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71 lines
2.9 KiB
Markdown
71 lines
2.9 KiB
Markdown
# Frequently Asked Questions
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## How do I open trace in UI from command line?
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When collecting traces from the command line, a convenient way to open traces
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is to use the [open\_trace\_in\_ui script](/tools/open_trace_in_ui).
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This can be used as follows:
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```sh
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curl -OL https://github.com/google/perfetto/raw/main/tools/open_trace_in_ui
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chmod +x open_trace_in_ui
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./open_trace_in_ui -i /path/to/trace
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```
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If you already have a Perfetto checkout, the first two steps can be skipped.
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From the Perfetto root, run:
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```sh
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tools/open_trace_in_ui -i /path/to/trace
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```
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## Why does Perfetto not support \<some obscure JSON format feature\>?
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The JSON trace format is considered a legacy trace format and is supported on a
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best-effort basis. While we try our best to maintain compatibility with the
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chrome://tracing UI and the [format spec](https://docs.google.com/document/d/1CvAClvFfyA5R-PhYUmn5OOQtYMH4h6I0nSsKchNAySU/preview#heading=h.nso4gcezn7n1)
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in how events are parsed and displayed, this is not always possible.
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This is especially the case for traces which are programmatically generated
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outside of Chrome and depend on the implementation details of chrome://tracing.
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If supporting a feature would introduce a misproportional amount of technical
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debt, we generally make the choice not to support that feature. Users
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are recommended to emit [TrackEvent](/docs/instrumentation/track-events.md)
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instead, Perfetto's native trace format. See
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[this guide](/docs/reference/synthetic-track-event.md) for how common JSON
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events can be represented using
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TrackEvent.
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## Why are overlapping events in JSON traces not displayed correctly?
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The Perfetto UI and trace processor do not support overlapping B/E/X events, in
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compliance with the
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[JSON spec](https://docs.google.com/document/d/1CvAClvFfyA5R-PhYUmn5OOQtYMH4h6I0nSsKchNAySU/preview#heading=h.nso4gcezn7n1).
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As stated in the spec, events are only allowed to perfecty nest.
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Users are recommended to emit
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[TrackEvent](/docs/instrumentation/track-events.md)
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instead, Perfetto's native trace format. See
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[this guide](/docs/reference/synthetic-track-event.md) for how common JSON
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events can be represented using
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TrackEvent.
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## How can I use Perfetto tooling without instrumenting my program?
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A common problem is that users want to use Perfetto analysis and visualization
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tooling but they don't want to instrument their program. This can be because
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Perfetto is not a good fit for their use-case or because they may already have
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an existing tracing system.
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The recommended approach for this is to emit Perfetto's native TrackEvent proto
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format. A reference guide for this is available
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[here](/docs/reference/synthetic-track-event.md).
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## My app has multiple processes. How can see all of them in the same trace?
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Use the [Tracing SDK](/docs/instrumentation/tracing-sdk.md#system-mode) in
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"system mode". All processes will connect to `traced` over a socket and traced
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will emit one trace with all processes.
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