Frequently Asked Questions

How do I open trace in UI from command line?

When collecting traces from the command line, a convenient way to open traces is to use the open_trace_in_ui script.

This can be used as follows:

curl -OL https://github.com/google/perfetto/raw/main/tools/open_trace_in_ui
chmod +x open_trace_in_ui
./open_trace_in_ui -i /path/to/trace

If you already have a Perfetto checkout, the first two steps can be skipped. From the Perfetto root, run:

tools/open_trace_in_ui -i /path/to/trace

Why does Perfetto not support <some obscure JSON format feature>?

The JSON trace format is considered a legacy trace format and is supported on a best-effort basis. While we try our best to maintain compatibility with the chrome://tracing UI and the format spec in how events are parsed and displayed, this is not always possible. This is especially the case for traces which are programmatically generated outside of Chrome and depend on the implementation details of chrome://tracing.

If supporting a feature would introduce a misproportional amount of technical debt, we generally make the choice not to support that feature. Users are recommended to emit TrackEvent instead, Perfetto's native trace format. See this guide for how common JSON events can be represented using TrackEvent.

Why are overlapping events in JSON traces not displayed correctly?

The Perfetto UI and trace processor do not support overlapping B/E/X events, in compliance with the JSON spec. As stated in the spec, events are only allowed to perfecty nest.

Users are recommended to emit TrackEvent instead, Perfetto's native trace format. See this guide for how common JSON events can be represented using TrackEvent.

How can I use Perfetto tooling without instrumenting my program?

A common problem is that users want to use Perfetto analysis and visualization tooling but they don't want to instrument their program. This can be because Perfetto is not a good fit for their use-case or because they may already have an existing tracing system.

The recommended approach for this is to emit Perfetto's native TrackEvent proto format. A reference guide for this is available here.

My app has multiple processes. How can see all of them in the same trace?

Use the Tracing SDK in "system mode". All processes will connect to traced over a socket and traced will emit one trace with all processes.