Fix BitrateProber to match the requested bitrate more precisely

webrtc/modules/pacing/bitrate_prober.cc

 /*
  *  Copyright (c) 2014 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 #include "webrtc/modules/pacing/bitrate_prober.h"
 
 #include <algorithm>
 
 #include "webrtc/base/checks.h"
 #include "webrtc/base/logging.h"
 #include "webrtc/modules/pacing/paced_sender.h"
 
 namespace webrtc {
 
 namespace {
 
-// Inactivity threshold above which probing is restarted.
-constexpr int kInactivityThresholdMs = 5000;
-
 // A minimum interval between probes to allow scheduling to be feasible.
 constexpr int kMinProbeDeltaMs = 1;
 
-int ComputeDeltaFromBitrate(size_t probe_size, uint32_t bitrate_bps) {
-  RTC_CHECK_GT(bitrate_bps, 0);
-  // Compute the time delta needed to send probe_size bytes at bitrate_bps
-  // bps. Result is in milliseconds.
-  return static_cast<int>((1000ll * probe_size * 8) / bitrate_bps);
-}
 }  // namespace
 
 BitrateProber::BitrateProber()
     : probing_state_(ProbingState::kDisabled),
-      probe_size_last_sent_(0),
-      time_last_probe_sent_ms_(-1),
+      next_probe_time_ms_(-1),
       next_cluster_id_(0) {
   SetEnabled(true);
 }
 
 void BitrateProber::SetEnabled(bool enable) {
   if (enable) {
     if (probing_state_ == ProbingState::kDisabled) {
       probing_state_ = ProbingState::kInactive;
       LOG(LS_INFO) << "Bandwidth probing enabled, set to inactive";
     }
   } else {
     probing_state_ = ProbingState::kDisabled;
     LOG(LS_INFO) << "Bandwidth probing disabled";
   }
 }
 
 bool BitrateProber::IsProbing() const {
   return probing_state_ == ProbingState::kActive;
 }
 
 void BitrateProber::OnIncomingPacket(size_t packet_size) {
   // Don't initialize probing unless we have something large enough to start
   // probing.
   if (probing_state_ == ProbingState::kInactive &&
       !clusters_.empty() &&
       packet_size >= PacedSender::kMinProbePacketSize) {
+    // Send next probe right away.
+    next_probe_time_ms_ = -1;
     probing_state_ = ProbingState::kActive;
   }
 }
 
 void BitrateProber::CreateProbeCluster(int bitrate_bps, int num_probes) {
   RTC_DCHECK(probing_state_ != ProbingState::kDisabled);
   ProbeCluster cluster;
   cluster.max_probes = num_probes;
   cluster.probe_bitrate_bps = bitrate_bps;
   cluster.id = next_cluster_id_++;
   clusters_.push(cluster);
   LOG(LS_INFO) << "Probe cluster (bitrate:probes): ("
                << cluster.probe_bitrate_bps << ":" << cluster.max_probes
                << ") ";
   if (probing_state_ != ProbingState::kActive)
     probing_state_ = ProbingState::kInactive;
 }
 
 void BitrateProber::ResetState() {
-  time_last_probe_sent_ms_ = -1;
-  probe_size_last_sent_ = 0;
-
   // Recreate all probing clusters.
   std::queue<ProbeCluster> clusters;
   clusters.swap(clusters_);
   while (!clusters.empty()) {
     CreateProbeCluster(clusters.front().probe_bitrate_bps,
                        clusters.front().max_probes);
     clusters.pop();
   }
   // If its enabled, reset to inactive.
   if (probing_state_ != ProbingState::kDisabled)
     probing_state_ = ProbingState::kInactive;
 }
 
 int BitrateProber::TimeUntilNextProbe(int64_t now_ms) {
   // Probing is not active or probing is already complete.
   if (probing_state_ != ProbingState::kActive || clusters_.empty())
     return -1;
-  // time_last_probe_sent_ms_ of -1 indicates no probes have yet been sent.
-  int64_t elapsed_time_ms;
-  if (time_last_probe_sent_ms_ == -1) {
-    elapsed_time_ms = 0;
-  } else {
-    elapsed_time_ms = now_ms - time_last_probe_sent_ms_;
-  }
-  // If no probes have been sent for a while, abort current probing and
-  // reset.
-  if (elapsed_time_ms > kInactivityThresholdMs) {

[Sergey Ulanov, 2017/01/03 23:28:59]

I couldn't see any case when this condition would be useful. As far as I can
tell probes would be suspended below by kMaxProbeDelayMs before reaching 5000 ms
here. Am I missing something?

-    ResetState();
-    return -1;
-  }
-  // We will send the first probe packet immediately if no packet has been
-  // sent before.
+
   int time_until_probe_ms = 0;
-  if (probe_size_last_sent_ != 0 && probing_state_ == ProbingState::kActive) {
-    int next_delta_ms = ComputeDeltaFromBitrate(
-        probe_size_last_sent_, clusters_.front().probe_bitrate_bps);
-    time_until_probe_ms = next_delta_ms - elapsed_time_ms;
-    // If we have waited more than 3 ms for a new packet to probe with we will
-    // consider this probing session over.
+  if (next_probe_time_ms_ >= 0) {
+    time_until_probe_ms = next_probe_time_ms_ - now_ms;
+    // If we have waited more than 3 ms for a new packet we will restart probing
+    // again later.
     const int kMaxProbeDelayMs = 3;
-    if (next_delta_ms < kMinProbeDeltaMs ||
-        time_until_probe_ms < -kMaxProbeDelayMs) {
-      probing_state_ = ProbingState::kSuspended;

[Sergey Ulanov, 2017/01/03 23:28:59]

This logic seems to be incorrect. It could suspend probing in a middle of a
cluster and leave that cluster in that state. The remaining part of that cluster
would be restarted when probing is requested again, which makes little sense. I
changed this code to call ResetState() when probe is delayed by more than 3 ms.

-      LOG(LS_INFO) << "Delta too small or missed probing accurately, suspend";
-      time_until_probe_ms = 0;
+    if (time_until_probe_ms < -kMaxProbeDelayMs) {
+      ResetState();
+      return -1;
     }
   }
+
   return std::max(time_until_probe_ms, 0);
 }
 
 int BitrateProber::CurrentClusterId() const {
   RTC_DCHECK(!clusters_.empty());
   RTC_DCHECK(ProbingState::kActive == probing_state_);
   return clusters_.front().id;
 }
 
 // Probe size is recommended based on the probe bitrate required. We choose
 // a minimum of twice |kMinProbeDeltaMs| interval to allow scheduling to be
 // feasible.
 size_t BitrateProber::RecommendedMinProbeSize() const {
   RTC_DCHECK(!clusters_.empty());
-  return clusters_.front().probe_bitrate_bps * 2 * kMinProbeDeltaMs /
+  return (clusters_.front().probe_bitrate_bps * 2 * kMinProbeDeltaMs) /
          (8 * 1000);
 }
 
 void BitrateProber::ProbeSent(int64_t now_ms, size_t bytes) {
   RTC_DCHECK(probing_state_ == ProbingState::kActive);
   RTC_DCHECK_GT(bytes, 0);
-  probe_size_last_sent_ = bytes;
-  time_last_probe_sent_ms_ = now_ms;
+
   if (!clusters_.empty()) {
     ProbeCluster* cluster = &clusters_.front();
+    if (cluster->sent_probes == 0) {
+      RTC_DCHECK_EQ(cluster->time_started_ms, -1);
+      cluster->time_started_ms = now_ms;
+    }
     ++cluster->sent_probes;
+    cluster->bytes_sent += bytes;
+    next_probe_time_ms_ = GetNextProbeTime(clusters_.front());
     if (cluster->sent_probes == cluster->max_probes)
       clusters_.pop();
     if (clusters_.empty())

[philipel, 2017/01/04 10:13:18]

if (clusters_.empty()) {
  probing_state_ = ProbingState::kSuspended;
  next_probe_time_ms_ = -1;
}

[Sergey Ulanov, 2017/01/05 23:31:53]

next_probe_time_ms_ is only used in kActive state and it's reset when probing is
activated, so resetting it here would not make any difference

[philipel, 2017/01/09 09:34:34]

Acknowledged.

       probing_state_ = ProbingState::kSuspended;
   }
 }
+
+int64_t BitrateProber::GetNextProbeTime(const ProbeCluster& cluster) {
+  RTC_CHECK_GT(cluster.probe_bitrate_bps, 0);
+  RTC_CHECK_GE(cluster.time_started_ms, 0);
+  // Compute the time delta needed to send the probe at the target bitrate.
+  // Result is in milliseconds.
+  return cluster.time_started_ms +

[philipel, 2017/01/04 10:13:18]

Not super important, but can you break this calculation into maybe two steps? I
think that would be cleaner.

Also, I can't see why you need to cast to int?

[Sergey Ulanov, 2017/01/05 23:31:53]

Will do.
yes, the cast is redundant

+         static_cast<int>(
+             (1000ll * cluster.bytes_sent * 8 + cluster.probe_bitrate_bps / 2) /
+             cluster.probe_bitrate_bps);
+}
+
+
 }  // namespace webrtc