Design Review: Prioritized Task Scheduling (big picture)

[shaseley]

こんにちは TAG-さん!

I'm requesting a TAG review of Prioritized Task Scheduling (big picture).

Scheduling is an important tool for improving website performance and user experience, particularly on interactive pages with a lot of JavaScript. Two important aspects of this are:

1. Yielding, or breaking up long tasks. Long tasks limit scheduling opportunities because JS tasks are not preemptable. Long tasks can block handling input or updating the UI in response to input, which is often a cause of poor responsiveness.
2. Prioritization, or running the most important work first. The (task/event loop) scheduler determines which task runs next on the event loop. Running higher priority work sooner can improve user experience by minimizing user-perceived latency of the associated user interaction.

The proposal consists of:

• a small number of semantic priorities, which are used to prioritize tasks and continuations, and are used by UAs to prioritize these tasks within the event loop
• several new APIs for scheduling and breaking up JavaScript tasks, with an associated priority
• modifications to a few existing async APIs to support prioritization

---

• Explainer¹ (minimally containing user needs and example code): https://github.com/WICG/scheduling-apis/blob/main/explainers/prioritized-task-scheduling.md
• Specification: https://wicg.github.io/scheduling-apis/ (this covers scheduler.postTask(), scheduler.yield(), TaskController, and TaskSignal. Other APIs in the explainer are at earlier stages of thought/design).
• User research: N/A
• Security and Privacy self-review²:
  • postTask(): https://github.com/WICG/scheduling-apis/blob/main/explainers/security-privacy-questionnaire-post-task.md
  • TaskSignal.any(): https://github.com/shaseley/abort-signal-any/blob/main/tag-questionnaire.md
  • yield(): https://github.com/WICG/scheduling-apis/blob/main/explainers/yield-and-continuation.md#self-review-questionnaire-security-and-privacy
• GitHub repo: https://github.com/WICG/scheduling-apis/
• Primary contacts (and their relationship to the specification):
  • Scott Haseley (@shaseley), Google (feature/spec author)
• Organization/project driving the design: Google
• External status/issue trackers for this feature (publicly visible, e.g. Chrome Status):
  • scheduler.postTask(): https://chromestatus.com/feature/6031161734201344
  • TaskSignal.any(): https://chromestatus.com/feature/5202879349522432
  • scheduler.yield(): https://chromestatus.com/feature/6266249336586240

Further details:

• [X ] I have reviewed the TAG's Web Platform Design Principles
• The group where the incubation/design work on this is being done (or is intended to be done in the future): WICG
• The group where standardization of this work is intended to be done ("unknown" if not known): WHATWG
• Existing major pieces of multi-implementer review or discussion of this design:
• Major unresolved issues with or opposition to this design:
• This work is being funded by: Google

You should also know that...

This review request is in response to a request for a "bigger picture" Scheduling APIs explainer (cc: @hober @plinss), and there is overlap with scheduler.yield()'s specification review in #966. Some of the work here (TaskController, TaskSignal (except .any()), and scheduler.postTask()) was reviewed in #647.