Intent to Extend Origin Trial: scheduler.postTask
Scott Haseley
Contact emails
shas...@chromium.org, jap...@chromium.org
Explainer
https://github.com/WICG/main-thread-scheduling/blob/master/PrioritizedPostTask.md
Specification
None
Notes:
1. Please note the spec requirement wasn’t yet in place for the original I2E, but the explainer is detailed with regard to the API details, including how the priorities work and will be specified.
2. This might fall into the category of “early experimentation”, where we expect some iteration based on partner feedback (more below). We hope to start working on a proper spec soon, but we’ve put that on hold since we’re not entirely sure what will be included in V0, and OT feedback will likely help shape that.
Design docs
https://docs.google.com/document/d/1Apz-SD-pOagGeyWxIpgOi0ARNkrCrELhPdm18eeu9tw/edit?usp=sharing
Summary
Userspace tasks often have varying degrees of importance (related to user experience), but the Platform lacks a unified API to schedule prioritized work. The postTask API allows developers to schedule tasks (javascript callbacks) with a native browser scheduler at 3 levels of priority: user-blocking, user-visible, and background. It also exposes a TaskController, which can be used to dynamically cancel tasks and change their priority.
Blink component
Search tags
scheduling, scheduler, main thread, main-thread
TAG review
https://github.com/w3ctag/design-reviews/issues/338
TAG review status
Pending
Risks
Interoperability and Compatibility
If other browsers do not implement the APIs, the situation is no worse than today; it will be possible to feature detect and take advantage of improved scheduling on browsers that do ship the API. This API was recently discussed at the WebPerfWG F2F (https://docs.google.com/document/d/1uQ7pXwuBv-1jitYou7TALJxV0tllXLxTyEjA2n1mSzY/edit?ts=5d0774e5#heading=h.ptau5ck3jdlk). The overall sentiment was positive, especially from developers present at the meeting. Browser vendors appear generally cautious, but supportive.
Gecko: No signal
WebKit: No signal Priority inversion and dependency tracking are concerns. We plan to flesh these concerns out and address them as the API progresses.
Web developers: Strongly positive Positive feedback at WebPerf WG F2F (https://docs.google.com/document/d/1uQ7pXwuBv-1jitYou7TALJxV0tllXLxTyEjA2n1mSzY/edit?ts=5d0774e5#heading=h.ptau5ck3jdlk). We are also closely working with React and Airbnb, who are strongly positive about the API.
Ergonomics
Could the default usage of this API make it hard for Chrome to maintain good performance? Unlikely. There is always concern around starvation and abuse when introducing (higher) priorities. But it is always a possibility that script just won't yield (or queue microtasks, or abuse rAF) -- which are as high or higher than priorities we are planning to introduce. We will continue, however, to think about abuse and mitigation as we develop the API.
Activation
Will it be challenging for developers to take advantage of this feature immediately, as-is? No. Would this feature benefit from having polyfills, significant documentation and outreach, and/or libraries built on top of it to make it easier to use? Yes.
Security
Goals for experimentation
API shape:
- We want to validate that the new Signal/Controller-based API is ergonomic for developers (the shape has evolved quite a bit). Is it easy to use/integrate with existing schedulers or app code?
- We also want feedback on the priorities: are the priorities intuitive/useful? Are there (general) use cases that aren’t captured by these priorities? (See explainer for more discussion around number of priorities).
Performance: We’re relying on site-specific latency and responsiveness metrics for measuring performance. Specifically, we’re looking to find out if site-specific performance improves, regresses, or stays the same. The outcome can depend on a lot of factors, e.g. does the site already have a scheduler? how much of the page does the site own? how well does the implementation’s approach to integrating postTask tasks and tasks from other task sources work? Our hypothesis is that performance shouldn’t regress, but we want to validate this. And if it does regress performance, we want to understand why and possibly vary the implementation.
Usefulness of the MVP: We’re looking to answer a couple questions here:
1. Does the API simplify scheduling?
2. Is the postTask MVP useful enough on its own, or are there vital features missing? For example, are there enough guarantees/predictability around postTask priorities to be useful, or are priorities needed elsewhere (e.g. fetch, IDB)?
Experimental timeline
M86 -- M87
Further rationale and notes:
- We have partners ready to experiment immediately and soon (see below). Given this current engagement, we’d like to continue experimenting now
- Please note that the OT wasn’t enabled in M85 (we are aware of the burn-in risk)
- We will likely need to extend the OT again once we integrate partner feedback. Our alternative plan was to extend the OT for 3 milestones now, but given the branch point for targeting M88 is Nov. 12, we don’t feel that would be enough time to incorporate feedback. We’re calling this out here since that could potentially take us over 6 total milestones, albeit with gaps.
Reason this experiment is being extended
Previous I2E: https://groups.google.com/a/chromium.org/g/blink-dev/c/31qZ-bMToM8/m/k1BefOpMAwAJ
We were unable to achieve our experimentation goals in the first OT, largely due to COVID-related partner delays. Our initial OT also only ran for 2 milestones since M82 was skipped. We have one partner that would like to continue experimenting immediately (React / Facebook -- they turned down the experiment after it exposed an internal bug, and are ready to resume), and one that wants to start in this new timeframe (Airbnb).
Ongoing technical constraints
None.
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
But it’s fully tested by Chromium wpt_internal tests: https://source.chromium.org/chromium/chromium/src/+/master:third_party/blink/web_tests/wpt_internal/scheduler/
Tracking bug
https://bugs.chromium.org/p/chromium/issues/detail?id=979017
Launch bug
https://bugs.chromium.org/p/chromium/issues/detail?id=1048941
Link to entry on the Chrome Platform Status
https://chromestatus.com/feature/6031161734201344
Links to previous Intent discussions
Intent to prototype: https://groups.google.com/a/chromium.org/g/blink-dev/c/HvqbB7TGJKU/m/Kd0WUGXkAQAJ
Intent to Experiment: https://groups.google.com/a/chromium.org/g/blink-dev/c/31qZ-bMToM8/m/k1BefOpMAwAJ
This intent message was generated by Chrome Platform Status.
Yoav Weiss
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