dev-bench
^{_{PERFORMANCE MEASUREMENT FOR RUST}}

Performance measurement and regression detection.
Part of the dev-* verification suite.

---

What it does

Measures performance and produces verdicts the same way the rest of the dev-* suite does: machine-readable, stored as dev-report results.

Designed for AI agents and CI gating, not for interactive profiling. For interactive profiling, use criterion or divan.

Quick start

[dependencies]
dev-bench = "0.9"

use dev_bench::{Benchmark, Threshold};

let mut b = Benchmark::new("parse_query");
for _ in 0..1000 {
    b.iter(|| std::hint::black_box(40 + 2));
}

let result = b.finish();
let threshold = Threshold::regression_pct(10.0);

// If you have a stored baseline mean from a previous run, pass it in.
// Returns a CheckResult that can be pushed into a dev_report::Report.
let _check = result.compare_against_baseline(None, threshold);

The returned CheckResult carries the bench tag and numeric Evidence for mean_ns, baseline_ns, p50_ns, p99_ns, cv, ops_per_sec, samples, and iterations_recorded — no parsing of free-form detail strings.

Variance-aware verdicts

Noisy machines produce false regressions. dev-bench computes the coefficient of variation alongside the mean and downgrades regressions within the noise band from Fail to Warn:

use dev_bench::{Benchmark, CompareOptions, Threshold};
use std::time::Duration;

let mut b = Benchmark::new("hot");
for _ in 0..30 { b.iter(|| std::hint::black_box(1 + 1)); }
let r = b.finish();

let opts = CompareOptions {
    baseline_mean: Some(Duration::from_nanos(1000)),
    threshold: Threshold::regression_pct(10.0),
    min_samples: 30,
    allow_cv_noise_band: true,
};
let _check = r.compare_with_options(&opts);

Throughput

For sub-microsecond operations where Instant::now() overhead would dominate per-iter timing, batch with iter_with_count:

use dev_bench::{Benchmark, Threshold};

let mut b = Benchmark::new("hot");
b.iter_with_count(100_000, || {
    std::hint::black_box(1 + 1);
});
let r = b.finish();
println!("{} ops/sec", r.ops_per_sec());

Baseline storage

Baselines persist via the BaselineStore trait. A JSON file backend ships out of the box:

use dev_bench::{Baseline, BaselineStore, JsonFileBaselineStore};

let store = JsonFileBaselineStore::new("./baselines");
let baseline = Baseline {
    name: "parse_query".into(),
    mean_ns: 1234,
    samples: 1000,
    ops_per_sec: 810_000.0,
};
store.save("main", &baseline).unwrap();

if let Some(b) = store.load("main", "parse_query").unwrap() {
    // use b.mean() in compare_against_baseline(...)
}

Saves are atomic (write-temp-rename); loads tolerate missing files.

Producer trait

To plug into dev-report::MultiReport aggregation alongside other producers, wrap a benchmark in BenchProducer:

use dev_bench::{BenchProducer, Benchmark, BenchmarkResult, Threshold};
use dev_report::Producer;

fn run() -> BenchmarkResult {
    let mut b = Benchmark::new("parse");
    for _ in 0..100 { b.iter(|| std::hint::black_box(1 + 1)); }
    b.finish()
}

let producer = BenchProducer::new(run, "0.1.0", None, Threshold::regression_pct(10.0));
let report = producer.produce();   // dev_report::Report

Allocation tracking (opt-in)

[dependencies]
dev-bench = { version = "0.9", features = ["alloc-tracking"] }

#[global_allocator]
static ALLOC: dhat::Alloc = dhat::Alloc;

let _profiler = dhat::Profiler::new_heap();
// ... run benchmarked code ...
let stats = dev_bench::alloc::AllocationStats::snapshot();
let check = stats.compare_against_baseline("parse", baseline_alloc, 10.0);

The dhat allocator changes timing characteristics, so do not combine timing thresholds with allocation thresholds in the same invocation.

Design choices

• Output is a CheckResult, not a stdout dump. Agents can read it.
• Regression-first, not absolute-perf-first. The interesting question is "did this change make it slower," not "is this the fastest in the world."
• Configurable thresholds: percent-based, absolute-nanosecond, or throughput-drop-percent.
• Variance-aware: regressions within the CV noise band are flagged as Warn, not Fail.

The dev-* suite

dev-bench is one of the producer crates. See dev-tools for the umbrella crate and dev-report for the schema all results conform to.

Status

v0.9.x is the pre-1.0 stabilization line. APIs are expected to be near-final; minor adjustments may still happen ahead of 1.0. The statistic definitions (mean, p50, p99) are pinned and will not change.

Minimum supported Rust version

1.85 — pinned in Cargo.toml via rust-version and verified by the MSRV job in CI. (Bumped from 1.75 because the alloc-tracking feature pulls dhat → addr2line, which requires Rust 1.81+, and sibling crates require edition2024 (1.85+).)

License

Apache-2.0. See LICENSE.