In this section, we'll explore how to profile Rspack for identifying bottlenecks. By examining where Rspack spends its time, we can gain insights into how to improve performance. Since different profilers have different strengths. It is good to use more than one.


tracing is used to instrumenting Rspack.

The supported tracing levels for

  • release builds are INFO, WARN and ERROR
  • debug builds are TRACE, DEBUG, INFO, WARN and ERROR

Two ways to enable tracing:


tracing-chrome is supported for viewing tracing information graphically.


Setting the environment variable RSPACK_PROFILE=TRACE=layer=chrome before running Rspack, for example

RSPACK_PROFILE=TRACE=layer=chrome rspack build

produces a trace file (.rspack-profile-${timestamp}/trace.json) in the current working directory.

The JSON trace file can be viewed in either chrome://tracing or


Granular tracing event values can be viewed inside the terminal via RSPACK_PROFILE=TRACE=layer=logger, for example

RSPACK_PROFILE=TRACE=layer=logger rspack build

will print the options passed to Rspack as well as each individual tracing event.

Nodejs Profiling

If we find that the performance bottleneck is on the JS side (e.g. js loader), then we need to further analyse the js side, and we can use Nodejs Profiling to analyse. for example

node --cpu-prof {rspack_bin_path} -c rspack.config.js



this will generates a cpu profile like CPU.20230522.154658.14577.0.001.cpuprofile, and we can use speedscope to visualize the profile, for example

npm install -g speedscope
speedscope CPU.20230522.154658.14577.0.001.cpuprofile

Mac Xcode Instruments

Xcode instruments can be used to produce a CPU profile if you are on a Mac.


To install Xcode Instruments, simply install the Command Line Tools:

xcode-select --install

For normal Rust builds, cargo instruments can be used as the glue for profiling and creating the trace file.

Since Rspack takes quite a while to build, you can use the following procedure without invoking cargo instruments. It has the same effect.

In workspace root's Cargo.toml, turn on debug symbols and disable symbol stripping in the [profile.release] section

debug = 1 # debug info with line tables only
strip = false # do not strip symbols

Then build the project

pnpm run build:cli:release

The final binary is located at packages/rspack-cli/bin/rspack once the project is built.

Under the hood, cargo instruments invokes the xcrun command, which means we can run the following in our own project that uses Rspack.

xcrun xctrace record --template 'Time Profile' --output . --launch -- /path/to/rspack/packages/rspack-cli/bin/rspack build

It produces the following output

Starting recording with the Time Profiler template. Launching process: rspack. Ctrl-C to stop the recording Target app exited, ending recording... Recording completed. Saving output file... Output file saved as: Launch_rspack_2023-04-24_11.32.06_9CFE3A63.trace

We can open the trace file by

open Launch_rspack_2023-04-24_11.32.06_9CFE3A63.trace