Push back on the video encoder to avoid building queues in the pacer.

modules/congestion_controller/send_side_congestion_controller.cc

 /*
  *  Copyright (c) 2012 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.
  */
 
@@ skipping 14 matching lines @@
 #include "rtc_base/ptr_util.h"
 #include "rtc_base/rate_limiter.h"
 #include "rtc_base/socket.h"
 #include "rtc_base/timeutils.h"
 #include "system_wrappers/include/field_trial.h"
 
 namespace webrtc {
 namespace {
 
 const char kCwndExperiment[] = "WebRTC-CwndExperiment";
+const char kPacerPushbackExperiment[] = "WebRTC-PacerPushbackExperiment";
 const int64_t kDefaultAcceptedQueueMs = 250;
 
 bool CwndExperimentEnabled() {
   std::string experiment_string =
       webrtc::field_trial::FindFullName(kCwndExperiment);
   // The experiment is enabled iff the field trial string begins with "Enabled".
   return experiment_string.find("Enabled") == 0;
 }
 
 bool ReadCwndExperimentParameter(int64_t* accepted_queue_ms) {
@@ skipping 70 matching lines @@
       last_reported_bitrate_bps_(0),
       last_reported_fraction_loss_(0),
       last_reported_rtt_(0),
       network_state_(kNetworkUp),
       pause_pacer_(false),
       pacer_paused_(false),
       min_bitrate_bps_(congestion_controller::GetMinBitrateBps()),
       delay_based_bwe_(new DelayBasedBwe(event_log_, clock_)),
       in_cwnd_experiment_(CwndExperimentEnabled()),
       accepted_queue_ms_(kDefaultAcceptedQueueMs),
-      was_in_alr_(0) {
+      was_in_alr_(false),
+      pacer_pushback_experiment_(
+          webrtc::field_trial::IsEnabled(kPacerPushbackExperiment)) {
   delay_based_bwe_->SetMinBitrate(min_bitrate_bps_);
   if (in_cwnd_experiment_ &&
       !ReadCwndExperimentParameter(&accepted_queue_ms_)) {
     LOG(LS_WARNING) << "Failed to parse parameters for CwndExperiment "
                        "from field trial string. Experiment disabled.";
     in_cwnd_experiment_ = false;
   }
 }
 
 SendSideCongestionController::SendSideCongestionController(
@@ skipping 16 matching lines @@
       last_reported_bitrate_bps_(0),
       last_reported_fraction_loss_(0),
       last_reported_rtt_(0),
       network_state_(kNetworkUp),
       pause_pacer_(false),
       pacer_paused_(false),
       min_bitrate_bps_(congestion_controller::GetMinBitrateBps()),
       delay_based_bwe_(new DelayBasedBwe(event_log_, clock_)),
       in_cwnd_experiment_(CwndExperimentEnabled()),
       accepted_queue_ms_(kDefaultAcceptedQueueMs),
-      was_in_alr_(0) {
+      was_in_alr_(false),
+      pacer_pushback_experiment_(
+          webrtc::field_trial::IsEnabled(kPacerPushbackExperiment)) {
   delay_based_bwe_->SetMinBitrate(min_bitrate_bps_);
   if (in_cwnd_experiment_ &&
       !ReadCwndExperimentParameter(&accepted_queue_ms_)) {
     LOG(LS_WARNING) << "Failed to parse parameters for CwndExperiment "
                        "from field trial string. Experiment disabled.";
     in_cwnd_experiment_ = false;
   }
 }
 
 SendSideCongestionController::~SendSideCongestionController() {}
@@ skipping 236 matching lines @@
   uint8_t fraction_loss;
   int64_t rtt;
   bool estimate_changed = bitrate_controller_->GetNetworkParameters(
       &bitrate_bps, &fraction_loss, &rtt);
   if (estimate_changed) {
     pacer_->SetEstimatedBitrate(bitrate_bps);
     probe_controller_->SetEstimatedBitrate(bitrate_bps);
     retransmission_rate_limiter_->SetMaxRate(bitrate_bps);
   }
 
-  bitrate_bps = IsNetworkDown() || IsSendQueueFull() ? 0 : bitrate_bps;
+  if (!pacer_pushback_experiment_) {
+    bitrate_bps = IsNetworkDown() || IsSendQueueFull() ? 0 : bitrate_bps;
+  } else {
+    if (IsNetworkDown()) {
+      bitrate_bps = 0;
+    } else {
+      int64_t queue_length_ms = pacer_->ExpectedQueueTimeMs();
+
+      if (queue_length_ms == 0) {
+        encoding_rate_ = 1.0;
+      } else if (queue_length_ms > 50) {
+        float encoding_rate = 1.0 - queue_length_ms / 1000.0;
+        encoding_rate_ = std::min(encoding_rate_, encoding_rate);
+        encoding_rate_ = std::max(encoding_rate_, 0.0f);
+      }
+
+      bitrate_bps *= encoding_rate_;
+      bitrate_bps = bitrate_bps < 50000 ? 0 : bitrate_bps;
+    }
+  }
 
   if (HasNetworkParametersToReportChanged(bitrate_bps, fraction_loss, rtt)) {
     int64_t probing_interval_ms;
     {
       rtc::CritScope cs(&bwe_lock_);
       probing_interval_ms = delay_based_bwe_->GetExpectedBwePeriodMs();
     }
     {
       rtc::CritScope cs(&observer_lock_);
       if (observer_) {
@@ skipping 26 matching lines @@
 bool SendSideCongestionController::IsSendQueueFull() const {
   return pacer_->ExpectedQueueTimeMs() > PacedSender::kMaxQueueLengthMs;
 }
 
 bool SendSideCongestionController::IsNetworkDown() const {
   rtc::CritScope cs(&network_state_lock_);
   return network_state_ == kNetworkDown;
 }
 
 }  // namespace webrtc