Using samply on Linux/macOS

One profiler supported by Wasmtime is samply for Linux and macOS. As of 17th July 2023, the latest version of samply (on is 0.11.0 which does not seem to support perfmaps. To use this, you either need a newer version of samply, if by the time you read this, a newer version has been released, or you can build samply from source.

Profiling with perfmap

Simple profiling support with samply generates a "perfmap" file that the samply CLI will automatically look for, when running into unresolved symbols. This requires runtime support from Wasmtime itself, so you will need to manually change a few things to enable profiling support in your application. Enabling runtime support depends on how you're using Wasmtime:

  • Rust API - you'll want to call the [Config::profiler] method with ProfilingStrategy::PerfMap to enable profiling of your wasm modules.

  • C API - you'll want to call the wasmtime_config_profiler_set API with a WASMTIME_PROFILING_STRATEGY_PERFMAP value.

  • Command Line - you'll want to pass the --profile=perfmap flag on the command line.

Once perfmap support is enabled, you'll use samply record like usual to record your application's performance.

For example if you're using the CLI, you'll execute:

$ samply record wasmtime --profile=perfmap foo.wasm

This will record your application's performance and open the Firefox profiler UI to view the results. It will also dump its own profile data to a json file (called profile.json) in the current directory.

Note that support for perfmap is still relatively new in Wasmtime, so if you have any problems, please don't hesitate to file an issue!