Compact machine worker loops

[SandroMaglione]

Summary

• keep each running machine in one generator loop instead of retaining a nested Effect.whileLoop body generator and its closures
• preserve all mailbox, state publication, invoke lifecycle, command, and emission ordering boundaries
• allocate and cache the live emitted-event runtime only after a machine actually emits
• continue relying on Effect's scheduler operation budget for cooperative yielding

Heap-retainer analysis attributed approximately 1.5 KiB per idle statechart to the nested loop and eager emission closures. Four order-balanced local process pairs reduced idle heap by 7.2% and parent-family heap by 6.0%, while improving the measured runtime, observed, and child-send hot paths by 17–26%.

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; local budgets are unchanged and no public API changed
• 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)	0.10s	0.09s
Machine.make (3 states, 2 events)	0.15s	0.15s
machine.handle (3 states, 2 transitions)	0.21s	0.20s
machine.handle (depth 8)	0.49s	0.48s
machine.handle (depth 12)	0.53s	0.51s
machine.handle (depth 16)	0.57s	0.57s
machine.handle (depth 24)	0.66s	0.64s
machine.handle (wide depth 16)	0.72s	0.70s
machine.handle (parallel/history/choice)	0.58s	0.56s
machine.handle (4 successive calls)	0.54s	0.54s
machine exact input/output/error/services	0.48s	0.47s
execution adapter readiness	0.57s	0.52s

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 INTEL(R) XEON(R) PLATINUM 8573C with Node v24.18.0.

Pull request baseline

Scenario	Effect Machine	XState 5	XState 6 alpha
Plan counter transitions	161,833 transitions/s	194,540 transitions/s	158,528 transitions/s
Drain burst with terminal fence	121,720 increments/s	582,229 increments/s	149,906 increments/s
Drain burst with a change observer	100,090 increments/s	581,575 increments/s	149,573 increments/s
Lookup and send to one child	108,143 increments/s	577,951 increments/s	143,283 increments/s
Start and stop a machine	31,093 machines/s	263,505 machines/s	107,089 machines/s
Start and stop a parent with one child	9,531 families/s	100,857 families/s	43,375 families/s

Memory profile	Effect Machine	XState 5	XState 6 alpha	Effect runtime primitives
Idle machine	16.6 KiB	3.8 KiB	6.1 KiB	—
Raw managed process	13.1 KiB	—	—	—
Two independent idle machines	32.3 KiB	7.3 KiB	11.8 KiB	—
Idle parent with one child	38.5 KiB	5.5 KiB	8.8 KiB	—
Parent with observed child registry	49.9 KiB	—	—	—
Parent with observed invoked child snapshots	38.7 KiB	—	—	—
Suspended Effect fiber	—	—	—	0.7 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	165,896 transitions/s	0.6% MAD	161,833 transitions/s	2.7% MAD	-2.4%
Drain burst with terminal fence	100,165 increments/s	1.1% MAD	121,720 increments/s	0.8% MAD	+21.5%
Drain burst with a change observer	84,781 increments/s	1.5% MAD	100,090 increments/s	0.6% MAD	+18.1%
Lookup and send to one child	90,740 increments/s	1.9% MAD	108,143 increments/s	0.9% MAD	+19.2%
Start and stop a machine	29,024 machines/s	0.2% MAD	31,093 machines/s	1.4% MAD	+7.1%
Start and stop a parent with one child	9,160 families/s	0.2% MAD	9,531 families/s	1.0% MAD	+4.1%
Idle machine heap per unit	17.8 KiB	0.0% MAD	16.6 KiB	0.0% MAD	-7.2%
Raw managed process heap per unit	13.1 KiB	0.0% MAD	13.1 KiB	0.0% MAD	-0.0%
Two independent idle machines heap per unit	34.9 KiB	0.0% MAD	32.3 KiB	0.0% MAD	-7.3%
Idle parent with one child heap per unit	41.0 KiB	0.0% MAD	38.5 KiB	0.0% MAD	-6.0%
Parent with observed child registry heap per unit	52.4 KiB	0.0% MAD	49.9 KiB	0.0% MAD	-4.7%
Parent with observed invoked child snapshots heap per unit	41.2 KiB	0.0% MAD	38.7 KiB	0.0% MAD	-6.0%

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.