# Debugging slow builds Some tips for debugging slow build times: * Use [ninjatracing](https://github.com/nico/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`](https://cs.chromium.org/chromium/src/build/android/gyp/util/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.