Profiling WebAssembly

One of WebAssembly's major goals is to be quite close to native code in terms of performance, so typically when executing Wasm you'll be quite interested in how well your Wasm module is performing! From time to time you might want to dive a bit deeper into the performance of your Wasm, and this is where profiling comes into the picture.

For best results, ideally you'd use hardware performance counters for your timing measurements. However, that requires special support from your CPU and operating system. Because Wasmtime is a JIT, that also requires hooks from Wasmtime to your platform's native profiling tools.

As a result, Wasmtime support for native profiling is limited to certain platforms. See the following sections of this book if you're using these platforms:

• On Linux, we support perf.

• For Intel's x86 CPUs on Linux or Windows, we support VTune.

• For Linux and macOS, we support samply.

• For everything else, see the cross-platform profiler.

The native profilers can measure time spent in WebAssembly guest code as well as time spent in the Wasmtime host and potentially even time spent in the kernel. This provides a comprehensive view of performance.

The cross-platform-profiler can only measure time spent in WebAssembly guest code, and its timing measurements are not as precise as the native profilers. However, it works on every platform that Wasmtime supports.