Recipes

Two situations come up often and need no new command.

Compare build variants

To see what a build setting (opt-level, wasm-opt flags, a feature) does to size and memory, list each build as a module of its own. peakmark does not build anything: produce the variants yourself, then point one module at each artifact.

[[module]]
name     = "engine-size"
artifact = "target/variants/size/engine_bg.wasm"
glue     = "target/variants/size/engine.js"

[module.budget]
brotli = "170 kB"

[[module]]
name     = "engine-speed"
artifact = "target/variants/speed/engine_bg.wasm"
glue     = "target/variants/speed/engine.js"

[module.budget]
brotli = "190 kB"

[[scenario]]
module = "engine-size"
file   = "scripts/engine.mjs"

[[scenario]]
module = "engine-speed"
file   = "scripts/engine.mjs"

Both modules run the same scenario file, which is allowed: a scenario file may be listed once per module. Run peakmark check --details and read the values side by side. Accept only the variant you keep, or give each its own budget and accept them separately.

What this does not do: peakmark does not rank the variants, and it does not say which setting made the difference. Compare the numbers yourself, and compare modules built from the same source.

Check that a binding survives memory growth

Linear memory can move when it grows, and a JavaScript view created before the growth is then empty. peakmark cannot see whether a binding keeps using such a view, so it does not claim to detect it. Assert the behavior in the scenario instead: do the same read before and after an operation that grows memory, and fail when the result differs.

The method names below stand for your own binding.

export default async function (engine) {
  const probe = engine.summarize("a,b\n1,2\n");
  const before = JSON.stringify(probe);

  const big = engine.load(largeInput()); // grows linear memory
  const after = JSON.stringify(engine.summarize("a,b\n1,2\n"));
  big.free();

  if (after !== before) {
    throw new Error(`result changed after memory growth: ${before} != ${after}`);
  }
}

A scenario that throws stops the check with exit code 2 and shows the message. The scenario only covers the calls it makes: it shows that these calls survive growth, not that every binding does. Make sure the input is large enough to grow the memory, which the per-scenario trace in --json (after_run) confirms.