Runtimes

A benchmark number is only meaningful next to the runtime it was measured on. as-bench runs the same bench file on Node or any external WASI runtime, so you can measure on what you actually ship to.

asb run                      # node (default)
asb run --runtime wasmtime   # pure-WASI build under wasmtime
asb run --runtime wasmer
asb run --runtime wazero

How external runtimes work

The default node host imports a clock and reporting callbacks directly. External runtimes can't, so for them as-bench compiles a pure-WASI build: the module's only imports are wasi_snapshot_preview1, and it streams every engine event as framed binary messages over stdout (WIPC), which the CLI parses and feeds to the same renderer. Any WASI runtime can run it.

• Settings reach the guest as AS_BENCH_TUNE_* environment variables.
• Non-frame stdout (your console.log) passes straight through.
• --save-baseline works on external runtimes; --baseline comparison and --deterministic are node-host only (they need request/reply).

Custom runtime commands

For anything beyond the three shorthands, give a command template — on the CLI or in config:

asb run --runtime "wazero run <env:-env> <file>"

Placeholder	Expands to
<file>	the bench wasm path (appended as the last arg if you omit it)
<env:PREFIX>	the AS_BENCH_TUNE_* pairs as runtime env flags; a trailing = fuses them (<env:--env=> → --env=K=V)

The same shape is available as runOptions.runtime.cmd in config. Command parsing is quote-aware.

Pages

• Multiple Runtimes & Modes — compare runtimes in one table.
• Deterministic Mode — neutralize host nondeterminism.