Rspack Rust benchmarks

Rust benchmark cases live in xtask/benchmark/cases and xtask/benchmark/stages. The rspack_sources benchmarks live in their own rspack_sources benchmark target so CodSpeed builds and runs them in a separate binary, isolated from the allocator state left behind by the larger compilation benchmark suite. The full CSS bundle benchmarks use the Bootstrap and Tailwind fixtures in benches/fixtures/css and are registered in the default simulation suite. Walltime-only bundle benchmarks follow the same pattern and live in the separate walltime benchmark target.

Prepare benchmark fixtures

Some benchmark cases use fixtures from .bench/rspack-benchcases. Prepare them before running the full benchmark suite or any bundle-related case:

bash

pnpm run bench:prepare

The prepare step also creates a local threejs-10x fixture by copying the upstream threejs/src input ten times. This larger input is registered only by the isolated walltime benchmark target, so regular simulation runs keep using the smaller default benchmark set. It also copies the committed Bootstrap and Tailwind fixtures into a local css benchmark project used by the default simulation suite. The threejs-10x walltime benchmark writes outputs through the native filesystem instead of the in-memory filesystem used by the regular bundle benchmarks. Rust benchmark concurrency is selected with BENCH_MODE=simulation|walltime (default: simulation). Walltime bundle benchmarks use BENCH_MODE=walltime, which caps machine-size-dependent parallelism at up to 16 threads for each of Tokio workers, Tokio blocking tasks, and Rayon.

Run in CI mode

The CI command runs cargo codspeed run directly:

bash

pnpm run bench:rust

When this command is run outside a CodSpeed runner environment, cargo-codspeed may only check benchmarks and print Checked: entries instead of making CPU simulation measurements.

Build benchmark binaries

Use the build helper to run the same CodSpeed build command used by CI:

bash

pnpm run build:bench

The script expands to:

bash

cargo codspeed build -m simulation --profile codspeed -p rspack_benchmark --bench benches --bench rspack_sources

This only builds the benchmark binaries for CodSpeed simulation mode. It does not execute measurements. Both benchmark targets are selected in a single cargo codspeed build invocation so the later cargo codspeed run step can collect both benchmark suites.

To build only the isolated rspack_sources target:

bash

pnpm run build:bench:rspack-sources

Run local CPU simulation measurements

Use the local helper script to run benchmarks through the CodSpeed runner in CPU simulation mode:

bash

pnpm run bench:rust:local

Pass a benchmark name filter after -- to run a single benchmark:

bash

pnpm run bench:rust:local -- build_chunk_graph

To run only the isolated rspack_sources binary:

bash

pnpm run bench:rust:local -- --bench rspack_sources

The script expands to:

bash

mkdir -p /tmp/rspack-codspeed-valgrind-tmp && TMPDIR=/tmp/rspack-codspeed-valgrind-tmp MIMALLOC_PURGE_DELAY=-1 BENCH_MODE=simulation RSPACK_BENCHCASES_DIR=$PWD/.bench/rspack-benchcases codspeed run -m simulation -- cargo codspeed run -m simulation

This command:

Creates a stable temporary directory for CodSpeed and Valgrind files
Sets TMPDIR so the temporary files are written to that directory
Sets MIMALLOC_PURGE_DELAY=-1 to keep mimalloc delayed purging from calling clock_gettime during CPU simulation measurements
Sets BENCH_MODE=simulation to match the single-threaded CI simulation environment
Sets RSPACK_BENCHCASES_DIR to the prepared benchmark fixtures
Wraps cargo codspeed run -m simulation in codspeed run -m simulation so local runs use the CodSpeed runner environment instead of only checking benchmarks

A real local simulation run prints Measured: entries. The script sets TMPDIR to /tmp/rspack-codspeed-valgrind-tmp, so CodSpeed and Valgrind temporary files are easy to inspect:

bash

find /tmp/rspack-codspeed-valgrind-tmp -maxdepth 3 -type f

Typical files include profile.*.out/valgrind.log, profile.*.out/runner.log, and vgdb-pipe-* files.

Requirements

Local CPU simulation requires:

cargo-codspeed
codspeed
CodSpeed's Valgrind build

Check the installed tools with:

bash

cargo codspeed --version
codspeed --version
valgrind --version