Optimize fiber message queue draining

[coyaSONG]

Type

• Refactor
• Feature
• Bug Fix
• Optimization
• Documentation Update

Description

FiberRuntime stored pending fiber messages in an array and removed the first item with splice(0, 1). Each dequeue shifted the remaining items, making a burst of N queued messages quadratic to drain.

This replaces that array with Effect's existing unbounded MutableQueue and uses offer, poll, and isEmpty throughout the runtime. FIFO behavior and the existing scheduling logic are preserved, while enqueue and dequeue are constant-time. There is no public API change. A patch changeset is included.

A local median benchmark for draining 60,000 no-op stateful messages changed from 274.125 ms on the untouched base to 3.563 ms with this patch (about 77x faster).

Validation:

• corepack pnpm lint-fix
• corepack pnpm check
• corepack pnpm build
• corepack pnpm docgen
• pnpm test run packages/effect/test/Fiber.test.ts packages/effect/test/Runtime.test.ts — 24 passed
• pnpm test run packages/effect/test — 6,210 passed, 7 skipped, 3 todo; one unrelated BigDecimal/clampBigDecimal assertion failed locally under both Node 25 and Node 24. The test, helper, and BigDecimal source are unchanged by this PR, and the current upstream base Check run is green.

Development note: the implementation and PR text were prepared with AI assistance and validated with the repository checks above; no human review is claimed.

Related

• Related Issue Fiber message queue dequeues with Array.splice(0,1) — O(N²) to drain N queued messages #6309
• Closes Fiber message queue dequeues with Array.splice(0,1) — O(N²) to drain N queued messages #6309

[changeset-bot]

🦋 Changeset detected

Latest commit: a8a4717

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
effect	Patch

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