motoko-benchmarks-generation

name	motoko-benchmarks-generation
description	How to write benchmarks in Motoko using bench‑helper. Covers project setup (mops.toml), bench file layout in bench/*.bench.mo, the Bench.Schema rows/cols model, and safe patterns for encode/decode, hashing, crypto, and allocation benches.

Motoko Benchmarks with bench‑helper

What This Is

bench-helper is a tiny Motoko library that standardizes how to write benchmarks. You describe a benchmark using a small schema (name, rows, cols), provide a run(row, col) function, and return a versioned bench record. Each file under bench/*.bench.mo defines one benchmark module. A runner can then discover and execute all benches consistently.

Prerequisites

mops.toml (add dependencies and toolchain)

If you already have a mops.toml, just add bench-helper under [dev-dependencies]. If your project still uses mo:base instead of mo:core, you can keep it — benches themselves can be written with mo:base without affecting your runtime canisters.

Directory & File Conventions

• Put benches under bench/ at the repo root.
• Name files with the suffix .bench.mo, one benchmark per file; for example: bench/base64.bench.mo.
• Each bench file is a Motoko module { ... } that exposes a single public func init() : Bench.V1 function.
• Inside init, construct a Bench.Schema and return Bench.V1(schema, run) where run : (rowIndex : Nat, colIndex : Nat) -> () performs the measured operation.

Minimal skeleton

import Array "mo:core/Array";
import Bench "mo:bench-helper";

module {
  /// One benchmark per file; `mops bench` calls `init()` to obtain the
  /// schema and the run function.
  public func init() : Bench.V1 {
    let schema : Bench.Schema = {
      name = "My bench";
      description = "What this bench measures";
      rows = ["size 16", "size 64", "size 256"]; // input categories
      cols = ["operation A", "operation B"];     // operations to measure
    };

    // Prepare inputs once in `init` so they are not re-created on every call.
    // `Array.tabulate` returns an immutable `[Nat8]`; use `VarArray.repeat`
    // (or `VarArray.tabulate`) if you need a `[var Nat8]` instead.
    let inputs : [[Nat8]] = [
      Array.tabulate<Nat8>(16, func _ = 0),
      Array.tabulate<Nat8>(64, func _ = 0),
      Array.tabulate<Nat8>(256, func _ = 0),
    ];

    // Build a table of routines: routines[ri][ci] : () -> ()
    let routines : [[() -> ()]] = Array.tabulate<[() -> ()]>(
      schema.rows.size(),
      func(ri) {
        let input = inputs[ri]; // capture precomputed input
        [
          func() { ignore input.size() }, // operation A @ inputs[ri]
          func() { ignore input[0] },     // operation B @ inputs[ri]
        ];
      },
    );

    // `Bench.V1` is a *class* (instance type), not a record alias.
    // The runner calls the second argument many times; keep it tiny
    // and branch-free.
    Bench.V1(schema, func(ri : Nat, ci : Nat) = routines[ri][ci]());
  };
};

Notes:

• The Schema field is cols, not columns. Same for rows.size() — you must say schema.rows.size() (the bare rows identifier is not in scope).
• mo:core/Array has no init function. To build an immutable [T], use Array.tabulate<T>(size, gen) or Array.repeat<T>(val, size). For a mutable [var T], import VarArray "mo:core/VarArray" and use VarArray.tabulate<T>(size, gen) or VarArray.repeat<T>(val, size); annotate such inputs as [var T] (e.g. [[var Nat8]] for a table of them).
• If you're not using mo:core, replace mo:core imports with mo:base. Note that mo:base/Array covers both immutable and mutable arrays — there is no separate VarArray module. The interface is not identical to mo:core; for the Array API the relevant equivalences are:
  • mo:core Array.tabulate(size, gen) → mo:base Array.tabulate(size, gen)
  • mo:core Array.repeat(val, size) → mo:base Array.freeze(Array.init(size, val)) (no direct immutable repeat)
  • mo:core VarArray.tabulate(size, gen) → mo:base Array.tabulateVar(size, gen)
  • mo:core VarArray.repeat(val, size) → mo:base Array.init(size, val) (args reversed) See the base-core-migration skill for the full mapping.

How It Works

• Schema
  • name and description describe the bench.
  • rows enumerate different operations or variants you measure (e.g., "encode", "decode").
  • cols enumerate different input categories (e.g., message sizes).
• Runner contract
  • You return a versioned record Bench.V1(schema, run); the runner calls run(rowIndex, colIndex) many times to record timings.
  • Side effects/results inside run should be consumed (e.g., ignore ...) to prevent dead‑code elimination.

Common Pitfalls

1. Rows/cols mismatch
   • Ensure your routines table has dimensions rows.size() x cols.size(). If you add or remove a row/col label, update how you build routines; otherwise some cells will be no‑ops.
2. Doing expensive setup inside run
   • Generate inputs once in init and capture them in closures. Only do the core operation in run.
3. Forgetting to consume results
   • Use ignore to consume return values; otherwise the compiler might drop the call as dead code.
4. Non‑determinism and timing noise
   • Keep run free of logging/printing and random allocation; keep GC pressure comparable across rows.
5. Repeated runs and mutating operations
   • bench-helper calls run(ri, ci) many times per cell to gather timing samples. If the measured operation mutates the captured input (e.g. an in-place sort, an in-place hash update), the second call onwards sees the post-operation state — for an in-place sort, the already-sorted array from the previous call; for an in-place hash update, the accumulator left behind by the previous call. Either rebuild the input inside run (and accept the allocation cost as part of the measurement) or document that the bench measures repeated runs on a mutated/post-operation state rather than on a freshly prepared input.

More examples

bench-helper reference benches: https://github.com/research-ag/bench-helper/tree/main/bench

Verify It Works

The exact runner/command may vary depending on your environment. After adding bench-helper to mops.toml and writing .bench.mo files:

• Ensure your project resolves dependencies:

  mops install
  

• Run benchmarks:

  mops bench
  

• Consult the bench-helper package README for the latest recommended runner command for your toolchain/version.