core(speedindex): scale coefficients according to throttling

lighthouse-core/computed/metrics/lantern-metric.js

@@ -42,6 +42,19 @@ class LanternMetricArtifact {
     throw new Error('COEFFICIENTS unimplemented!');
   }
 
+  /**
+   * Returns the coefficients, scaled by the throttling settings if needed by the metric.
+   * Some lantern metrics (speed-index) use components in their estimate that are not
+   * from the simulator. In this case, we need to adjust the coefficients as the target throttling
+   * settings change.
+   *
+   * @param {number} rttMs
+   * @return {LH.Gatherer.Simulation.MetricCoefficients}
+   */
+  static getScaledCoefficients(rttMs) { // eslint-disable-line no-unused-vars
+    return this.COEFFICIENTS;
+  }
+
   /**
    * @param {Node} dependencyGraph
    * @param {LH.Artifacts.TraceOfTab} traceOfTab
@@ -107,13 +120,14 @@ class LanternMetricArtifact {
       Object.assign({}, extras, {optimistic: false})
     );
 
+    const coefficients = this.getScaledCoefficients(simulator.rtt);
     // Estimates under 1s don't really follow the normal curve fit, minimize the impact of the intercept
-    const interceptMultiplier = this.COEFFICIENTS.intercept > 0 ?
+    const interceptMultiplier = coefficients.intercept > 0 ?
       Math.min(1, optimisticEstimate.timeInMs / 1000) : 1;
     const timing =
-      this.COEFFICIENTS.intercept * interceptMultiplier +
-      this.COEFFICIENTS.optimistic * optimisticEstimate.timeInMs +
-      this.COEFFICIENTS.pessimistic * pessimisticEstimate.timeInMs;
+      coefficients.intercept * interceptMultiplier +
+      coefficients.optimistic * optimisticEstimate.timeInMs +
+      coefficients.pessimistic * pessimisticEstimate.timeInMs;
 
     return {
       timing,

lighthouse-core/computed/metrics/lantern-speed-index.js

@@ -10,6 +10,7 @@ const LanternMetric = require('./lantern-metric.js');
 const BaseNode = require('../../lib/dependency-graph/base-node.js');
 const Speedline = require('../speedline.js');
 const LanternFirstContentfulPaint = require('./lantern-first-contentful-paint.js');
+const defaultThrottling = require('../../config/constants.js').throttling;
 
 /** @typedef {BaseNode.Node} Node */
 
@@ -28,6 +29,33 @@ class LanternSpeedIndex extends LanternMetric {
     };
   }
 
+  /**
+   * @param {number} rttMs
+   * @return {LH.Gatherer.Simulation.MetricCoefficients}
+   */
+  static getScaledCoefficients(rttMs) { // eslint-disable-line no-unused-vars
+    // We want to scale our default coefficients based on the speed of the connection.
+    // We will linearly interpolate coefficients for the passed-in rttMs based on two pre-determined points:
+    //   1. Baseline point of 30 ms RTT where Speed Index should be a ~50/50 blend of optimistic/pessimistic.
+    //      30 ms was based on a typical home WiFi connection's actual RTT.
+    //      Coefficients here follow from the fact that the optimistic estimate should be very close
+    //      to reality at this connection speed and the pessimistic estimate compensates for minor
+    //      connection speed differences.
+    //   2. Default throttled point of 150 ms RTT where the default coefficients have been determined to be most accurate.
+    //      Coefficients here were determined through thorough analysis and linear regression on the
+    //      lantern test data set. See lighthouse-core/scripts/test-lantern.sh for more detail.
+    // While the coefficients haven't been analyzed at the interpolated points, it's our current best effort.
+    const defaultCoefficients = this.COEFFICIENTS;
+    const defaultRttExcess = defaultThrottling.mobileSlow4G.rttMs - 30;
+    const multiplier = Math.max((rttMs - 30) / defaultRttExcess, 0);
+
+    return {
+      intercept: defaultCoefficients.intercept * multiplier,
+      optimistic: 0.5 + (defaultCoefficients.optimistic - 0.5) * multiplier,
+      pessimistic: 0.5 + (defaultCoefficients.pessimistic - 0.5) * multiplier,
+    };
+  }
+
   /**
    * @param {Node} dependencyGraph
    * @return {Node}

lighthouse-core/lib/dependency-graph/simulator/simulator.js

@@ -75,6 +75,11 @@ class Simulator {
     this._connectionPool = /** @type {ConnectionPool} */ (null);
   }
 
+  /** @return {number} */
+  get rtt() {
+    return this._rtt;
+  }
+
   /**
    * @param {Node} graph
    */

lighthouse-core/test/computed/metrics/lantern-speed-index-test.js

@@ -5,6 +5,7 @@
  */
 'use strict';
 
+const defaultThrottling = require('../../../config/constants.js').throttling.mobileSlow4G;
 const LanternSpeedIndex = require('../../../computed/metrics/lantern-speed-index.js');
 
 const trace = require('../../fixtures/traces/progressive-app-m60.json');
@@ -13,14 +14,59 @@ const devtoolsLog = require('../../fixtures/traces/progressive-app-m60.devtools.
 /* eslint-env jest */
 describe('Metrics: Lantern Speed Index', () => {
   it('should compute predicted value', async () => {
-    const settings = {};
+    const settings = {throttlingMethod: 'simulate', throttling: defaultThrottling};
     const context = {settings, computedCache: new Map()};
     const result = await LanternSpeedIndex.request({trace, devtoolsLog, settings}, context);
 
     expect({
       timing: Math.round(result.timing),
       optimistic: Math.round(result.optimisticEstimate.timeInMs),
       pessimistic: Math.round(result.pessimisticEstimate.timeInMs),
-    }).toMatchSnapshot();
+    }).toMatchInlineSnapshot(`
+Object {
+  "optimistic": 605,
+  "pessimistic": 1631,
+  "timing": 1657,
+}
+`);
+  });
+
+  it('should compute predicted value for different settings', async () => {
+    const settings = {throttlingMethod: 'simulate', throttling: {...defaultThrottling, rttMs: 300}};
+    const context = {settings, computedCache: new Map()};
+    const result = await LanternSpeedIndex.request({trace, devtoolsLog, settings}, context);
+
+    expect({
+      timing: Math.round(result.timing),
+      optimistic: Math.round(result.optimisticEstimate.timeInMs),
+      pessimistic: Math.round(result.pessimisticEstimate.timeInMs),
+    }).toMatchInlineSnapshot(`
+Object {
+  "optimistic": 605,
+  "pessimistic": 2381,
+  "timing": 2958,
+}
+`);
+  });
+
+  it('should not scale coefficients at default', async () => {
+    const result = LanternSpeedIndex.getScaledCoefficients(defaultThrottling.rttMs);
+    expect(result).toEqual(LanternSpeedIndex.COEFFICIENTS);
+  });
+
+  it('should scale coefficients back', async () => {
+    const result = LanternSpeedIndex.getScaledCoefficients(5);
+    expect(result).toEqual({intercept: -0, pessimistic: 0.5, optimistic: 0.5});
+  });
+
+  it('should scale coefficients forward', async () => {
+    const result = LanternSpeedIndex.getScaledCoefficients(300);
+    expect(result).toMatchInlineSnapshot(`
+Object {
+  "intercept": -562.5,
+  "optimistic": 2.525,
+  "pessimistic": 0.8375,
+}
+`);
   });
 });