CARGO-BENCH(1)

📦 version 1.79.0, in ubuntu jammy, ubuntu noble

💖 cargo-bench 💖

okay so bestie, this is the manual page for cargo-bench — the one and only "Execute benchmarks of a package" command. sit down, grab a boba, we're doing this ✨

✨ SYNOPSIS ✨

cargo bench [options] [benchname] [-- bench-options]

🦋 DESCRIPTION 🦋

So basically: this compiles your benchmarks and then runs them. That's the whole vibe. 🎀

Here's the tea on how arguments work: the benchmark filtering argument benchname plus everything after the two dashes (--) gets passed straight to the benchmark binaries, and from there to libtest (which is rustc's built-in unit-test and micro-benchmarking framework, iykyk). So if you're passing args to both Cargo and the binary, remember the split: stuff after -- goes to the binary, stuff before it goes to Cargo. If you wanna know all of libtest's arguments, just run cargo bench -- --help and also go read the rustc book's chapter on how tests work at https://doc.rust-lang.org/rustc/tests/index.html 📖

Example time! This one runs only the benchmark named foo, and it will skip other similarly named benchmarks like foobar. iconic behavior honestly:

cargo bench -- foo --exact

Benchmarks get built with the --test option to rustc, which makes a special executable by linking your code with libtest. That executable automatically runs every function annotated with the #[bench] attribute. Cargo passes the --bench flag to the test harness to tell it "hey, run only benchmarks please" — and it does this regardless of whether the harness is libtest or a custom harness. 💅

The libtest harness can be disabled by setting harness = false in the target manifest settings — and if you do that, your code has to provide its own main function to handle running benchmarks. no harness, no hand-holding, you're on your own 🫶

Note: the #[bench] attribute (https://doc.rust-lang.org/nightly/unstable-book/library-features/test.html) is currently unstable and only available on the nightly channel (https://doc.rust-lang.org/book/appendix-07-nightly-rust.html). There are some packages on crates.io (https://crates.io/keywords/benchmark) that may help with running benchmarks on the stable channel, such as Criterion (https://crates.io/crates/criterion). 🌈

By default, cargo bench uses the bench profile (https://doc.rust-lang.org/cargo/reference/profiles.html#bench), which enables optimizations and disables debugging information. If you need to actually debug a benchmark, you can use the --profile=dev command-line option to switch to the dev profile — and then run your debug-enabled benchmark inside a debugger. slay 🔥

🌸 Working directory of benchmarks 🌸

The working directory of every benchmark is set to the root directory of the package the benchmark belongs to. Doing it this way means benchmarks can reliably access the package's files using relative paths, no matter where you ran cargo bench from. lowkey thoughtful of them 💜

👑 OPTIONS 👑

🎀 Benchmark Options 🎀

  • --no-run — compile it, but don't run the benchmarks. build only, bestie.
  • --no-fail-fast — run all the benchmarks regardless of failure. Without this flag, Cargo will exit after the first executable fails. (Heads up: the Rust test harness will run all benchmarks within the executable to completion anyway — this flag only applies to the executable as a whole.)

🍓 Package Selection 🍓

By default, when you don't give any package selection options, which packages get selected depends on the selected manifest file (which is based on the current working directory if --manifest-path isn't given). If the manifest is the root of a workspace, then the workspace's default members are selected. Otherwise, only the package defined by the manifest gets selected.

The default members of a workspace can be set explicitly with the workspace.default-members key in the root manifest. And if that's not set: a virtual workspace will include all workspace members (equivalent to passing --workspace), while a non-virtual workspace will include only the root crate itself. 💫

  • -p spec…, --package spec… — benchmark only the specified packages. See cargo-pkgid(1) for the SPEC format. This flag may be specified multiple times and supports common Unix glob patterns like *, ? and []. But bestie, listen: to avoid your shell accidentally expanding glob patterns before Cargo gets to handle them, you must use single quotes or double quotes around each pattern. 🙈
  • --workspace — benchmark all members in the workspace. everybody's invited.
  • --all — deprecated alias for --workspace. 👵
  • --exclude *SPEC*… — exclude the specified packages. Must be used in conjunction with the --workspace flag. This flag may be specified multiple times and supports common Unix glob patterns like *, ? and []. And again: to avoid your shell expanding glob patterns before Cargo handles them, you must wrap each pattern in single quotes or double quotes.

🧚 Target Selection 🧚

When no target selection options are given, cargo bench will build these targets of the selected packages:

  • lib — used to link with binaries and benchmarks
  • bins (only if benchmark targets are built and required features are available)
  • lib as a benchmark
  • bins as benchmarks
  • benchmark targets

The default behavior can be changed by setting the bench flag for the target in the manifest settings. Setting examples to bench = true will build and run the example as a benchmark, replacing the example's main function with the libtest harness. 😌

Setting targets to bench = false will stop them from being bencharmked by default (that typo is theirs, not mine, bestie 💀). Target selection options that take a target by name (such as --example foo) ignore the bench flag and will always benchmark the given target.

See Configuring a target (https://doc.rust-lang.org/cargo/reference/cargo-targets.html#configuring-a-target) for more info on per-target settings.

Binary targets are automatically built if there's an integration test or benchmark being selected to benchmark — this lets an integration test execute the binary to exercise and test its behavior. The CARGO_BIN_EXE_<name> environment variable (https://doc.rust-lang.org/cargo/reference/environment-variables.html#environment-variables-cargo-sets-for-crates) is set when the integration test is built so that it can use the env macro (https://doc.rust-lang.org/std/macro.env.html) to locate the executable. 🔍

Passing target selection flags will benchmark only the specified targets.

Note that --bin, --example, --test and --bench flags also support common Unix glob patterns like *, ? and []. However, to avoid your shell accidentally expanding glob patterns before Cargo handles them, you must use single quotes or double quotes around each glob pattern. 🫶
  • --lib — benchmark the package's library.
  • --bin *name*… — benchmark the specified binary. This flag may be specified multiple times and supports common Unix glob patterns.
  • --bins — benchmark all binary targets.
  • --example *name*… — benchmark the specified example. This flag may be specified multiple times and supports common Unix glob patterns.
  • --examples — benchmark all example targets.
  • --test *name*… — benchmark the specified integration test. This flag may be specified multiple times and supports common Unix glob patterns.
  • --tests — benchmark all targets in test mode that have the test = true manifest flag set. By default this includes the library and binaries built as unittests, plus integration tests. Be aware: this will also build any required dependencies, so the lib target may be built twice (once as a unittest, and once as a dependency for binaries, integration tests, etc.). Targets may be enabled or disabled by setting the test flag in the manifest settings for the target.
  • --bench *name*… — benchmark the specified benchmark. This flag may be specified multiple times and supports common Unix glob patterns.
  • --benches — benchmark all targets in benchmark mode that have the bench = true manifest flag set. By default this includes the library and binaries built as benchmarks, plus bench targets. And again, be aware: this will also build any required dependencies, so the lib target may be built twice (once as a benchmark, and once as a dependency for binaries, benchmarks, etc.). Targets may be enabled or disabled by setting the bench flag in the manifest settings for the target.
  • --all-targets — benchmark all targets. This is equivalent to specifying --lib --bins --tests --benches --examples. go big or go home ✨

💅 Feature Selection 💅

The feature flags let you control which features are enabled. When no feature options are given, the default feature is activated for every selected package.

See the features documentation (https://doc.rust-lang.org/cargo/reference/features.html#command-line-feature-options) for more details.

  • -F features, --features features — space or comma separated list of features to activate. Features of workspace members may be enabled with package-name/feature-name syntax. This flag may be specified multiple times, which enables all specified features.
  • --all-features — activate all available features of all selected packages. all of them. every last one.
  • --no-default-features — do not activate the default feature of the selected packages. 🙅

🌈 Compilation Options 🌈

  • --target triple — benchmark for the given architecture. The default is the host architecture. The general format of the triple is <arch><sub>-<vendor>-<sys>-<abi>. Run rustc --print target-list for a list of supported targets. This flag may be specified multiple times.

This may also be specified with the build.target config value (https://doc.rust-lang.org/cargo/reference/config.html).

Note that specifying this flag makes Cargo run in a different mode where the target artifacts are placed in a separate directory. See the build cache documentation (https://doc.rust-lang.org/cargo/guide/build-cache.html) for more details.

  • --profile name — benchmark with the given profile. See the reference (https://doc.rust-lang.org/cargo/reference/profiles.html) for more details on profiles.
  • --timings=fmts — output information about how long each compilation takes, and track concurrency information over time. Accepts an optional comma-separated list of output formats; --timings without an argument will default to --timings=html. Specifying an output format (rather than the default) is unstable and requires -Zunstable-options. Valid output formats:

(part 2 incoming bestie, don't go anywhere 💖)

  • html (unstable, needs -Zunstable-options) — writes a whole human-readable file called cargo-timing.html into the target/cargo-timings directory, with a full report of the compilation. And if you're the type to go back and stare at older runs, it also drops a report in that same directory with a timestamp in the filename. Heads up though: HTML output is for human eyeballs only, it does not give you machine-readable timing data.
  • json (unstable, needs -Zunstable-options) — emits machine-readable JSON information about timing information. That's it, that's the vibe 💅

🎀 Output Options 🎀

  • --target-dir directory — the directory where all generated artifacts and intermediate files go. You can also set this with the CARGO_TARGET_DIR environment variable, or with the build.target-dir config value at https://doc.rust-lang.org/cargo/reference/config.html. Defaults to target in the root of the workspace.

📺 Display Options 📺

okay so by default the Rust test harness is a lil shy and hides output from benchmark execution, all so your results stay readable. but bestie, you can totally recover that benchmark output (like, for debugging, you know) by passing --nocapture to the benchmark binaries:

cargo bench -- --nocapture
  • -v, --verbose — use verbose output. Pass it twice for "very verbose" output, which throws in extra stuff like dependency warnings and build script output. You can also set this with the term.verbose config value at https://doc.rust-lang.org/cargo/reference/config.html.
  • -q, --quiet — do not print cargo log messages. Also settable with the term.quiet config value at https://doc.rust-lang.org/cargo/reference/config.html.
  • --color when — controls when colored output gets used. Valid values are:
    • auto (the default): automatically detect if the terminal supports color.
    • always: always display colors.
    • never: never display colors.

Also settable with the term.color config value at https://doc.rust-lang.org/cargo/reference/config.html.

  • --message-format fmt — the output format for diagnostic messages. You can pass this multiple times, and it's made of comma-separated values. Valid values:
    • human (the default): display in a human-readable text format. Conflicts with short and json.
    • short: emit shorter, human-readable text messages. Conflicts with human and json.
    • json: emit JSON messages to stdout. See the reference at https://doc.rust-lang.org/cargo/reference/external-tools.html#json-messages for more details. Conflicts with human and short.
    • json-diagnostic-short: makes sure the rendered field of JSON messages contains the "short" rendering from rustc. Cannot be used with human or short.
    • json-diagnostic-rendered-ansi: makes sure the rendered field of JSON messages contains embedded ANSI color codes, so it respects rustc's default color scheme. Cannot be used with human or short.
    • json-render-diagnostics: tells Cargo not to include rustc diagnostics in the JSON messages it prints, and instead have Cargo itself render the JSON diagnostics coming from rustc. Cargo's own JSON diagnostics, and others coming from rustc, are still emitted. Cannot be used with human or short.

📜 Manifest Options 📜

  • --manifest-path path — path to the Cargo.toml file. By default, Cargo goes looking for the Cargo.toml file in the current directory or any parent directory.
  • --ignore-rust-version — ignore the rust-version specification in packages.
  • --locked — asserts that the exact same dependencies and versions are used as when the existing Cargo.lock file was originally generated. Cargo will exit with an error when either of these scenarios comes up:
    • The lock file is missing.
    • Cargo attempted to change the lock file due to a different dependency resolution.

It may be used in environments where deterministic builds are desired, like in CI pipelines.

  • --offline — stops Cargo from accessing the network for any reason. Without this flag, Cargo just stops with an error if it needs the network and the network isn't available. With this flag, Cargo will attempt to proceed without the network if it possibly can.
⚠️ Beware, bestie — this may result in different dependency resolution than online mode. Cargo will restrict itself to crates that are downloaded locally, even if there might be a newer version as indicated in the local copy of the index. See the cargo-fetch(1) command to download dependencies before going offline.

Also settable with the net.offline config value at https://doc.rust-lang.org/cargo/reference/config.html.

  • --frozen — equivalent to specifying both --locked and --offline at the same time. two-for-one, iconic.

🧰 Common Options 🧰

  • +toolchain — if Cargo has been installed with rustup, and the first argument to cargo begins with +, it gets interpreted as a rustup toolchain name (like +stable or +nightly). See the rustup documentation at https://rust-lang.github.io/rustup/overrides.html for more info about how toolchain overrides work.
  • --config KEY=VALUE or PATH — overrides a Cargo configuration value. The argument should be in TOML syntax of KEY=VALUE, or given as a path to an extra configuration file. This flag can be specified multiple times. See the command-line overrides section at https://doc.rust-lang.org/cargo/reference/config.html#command-line-overrides for more.
  • -C PATH — changes the current working directory before executing any specified operations. This affects things like where cargo looks by default for the project manifest (Cargo.toml), and the directories searched for discovering .cargo/config.toml, for example. This option must appear before the command name, like cargo -C path/to/my-project build.
💀 This option is only available on the nightly channel (https://doc.rust-lang.org/book/appendix-07-nightly-rust.html) and requires the -Z unstable-options flag to enable (see #10098 at https://github.com/rust-lang/cargo/issues/10098).
  • -h, --help — prints help information. classic.
  • -Z flag — unstable (nightly-only) flags to Cargo. Run cargo -Z help for details.

🎲 Miscellaneous Options 🎲

okay so important tea: the --jobs argument affects the building of the benchmark executable, but it does not affect how many threads are used when running the benchmarks. The Rust test harness runs benchmarks serially, in a single thread. periodt.

  • -j N, --jobs N — number of parallel jobs to run. Also settable with the build.jobs config value at https://doc.rust-lang.org/cargo/reference/config.html. Defaults to the number of logical CPUs. If negative, it sets the maximum number of parallel jobs to the number of logical CPUs plus the provided value. If a string default is provided, it sets the value back to defaults. Should not be 0.

While cargo bench involves compilation, it does not provide a --keep-going flag. Use --no-fail-fast to run as many benchmarks as possible without stopping at the first failure. And to "compile" as many benchmarks as possible, use --benches to build benchmark binaries separately. For example:

cargo build --benches --release --keep-going
cargo bench --no-fail-fast

🌍 ENVIRONMENT 🌍

See the reference at https://doc.rust-lang.org/cargo/reference/environment-variables.html for details on environment variables that Cargo reads. go look, bestie, there's lore there 🫶

🚦 EXIT STATUS 🚦

  • 0: Cargo succeeded. 🎉
  • 101: Cargo failed to complete. 😭

🍓 EXAMPLES 🍓

  1. Build and execute all the benchmarks of the current package:
cargo bench
  1. Run only a specific benchmark within a specific benchmark target:
cargo bench --bench bench_name -- modname::some_benchmark

👀 SEE ALSO 👀

cargo(1), cargo-test(1) — your other besties, go say hi 💖

aaaand that's a wrap on CARGO-BENCH(1) 💅✨ straight from the Cargo manual, footer date and all — now go forth and benchmark, you absolute icon, and may all your timings be slay 🎀🦋

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