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core(interactive): also use LCP as bounds in observed metric #16173

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core(interactive): also use LCP as bounds in observed metric #16173

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0a5abc4
misc: fix doc comment for InteractiveMetric audit
connorjclark Sep 3, 2024
d4fea90
fix findOverlappingQuietPeriods
connorjclark Sep 3, 2024
2d7d726
Merge branch 'main' into fix-interactive-doccomment
connorjclark Sep 5, 2024
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2 changes: 1 addition & 1 deletion core/audits/metrics/interactive.js
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Expand Up @@ -18,7 +18,7 @@ const str_ = i18n.createIcuMessageFn(import.meta.url, UIStrings);

/**
* @fileoverview This audit identifies the time the page is "consistently interactive".
* Looks for the first period of at least 5 seconds after FCP where both CPU and network were quiet,
* Looks for the first period of at least 5 seconds after LCP where both CPU and network were quiet,
* and returns the timestamp of the beginning of the CPU quiet period.
* @see https://docs.google.com/document/d/1GGiI9-7KeY3TPqS3YT271upUVimo-XiL5mwWorDUD4c/edit#
*/
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6 changes: 4 additions & 2 deletions core/computed/metrics/interactive.js
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Expand Up @@ -86,11 +86,13 @@ class Interactive extends NavigationMetric {
* @return {{cpuQuietPeriod: TimePeriod, networkQuietPeriod: TimePeriod, cpuQuietPeriods: Array<TimePeriod>, networkQuietPeriods: Array<TimePeriod>}}
*/
static findOverlappingQuietPeriods(longTasks, networkRecords, processedNavigation) {
const FcpTsInMs = processedNavigation.timestamps.firstContentfulPaint / 1000;
const minTime = processedNavigation.timestamps.largestContentfulPaint !== undefined ?
processedNavigation.timestamps.largestContentfulPaint / 1000 :
processedNavigation.timestamps.firstContentfulPaint / 1000;
Comment on lines +89 to +91
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Not sure if it matters since this is TTI we're talking about (but that applies to making any changes at all, so... :), but seems bad to have this fallback? Makes the metric harder to reason about, discontinuous for an otherwise identical trace depending on if LCP was captured or not, and diverges from the lantern version, which throws if there was no LCP.

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I agree that the fallback in TTI can introduce inconsistencies. While it might seem like a minor detail, the discontinuity can complicate analysis and make it difficult to accurately compare performance across different scenarios. Perhaps we could explore alternative approaches, such as using a default value: If LCP is not captured, we could set TTI to a default value (e.g., the maximum possible value) to indicate that the metric is incomplete or even introducing a new metric: We might consider creating a separate metric that specifically measures time to first interaction without relying on LCP.

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So alternatively, we can throw if there's no LCP

and ignore this FCP fallback


/** @type {function(TimePeriod):boolean} */
const isLongEnoughQuietPeriod = period =>
period.end > FcpTsInMs + REQUIRED_QUIET_WINDOW &&
period.end > minTime + REQUIRED_QUIET_WINDOW &&
period.end - period.start >= REQUIRED_QUIET_WINDOW;
const networkQuietPeriods = this._findNetworkQuietPeriods(networkRecords, processedNavigation)
.filter(isLongEnoughQuietPeriod);
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26 changes: 20 additions & 6 deletions core/test/computed/metrics/interactive-test.js
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Expand Up @@ -93,10 +93,10 @@ describe('Metrics: TTI', () => {
describe('#findOverlappingQuietPeriods', () => {
it('should return entire range when no activity is present', () => {
const timeOrigin = 220023532;
const firstContentfulPaint = 2500 * 1000 + timeOrigin;
const largestContentfulPaint = 2500 * 1000 + timeOrigin;
const traceEnd = 10000 * 1000 + timeOrigin;
const processedTrace = /** @type {LH.Artifacts.ProcessedNavigation} */ (
{timestamps: {timeOrigin, firstContentfulPaint, traceEnd}}
{timestamps: {timeOrigin, largestContentfulPaint, traceEnd}}
);
const network = generateNetworkRecords([], timeOrigin);

Expand All @@ -105,7 +105,21 @@ describe('Metrics: TTI', () => {
assert.deepEqual(result.networkQuietPeriod, {start: 0, end: traceEnd / 1000});
});

it('should throw when trace ended too soon after FCP', () => {
it('should throw when trace ended too soon after LCP', () => {
const timeOrigin = 220023532;
const largestContentfulPaint = 2500 * 1000 + timeOrigin;
const traceEnd = 5000 * 1000 + timeOrigin;
const processedTrace = /** @type {LH.Artifacts.ProcessedNavigation} */ (
{timestamps: {timeOrigin, largestContentfulPaint, traceEnd}}
);
const network = generateNetworkRecords([], timeOrigin);

assert.throws(() => {
Interactive.findOverlappingQuietPeriods([], network, processedTrace);
}, /NO.*IDLE_PERIOD/);
});

it('should throw when trace ended too soon after FCP (when LCP is missing)', () => {
const timeOrigin = 220023532;
const firstContentfulPaint = 2500 * 1000 + timeOrigin;
const traceEnd = 5000 * 1000 + timeOrigin;
Expand Down Expand Up @@ -183,14 +197,14 @@ describe('Metrics: TTI', () => {

it('should find first overlapping quiet period', () => {
const timeOrigin = 220023532;
const firstContentfulPaint = 10000 * 1000 + timeOrigin;
const largestContentfulPaint = 10000 * 1000 + timeOrigin;
const traceEnd = 45000 * 1000 + timeOrigin;
const processedTrace = /** @type {LH.Artifacts.ProcessedNavigation} */ (
{timestamps: {timeOrigin, firstContentfulPaint, traceEnd}}
{timestamps: {timeOrigin, largestContentfulPaint, traceEnd}}
);

const cpu = [
// quiet period before FCP
// quiet period before LCP
{start: 9000, end: 9900},
{start: 11000, end: 13000},
// quiet period during network activity
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