Let Effect schedule machine event loops

[SandroMaglione]

Summary

• Let Effect's runtime scheduler control cooperative yielding while draining machine event bursts.
• Remove the forced scheduler turn after every processed or ignored event.
• Verify that scheduler-configured fairness still allows concurrent stops and invoked children to progress during sustained backlogs.

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
• Reviewed the automated type-performance report, when the public TypeScript API or inference changed
• Reviewed the automated runtime-performance report, when runtime behavior changed

Local validation also included pnpm perf:types and five interleaved benchmark processes comparing main, a fixed 32-event yield cadence, and Effect scheduler-only behavior. Scheduler-only behavior matched cadence 32 while improving the event throughput scenarios by approximately 54–62% over main.

[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)	8,731	8,731	0 (0.0%)
machine.handle (3 states, 2 transitions)	23,634	23,634	0 (0.0%)
machine.handle (depth 8)	113,023	113,023	0 (0.0%)
machine.handle (depth 12)	125,661	125,661	0 (0.0%)
machine.handle (depth 16)	141,003	141,003	0 (0.0%)
machine.handle (depth 24)	179,799	179,799	0 (0.0%)
machine.handle (wide depth 16)	210,137	210,137	0 (0.0%)
machine.handle (parallel/history/choice)	128,876	128,876	0 (0.0%)
machine.handle (4 successive calls)	129,498	129,498	0 (0.0%)
machine exact input/output/error/services	107,371	107,371	0 (0.0%)
execution adapter readiness	122,890	122,890	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)	5,758	5,758	0 (0.0%)
machine.handle (3 states, 2 transitions)	14,903	14,903	0 (0.0%)
machine.handle (depth 8)	105,546	105,546	0 (0.0%)
machine.handle (depth 12)	116,984	116,984	0 (0.0%)
machine.handle (depth 16)	131,126	131,126	0 (0.0%)
machine.handle (depth 24)	167,522	167,522	0 (0.0%)
machine.handle (wide depth 16)	198,169	198,169	0 (0.0%)
machine.handle (parallel/history/choice)	110,514	110,514	0 (0.0%)
machine.handle (4 successive calls)	114,497	114,497	0 (0.0%)
machine exact input/output/error/services	95,819	95,819	0 (0.0%)
execution adapter readiness	96,968	96,968	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)	0.09s	0.09s
Machine.make (3 states, 2 events)	0.15s	0.14s
machine.handle (3 states, 2 transitions)	0.22s	0.22s
machine.handle (depth 8)	0.47s	0.46s
machine.handle (depth 12)	0.53s	0.51s
machine.handle (depth 16)	0.54s	0.53s
machine.handle (depth 24)	0.63s	0.61s
machine.handle (wide depth 16)	0.68s	0.68s
machine.handle (parallel/history/choice)	0.57s	0.57s
machine.handle (4 successive calls)	0.53s	0.53s
machine exact input/output/error/services	0.49s	0.48s
execution adapter readiness	0.51s	0.51s

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	27,376 transitions/s	133,039 transitions/s	121,363 transitions/s
Drain burst with terminal fence	18,444 increments/s	398,728 increments/s	110,674 increments/s
Drain burst with a change observer	16,719 increments/s	395,699 increments/s	110,056 increments/s
Lookup and send to one child	16,352 increments/s	394,658 increments/s	105,734 increments/s
Start and stop a machine	6,296 machines/s	183,824 machines/s	72,696 machines/s
Start and stop a parent with one child	2,535 families/s	67,622 families/s	26,503 families/s

Memory profile	Effect Machine	XState 5	XState 6 alpha
Idle machine	25.0 KiB	3.8 KiB	6.1 KiB
Idle parent with one child	74.8 KiB	5.5 KiB	8.8 KiB

Effect Machine change from base

Metric	Base	Base variability	PR	PR variability	Difference
Plan counter transitions	26,698 transitions/s	3.3% MAD	27,376 transitions/s	2.5% MAD	+2.5%
Drain burst with terminal fence	15,432 increments/s	4.4% MAD	18,444 increments/s	0.4% MAD	+19.5%
Drain burst with a change observer	14,484 increments/s	0.2% MAD	16,719 increments/s	2.0% MAD	+15.4%
Lookup and send to one child	14,269 increments/s	2.8% MAD	16,352 increments/s	1.5% MAD	+14.6%
Start and stop a machine	6,230 machines/s	1.4% MAD	6,296 machines/s	0.8% MAD	+1.1%
Start and stop a parent with one child	2,583 families/s	2.3% MAD	2,535 families/s	0.1% MAD	-1.9%
Idle machine heap per unit	25.0 KiB	0.0% MAD	25.0 KiB	0.0% MAD	+0.0%
Idle parent with one child heap per unit	74.8 KiB	0.0% MAD	74.8 KiB	0.0% MAD	-0.0%

Versions and interpretation

• Effect Machine: 0.3.0
• XState 5: 5.32.5
• XState 6 alpha: 6.0.0-alpha.27

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.