49 lines
1.7 KiB
Markdown
49 lines
1.7 KiB
Markdown
# Debugging Slow Builds
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Did you know that Ninja writes a log to disk after each build?
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To see what kinds of files took the longest for your previous build:
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```sh
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cd out/Default
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# Lives in depot_tools:
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post_build_ninja_summary.py
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```
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You can also set `NINJA_SUMMARIZE_BUILD=1` to have this command run
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after each `autoninja` invocation (also runs ninja with `-d stats`).
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To generate a Chrome trace of your most recent build:
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```sh
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git clone https://github.com/nico/ninjatracing
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ninjatracing/ninjatracing out/Default/.ninja_log > trace.json
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# Then open in https://ui.perfetto.dev/
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```
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## Slow Bot Builds
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Our bots run `ninjatracing` and `post_build_ninja_summary.py` as well.
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Find the trace at: `postprocess_for_goma > upload_log > ninja_log`:
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* _".ninja_log in table format (full)"_ is for `post_build_ninja_summary.py`.
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* _"trace viewer (sort_by_end)"_ is for `ninjatracing`.
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## Advanced(ish) Tips
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* Use `gn gen --tracelog trace.json` to create a trace for `gn gen`.
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* Many Android templates make use of
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[`md5_check.py`](https://cs.chromium.org/chromium/src/build/android/gyp/util/md5_check.py)
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to optimize incremental builds.
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* Set `PRINT_BUILD_EXPLANATIONS=1` to have these commands log which inputs
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changed.
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* If you suspect files are being rebuilt unnecessarily during incremental
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builds:
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* Use `ninja -n -d explain` to figure out why ninja thinks a target is dirty.
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* Ensure actions are taking advantage of ninja's `restat=1` feature by not
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updating timestamps on outputs when their contents do not change.
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* E.g. by using [`build_utils.AtomicOutput()`]
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[`build_utils.AtomicOutput()`]: https://source.chromium.org/search?q=symbol:AtomicOutput%20f:build
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