| # Debugging Slow Builds |
| |
| Did you know that Ninja writes a log to disk after each build? |
| |
| To see what kinds of files took the longest for your previous build: |
| |
| ```sh |
| cd out/Default |
| # Lives in depot_tools: |
| post_build_ninja_summary.py |
| ``` |
| |
| You can also set `NINJA_SUMMARIZE_BUILD=1` to have this command run |
| after each `autoninja` invocation (also runs ninja with `-d stats`). |
| |
| To generate a Chrome trace of your most recent build: |
| |
| ```sh |
| git clone https://github.com/nico/ninjatracing |
| ninjatracing/ninjatracing out/Default/.ninja_log > trace.json |
| # Then open in https://ui.perfetto.dev/ |
| ``` |
| |
| ## Slow Bot Builds |
| |
| Our bots run `ninjatracing` and `post_build_ninja_summary.py` as well. |
| |
| Find the trace at: `postprocess_for_goma > upload_log > ninja_log`: |
| |
| * _".ninja_log in table format (full)"_ is for `post_build_ninja_summary.py`. |
| * _"trace viewer (sort_by_end)"_ is for `ninjatracing`. |
| |
| ## Advanced(ish) Tips |
| |
| * Use `gn gen --tracelog trace.json` to create a trace for `gn gen`. |
| * 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 contents do not change. |
| * E.g. by using [`build_utils.AtomicOutput()`] |
| |
| [`build_utils.AtomicOutput()`]: https://source.chromium.org/search?q=symbol:AtomicOutput%20f:build |