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![Yuchung Cheng](/assets/img/avatar_default.png)
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>
315 lines
7.4 KiB
C
315 lines
7.4 KiB
C
/*
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* H-TCP congestion control. The algorithm is detailed in:
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* R.N.Shorten, D.J.Leith:
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* "H-TCP: TCP for high-speed and long-distance networks"
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* Proc. PFLDnet, Argonne, 2004.
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* http://www.hamilton.ie/net/htcp3.pdf
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*/
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#include <linux/mm.h>
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#include <linux/module.h>
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#include <net/tcp.h>
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#define ALPHA_BASE (1<<7) /* 1.0 with shift << 7 */
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#define BETA_MIN (1<<6) /* 0.5 with shift << 7 */
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#define BETA_MAX 102 /* 0.8 with shift << 7 */
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static int use_rtt_scaling __read_mostly = 1;
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module_param(use_rtt_scaling, int, 0644);
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MODULE_PARM_DESC(use_rtt_scaling, "turn on/off RTT scaling");
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static int use_bandwidth_switch __read_mostly = 1;
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module_param(use_bandwidth_switch, int, 0644);
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MODULE_PARM_DESC(use_bandwidth_switch, "turn on/off bandwidth switcher");
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struct htcp {
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u32 alpha; /* Fixed point arith, << 7 */
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u8 beta; /* Fixed point arith, << 7 */
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u8 modeswitch; /* Delay modeswitch
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until we had at least one congestion event */
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u16 pkts_acked;
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u32 packetcount;
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u32 minRTT;
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u32 maxRTT;
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u32 last_cong; /* Time since last congestion event end */
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u32 undo_last_cong;
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u32 undo_maxRTT;
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u32 undo_old_maxB;
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/* Bandwidth estimation */
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u32 minB;
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u32 maxB;
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u32 old_maxB;
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u32 Bi;
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u32 lasttime;
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};
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static inline u32 htcp_cong_time(const struct htcp *ca)
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{
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return jiffies - ca->last_cong;
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}
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static inline u32 htcp_ccount(const struct htcp *ca)
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{
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return htcp_cong_time(ca) / ca->minRTT;
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}
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static inline void htcp_reset(struct htcp *ca)
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{
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ca->undo_last_cong = ca->last_cong;
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ca->undo_maxRTT = ca->maxRTT;
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ca->undo_old_maxB = ca->old_maxB;
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ca->last_cong = jiffies;
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}
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static u32 htcp_cwnd_undo(struct sock *sk)
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{
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const struct tcp_sock *tp = tcp_sk(sk);
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struct htcp *ca = inet_csk_ca(sk);
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if (ca->undo_last_cong) {
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ca->last_cong = ca->undo_last_cong;
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ca->maxRTT = ca->undo_maxRTT;
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ca->old_maxB = ca->undo_old_maxB;
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ca->undo_last_cong = 0;
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}
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return max(tp->snd_cwnd, (tp->snd_ssthresh << 7) / ca->beta);
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}
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static inline void measure_rtt(struct sock *sk, u32 srtt)
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{
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const struct inet_connection_sock *icsk = inet_csk(sk);
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struct htcp *ca = inet_csk_ca(sk);
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/* keep track of minimum RTT seen so far, minRTT is zero at first */
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if (ca->minRTT > srtt || !ca->minRTT)
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ca->minRTT = srtt;
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/* max RTT */
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if (icsk->icsk_ca_state == TCP_CA_Open) {
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if (ca->maxRTT < ca->minRTT)
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ca->maxRTT = ca->minRTT;
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if (ca->maxRTT < srtt &&
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srtt <= ca->maxRTT + msecs_to_jiffies(20))
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ca->maxRTT = srtt;
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}
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}
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static void measure_achieved_throughput(struct sock *sk, u32 pkts_acked, s32 rtt)
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{
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const struct inet_connection_sock *icsk = inet_csk(sk);
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const struct tcp_sock *tp = tcp_sk(sk);
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struct htcp *ca = inet_csk_ca(sk);
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u32 now = tcp_time_stamp;
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if (icsk->icsk_ca_state == TCP_CA_Open)
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ca->pkts_acked = pkts_acked;
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if (rtt > 0)
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measure_rtt(sk, usecs_to_jiffies(rtt));
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if (!use_bandwidth_switch)
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return;
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/* achieved throughput calculations */
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if (!((1 << icsk->icsk_ca_state) & (TCPF_CA_Open | TCPF_CA_Disorder))) {
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ca->packetcount = 0;
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ca->lasttime = now;
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return;
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}
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ca->packetcount += pkts_acked;
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if (ca->packetcount >= tp->snd_cwnd - (ca->alpha >> 7 ? : 1) &&
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now - ca->lasttime >= ca->minRTT &&
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ca->minRTT > 0) {
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__u32 cur_Bi = ca->packetcount * HZ / (now - ca->lasttime);
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if (htcp_ccount(ca) <= 3) {
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/* just after backoff */
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ca->minB = ca->maxB = ca->Bi = cur_Bi;
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} else {
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ca->Bi = (3 * ca->Bi + cur_Bi) / 4;
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if (ca->Bi > ca->maxB)
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ca->maxB = ca->Bi;
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if (ca->minB > ca->maxB)
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ca->minB = ca->maxB;
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}
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ca->packetcount = 0;
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ca->lasttime = now;
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}
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}
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static inline void htcp_beta_update(struct htcp *ca, u32 minRTT, u32 maxRTT)
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{
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if (use_bandwidth_switch) {
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u32 maxB = ca->maxB;
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u32 old_maxB = ca->old_maxB;
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ca->old_maxB = ca->maxB;
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if (!between(5 * maxB, 4 * old_maxB, 6 * old_maxB)) {
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ca->beta = BETA_MIN;
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ca->modeswitch = 0;
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return;
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}
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}
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if (ca->modeswitch && minRTT > msecs_to_jiffies(10) && maxRTT) {
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ca->beta = (minRTT << 7) / maxRTT;
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if (ca->beta < BETA_MIN)
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ca->beta = BETA_MIN;
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else if (ca->beta > BETA_MAX)
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ca->beta = BETA_MAX;
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} else {
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ca->beta = BETA_MIN;
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ca->modeswitch = 1;
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}
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}
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static inline void htcp_alpha_update(struct htcp *ca)
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{
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u32 minRTT = ca->minRTT;
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u32 factor = 1;
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u32 diff = htcp_cong_time(ca);
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if (diff > HZ) {
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diff -= HZ;
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factor = 1 + (10 * diff + ((diff / 2) * (diff / 2) / HZ)) / HZ;
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}
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if (use_rtt_scaling && minRTT) {
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u32 scale = (HZ << 3) / (10 * minRTT);
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/* clamping ratio to interval [0.5,10]<<3 */
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scale = min(max(scale, 1U << 2), 10U << 3);
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factor = (factor << 3) / scale;
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if (!factor)
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factor = 1;
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}
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ca->alpha = 2 * factor * ((1 << 7) - ca->beta);
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if (!ca->alpha)
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ca->alpha = ALPHA_BASE;
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}
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/*
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* After we have the rtt data to calculate beta, we'd still prefer to wait one
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* rtt before we adjust our beta to ensure we are working from a consistent
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* data.
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*
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* This function should be called when we hit a congestion event since only at
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* that point do we really have a real sense of maxRTT (the queues en route
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* were getting just too full now).
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*/
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static void htcp_param_update(struct sock *sk)
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{
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struct htcp *ca = inet_csk_ca(sk);
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u32 minRTT = ca->minRTT;
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u32 maxRTT = ca->maxRTT;
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htcp_beta_update(ca, minRTT, maxRTT);
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htcp_alpha_update(ca);
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/* add slowly fading memory for maxRTT to accommodate routing changes */
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if (minRTT > 0 && maxRTT > minRTT)
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ca->maxRTT = minRTT + ((maxRTT - minRTT) * 95) / 100;
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}
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static u32 htcp_recalc_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 htcp *ca = inet_csk_ca(sk);
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htcp_param_update(sk);
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return max((tp->snd_cwnd * ca->beta) >> 7, 2U);
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}
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static void htcp_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 htcp *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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/* In dangerous area, increase slowly.
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* In theory this is tp->snd_cwnd += alpha / tp->snd_cwnd
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*/
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if ((tp->snd_cwnd_cnt * ca->alpha)>>7 >= tp->snd_cwnd) {
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if (tp->snd_cwnd < tp->snd_cwnd_clamp)
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tp->snd_cwnd++;
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tp->snd_cwnd_cnt = 0;
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htcp_alpha_update(ca);
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} else
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tp->snd_cwnd_cnt += ca->pkts_acked;
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ca->pkts_acked = 1;
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}
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}
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static void htcp_init(struct sock *sk)
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{
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struct htcp *ca = inet_csk_ca(sk);
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memset(ca, 0, sizeof(struct htcp));
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ca->alpha = ALPHA_BASE;
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ca->beta = BETA_MIN;
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ca->pkts_acked = 1;
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ca->last_cong = jiffies;
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}
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static void htcp_state(struct sock *sk, u8 new_state)
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{
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switch (new_state) {
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case TCP_CA_Open:
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{
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struct htcp *ca = inet_csk_ca(sk);
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if (ca->undo_last_cong) {
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ca->last_cong = jiffies;
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ca->undo_last_cong = 0;
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}
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}
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break;
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case TCP_CA_CWR:
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case TCP_CA_Recovery:
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case TCP_CA_Loss:
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htcp_reset(inet_csk_ca(sk));
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break;
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}
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}
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static struct tcp_congestion_ops htcp __read_mostly = {
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.init = htcp_init,
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.ssthresh = htcp_recalc_ssthresh,
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.cong_avoid = htcp_cong_avoid,
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.set_state = htcp_state,
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.undo_cwnd = htcp_cwnd_undo,
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.pkts_acked = measure_achieved_throughput,
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.owner = THIS_MODULE,
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.name = "htcp",
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};
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static int __init htcp_register(void)
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{
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BUILD_BUG_ON(sizeof(struct htcp) > ICSK_CA_PRIV_SIZE);
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BUILD_BUG_ON(BETA_MIN >= BETA_MAX);
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return tcp_register_congestion_control(&htcp);
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}
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static void __exit htcp_unregister(void)
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{
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tcp_unregister_congestion_control(&htcp);
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}
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module_init(htcp_register);
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module_exit(htcp_unregister);
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MODULE_AUTHOR("Baruch Even");
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MODULE_LICENSE("GPL");
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MODULE_DESCRIPTION("H-TCP");
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