Field trial to initialize throughput estimate faster.

The current acknowledged bitrate (i.e. throughput) estimator takes 500ms
to initialize. This CL creates a field trial to experiment with this
arbitrary initialization time.

Bug: webrtc:9492, webrtc:7746
Change-Id: I8a803f7bc0ee78856e808e289f37bab57d763efa
Reviewed-on: https://webrtc-review.googlesource.com/87145
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Commit-Queue: Björn Terelius <terelius@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23852}

modules/congestion_controller/goog_cc/bitrate_estimator.cc

@@ -11,23 +11,57 @@
 #include "modules/congestion_controller/goog_cc/bitrate_estimator.h"
 
 #include <cmath>
+#include <string>
 
 #include "modules/remote_bitrate_estimator/test/bwe_test_logging.h"
 #include "modules/rtp_rtcp/include/rtp_rtcp_defines.h"
+#include "rtc_base/logging.h"
+#include "system_wrappers/include/field_trial.h"
 
 namespace webrtc {
 
 namespace {
 constexpr int kInitialRateWindowMs = 500;
 constexpr int kRateWindowMs = 150;
+
+const char kBweInitialThroughputWindowExperiment[] =
+    "WebRTC-BweInitialThroughputWindowExperiment";
+
+int ReadInitialThroughputWindowSizeMs() {
+  std::string experiment_string =
+      webrtc::field_trial::FindFullName(kBweInitialThroughputWindowExperiment);
+  int initial_window_ms = kInitialRateWindowMs;
+  int parsed_values =
+      sscanf(experiment_string.c_str(), "Enabled-%d", &initial_window_ms);
+  if (parsed_values != 1) {
+    RTC_LOG(LS_ERROR) << "Incorrectly formatted field trial string for "
+                      << kBweInitialThroughputWindowExperiment;
+    return kInitialRateWindowMs;
+  }
+  if (kRateWindowMs <= initial_window_ms && initial_window_ms <= 1000) {
+    RTC_LOG(LS_INFO) << kBweInitialThroughputWindowExperiment
+                     << " enabled with " << initial_window_ms << " ms window.";
+    return initial_window_ms;
+  }
+  RTC_LOG(LS_ERROR)
+      << "Initial window for throughput estimation must be between "
+      << kRateWindowMs << " and 1000 ms.";
+  return kInitialRateWindowMs;
+}
+
 }  // namespace
 
 BitrateEstimator::BitrateEstimator()
     : sum_(0),
-      current_win_ms_(0),
+      initial_window_ms_(kInitialRateWindowMs),
+      current_window_ms_(0),
       prev_time_ms_(-1),
       bitrate_estimate_(-1.0f),
-      bitrate_estimate_var_(50.0f) {}
+      bitrate_estimate_var_(50.0f) {
+  if (field_trial::IsEnabled(kBweInitialThroughputWindowExperiment)) {
+    initial_window_ms_ = ReadInitialThroughputWindowSizeMs();
+  }
+}
 
 BitrateEstimator::~BitrateEstimator() = default;
 
@@ -36,7 +70,7 @@ void BitrateEstimator::Update(int64_t now_ms, int bytes) {
   // We use a larger window at the beginning to get a more stable sample that
   // we can use to initialize the estimate.
   if (bitrate_estimate_ < 0.f)
-    rate_window_ms = kInitialRateWindowMs;
+    rate_window_ms = initial_window_ms_;
   float bitrate_sample = UpdateWindow(now_ms, bytes, rate_window_ms);
   if (bitrate_sample < 0.0f)
     return;
@@ -71,21 +105,21 @@ float BitrateEstimator::UpdateWindow(int64_t now_ms,
   if (now_ms < prev_time_ms_) {
     prev_time_ms_ = -1;
     sum_ = 0;
-    current_win_ms_ = 0;
+    current_window_ms_ = 0;
   }
   if (prev_time_ms_ >= 0) {
-    current_win_ms_ += now_ms - prev_time_ms_;
+    current_window_ms_ += now_ms - prev_time_ms_;
     // Reset if nothing has been received for more than a full window.
     if (now_ms - prev_time_ms_ > rate_window_ms) {
       sum_ = 0;
-      current_win_ms_ %= rate_window_ms;
+      current_window_ms_ %= rate_window_ms;
     }
   }
   prev_time_ms_ = now_ms;
   float bitrate_sample = -1.0f;
-  if (current_win_ms_ >= rate_window_ms) {
+  if (current_window_ms_ >= rate_window_ms) {
     bitrate_sample = 8.0f * sum_ / static_cast<float>(rate_window_ms);
-    current_win_ms_ -= rate_window_ms;
+    current_window_ms_ -= rate_window_ms;
     sum_ = 0;
   }
   sum_ += bytes;

modules/congestion_controller/goog_cc/bitrate_estimator.h

@@ -35,7 +35,8 @@ class BitrateEstimator {
  private:
   float UpdateWindow(int64_t now_ms, int bytes, int rate_window_ms);
   int sum_;
-  int64_t current_win_ms_;
+  int64_t initial_window_ms_;
+  int64_t current_window_ms_;
   int64_t prev_time_ms_;
   float bitrate_estimate_;
   float bitrate_estimate_var_;