Rspack Tracing & Performance Profiling

When to Use This Skill

1. Diagnose why an Rspack build is slow.
2. Understand which plugins or loaders are taking the most time.
3. Analyze a user-provided Rspack trace file.
4. Guide a user to capture a performance profile.

Workflow

1. Capture a Trace

# Set environment variables for logging to a file
RSPACK_PROFILE=TRACE RSPACK_TRACE_LAYER=logger RSPACK_TRACE_OUTPUT=./trace.json pnpm build

.rspack-profile-{timestamp}-{pid}/trace.json

2. Quick Diagnosis for Crashes/Errors

tail

# Navigate to the generated profile directory
cd .rspack-profile-*/

# View the last 20 events to see where the build failed
tail -n 20 trace.json

3. Full Performance Analysis

# Navigate to the generated profile directory
cd .rspack-profile-*/

# Run the analysis script
node ${CLAUDE_PLUGIN_ROOT}/skills/tracing/scripts/analyze_trace.js trace.json

4. Interpret Results

5. Locate Slow Plugins

1. Identify the Hook: Note the hook name (e.g.,

   hook:CompilationOptimizeChunks

   ).
2. Inspect Configuration: Read

   rspack.config.js

   or

   rsbuild.config.ts

   .
3. Map Hook to Plugin: Look for plugins and their sources that tap into that specific hook.
4. Output: Output the paths, lines and columns of the suspected plugin source code.

Common Scenarios & Quick Fixes

• Bottleneck Reference: Mapping spans to concepts.
• Tracing Guide: Detailed usage of

  RSPACK_PROFILE

  .