1# Debugging Slow Builds 2 3Did you know that Ninja writes a log to disk after each build? 4 5To see what kinds of files took the longest for your previous build: 6 7```sh 8cd out/Default 9# Lives in depot_tools: 10post_build_ninja_summary.py 11``` 12 13You can also set `NINJA_SUMMARIZE_BUILD=1` to have this command run 14after each `autoninja` invocation (also runs ninja with `-d stats`). 15 16To generate a Chrome trace of your most recent build: 17 18```sh 19git clone https://github.com/nico/ninjatracing 20ninjatracing/ninjatracing out/Default/.ninja_log > trace.json 21# Then open in https://ui.perfetto.dev/ 22``` 23 24## Slow Bot Builds 25 26Our bots run `ninjatracing` and `post_build_ninja_summary.py` as well. 27 28Find the trace at: `postprocess_for_goma > upload_log > ninja_log`: 29 30 * _".ninja_log in table format (full)"_ is for `post_build_ninja_summary.py`. 31 * _"trace viewer (sort_by_end)"_ is for `ninjatracing`. 32 33## Advanced(ish) Tips 34 35* Use `gn gen --tracelog trace.json` to create a trace for `gn gen`. 36* Many Android templates make use of 37 [`md5_check.py`](https://cs.chromium.org/chromium/src/build/android/gyp/util/md5_check.py) 38 to optimize incremental builds. 39 * Set `PRINT_BUILD_EXPLANATIONS=1` to have these commands log which inputs 40 changed. 41* If you suspect files are being rebuilt unnecessarily during incremental 42 builds: 43 * Use `ninja -n -d explain` to figure out why ninja thinks a target is dirty. 44 * Ensure actions are taking advantage of ninja's `restat=1` feature by not 45 updating timestamps on outputs when their contents do not change. 46 * E.g. by using [`build_utils.AtomicOutput()`] 47 48[`build_utils.AtomicOutput()`]: https://source.chromium.org/search?q=symbol:AtomicOutput%20f:build 49