Source: lib/abr/ewma_bandwidth_estimator.js

  1. /*! @license
  2.  * Shaka Player
  3.  * Copyright 2016 Google LLC
  4.  * SPDX-License-Identifier: Apache-2.0
  5.  */
  7. goog.provide('shaka.abr.EwmaBandwidthEstimator');
  9. goog.require('shaka.abr.Ewma');
  12. /**
  13.  * @summary
  14.  * This class tracks bandwidth samples and estimates available bandwidth.
  15.  * Based on the minimum of two exponentially-weighted moving averages with
  16.  * different half-lives.
  17.  *
  18.  */
  19. shaka.abr.EwmaBandwidthEstimator = class {
  20.   /** */
  21.   constructor() {
  22.     /**
  23.      * A fast-moving average.
  24.      * Half of the estimate is based on the last 2 seconds of sample history.
  25.      * @private {!shaka.abr.Ewma}
  26.      */
  27.     this.fast_ = new shaka.abr.Ewma(2);
  29.     /**
  30.      * A slow-moving average.
  31.      * Half of the estimate is based on the last 5 seconds of sample history.
  32.      * @private {!shaka.abr.Ewma}
  33.      */
  34.     this.slow_ = new shaka.abr.Ewma(5);
  36.     /**
  37.      * Number of bytes sampled.
  38.      * @private {number}
  39.      */
  40.     this.bytesSampled_ = 0;
  43.     /**
  44.      * Minimum number of bytes sampled before we trust the estimate.  If we have
  45.      * not sampled much data, our estimate may not be accurate enough to trust.
  46.      * If bytesSampled_ is less than minTotalBytes_, we use defaultEstimate_.
  47.      * This specific value is based on experimentation.
  48.      *
  49.      * @private {number}
  50.      * @const
  51.      */
  52.     this.minTotalBytes_ = 128e3;  // 128kB
  54.     /**
  55.      * Minimum number of bytes, under which samples are discarded.  Our models
  56.      * do not include latency information, so connection startup time (time to
  57.      * first byte) is considered part of the download time.  Because of this, we
  58.      * should ignore very small downloads which would cause our estimate to be
  59.      * too low.
  60.      * This specific value is based on experimentation.
  61.      *
  62.      * @private {number}
  63.      * @const
  64.      */
  65.     this.minBytes_ = 16e3;  // 16kB
  66.   }
  68.   /**
  69.    * Takes a bandwidth sample.
  70.    *
  71.    * @param {number} durationMs The amount of time, in milliseconds, for a
  72.    *   particular request.
  73.    * @param {number} numBytes The total number of bytes transferred in that
  74.    *   request.
  75.    */
  76.   sample(
  77.       durationMs, numBytes) {
  78.     if (numBytes < this.minBytes_) {
  79.       return;
  80.     }
  82.     const bandwidth = 8000 * numBytes / durationMs;
  83.     const weight = durationMs / 1000;
  85.     this.bytesSampled_ += numBytes;
  86.     this.fast_.sample(weight, bandwidth);
  87.     this.slow_.sample(weight, bandwidth);
  88.   }
  91.   /**
  92.    * Gets the current bandwidth estimate.
  93.    *
  94.    * @param {number} defaultEstimate
  95.    * @return {number} The bandwidth estimate in bits per second.
  96.    */
  97.   getBandwidthEstimate(defaultEstimate) {
  98.     if (this.bytesSampled_ < this.minTotalBytes_) {
  99.       return defaultEstimate;
  100.     }
  102.     // Take the minimum of these two estimates.  This should have the effect
  103.     // of adapting down quickly, but up more slowly.
  104.     return Math.min(this.fast_.getEstimate(), this.slow_.getEstimate());
  105.   }
  108.   /**
  109.    * @return {boolean} True if there is enough data to produce a meaningful
  110.    *   estimate.
  111.    */
  112.   hasGoodEstimate() {
  113.     return this.bytesSampled_ >= this.minTotalBytes_;
  114.   }
  115. };