Source: lib/abr/ewma_bandwidth_estimator.js

  1. /**
  2.  * @license
  3.  * Copyright 2016 Google Inc.
  4.  *
  5.  * Licensed under the Apache License, Version 2.0 (the "License");
  6.  * you may not use this file except in compliance with the License.
  7.  * You may obtain a copy of the License at
  8.  *
  9.  *     http://www.apache.org/licenses/LICENSE-2.0
  10.  *
  11.  * Unless required by applicable law or agreed to in writing, software
  12.  * distributed under the License is distributed on an "AS IS" BASIS,
  13.  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  14.  * See the License for the specific language governing permissions and
  15.  * limitations under the License.
  16.  */
  18. goog.provide('shaka.abr.EwmaBandwidthEstimator');
  20. goog.require('shaka.abr.Ewma');
  23. /**
  24.  * Tracks bandwidth samples and estimates available bandwidth.
  25.  * Based on the minimum of two exponentially-weighted moving averages with
  26.  * different half-lives.
  27.  *
  28.  * @constructor
  29.  * @struct
  30.  */
  31. shaka.abr.EwmaBandwidthEstimator = function() {
  32.   /**
  33.    * A fast-moving average.
  34.    * Half of the estimate is based on the last 2 seconds of sample history.
  35.    * @private {!shaka.abr.Ewma}
  36.    */
  37.   this.fast_ = new shaka.abr.Ewma(2);
  39.   /**
  40.    * A slow-moving average.
  41.    * Half of the estimate is based on the last 5 seconds of sample history.
  42.    * @private {!shaka.abr.Ewma}
  43.    */
  44.   this.slow_ = new shaka.abr.Ewma(5);
  46.   /**
  47.    * Number of bytes sampled.
  48.    * @private {number}
  49.    */
  50.   this.bytesSampled_ = 0;
  53.   /**
  54.    * Minimum number of bytes sampled before we trust the estimate.  If we have
  55.    * not sampled much data, our estimate may not be accurate enough to trust.
  56.    * If bytesSampled_ is less than minTotalBytes_, we use defaultEstimate_.
  57.    * This specific value is based on experimentation.
  58.    *
  59.    * @private {number}
  60.    * @const
  61.    */
  62.   this.minTotalBytes_ = 128e3;  // 128kB
  64.   /**
  65.    * Minimum number of bytes, under which samples are discarded.  Our models do
  66.    * not include latency information, so connection startup time (time to first
  67.    * byte) is considered part of the download time.  Because of this, we should
  68.    * ignore very small downloads which would cause our estimate to be too low.
  69.    * This specific value is based on experimentation.
  70.    *
  71.    * @private {number}
  72.    * @const
  73.    */
  74.   this.minBytes_ = 16e3;  // 16kB
  75. };
  78. /**
  79.  * Takes a bandwidth sample.
  80.  *
  81.  * @param {number} durationMs The amount of time, in milliseconds, for a
  82.  *   particular request.
  83.  * @param {number} numBytes The total number of bytes transferred in that
  84.  *   request.
  85.  */
  86. shaka.abr.EwmaBandwidthEstimator.prototype.sample = function(
  87.     durationMs, numBytes) {
  88.   if (numBytes < this.minBytes_) {
  89.     return;
  90.   }
  92.   let bandwidth = 8000 * numBytes / durationMs;
  93.   let weight = durationMs / 1000;
  95.   this.bytesSampled_ += numBytes;
  96.   this.fast_.sample(weight, bandwidth);
  97.   this.slow_.sample(weight, bandwidth);
  98. };
  101. /**
  102.  * Gets the current bandwidth estimate.
  103.  *
  104.  * @param {number} defaultEstimate
  105.  * @return {number} The bandwidth estimate in bits per second.
  106.  */
  107. shaka.abr.EwmaBandwidthEstimator.prototype.getBandwidthEstimate =
  108.     function(defaultEstimate) {
  109.   if (this.bytesSampled_ < this.minTotalBytes_) {
  110.     return defaultEstimate;
  111.   }
  113.   // Take the minimum of these two estimates.  This should have the effect of
  114.   // adapting down quickly, but up more slowly.
  115.   return Math.min(this.fast_.getEstimate(), this.slow_.getEstimate());
  116. };
  119. /**
  120.  * @return {boolean} True if there is enough data to produce a meaningful
  121.  *   estimate.
  122.  */
  123. shaka.abr.EwmaBandwidthEstimator.prototype.hasGoodEstimate = function() {
  124.   return this.bytesSampled_ >= this.minTotalBytes_;
  125. };