From b248af5ee14b6910c6b21a29fb8ebe2126a8f68e Mon Sep 17 00:00:00 2001 From: Dimitri Staessens Date: Sat, 19 Mar 2022 21:47:01 +0100 Subject: lib: Non-configurable delayed acks in FRCP It doesn't really make sense to manually and one-sidedly configure the timeout of delayed acknowledgements, as setting it too high upsets the peer's sRTT estimates. Even worse, it also causes a lot of spurious retransmissions if it exceeds the sRTT mean deviation calculated by the receiver. Compensating on bare acknowledgment for the ack delay could improve the RTT estimate deviation, but not the spurious retransmissions if it was set too high. This sets the delayed ack to wait for a single RTT mean deviation. Probably needs more tweaking to further reduce differences between the RTT estimates at the sender and receiver, e.g. compensate the RTT estimate for delayed acks, or increase the RTO to add 8 mdevs to sRTT instead of 4. However, it looks like the mdev estimate is the trickiest one to get to sync, not the RTT average. Linux reduces the sample weight for mdev from 1/4 to 1/32 in some cases, will give that a shot some day too to see if that further align sRTT estimates. In any case, this patch already improves things a lot. Also fixes a bug where the sender was sending acknowlegments on the first packets in flight for the 0 sequence number. The receiver activity was measured in seconds but compared to a timeout value in nanoseconds. There's still a lot of spurious retransmissions that start after actual packet loss occurs, I'm still investigating what causes it. Signed-off-by: Dimitri Staessens Signed-off-by: Sander Vrijders --- src/lib/frct.c | 26 +++++++++++--------------- 1 file changed, 11 insertions(+), 15 deletions(-) (limited to 'src/lib/frct.c') diff --git a/src/lib/frct.c b/src/lib/frct.c index e7193da0..99962868 100644 --- a/src/lib/frct.c +++ b/src/lib/frct.c @@ -279,9 +279,11 @@ static void send_frct_pkt(struct frcti * frcti) assert(frcti); - pthread_rwlock_rdlock(&frcti->lock); + clock_gettime(PTHREAD_COND_CLOCK, &now); - if (frcti->rcv_cr.lwe == frcti->rcv_cr.seqno) { + pthread_rwlock_wrlock(&frcti->lock); + + if (!after(frcti->rcv_cr.lwe, frcti->rcv_cr.seqno)) { pthread_rwlock_unlock(&frcti->lock); return; } @@ -290,24 +292,19 @@ static void send_frct_pkt(struct frcti * frcti) ackno = frcti->rcv_cr.lwe; rwe = frcti->rcv_cr.rwe; - clock_gettime(PTHREAD_COND_CLOCK, &now); - diff = ts_diff_ns(&frcti->rcv_cr.act, &now); - pthread_rwlock_unlock(&frcti->lock); - - if (diff > frcti->a || diff < DELT_ACK) + if (diff > frcti->a || diff < frcti->mdev) { + pthread_rwlock_unlock(&frcti->lock); return; + } - __send_frct_pkt(fd, FRCT_ACK | FRCT_FC, ackno, rwe); - - pthread_rwlock_wrlock(&frcti->lock); - - if (after(frcti->rcv_cr.lwe, frcti->rcv_cr.seqno)) - frcti->rcv_cr.seqno = frcti->rcv_cr.lwe; + frcti->rcv_cr.seqno = frcti->rcv_cr.lwe; pthread_rwlock_unlock(&frcti->lock); + __send_frct_pkt(fd, FRCT_ACK | FRCT_FC, ackno, rwe); + } static void __send_rdv(int fd) @@ -734,8 +731,7 @@ static int __frcti_snd(struct frcti * frcti, frcti->t_probe = now; frcti->probe = true; } - - if (now.tv_sec - rcv_cr->act.tv_sec <= frcti->a) { + if ((now.tv_sec - rcv_cr->act.tv_sec) * BILLION <= frcti->a) { pci->flags |= FRCT_ACK; pci->ackno = hton32(rcv_cr->lwe); rcv_cr->seqno = rcv_cr->lwe; -- cgit v1.2.3