| Message ID | 1391731030.10160.36.camel@edumazet-glaptop2.roam.corp.google.com |
|---|---|
| State | Accepted, archived |
| Delegated to: | David Miller |
| Headers | show |
On Thu, Feb 6, 2014 at 6:57 PM, Eric Dumazet <eric.dumazet@gmail.com> wrote: > From: Eric Dumazet <edumazet@google.com> > > TCP pacing depends on an accurate srtt estimation. > > Current srtt estimation is using jiffie resolution, > and has an artificial offset of at least 1 ms, which can produce > slowdowns when FQ/pacing is used, especially in DC world, > where typical rtt is below 1 ms. > > We are planning a switch to usec resolution for linux-3.15, > but in the meantime, this patch removes the 1 ms offset. > > All we need is to have tp->srtt minimal value of 1 to differentiate > the case of srtt being initialized or not, not 8. > > The problematic behavior was observed on a 40Gbit testbed, > where 32 concurrent netperf were reaching 12Gbps of aggregate > speed, instead of line speed. > > This patch also has the effect of reporting more accurate srtt and send > rates to iproute2 ss command as in : > > $ ss -i dst cca2 > Netid State Recv-Q Send-Q Local Address:Port > Peer Address:Port > tcp ESTAB 0 0 10.244.129.1:56984 > 10.244.129.2:12865 > cubic wscale:6,6 rto:200 rtt:0.25/0.25 ato:40 mss:1448 cwnd:10 send > 463.4Mbps rcv_rtt:1 rcv_space:29200 > tcp ESTAB 0 390960 10.244.129.1:60247 > 10.244.129.2:50204 > cubic wscale:6,6 rto:200 rtt:0.875/0.75 mss:1448 cwnd:73 ssthresh:51 > send 966.4Mbps unacked:73 retrans:0/121 rcv_space:29200 > > Reported-by: Vytautas Valancius <valas@google.com> > Signed-off-by: Eric Dumazet <edumazet@google.com> > Cc: Yuchung Cheng <ycheng@google.com> > Cc: Neal Cardwell <ncardwell@google.com> > --- > net/ipv4/tcp_input.c | 18 ++++++++++-------- > net/ipv4/tcp_output.c | 2 +- > 2 files changed, 11 insertions(+), 9 deletions(-) Acked-by: Neal Cardwell <ncardwell@google.com> Thanks, Eric! 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: Eric Dumazet <eric.dumazet@gmail.com> Date: Thu, 06 Feb 2014 15:57:10 -0800 > From: Eric Dumazet <edumazet@google.com> > > TCP pacing depends on an accurate srtt estimation. > > Current srtt estimation is using jiffie resolution, > and has an artificial offset of at least 1 ms, which can produce > slowdowns when FQ/pacing is used, especially in DC world, > where typical rtt is below 1 ms. > > We are planning a switch to usec resolution for linux-3.15, > but in the meantime, this patch removes the 1 ms offset. > > All we need is to have tp->srtt minimal value of 1 to differentiate > the case of srtt being initialized or not, not 8. > > The problematic behavior was observed on a 40Gbit testbed, > where 32 concurrent netperf were reaching 12Gbps of aggregate > speed, instead of line speed. > > This patch also has the effect of reporting more accurate srtt and send > rates to iproute2 ss command as in : > > $ ss -i dst cca2 > Netid State Recv-Q Send-Q Local Address:Port > Peer Address:Port > tcp ESTAB 0 0 10.244.129.1:56984 > 10.244.129.2:12865 > cubic wscale:6,6 rto:200 rtt:0.25/0.25 ato:40 mss:1448 cwnd:10 send > 463.4Mbps rcv_rtt:1 rcv_space:29200 > tcp ESTAB 0 390960 10.244.129.1:60247 > 10.244.129.2:50204 > cubic wscale:6,6 rto:200 rtt:0.875/0.75 mss:1448 cwnd:73 ssthresh:51 > send 966.4Mbps unacked:73 retrans:0/121 rcv_space:29200 > > Reported-by: Vytautas Valancius <valas@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
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 65cf90e063d5..227cba79fa6b 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -671,6 +671,7 @@ static void tcp_rtt_estimator(struct sock *sk, const __u32 mrtt) { struct tcp_sock *tp = tcp_sk(sk); long m = mrtt; /* RTT */ + u32 srtt = tp->srtt; /* The following amusing code comes from Jacobson's * article in SIGCOMM '88. Note that rtt and mdev @@ -688,11 +689,9 @@ static void tcp_rtt_estimator(struct sock *sk, const __u32 mrtt) * does not matter how to _calculate_ it. Seems, it was trap * that VJ failed to avoid. 8) */ - if (m == 0) - m = 1; - if (tp->srtt != 0) { - m -= (tp->srtt >> 3); /* m is now error in rtt est */ - tp->srtt += m; /* rtt = 7/8 rtt + 1/8 new */ + if (srtt != 0) { + m -= (srtt >> 3); /* m is now error in rtt est */ + srtt += m; /* rtt = 7/8 rtt + 1/8 new */ if (m < 0) { m = -m; /* m is now abs(error) */ m -= (tp->mdev >> 2); /* similar update on mdev */ @@ -723,11 +722,12 @@ static void tcp_rtt_estimator(struct sock *sk, const __u32 mrtt) } } else { /* no previous measure. */ - tp->srtt = m << 3; /* take the measured time to be rtt */ + srtt = m << 3; /* take the measured time to be rtt */ tp->mdev = m << 1; /* make sure rto = 3*rtt */ tp->mdev_max = tp->rttvar = max(tp->mdev, tcp_rto_min(sk)); tp->rtt_seq = tp->snd_nxt; } + tp->srtt = max(1U, srtt); } /* Set the sk_pacing_rate to allow proper sizing of TSO packets. @@ -746,8 +746,10 @@ static void tcp_update_pacing_rate(struct sock *sk) rate *= max(tp->snd_cwnd, tp->packets_out); - /* Correction for small srtt : minimum srtt being 8 (1 jiffy << 3), - * be conservative and assume srtt = 1 (125 us instead of 1.25 ms) + /* Correction for small srtt and scheduling constraints. + * For small rtt, consider noise is too high, and use + * the minimal value (srtt = 1 -> 125 us for HZ=1000) + * * We probably need usec resolution in the future. * Note: This also takes care of possible srtt=0 case, * when tcp_rtt_estimator() was not yet called. diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c index 03d26b85eab8..10435b3b9d0f 100644 --- a/net/ipv4/tcp_output.c +++ b/net/ipv4/tcp_output.c @@ -1977,7 +1977,7 @@ bool tcp_schedule_loss_probe(struct sock *sk) /* Schedule a loss probe in 2*RTT for SACK capable connections * in Open state, that are either limited by cwnd or application. */ - if (sysctl_tcp_early_retrans < 3 || !rtt || !tp->packets_out || + if (sysctl_tcp_early_retrans < 3 || !tp->srtt || !tp->packets_out || !tcp_is_sack(tp) || inet_csk(sk)->icsk_ca_state != TCP_CA_Open) return false;