Phase A single GC mutator on scheduler threads

Platform spec ADR

Phase A single GC mutator on scheduler threads

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Spec standingStandard

Owner

Piotr Mikstacki pmikstacki@cybernomad.it

Submitter

Piotr Mikstacki pmikstacki@cybernomad.it

Current document

Articles

• Fiber scheduler and stacks - Design model Scheduler loops, parking, and stack switching implementation expectations. article Standard Reviewed Wed May 20
• Fiber scheduler and stacks - Verification and traceability Tests and ABI anchors for fiber scheduler work. article Standard Reviewed Wed May 20

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Completeness

OKfeature · 9 SpecSection(s) · 1 H2 heading(s)

Section id	Required	Found
what-this-feature-specifies	yes	yes
implementation-anchors	yes	yes

Full tree: run pnpm verify:platform-spec-layout (writes src/generated/platform-spec-layout-report.json).

Related spec docs

• Feature
  Fiber scheduler and stacks

  Parent feature hub

• Feature
  Memory and GC runtime contract

  Phase B parallel mutators

Context

Fibers may run on multiple OS threads while Phase A GC still uses one process-wide arena (enter_runtime_scope). Syscall pool workers must not become second mutators.

Decision

Rule	Detail
Phase A (default)	One thread at a time may execute Beskid allocations as GC mutator
Scheduler	Transfers mutator execution between fibers on that contract
Syscall pool	Workers run blocking host work without arbitrary Beskid mutator code or generated allocation; runtime tags them and traps stray allocations (assert_mutator_allowed)
Phase B (opt-in, v0.3)	Multiple Beskid mutators may share one heap by holding a MutatorAttachGuard from attach_phase_b_mutator; pointer-payload channel ops apply gc_write_barrier on send and receive
Future	Phase B becomes the default once preemption code emission and full concurrent-mark stress coverage land

Aligns with D-CORE-CONC-0007.

Consequences

run_blocking paths park fibers and resume on scheduler threads for mutator work.

Verification anchors

beskid_runtime scheduler + enter_runtime_scope; concurrency runtime tests (tests/concurrency.rs, tests/gc_concurrency.rs); Phase B opt-in coverage in tests/phase_b_concurrency.rs exercises multi-mutator allocation, pointer-payload channel write barriers, and the syscall-pool allocation guard.