dccp ccid-3: A lower bound for the inter-packet scheduling algorithm
This fixes a subtle bug in the calculation of the inter-packet gap and shows that t_delta, as it is currently used, is not needed. The algorithm from RFC 5348, 8.3 below continually computes a send time t_nom, which is initialised with the current time t_now; t_gran = 1E6 / HZ specifies the scheduling granularity, s the packet size, and X the sending rate: t_distance = t_nom - t_now; // in microseconds t_delta = min(t_ipi, t_gran) / 2; // `delta' parameter in microseconds if (t_distance >= t_delta) { reschedule after (t_distance / 1000) milliseconds; } else { t_ipi = s / X; // inter-packet interval in usec t_nom += t_ipi; // compute the next send time send packet now; } Problem: -------- Rescheduling requires a conversion into milliseconds (sk_reset_timer()). The highest jiffy resolution with HZ=1000 is 1 millisecond, so using a higher granularity does not make much sense here. As a consequence, values of t_distance < 1000 are truncated to 0. This issue has so far been resolved by using instead if (t_distance >= t_delta + 1000) reschedule after (t_distance / 1000) milliseconds; This is unnecessarily large, a lower bound is t_delta' = max(t_delta, 1000). And it implies a further simplification: a) when HZ >= 500, then t_delta <= t_gran/2 = 10^6/(2*HZ) <= 1000, so that t_delta' = MAX(1000, t_delta) = 1000 (constant value); b) when HZ < 500, then t_delta = 1/2*MIN(rtt, t_ipi, t_gran) <= t_gran/2, so that 1000 <= t_delta' <= t_gran/2. The maximum error of using a constant t_delta in (b) is less than half a jiffy. Fix: ---- The patch replaces t_delta with a constant, whose value depends on CONFIG_HZ, changing the above algorithm to: if (t_distance >= t_delta') reschedule after (t_distance / 1000) milliseconds; where t_delta' = 10^6/(2*HZ) if HZ < 500, and t_delta' = 1000 otherwise. Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
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Родитель
068e8a3032
Коммит
20cbd3e120
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@ -91,19 +91,16 @@ static inline u64 rfc3390_initial_rate(struct sock *sk)
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return scaled_div(w_init << 6, hc->tx_rtt);
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}
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/*
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* Recalculate t_ipi and delta (should be called whenever X changes)
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/**
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* ccid3_update_send_interval - Calculate new t_ipi = s / X_inst
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* This respects the granularity of X_inst (64 * bytes/second).
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*/
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static void ccid3_update_send_interval(struct ccid3_hc_tx_sock *hc)
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{
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/* Calculate new t_ipi = s / X_inst (X_inst is in 64 * bytes/second) */
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hc->tx_t_ipi = scaled_div32(((u64)hc->tx_s) << 6, hc->tx_x);
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/* Calculate new delta by delta = min(t_ipi / 2, t_gran / 2) */
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hc->tx_delta = min_t(u32, hc->tx_t_ipi / 2, TFRC_OPSYS_HALF_TIME_GRAN);
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ccid3_pr_debug("t_ipi=%u, delta=%u, s=%u, X=%u\n", hc->tx_t_ipi,
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hc->tx_delta, hc->tx_s, (unsigned)(hc->tx_x >> 6));
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ccid3_pr_debug("t_ipi=%u, s=%u, X=%u\n", hc->tx_t_ipi,
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hc->tx_s, (unsigned)(hc->tx_x >> 6));
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}
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static u32 ccid3_hc_tx_idle_rtt(struct ccid3_hc_tx_sock *hc, ktime_t now)
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@ -332,15 +329,15 @@ static int ccid3_hc_tx_send_packet(struct sock *sk, struct sk_buff *skb)
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delay = ktime_us_delta(hc->tx_t_nom, now);
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ccid3_pr_debug("delay=%ld\n", (long)delay);
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/*
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* Scheduling of packet transmissions [RFC 3448, 4.6]
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* Scheduling of packet transmissions (RFC 5348, 8.3)
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*
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* if (t_now > t_nom - delta)
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* // send the packet now
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* else
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* // send the packet in (t_nom - t_now) milliseconds.
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*/
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if (delay - (s64)hc->tx_delta >= 1000)
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return (u32)delay / 1000L;
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if (delay >= TFRC_T_DELTA)
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return (u32)delay / USEC_PER_MSEC;
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ccid3_hc_tx_update_win_count(hc, now);
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break;
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@ -45,12 +45,22 @@
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/* Two seconds as per RFC 5348, 4.2 */
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#define TFRC_INITIAL_TIMEOUT (2 * USEC_PER_SEC)
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/* In usecs - half the scheduling granularity as per RFC3448 4.6 */
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#define TFRC_OPSYS_HALF_TIME_GRAN (USEC_PER_SEC / (2 * HZ))
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/* Parameter t_mbi from [RFC 3448, 4.3]: backoff interval in seconds */
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#define TFRC_T_MBI 64
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/*
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* The t_delta parameter (RFC 5348, 8.3): delays of less than %USEC_PER_MSEC are
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* rounded down to 0, since sk_reset_timer() here uses millisecond granularity.
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* Hence we can use a constant t_delta = %USEC_PER_MSEC when HZ >= 500. A coarse
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* resolution of HZ < 500 means that the error is below one timer tick (t_gran)
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* when using the constant t_delta = t_gran / 2 = %USEC_PER_SEC / (2 * HZ).
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*/
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#if (HZ >= 500)
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# define TFRC_T_DELTA USEC_PER_MSEC
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#else
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# define TFRC_T_DELTA (USEC_PER_SEC / (2 * HZ))
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#endif
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enum ccid3_options {
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TFRC_OPT_LOSS_EVENT_RATE = 192,
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TFRC_OPT_LOSS_INTERVALS = 193,
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@ -90,7 +100,6 @@ enum ccid3_hc_tx_states {
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* @tx_no_feedback_timer: Handle to no feedback timer
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* @tx_t_ld: Time last doubled during slow start
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* @tx_t_nom: Nominal send time of next packet
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* @tx_delta: Send timer delta (RFC 3448, 4.6) in usecs
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* @tx_hist: Packet history
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* @tx_options_received: Parsed set of retrieved options
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*/
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@ -109,7 +118,6 @@ struct ccid3_hc_tx_sock {
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struct timer_list tx_no_feedback_timer;
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ktime_t tx_t_ld;
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ktime_t tx_t_nom;
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u32 tx_delta;
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struct tfrc_tx_hist_entry *tx_hist;
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struct ccid3_options_received tx_options_received;
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};
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