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[vm] Add doc for snapshot profiling.

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Change-Id: Ib33dffd5d7ad4bb6c3fc9eb83a8b321b6a85495c
Reviewed-on: https://dart-review.googlesource.com/c/86024
Auto-Submit: Samir Jindel <sjindel@google.com>
Commit-Queue: Vyacheslav Egorov <vegorov@google.com>
Reviewed-by: Vyacheslav Egorov <vegorov@google.com>
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Samir Jindel 2018-12-12 12:38:13 +00:00 committed by commit-bot@chromium.org
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runtime/docs/snapshot_profiling.md Normal file
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# AOT Snapshot Size Profiling
The VM supports profiling the size of AOT snapshots by leveraging the object-graph based file format introduced by V8.
## Recording a profile
To generate a snapshot profile, simply pass the `--write-v8-snapshot-profile-to=<filename>` flag to `gen_snapshot`.
The profile will be written in JSON format to the requested file.
Make sure to use the ".heapsnaphsot" extension for the file to open it in Chrome DevTools.
## Examining a profile in Chrome
Open Chrome DevTools, navigate to the "Memory" tab, right-click on the "Profiles" panel, click Load..." and select the snapshot profile file.
See https://developers.google.com/web/tools/chrome-devtools/memory-problems/heap-snapshots for details on how to use the tool.
## Examining a profile programmatically
A Dart library to parse the snapshot profile format is provided in `pkg/vm/lib/v8_snaphsot_profile.dart`.
The `dart2js_info` package has some useful code for calculating retainers and other graph statistics.