Intent to Experiment: scheduler.yield()
Scott Haseley
Contact emails
shas...@chromium.orgExplainer
https://github.com/WICG/scheduling-apis/blob/main/explainers/yield-and-continuation.mdSpecification
NoneSummary
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>APIsTAG review
https://github.com/w3ctag/design-reviews/issues/827TAG review status
PendingRisks
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
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)?
YesIs this feature fully tested by web-platform-tests?
YesDevTrial instructions
https://github.com/WICG/scheduling-apis/blob/main/implementation-status.mdFlag name
--enable-blink-features=SchedulerYieldRequires code in //chrome?
FalseTracking bug
https://bugs.chromium.org/p/chromium/issues/detail?id=979020Estimated milestones
| OriginTrial desktop last | 118 |
| OriginTrial desktop first | 115 |
| DevTrial on desktop | 113 |
| OriginTrial Android last | 118 |
| OriginTrial Android first | 115 |
| DevTrial on Android | 113 |
| OriginTrial webView last | 118 |
| OriginTrial webView first | 115 |
Link to entry on the Chrome Platform Status
https://chromestatus.com/feature/6266249336586240Links to previous Intent discussions
Intent to prototype: https://groups.google.com/a/chromium.org/d/msgid/blink-dev/CAKXGoJ1SBQP-ABM3%2BsDtKzUZiPoSCWqW2mLOjMrUfFBx4TomSw%40mail.gmail.com
Mike Taylor
LGTM to experiment from 115 to 118 inclusive.
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Domenic Denicola
> The signal inheritance bit [1], however, would need transpilation support to propagate the current signal across async (Promise) boundaries.
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Scott Haseley
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
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
To view this discussion on the web visit https://groups.google.com/a/chromium.org/d/msgid/blink-dev/CAKXGoJ3kOKppN1cEFr8RkN1T%2ByoEj%2BtLmUTMs5Z9hnLJh6wYUQ%40mail.gmail.com.