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. 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. 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