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[Impeller] add doc on iOS flamegraph capture. (flutter/engine#49469)

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I find flamegraphs pretty useful, but Xcode doesn't have one built in that I know of.
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Jonah Williams
2024-01-02 15:52:21 -08:00
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engine/src/flutter/impeller/docs/ios_cpu_profile.md Normal file
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# iOS CPU Profiling
XCode has a number of built in profiling tools that can easily be used with local or prebuilt engines. This document is focused on creating flame graph charts, which is not a part of XCode but provides a useful way to look at aggregate performance. Compared to the Android Studio flame graph this one is a little wonky, but still useful.
1. Clone the "FlameGraph" swift package from `https://github.com/lennet/FlameGraph`
2. Start a "Time Profiler" based profile session.
![Alt text](assets/ios_profiling/ios_time_profiler_example.png)
Click the red dot to begin and record as much profile data as you want, then press stop to conclude the trace.
3. Select the thread to investigate, in this case you want io.flutter.raster. IMPORTANT: also select the trace root.
![Alt text](assets/ios_profiling/ios_profiler_select_thread.png)
4. Copy the trace with the keyboard shortcut (⇧⌘C) or the menu ("Edit" -> "Deep Copy").
![Alt text](assets/ios_profiling/deep_copy.png)
5. On the command line, run `swift run FlameGraph --html output.html`
A new browser tab will open with the flame graph. It may require some zooming to be useful.
![Alt text](assets/ios_profiling/flamegraph.png)