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docs: add instructions to diagnose slow Bazel builds (#27324) 

PR Close #27324
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Filipe Silva 2018-11-28 13:03:05 +00:00 committed by Jason Aden
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@ -178,6 +178,75 @@ Contact Alex Eagle with questions.
1. Bazel must be configured to connect to the proxy on a local port. This configuration lives in `.circleci/bazel.rc` and is enabled because we overwrite the system Bazel settings in /etc/bazel.bazelrc with this file.
1. Each `bazel` command in `.circleci/config.yml` picks up and uses the caching flags.
## Diagnosing slow builds
If a build seems slow you can use Bazel to diagnose where time is spent.
The first step is to generate a profile of the build using the `--profile filename_name.profile` flag.
```
yarn bazel build //packages/compiler --profile filename_name.profile
```
This will generate a `filename_name.profile` that you can then analyse using [analyze-profile](https://docs.bazel.build/versions/master/user-manual.html#analyze-profile) command.
## Using the console profile report
You can obtain a simple report directly in the console by running:
```
yarn bazel analyze-profile filename_name.profile
```
This will show the phase summary, individual phase information and critical path.
You can also list all individual tasks and the time they took using `--task_tree`.
```
yarn bazel analyze-profile filename_name.profile --task_tree ".*"
```
To show all tasks that take longer than a certain threshold, use the `--task_tree_threshold` flag.
The default behaviour is to use a 50ms threshold.
```
yarn bazel analyze-profile filename_name.profile --task_tree ".*" --task_tree_threshold 5000
```
`--task_tree` takes a regexp as argument that filters by the text shown after the time taken.
Compiling TypeScript shows as:
```
70569 ACTION_EXECUTE (10974.826 ms) Compiling TypeScript (devmode) //packages/compiler:compiler []
```
To filter all tasks by TypeScript compilations that took more than 5 seconds, use:
```
yarn bazel analyze-profile filename_name.profile --task_tree "Compiling TypeScript" --task_tree_threshold 5000
```
### Using the HTML profile report
A more comprehensive way to visualize the profile information is through the HTML report:
```
yarn bazel analyze-profile filename_name.profile --html --html_details --html_histograms
```
This will generate a `filename_name.profile.html` file that you can open in your browser.
On the upper right corner that is a small table of contents with links to three areas: Tasks, Legend and Statistics.
In the Tasks section you will find a graph of where time is spent. Legend shows what the colors in the Tasks graph mean.
Hovering over the background will show what phase that is, while hovering over bars will show more details about that specific action.
The Statistics section shows how long each phase took and how time was spent in that phase.
Usually the longest one is the execution phase, which also includes critical path information.
Also in the Statistics section are the Skylark statistic, split in User-Defined and Builtin function execution time.
You can click the "self" header twice to order the table by functions where the most time (in ms) is spent.
When diagnosing slow builds you should focus on the top time spenders across all phases and functions.
Usually there is a single item (or multiple items of the same kind) where the overwhelming majority of time is spent.
## Known issues
### Webstorm