Intent to Experiment: scheduler.yield()

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Scott Haseley

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Jun 13, 2023, 1:53:23 PM6/13/23
to blink-dev

Contact emails

shas...@chromium.org

Explainer

https://github.com/WICG/scheduling-apis/blob/main/explainers/yield-and-continuation.md

Specification

None

Summary

Provides a method for yielding control to the browser, which can be used to break up long tasks. Awaiting the promise returned by scheduler.yield() causes the current task to yield, continuing in a new browser task. This can be used to improve responsiveness issues caused by long tasks. Continuations are prioritized to mitigate performance problems of existing alternatives.



Blink component

Blink>Scheduling>APIs

TAG review

https://github.com/w3ctag/design-reviews/issues/827

TAG review status

Pending

Risks



Interoperability and Compatibility

This is a new feature and will not change existing event loop task scheduling, so the main risk is that other browsers might not implement the feature. There is an interop challenge, however, that comes with prioritization: we want to be specific enough to provide developers guarantees and interoperable implementations, but provide enough scheduling flexibility for UAs (like the HTML specification does with task sources/task queues), which we'll keep in mind while drafting the spec (see also https://github.com/WICG/scheduling-apis/issues/67).



Gecko: No signal

WebKit: No signal

Web developers: No signals

Other signals:

Ergonomics

The default use (inserting yield points in long tasks) should enable Chrome to maintain better performance (responsiveness). There is a risk of continuations starving other work, but there are reasonable mitigations, e.g. bounding total of prioritized continuations (see also https://github.com/WICG/scheduling-apis/blob/main/explainers/yield-and-continuation.md#preventing-task-starvation-by-continuations).



Activation

The feature would benefit from a polyfill so that tasks still yield in the case the feature is unavailable. The behavior can be approximated by awaiting `scheduler.postTask()` or wrapping `setTimeout(0)` in a promise. The signal inheritance bit [1], however, would need transpilation support to propagate the current signal across async (Promise) boundaries. But developers can alternatively pass the appropriate priority/signal if necessary on browsers that don't support the feature.



Security

See https://github.com/WICG/scheduling-apis/blob/main/explainers/yield-and-continuation.md#self-review-questionnaire-security-and-privacy



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?

None



Goals for experimentation

The main goal is to evaluate yielding with prioritized continuations on site-specific metrics. More frequent yielding is known to improve responsiveness (should also be measured in experiments), but there's often a cost (latency) to regaining control of the thread. This API prioritizes continuations to mitigate this, and we want to measure the impact on site-specific metrics to evaluate the scheduling behavior.


Ongoing technical constraints

None.



Debuggability

This has basic new-API devtools support. We plan to work with the devtools team to see if we can integrate continuations into the performance panel in some way.



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?

Yes

DevTrial instructions

https://github.com/WICG/scheduling-apis/blob/main/implementation-status.md

Flag name

--enable-blink-features=SchedulerYield

Requires code in //chrome?

False

Tracking bug

https://bugs.chromium.org/p/chromium/issues/detail?id=979020

Estimated milestones

OriginTrial desktop last118
OriginTrial desktop first115
DevTrial on desktop113
OriginTrial Android last118
OriginTrial Android first115
DevTrial on Android113
OriginTrial webView last118
OriginTrial webView first115


Link to entry on the Chrome Platform Status

https://chromestatus.com/feature/6266249336586240

Links to previous Intent discussions

Intent to prototype: https://groups.google.com/a/chromium.org/d/msgid/blink-dev/CAKXGoJ1SBQP-ABM3%2BsDtKzUZiPoSCWqW2mLOjMrUfFBx4TomSw%40mail.gmail.com

This intent message was generated by Chrome Platform Status.

Mike Taylor

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Jun 13, 2023, 3:00:43 PM6/13/23
to Scott Haseley, blink-dev

LGTM to experiment from 115 to 118 inclusive.

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Domenic Denicola

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Jun 13, 2023, 10:03:00 PM6/13/23
to Scott Haseley, blink-dev
Exciting stuff!!


> The signal inheritance bit [1], however, would need transpilation support to propagate the current signal across async (Promise) boundaries.

I couldn't find where this [1] goes. I'm very interested in this general area, and questions like whether we propagate by default or not, and how robust the propagation is to different mechanisms (e.g., not just promises, but other web platform callbacks as well).

What's the latest source of information on that, either in our implementation or in the explainer/spec? I see from the explainer there's a non-default `signal: "inherit"`, but I couldn't find details on how "the current task"'s priority is determined and propagated.


--

Scott Haseley

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Jun 14, 2023, 6:03:20 PM6/14/23
to Domenic Denicola, blink-dev
On Tue, Jun 13, 2023 at 7:02 PM Domenic Denicola <dom...@chromium.org> wrote:
Exciting stuff!!

> The signal inheritance bit [1], however, would need transpilation support to propagate the current signal across async (Promise) boundaries.

I couldn't find where this [1] goes. I'm very interested in this general area, and questions like whether we propagate by default or not, and how robust the propagation is to different mechanisms (e.g., not just promises, but other web platform callbacks as well).

What's the latest source of information on that, either in our implementation or in the explainer/spec? I see from the explainer there's a non-default `signal: "inherit"`, but I couldn't find details on how "the current task"'s priority is determined and propagated.

Ah, sorry, not sure what happened there; that was probably pointing to the design discussion section.

It's still an open question of whether or not inheritance should be the default here, hoping to work that out soon. I just brain-dumped the current state of things in this doc, with details about what gets propagated and how that works, along with code pointers. I'm planning to begin the spec work on this soon, in parallel with OT.

Scott Haseley

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Sep 25, 2023, 12:55:19 PM9/25/23
to blink-dev
Hi API Owners,

I'd like to extend the initial OT for two milestones (through M120). We've fixed some bugs and have had some recent signups, so I'd like to make sure folks have enough time to experiment. The initial OT was configured for 4 milestones, so 2 additional would take this to the max of 6.

Thanks,
Scott

Chris Harrelson

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Sep 25, 2023, 1:10:08 PM9/25/23
to Scott Haseley, blink-dev
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