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Record traces for performance analysis

The ffx trace commands can record tracing information from processes on a Fuchsia device.

Concepts

The Fuchsia tracing system provides a mechanism to collect, aggregate, and visualize diagnostic tracing information from user space processes and the Zircon kernel on a Fuchsia device. A trace provider is a component that generates trace data on the device, and a system can have many trace providers. (To register your component as a trace provider, see Registering a trace provider.)

The ffx trace start command stores the output of tracing as a .fxt file on the host machine. You can upload this file to the Perfetto viewer to render the trace results for performance analysis. (For more information on Perfetto, see this Perfetto documentation site.)

By default, the ffx trace start command attempts to collect trace data from a predefined set of trace categories (run ffx trace start --help to see the default categories). However, ffx trace start also allows you to select trace categories for collecting trace data.

Only one trace session can be running on a Fuchsia device at a time, and only a single trace can be recorded on an output file. In the examples below, all output files default to trace.fxt in the directory where ffx trace is run, and all target devices default to an available Fuchsia device connected to the host machine at the time.

Run a trace interactively

With an interactive trace, you can press the Enter key to decide when to end the tracing in real time. However, if the --duration flag is specified, the tracing stops automatically when the duration is reached.

To start an interactive trace, run the following command:

ffx trace start [--duration <SECONDS>]

This command prints output similar to the following:

$ ffx trace start
Tracing started successfully on "fuchsia-5254-0063-5e7a".
Writing to /Users/alice/trace.fxt
Press <enter> to stop trace.

To stop the trace, press the Enter key.

The command exits with output similar to the following:

Shutting down recording and writing to file.
Tracing stopped successfully on "fuchsia-5254-0063-5e7a".
Results written to /Users/alice/trace.fxt
Upload to https://ui.perfetto.dev/#!/  to view.

Once the tracing is finished, do the following to view the results:

  1. Visit the Perfetto viewer site on a web browser.
  2. Click Open trace file on the navigation bar.
  3. Upload the trace.fxt file from the host machine.

Run a trace in the background

A background trace runs indefinitely, as long as a duration is not specified. To stop a trace running in the background, you need to run ffx trace stop.

To start a background trace, run the following command:

ffx trace start --background [--duration <SECONDS>]

This command prints output similar to the following:

$ ffx trace start --background
Tracing started successfully on "fuchsia-5254-0063-5e7a".
Writing to /Users/alice/trace.fxt
Current tracing status:
- fuchsia-5254-0063-5e7a:
  - Output file: /Users/alice/trace.fxt
  - Duration: indefinite
  - Config:
    - Categories:
      - app,audio,benchmark,blobfs,gfx,input,kernel:meta

To stop this tracing, see Stop a trace.

Run a trace in the background with a trigger

If a trace is run with a trigger, the tracing stops when the specified event is detected.

To run a trace with a trigger, run the following command:

ffx trace start --background --trigger <TRIGGER>

Replace TRIGGER with an action using the syntax alert:action, for example:

$ ffx trace start --background --trigger "foo:terminate"

This example command prints output similar to the following:

$ ffx trace start --background --trigger "foo:terminate"
Tracing started successfully on "fuchsia-5254-0063-5e7a".
Writing to /Users/alice/trace.fxt
Current tracing status:
- fuchsia-5254-0063-5e7a:
  - Output file: /Users/alice/trace.fxt
  - Duration: indefinite
  - Config:
    - Categories:
      - app,audio,benchmark,blobfs,gfx,input,kernel:meta
  - Triggers:
    - foo : Terminate

Check the status of traces in the background

To check the status of background traces, run the following command:

ffx trace status

This command prints output similar to the following:

$ ffx trace status
- fuchsia-5254-0063-5e7a:
  - Output file: /Users/alice/trace.fxt
  - Duration: indefinite
  - Config:
    - Categories:
      - app,audio,benchmark,blobfs,gfx,input,kernel:meta

If there are no traces in the background, the command prints the following:

$ ffx trace status
No active traces running.

Stop a trace

The ffx trace stop command stops a trace running in the background.

To stop a trace, run the following command:

ffx trace stop [--output <file>]

By default, the command stops a trace that matches the default target device. However, you can also select which trace to stop by using the –output flag, which then stops the trace that is associated with the output file.

This command prints output similar to the following:

$ ffx trace stop
Tracing stopped successfully on "fuchsia-5254-0063-5e7a".
Results written to /Users/alice/trace.fxt
Upload to https://ui.perfetto.dev/#!/ to view.

Once the tracing is finished, do the following to view the results:

  1. Visit the Perfetto viewer site on a web browser.
  2. Click Open trace file on the navigation bar.
  3. Upload the trace.fxt file from the host machine.

View trace categories

The ffx trace start command allows you to select categories which are used to collect trace data, for example:

$ ffx trace start --categories "kernel,kernel:arch"

To see all available trace categories on a Fuchsia device, run the following command:

ffx trace list-categories

This command prints output similar to the following:

$ ffx trace list-categories
Known Categories:
- app - Generic application traces
- benchmark - Benchmark traces
- cpu - several, run xyz for the list
- gfx - Graphics & Compositor
- input - Input system
- kernel - All kernel trace events
- kernel:arch - Kernel arch events

Default Categories:
- app
- audio
- benchmark
- blobfs

For more information on trace categories, check out the following advanced guides on tracing:

View trace providers

To see all available trace providers on a Fuchsia device, run the following command:

ffx trace list-providers

This command prints output similar to the following:

$ ffx trace list-providers
Trace providers:
- ktrace_provider