Optimize child process ownership

[SandroMaglione]

Summary

• consolidate child supervision and registry state into one compact owner
• track anonymous children directly instead of retaining per-child scope finalizers
• allocate child observation resources only when consumed while preserving ordered replay
• cover named/anonymous cleanup, initialization-stop races, and first-observation races

Changeset

• Added or updated for a library or package-metadata change
• Not required because this PR does not change src/ or package.json

Validation

• pnpm check
• Relevant example checks, when examples changed (not applicable)
• Reviewed the automated type-performance report, when the public TypeScript API or inference changed (public API unchanged; local budget passes)
• Reviewed the automated runtime-performance report, when runtime behavior changed

[github-actions]

Type performance

Measured with TypeScript 6.0.3 and skipLibCheck=true.

Scenario	Base	PR	Difference
Effect only	55	55	0 (0.0%)
Import effect-machine	55	55	0 (0.0%)
Machine.defineStates (3 states)	2,965	2,965	0 (0.0%)
Machine.make (3 states, 2 events)	8,715	8,715	0 (0.0%)
machine.handle (3 states, 2 transitions)	23,492	23,492	0 (0.0%)
machine.handle (depth 8)	117,075	117,075	0 (0.0%)
machine.handle (depth 12)	129,713	129,713	0 (0.0%)
machine.handle (depth 16)	145,055	145,055	0 (0.0%)
machine.handle (depth 24)	183,851	183,851	0 (0.0%)
machine.handle (wide depth 16)	214,189	214,189	0 (0.0%)
machine.handle (parallel/history/choice)	132,667	132,667	0 (0.0%)
machine.handle (4 successive calls)	128,674	128,674	0 (0.0%)
machine exact input/output/error/services	110,584	110,584	0 (0.0%)
execution adapter readiness	127,854	127,854	0 (0.0%)

Marginal instantiations are measured against the matching setup without that API call:

Scenario	Base	PR	Difference
Import effect-machine	0	0	0
Machine.defineStates (3 states)	2,910	2,910	0 (0.0%)
Machine.make (3 states, 2 events)	5,742	5,742	0 (0.0%)
machine.handle (3 states, 2 transitions)	14,777	14,777	0 (0.0%)
machine.handle (depth 8)	109,601	109,601	0 (0.0%)
machine.handle (depth 12)	121,039	121,039	0 (0.0%)
machine.handle (depth 16)	135,181	135,181	0 (0.0%)
machine.handle (depth 24)	171,577	171,577	0 (0.0%)
machine.handle (wide depth 16)	202,224	202,224	0 (0.0%)
machine.handle (parallel/history/choice)	114,331	114,331	0 (0.0%)
machine.handle (4 successive calls)	113,703	113,703	0 (0.0%)
machine exact input/output/error/services	100,670	100,670	0 (0.0%)
execution adapter readiness	102,092	102,092	0 (0.0%)

Check times (informational)

Scenario	Base	PR
Effect only	0.03s	0.03s
Import effect-machine	0.03s	0.03s
Machine.defineStates (3 states)	0.11s	0.11s
Machine.make (3 states, 2 events)	0.17s	0.16s
machine.handle (3 states, 2 transitions)	0.24s	0.23s
machine.handle (depth 8)	0.51s	0.53s
machine.handle (depth 12)	0.58s	0.59s
machine.handle (depth 16)	0.60s	0.63s
machine.handle (depth 24)	0.69s	0.71s
machine.handle (wide depth 16)	0.77s	0.80s
machine.handle (parallel/history/choice)	0.64s	0.66s
machine.handle (4 successive calls)	0.61s	0.62s
machine exact input/output/error/services	0.53s	0.53s
execution adapter readiness	0.57s	0.59s

Type instantiations are the comparison metric. Check time varies with runner load and is informational only.

[github-actions]

Runtime performance

Median of 5 independent benchmark processes on AMD EPYC 7763 64-Core Processor with Node v24.18.0.

Pull request baseline

Scenario	Effect Machine	XState 5	XState 6 alpha
Plan counter transitions	106,515 transitions/s	133,449 transitions/s	123,030 transitions/s
Drain burst with terminal fence	55,656 increments/s	401,210 increments/s	112,550 increments/s
Drain burst with a change observer	43,793 increments/s	398,436 increments/s	112,218 increments/s
Lookup and send to one child	51,560 increments/s	395,455 increments/s	107,534 increments/s
Start and stop a machine	10,850 machines/s	184,128 machines/s	73,448 machines/s
Start and stop a parent with one child	4,102 families/s	68,644 families/s	26,969 families/s

Memory profile	Effect Machine	XState 5	XState 6 alpha	Effect runtime primitives
Idle machine	17.8 KiB	3.8 KiB	6.1 KiB	—
Raw managed process	13.1 KiB	—	—	—
Two independent idle machines	34.8 KiB	7.3 KiB	11.8 KiB	—
Idle parent with one child	58.5 KiB	5.5 KiB	8.8 KiB	—
Parent with observed child registry	69.7 KiB	—	—	—
Suspended Effect fiber	—	—	—	0.6 KiB
Effect queue with waiting fiber	—	—	—	2.8 KiB
Effect mailbox actor shell	—	—	—	3.2 KiB

Effect Machine change from base

Metric	Base	Base variability	PR	PR variability	Difference
Plan counter transitions	107,136 transitions/s	2.5% MAD	106,515 transitions/s	0.7% MAD	-0.6%
Drain burst with terminal fence	54,216 increments/s	1.1% MAD	55,656 increments/s	1.0% MAD	+2.7%
Drain burst with a change observer	43,116 increments/s	2.1% MAD	43,793 increments/s	0.6% MAD	+1.6%
Lookup and send to one child	49,773 increments/s	1.5% MAD	51,560 increments/s	0.6% MAD	+3.6%
Start and stop a machine	10,866 machines/s	0.8% MAD	10,850 machines/s	1.1% MAD	-0.1%
Start and stop a parent with one child	3,797 families/s	0.7% MAD	4,102 families/s	0.8% MAD	+8.0%
Idle machine heap per unit	18.0 KiB	0.0% MAD	17.8 KiB	0.0% MAD	-1.2%
Raw managed process heap per unit	13.3 KiB	0.0% MAD	13.1 KiB	0.0% MAD	-1.5%
Two independent idle machines heap per unit	35.3 KiB	0.0% MAD	34.8 KiB	0.0% MAD	-1.2%
Idle parent with one child heap per unit	63.7 KiB	0.0% MAD	58.5 KiB	0.0% MAD	-8.1%
Parent with observed child registry heap per unit	72.5 KiB	0.0% MAD	69.7 KiB	0.0% MAD	-3.9%

Versions and interpretation

• Effect Machine: 0.3.0
• XState 5: 5.32.5
• XState 6 alpha: 6.0.0-alpha.27
• Effect runtime primitives: 4.0.0-beta.105

Higher throughput is better; lower heap is better. Variability is the median absolute deviation across independent processes, relative to their median. Runtime measurements on shared GitHub-hosted hardware remain informational, so small differences should be confirmed across multiple workflow runs.