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Release 4.14 net/ipv4/tcp_rate.c

Directory: net/ipv4
#include <net/tcp.h>

/* The bandwidth estimator estimates the rate at which the network
 * can currently deliver outbound data packets for this flow. At a high
 * level, it operates by taking a delivery rate sample for each ACK.
 * A rate sample records the rate at which the network delivered packets
 * for this flow, calculated over the time interval between the transmission
 * of a data packet and the acknowledgment of that packet.
 * Specifically, over the interval between each transmit and corresponding ACK,
 * the estimator generates a delivery rate sample. Typically it uses the rate
 * at which packets were acknowledged. However, the approach of using only the
 * acknowledgment rate faces a challenge under the prevalent ACK decimation or
 * compression: packets can temporarily appear to be delivered much quicker
 * than the bottleneck rate. Since it is physically impossible to do that in a
 * sustained fashion, when the estimator notices that the ACK rate is faster
 * than the transmit rate, it uses the latter:
 *    send_rate = #pkts_delivered/(last_snd_time - first_snd_time)
 *    ack_rate  = #pkts_delivered/(last_ack_time - first_ack_time)
 *    bw = min(send_rate, ack_rate)
 * Notice the estimator essentially estimates the goodput, not always the
 * network bottleneck link rate when the sending or receiving is limited by
 * other factors like applications or receiver window limits.  The estimator
 * deliberately avoids using the inter-packet spacing approach because that
 * approach requires a large number of samples and sophisticated filtering.
 * TCP flows can often be application-limited in request/response workloads.
 * The estimator marks a bandwidth sample as application-limited if there
 * was some moment during the sampled window of packets when there was no data
 * ready to send in the write queue.

/* Snapshot the current delivery information in the skb, to generate
 * a rate sample later when the skb is (s)acked in tcp_rate_skb_delivered().

void tcp_rate_skb_sent(struct sock *sk, struct sk_buff *skb) { struct tcp_sock *tp = tcp_sk(sk); /* In general we need to start delivery rate samples from the * time we received the most recent ACK, to ensure we include * the full time the network needs to deliver all in-flight * packets. If there are no packets in flight yet, then we * know that any ACKs after now indicate that the network was * able to deliver those packets completely in the sampling * interval between now and the next ACK. * * Note that we use packets_out instead of tcp_packets_in_flight(tp) * because the latter is a guess based on RTO and loss-marking * heuristics. We don't want spurious RTOs or loss markings to cause * a spuriously small time interval, causing a spuriously high * bandwidth estimate. */ if (!tp->packets_out) { tp->first_tx_mstamp = skb->skb_mstamp; tp->delivered_mstamp = skb->skb_mstamp; } TCP_SKB_CB(skb)->tx.first_tx_mstamp = tp->first_tx_mstamp; TCP_SKB_CB(skb)->tx.delivered_mstamp = tp->delivered_mstamp; TCP_SKB_CB(skb)->tx.delivered = tp->delivered; TCP_SKB_CB(skb)->tx.is_app_limited = tp->app_limited ? 1 : 0; }


Yuchung Cheng9084.11%150.00%
Soheil Hassas Yeganeh1715.89%150.00%

/* When an skb is sacked or acked, we fill in the rate sample with the (prior) * delivery information when the skb was last transmitted. * * If an ACK (s)acks multiple skbs (e.g., stretched-acks), this function is * called multiple times. We favor the information from the most recently * sent skb, i.e., the skb with the highest prior_delivered count. */
void tcp_rate_skb_delivered(struct sock *sk, struct sk_buff *skb, struct rate_sample *rs) { struct tcp_sock *tp = tcp_sk(sk); struct tcp_skb_cb *scb = TCP_SKB_CB(skb); if (!scb->tx.delivered_mstamp) return; if (!rs->prior_delivered || after(scb->tx.delivered, rs->prior_delivered)) { rs->prior_delivered = scb->tx.delivered; rs->prior_mstamp = scb->tx.delivered_mstamp; rs->is_app_limited = scb->tx.is_app_limited; rs->is_retrans = scb->sacked & TCPCB_RETRANS; /* Find the duration of the "send phase" of this window: */ rs->interval_us = tcp_stamp_us_delta( skb->skb_mstamp, scb->tx.first_tx_mstamp); /* Record send time of most recently ACKed packet: */ tp->first_tx_mstamp = skb->skb_mstamp; } /* Mark off the skb delivered once it's sacked to avoid being * used again when it's cumulatively acked. For acked packets * we don't need to reset since it'll be freed soon. */ if (scb->sacked & TCPCB_SACKED_ACKED) scb->tx.delivered_mstamp = 0; }


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Soheil Hassas Yeganeh106.41%133.33%
Eric Dumazet10.64%133.33%

/* Update the connection delivery information and generate a rate sample. */
void tcp_rate_gen(struct sock *sk, u32 delivered, u32 lost, struct rate_sample *rs) { struct tcp_sock *tp = tcp_sk(sk); u32 snd_us, ack_us; /* Clear app limited if bubble is acked and gone. */ if (tp->app_limited && after(tp->delivered, tp->app_limited)) tp->app_limited = 0; /* TODO: there are multiple places throughout tcp_ack() to get * current time. Refactor the code using a new "tcp_acktag_state" * to carry current time, flags, stats like "tcp_sacktag_state". */ if (delivered) tp->delivered_mstamp = tp->tcp_mstamp; rs->acked_sacked = delivered; /* freshly ACKed or SACKed */ rs->losses = lost; /* freshly marked lost */ /* Return an invalid sample if no timing information is available. */ if (!rs->prior_mstamp) { rs->delivered = -1; rs->interval_us = -1; return; } rs->delivered = tp->delivered - rs->prior_delivered; /* Model sending data and receiving ACKs as separate pipeline phases * for a window. Usually the ACK phase is longer, but with ACK * compression the send phase can be longer. To be safe we use the * longer phase. */ snd_us = rs->interval_us; /* send phase */ ack_us = tcp_stamp_us_delta(tp->tcp_mstamp, rs->prior_mstamp); /* ack phase */ rs->interval_us = max(snd_us, ack_us); /* Normally we expect interval_us >= min-rtt. * Note that rate may still be over-estimated when a spuriously * retransmistted skb was first (s)acked because "interval_us" * is under-estimated (up to an RTT). However continuously * measuring the delivery rate during loss recovery is crucial * for connections suffer heavy or prolonged losses. */ if (unlikely(rs->interval_us < tcp_min_rtt(tp))) { if (!rs->is_retrans) pr_debug("tcp rate: %ld %d %u %u %u\n", rs->interval_us, rs->delivered, inet_csk(sk)->icsk_ca_state, tp->rx_opt.sack_ok, tcp_min_rtt(tp)); rs->interval_us = -1; return; } /* Record the last non-app-limited or the highest app-limited bw */ if (!rs->is_app_limited || ((u64)rs->delivered * tp->rate_interval_us >= (u64)tp->rate_delivered * rs->interval_us)) { tp->rate_delivered = rs->delivered; tp->rate_interval_us = rs->interval_us; tp->rate_app_limited = rs->is_app_limited; } }


Yuchung Cheng24788.85%240.00%
Soheil Hassas Yeganeh248.63%120.00%
Eric Dumazet72.52%240.00%

/* If a gap is detected between sends, mark the socket application-limited. */
void tcp_rate_check_app_limited(struct sock *sk) { struct tcp_sock *tp = tcp_sk(sk); if (/* We have less than one packet to send. */ tp->write_seq - tp->snd_nxt < tp->mss_cache && /* Nothing in sending host's qdisc queues or NIC tx queue. */ sk_wmem_alloc_get(sk) < SKB_TRUESIZE(1) && /* We are not limited by CWND. */ tcp_packets_in_flight(tp) < tp->snd_cwnd && /* All lost packets have been retransmitted. */ tp->lost_out <= tp->retrans_out) tp->app_limited = (tp->delivered + tcp_packets_in_flight(tp)) ? : 1; }


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Overall Contributors

Yuchung Cheng48876.61%233.33%
Soheil Hassas Yeganeh13621.35%116.67%
Eric Dumazet81.26%233.33%
Dave Watson50.78%116.67%
Directory: net/ipv4
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