Intent to Experiment: Increased max nesting level for setTimeout(0)

[Etienne Pierre-doray]

Contact emails

etie...@chromium.org

Explainer

None

Specification

https://html.spec.whatwg.org/multipage/timers-and-user-prompts.html

Design docs

https://docs.google.com/document/d/1boT0k8BQjl7mXXzvI9SdN4XJPSza27vE8T0CNxmMhCI

Summary

Increase the nesting threshold before which setTimeout(..., <4ms) start being clamped, from 5 to 100. setTimeout(..., 0) is commonly used to break down long Javascript tasks and let other internal tasks run, which prevents the browser from hanging. setTimeouts and setIntervals with an interval < 4ms are not clamped as aggressively as they were before. This improves short horizon performance, but websites abusing the API will still eventually have their set setTimeouts clamped

Blink component

Blink

TAG review

TAG review status

Not applicable

Risks

Interoperability and Compatibility

setTimeout is a well established and mature API. This change poses a risk of breaking websites and tests that rely on the current timing caused by clamping and the subtle task ordering that it entails. As an example, this change breaks assumptions about the ordering between setTimeout(0) and unrelated tasks in at least one case in Chrome tests (crbug.com/1302309). On the flip side, the implementation in Chrome is already non compliant (crbug.com/1108877). There's also a similar experiment on beta that is ongoing (crbug.com/1263190). Devs can use chrome://flags#unthrottled-nested-timeout to test their sites for compatibility issues.

Gecko: No signal

WebKit: Shipped/Shipping

Web developers: No signals

Other signals:

WebView Application Risks

Does this intent deprecate or change behavior of existing APIs, such that it has potentially high risk for Android WebView-based applications?

Goals for experimentation

Our goals are to determine the extent of breakage caused by this feature to determine if it is safe to ship and to ensure we performance metrics don't regress, e.g. core web vitals. We've previously experimented at 50% on Canary/Dev/Beta without capturing any issues, and we would like to experiment on Stable to analyze the impact on the broader population.

Ongoing technical constraints

None

Debuggability

setTimeout() and setInterval() have an associated trace event in DevTools. https://developer.chrome.com/docs/devtools/evaluate-performance/performance-reference/

Will this feature be supported on all six Blink platforms (Windows, Mac, Linux, Chrome OS, Android, and Android WebView)?

Yes

Is this feature fully tested by web-platform-tests?

No

Flag name

unthrottled-nested-timeout

Requires code in //chrome?

False

Tracking bug

https://crbug.com/1108877

Estimated milestones

OriginTrial desktop last	105
OriginTrial desktop first	104
DevTrial on desktop	101

OriginTrial android last	105
OriginTrial android first	104
DevTrial on android	101

We plan to do a 1% Stable experiment for M104 and M105 stable.

Link to entry on the Chrome Platform Status

https://chromestatus.com/feature/5710690097561600

Links to previous Intent discussions

Ready for Trial: https://groups.google.com/a/chromium.org/g/blink-dev/c/-TjeYs7shTQ/m/FhJq0mQyDAAJ

I2E on Beta: https://groups.google.com/a/chromium.org/g/blink-dev/c/iT0CpRXs4TE/m/iAxA4RcuAQAJ

This intent message was generated by Chrome Platform Status.

[Yoav Weiss]

LGTM to experiment M104-M105.

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