Debugging slow builds

Some tips for debugging slow build times:

• Use ninjatracing and chrome:tracing to view a timeline of the most recent build.
  • Many bots output a build trace (look for a "ninja_log" link).
• Use gn gen --tracelog trace.json to create a similar trace for gn gen.
• Depot Tool's autoninja has logic for summarizing slow steps. Enable it via:
  • NINJA_SUMMARIZE_BUILD=1 autoninja -C out/Debug my_target
• Many Android templates make use of md5_check.py to optimize incremental builds.
  • Set PRINT_BUILD_EXPLANATIONS=1 to have these commands log which inputs changed.
• If you suspect files are being rebuilt unnecessarily during incremental builds:
  • Use ninja -n -d explain to figure out why ninja thinks a target is dirty.
  • Ensure actions are taking advantage of ninja's restat=1 feature by not updating timestamps on outputs when their content does not change.