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Message-ID: <CAK6E8=ekVXA9JrajwmJk88bUKcJSi=T5+SFXs0VoG5iQkWBcRw@mail.gmail.com>
Date: Tue, 18 Feb 2014 10:46:18 -0800
From: Yuchung Cheng <ycheng@...gle.com>
To: Per Hurtig <per.hurtig@....se>
Cc: netdev <netdev@...r.kernel.org>,
David Miller <davem@...emloft.net>,
Eric Dumazet <edumazet@...gle.com>,
Anna Brunstrom <anna.brunstrom@....se>,
Andreas Petlund <apetlund@...ula.no>,
Michael Welzl <michawe@....uio.no>,
Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
Subject: Re: [PATCH 1/1] net: tcp: RTO restart
On Tue, Feb 18, 2014 at 10:12 AM, Per Hurtig <per.hurtig@....se> wrote:
> This patch implements the RTO restart modification described in
> http://tools.ietf.org/html/draft-ietf-tcpm-rtorestart-02
>
> RTO Restart's goal is to provide quicker loss recovery for segments lost in the
> end of a burst/connection. To accomplish this the algorithm adjusts the RTO
> value on each rearm of the retransmission timer to be exactly RTO ms after the
> earliest outstanding segment was sent. The offsetting against the earliest
> outstanding segment is not done by the regular rearm algorithm, which causes
> RTOs to occur, on average, after RTO+RTT ms.
>
> As a faster timeout is only beneficial in scenarios where fast retransmit/early
> retransmit cannot be triggered the algorithm will only kick in when there is a
> small amount of outstanding segments.
(repost in plaine-text, sorry for the duplication)
I am not sure this works well with Linux min-RTO=200ms, and don't feel
comfortable this is default on without some real experiments.
I've implemented (a nearly identical version of) rto-restart on Google
web servers and found #timeouts increased by ~30%. Although the fast
timeout helps really short flows, it has a very negative side-effect:
resetting cwnd to 1. Thus the next HTTP response may start with a
small cwnd. In contrast, TCP loss probe has fast timeout (1.5RTT) to
do fast recovery to avoid the side-effect. In other words, I am
doubtful we need rto-restart with TCP loss probe, but applying
RTO-restart on TLP timer may be useful.
I've voiced this concern multiple times in ietf tcpm discussion when
we discuss this draft: that the idea itself is fine, but we'll need to
change Linux RTO algorithm together, not just the timer itself.
I am happy to post some more detailed data if people are interested.
>
> The RTO Restart proposal is accepted as a working group item in the IETF TCP
> Maintenance and Minor Extensions (tcpm) TCP wg and is intended for experimental
> RFC status.
>
> Signed-off-by: Per Hurtig <per.hurtig@....se>
> ---
> include/net/tcp.h | 1 +
> net/ipv4/sysctl_net_ipv4.c | 7 +++++++
> net/ipv4/tcp_input.c | 11 +++++++++++
> 3 files changed, 19 insertions(+)
>
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index 56fc366..575e82a 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -278,6 +278,7 @@ extern int sysctl_tcp_slow_start_after_idle;
> extern int sysctl_tcp_thin_linear_timeouts;
> extern int sysctl_tcp_thin_dupack;
> extern int sysctl_tcp_early_retrans;
> +extern int sysctl_tcp_rto_restart;
> extern int sysctl_tcp_limit_output_bytes;
> extern int sysctl_tcp_challenge_ack_limit;
> extern unsigned int sysctl_tcp_notsent_lowat;
> diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
> index 44eba05..c605f4f 100644
> --- a/net/ipv4/sysctl_net_ipv4.c
> +++ b/net/ipv4/sysctl_net_ipv4.c
> @@ -717,6 +717,13 @@ static struct ctl_table ipv4_table[] = {
> .extra2 = &four,
> },
> {
> + .procname = "tcp_rto_restart",
> + .data = &sysctl_tcp_rto_restart,
> + .maxlen = sizeof(int),
> + .mode = 0644,
> + .proc_handler = proc_dointvec,
> + },
> + {
> .procname = "tcp_min_tso_segs",
> .data = &sysctl_tcp_min_tso_segs,
> .maxlen = sizeof(int),
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 227cba7..450ee30 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -98,6 +98,7 @@ int sysctl_tcp_thin_dupack __read_mostly;
>
> int sysctl_tcp_moderate_rcvbuf __read_mostly = 1;
> int sysctl_tcp_early_retrans __read_mostly = 3;
> +int sysctl_tcp_rto_restart __read_mostly = 1;
>
> #define FLAG_DATA 0x01 /* Incoming frame contained data. */
> #define FLAG_WIN_UPDATE 0x02 /* Incoming ACK was a window update. */
> @@ -2972,6 +2973,16 @@ void tcp_rearm_rto(struct sock *sk)
> */
> if (delta > 0)
> rto = delta;
> + } else if (icsk->icsk_pending == ICSK_TIME_RETRANS &&
> + sysctl_tcp_rto_restart &&
> + sk->sk_send_head == NULL &&
> + tp->packets_out < 4) {
> + struct sk_buff *skb = tcp_write_queue_head(sk);
> + const u32 rto_time_stamp = TCP_SKB_CB(skb)->when;
> + s32 delta = (s32)(tcp_time_stamp - rto_time_stamp);
> +
> + if (delta > 0)
> + rto -= delta;
> }
> inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS, rto,
> TCP_RTO_MAX);
> --
> 1.7.9.5
>
> --
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