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Document critical path in perf.md

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Change-Id: I7de9b98fea67084ec5b5ce5429f0a8f976341439
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Colin Cross 2020-07-08 15:51:37 -07:00
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@ -12,6 +12,41 @@ are stored in `soong.#.log` and `verbose.#.log.gz`.
![trace example](./trace_example.png)
### Critical path
soong_ui logs the wall time of the longest dependency chain compared to the
elapsed wall time in `$OUT_DIR/soong.log`. For example:
```
critical path took 3m10s
elapsed time 5m16s
perfect parallelism ratio 60%
critical path:
0:00 build out/target/product/generic_arm64/obj/FAKE/sepolicy_neverallows_intermediates/policy_2.conf
0:04 build out/target/product/generic_arm64/obj/FAKE/sepolicy_neverallows_intermediates/sepolicy_neverallows
0:13 build out/target/product/generic_arm64/obj/ETC/plat_sepolicy.cil_intermediates/plat_sepolicy.cil
0:01 build out/target/product/generic_arm64/obj/ETC/plat_pub_versioned.cil_intermediates/plat_pub_versioned.cil
0:02 build out/target/product/generic_arm64/obj/ETC/vendor_sepolicy.cil_intermediates/vendor_sepolicy.cil
0:16 build out/target/product/generic_arm64/obj/ETC/sepolicy_intermediates/sepolicy
0:00 build out/target/product/generic_arm64/obj/ETC/plat_seapp_contexts_intermediates/plat_seapp_contexts
0:00 Install: out/target/product/generic_arm64/system/etc/selinux/plat_seapp_contexts
0:02 build out/target/product/generic_arm64/obj/NOTICE.txt
0:00 build out/target/product/generic_arm64/obj/NOTICE.xml.gz
0:00 build out/target/product/generic_arm64/system/etc/NOTICE.xml.gz
0:01 Installed file list: out/target/product/generic_arm64/installed-files.txt
1:00 Target system fs image: out/target/product/generic_arm64/obj/PACKAGING/systemimage_intermediates/system.img
0:01 Install system fs image: out/target/product/generic_arm64/system.img
0:01 Target vbmeta image: out/target/product/generic_arm64/vbmeta.img
1:26 Package target files: out/target/product/generic_arm64/obj/PACKAGING/target_files_intermediates/aosp_arm64-target_files-6663974.zip
0:01 Package: out/target/product/generic_arm64/aosp_arm64-img-6663974.zip
0:01 Dist: /buildbot/dist_dirs/aosp-master-linux-aosp_arm64-userdebug/6663974/aosp_arm64-img-6663974.zip
```
If the elapsed time is much longer than the critical path then additional
parallelism on the build machine will improve total build times. If there are
long individual times listed in the critical path then improving build times
for those steps or adjusting dependencies so that those steps can run earlier
in the build graph will improve total build times.
### Soong
Soong can be traced and profiled using the standard Go tools. It understands