arjun024 arjun024 replied Oct 28, 2015

👏

@chandlerding

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@chandlerding chandlerding replied Nov 16, 2015

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

@losywee

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@losywee losywee replied Mar 5, 2016

Thanks.

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