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| Original file line number | Diff line number | Diff line change |
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| /** | ||
| * @license | ||
| * Copyright 2024 Google LLC | ||
| * SPDX-License-Identifier: Apache-2.0 | ||
| */ | ||
|
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| import * as Lantern from '../types/lantern.js'; | ||
| import {Metric} from '../metric.js'; | ||
| import {BaseNode} from '../base-node.js'; | ||
| // TODO(15841): move this default config value into lib/lantern | ||
| import {throttling as defaultThrottling} from '../../../config/constants.js'; | ||
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| /** @typedef {import('../base-node.js').Node} Node */ | ||
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| class SpeedIndex extends Metric { | ||
| /** | ||
| * @return {LH.Gatherer.Simulation.MetricCoefficients} | ||
| */ | ||
| static get COEFFICIENTS() { | ||
| return { | ||
| // Negative intercept is OK because estimate is Math.max(FCP, Speed Index) and | ||
| // the optimistic estimate is based on the real observed speed index rather than a real | ||
| // lantern graph. | ||
| intercept: -250, | ||
| optimistic: 1.4, | ||
| pessimistic: 0.65, | ||
| }; | ||
| } | ||
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| /** | ||
| * @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 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); | ||
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||
| return { | ||
| intercept: defaultCoefficients.intercept * multiplier, | ||
| optimistic: 0.5 + (defaultCoefficients.optimistic - 0.5) * multiplier, | ||
| pessimistic: 0.5 + (defaultCoefficients.pessimistic - 0.5) * multiplier, | ||
| }; | ||
| } | ||
|
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| /** | ||
| * @param {Node} dependencyGraph | ||
| * @return {Node} | ||
| */ | ||
| static getOptimisticGraph(dependencyGraph) { | ||
| return dependencyGraph; | ||
| } | ||
|
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| /** | ||
| * @param {Node} dependencyGraph | ||
| * @return {Node} | ||
| */ | ||
| static getPessimisticGraph(dependencyGraph) { | ||
| return dependencyGraph; | ||
| } | ||
|
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| /** | ||
| * @param {LH.Gatherer.Simulation.Result} simulationResult | ||
| * @param {import('../metric.js').Extras} extras | ||
| * @return {LH.Gatherer.Simulation.Result} | ||
| */ | ||
| static getEstimateFromSimulation(simulationResult, extras) { | ||
| if (!extras.fcpResult) throw new Error('missing fcpResult'); | ||
| if (!extras.speedline) throw new Error('missing speedline'); | ||
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| const fcpTimeInMs = extras.fcpResult.pessimisticEstimate.timeInMs; | ||
| const estimate = extras.optimistic | ||
| ? extras.speedline.speedIndex | ||
| : SpeedIndex.computeLayoutBasedSpeedIndex(simulationResult.nodeTimings, fcpTimeInMs); | ||
| return { | ||
| timeInMs: estimate, | ||
| nodeTimings: simulationResult.nodeTimings, | ||
| }; | ||
| } | ||
|
|
||
| /** | ||
| * @param {Lantern.Simulation.MetricComputationDataInput} data | ||
| * @param {Omit<import('../metric.js').Extras, 'optimistic'>=} extras | ||
| * @return {Promise<LH.Artifacts.LanternMetric>} | ||
| */ | ||
| static async compute(data, extras) { | ||
| const fcpResult = extras?.fcpResult; | ||
| if (!fcpResult) { | ||
| throw new Error('FCP is required to calculate the SpeedIndex metric'); | ||
| } | ||
|
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| const metricResult = await super.compute(data, extras); | ||
| metricResult.timing = Math.max(metricResult.timing, fcpResult.timing); | ||
| return metricResult; | ||
| } | ||
|
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| /** | ||
| * Approximate speed index using layout events from the simulated node timings. | ||
| * The layout-based speed index is the weighted average of the endTime of CPU nodes that contained | ||
| * a 'Layout' task. log(duration) is used as the weight to stand for "significance" to the page. | ||
| * | ||
| * If no layout events can be found or the endTime of a CPU task is too early, FCP is used instead. | ||
| * | ||
| * This approach was determined after evaluating the accuracy/complexity tradeoff of many | ||
| * different methods. Read more in the evaluation doc. | ||
| * | ||
| * @see https://docs.google.com/document/d/1qJWXwxoyVLVadezIp_Tgdk867G3tDNkkVRvUJSH3K1E/edit# | ||
| * @param {LH.Gatherer.Simulation.Result['nodeTimings']} nodeTimings | ||
| * @param {number} fcpTimeInMs | ||
| * @return {number} | ||
| */ | ||
| static computeLayoutBasedSpeedIndex(nodeTimings, fcpTimeInMs) { | ||
| /** @type {Array<{time: number, weight: number}>} */ | ||
| const layoutWeights = []; | ||
| for (const [node, timing] of nodeTimings.entries()) { | ||
| if (node.type !== BaseNode.TYPES.CPU) continue; | ||
|
|
||
| if (node.childEvents.some(x => x.name === 'Layout')) { | ||
| const timingWeight = Math.max(Math.log2(timing.endTime - timing.startTime), 0); | ||
| layoutWeights.push({time: timing.endTime, weight: timingWeight}); | ||
| } | ||
| } | ||
|
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| const totalWeightedTime = layoutWeights | ||
| .map(evt => evt.weight * Math.max(evt.time, fcpTimeInMs)) | ||
| .reduce((a, b) => a + b, 0); | ||
| const totalWeight = layoutWeights.map(evt => evt.weight).reduce((a, b) => a + b, 0); | ||
|
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| if (!totalWeight) return fcpTimeInMs; | ||
| return totalWeightedTime / totalWeight; | ||
| } | ||
| } | ||
|
|
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| export {SpeedIndex}; |
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