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9f9843a751
Slow start now increases cwnd by 1 if an ACK acknowledges some packets, regardless the number of packets. Consequently slow start performance is highly dependent on the degree of the stretch ACKs caused by receiver or network ACK compression mechanisms (e.g., delayed-ACK, GRO, etc). But slow start algorithm is to send twice the amount of packets of packets left so it should process a stretch ACK of degree N as if N ACKs of degree 1, then exits when cwnd exceeds ssthresh. A follow up patch will use the remainder of the N (if greater than 1) to adjust cwnd in the congestion avoidance phase. In addition this patch retires the experimental limited slow start (LSS) feature. LSS has multiple drawbacks but questionable benefit. The fractional cwnd increase in LSS requires a loop in slow start even though it's rarely used. Configuring such an increase step via a global sysctl on different BDPS seems hard. Finally and most importantly the slow start overshoot concern is now better covered by the Hybrid slow start (hystart) enabled by default. Signed-off-by: Yuchung Cheng <ycheng@google.com> Signed-off-by: Neal Cardwell <ncardwell@google.com> Signed-off-by: Eric Dumazet <edumazet@google.com> Signed-off-by: David S. Miller <davem@davemloft.net>
188 lines
4.9 KiB
C
188 lines
4.9 KiB
C
/*
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* Sally Floyd's High Speed TCP (RFC 3649) congestion control
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*
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* See http://www.icir.org/floyd/hstcp.html
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*
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* John Heffner <jheffner@psc.edu>
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*/
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#include <linux/module.h>
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#include <net/tcp.h>
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/* From AIMD tables from RFC 3649 appendix B,
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* with fixed-point MD scaled <<8.
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*/
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static const struct hstcp_aimd_val {
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unsigned int cwnd;
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unsigned int md;
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} hstcp_aimd_vals[] = {
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{ 38, 128, /* 0.50 */ },
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{ 118, 112, /* 0.44 */ },
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{ 221, 104, /* 0.41 */ },
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{ 347, 98, /* 0.38 */ },
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{ 495, 93, /* 0.37 */ },
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{ 663, 89, /* 0.35 */ },
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{ 851, 86, /* 0.34 */ },
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{ 1058, 83, /* 0.33 */ },
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{ 1284, 81, /* 0.32 */ },
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{ 1529, 78, /* 0.31 */ },
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{ 1793, 76, /* 0.30 */ },
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{ 2076, 74, /* 0.29 */ },
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{ 2378, 72, /* 0.28 */ },
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{ 2699, 71, /* 0.28 */ },
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{ 3039, 69, /* 0.27 */ },
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{ 3399, 68, /* 0.27 */ },
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{ 3778, 66, /* 0.26 */ },
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{ 4177, 65, /* 0.26 */ },
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{ 4596, 64, /* 0.25 */ },
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{ 5036, 62, /* 0.25 */ },
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{ 5497, 61, /* 0.24 */ },
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{ 5979, 60, /* 0.24 */ },
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{ 6483, 59, /* 0.23 */ },
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{ 7009, 58, /* 0.23 */ },
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{ 7558, 57, /* 0.22 */ },
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{ 8130, 56, /* 0.22 */ },
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{ 8726, 55, /* 0.22 */ },
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{ 9346, 54, /* 0.21 */ },
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{ 9991, 53, /* 0.21 */ },
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{ 10661, 52, /* 0.21 */ },
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{ 11358, 52, /* 0.20 */ },
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{ 12082, 51, /* 0.20 */ },
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{ 12834, 50, /* 0.20 */ },
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{ 13614, 49, /* 0.19 */ },
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{ 14424, 48, /* 0.19 */ },
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{ 15265, 48, /* 0.19 */ },
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{ 16137, 47, /* 0.19 */ },
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{ 17042, 46, /* 0.18 */ },
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{ 17981, 45, /* 0.18 */ },
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{ 18955, 45, /* 0.18 */ },
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{ 19965, 44, /* 0.17 */ },
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{ 21013, 43, /* 0.17 */ },
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{ 22101, 43, /* 0.17 */ },
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{ 23230, 42, /* 0.17 */ },
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{ 24402, 41, /* 0.16 */ },
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{ 25618, 41, /* 0.16 */ },
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{ 26881, 40, /* 0.16 */ },
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{ 28193, 39, /* 0.16 */ },
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{ 29557, 39, /* 0.15 */ },
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{ 30975, 38, /* 0.15 */ },
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{ 32450, 38, /* 0.15 */ },
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{ 33986, 37, /* 0.15 */ },
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{ 35586, 36, /* 0.14 */ },
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{ 37253, 36, /* 0.14 */ },
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{ 38992, 35, /* 0.14 */ },
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{ 40808, 35, /* 0.14 */ },
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{ 42707, 34, /* 0.13 */ },
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{ 44694, 33, /* 0.13 */ },
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{ 46776, 33, /* 0.13 */ },
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{ 48961, 32, /* 0.13 */ },
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{ 51258, 32, /* 0.13 */ },
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{ 53677, 31, /* 0.12 */ },
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{ 56230, 30, /* 0.12 */ },
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{ 58932, 30, /* 0.12 */ },
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{ 61799, 29, /* 0.12 */ },
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{ 64851, 28, /* 0.11 */ },
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{ 68113, 28, /* 0.11 */ },
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{ 71617, 27, /* 0.11 */ },
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{ 75401, 26, /* 0.10 */ },
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{ 79517, 26, /* 0.10 */ },
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{ 84035, 25, /* 0.10 */ },
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{ 89053, 24, /* 0.10 */ },
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};
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#define HSTCP_AIMD_MAX ARRAY_SIZE(hstcp_aimd_vals)
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struct hstcp {
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u32 ai;
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};
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static void hstcp_init(struct sock *sk)
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{
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struct tcp_sock *tp = tcp_sk(sk);
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struct hstcp *ca = inet_csk_ca(sk);
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ca->ai = 0;
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/* Ensure the MD arithmetic works. This is somewhat pedantic,
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* since I don't think we will see a cwnd this large. :) */
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tp->snd_cwnd_clamp = min_t(u32, tp->snd_cwnd_clamp, 0xffffffff/128);
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}
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static void hstcp_cong_avoid(struct sock *sk, u32 ack, u32 acked, u32 in_flight)
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{
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struct tcp_sock *tp = tcp_sk(sk);
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struct hstcp *ca = inet_csk_ca(sk);
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if (!tcp_is_cwnd_limited(sk, in_flight))
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return;
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if (tp->snd_cwnd <= tp->snd_ssthresh)
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tcp_slow_start(tp, acked);
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else {
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/* Update AIMD parameters.
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*
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* We want to guarantee that:
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* hstcp_aimd_vals[ca->ai-1].cwnd <
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* snd_cwnd <=
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* hstcp_aimd_vals[ca->ai].cwnd
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*/
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if (tp->snd_cwnd > hstcp_aimd_vals[ca->ai].cwnd) {
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while (tp->snd_cwnd > hstcp_aimd_vals[ca->ai].cwnd &&
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ca->ai < HSTCP_AIMD_MAX - 1)
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ca->ai++;
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} else if (ca->ai && tp->snd_cwnd <= hstcp_aimd_vals[ca->ai-1].cwnd) {
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while (ca->ai && tp->snd_cwnd <= hstcp_aimd_vals[ca->ai-1].cwnd)
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ca->ai--;
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}
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/* Do additive increase */
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if (tp->snd_cwnd < tp->snd_cwnd_clamp) {
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/* cwnd = cwnd + a(w) / cwnd */
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tp->snd_cwnd_cnt += ca->ai + 1;
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if (tp->snd_cwnd_cnt >= tp->snd_cwnd) {
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tp->snd_cwnd_cnt -= tp->snd_cwnd;
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tp->snd_cwnd++;
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}
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}
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}
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}
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static u32 hstcp_ssthresh(struct sock *sk)
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{
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const struct tcp_sock *tp = tcp_sk(sk);
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const struct hstcp *ca = inet_csk_ca(sk);
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/* Do multiplicative decrease */
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return max(tp->snd_cwnd - ((tp->snd_cwnd * hstcp_aimd_vals[ca->ai].md) >> 8), 2U);
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}
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static struct tcp_congestion_ops tcp_highspeed __read_mostly = {
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.init = hstcp_init,
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.ssthresh = hstcp_ssthresh,
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.cong_avoid = hstcp_cong_avoid,
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.min_cwnd = tcp_reno_min_cwnd,
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.owner = THIS_MODULE,
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.name = "highspeed"
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};
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static int __init hstcp_register(void)
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{
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BUILD_BUG_ON(sizeof(struct hstcp) > ICSK_CA_PRIV_SIZE);
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return tcp_register_congestion_control(&tcp_highspeed);
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}
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static void __exit hstcp_unregister(void)
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{
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tcp_unregister_congestion_control(&tcp_highspeed);
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}
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module_init(hstcp_register);
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module_exit(hstcp_unregister);
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MODULE_AUTHOR("John Heffner");
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MODULE_LICENSE("GPL");
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MODULE_DESCRIPTION("High Speed TCP");
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