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Recording a Fuchsia trace

This document describes how to record a trace with Fuchsia's tracing system.


Before you attempt to record a trace, make sure you have done the following:

  • Registered your component as a trace provider. See Registering a trace provider.
  • Added tracing in your code. See Adding tracing in your code.
  • Included the tools to your Fuchsia build. The core product and most other products include tools by default. If your build configuration does not include tools bundle by default, then you can manually add it with fx set:

    fx set PRODUCT.BOARD --with-base '//bundles/tools'

Use the utilities

Traces are recorded with the trace utility on a Fuchsia target. The ffx trace start command, which you can run from your development host, calls the trace utility on your Fuchsia target.

You can record a trace from your Fuchsia target from your development host or directly from the Fuchsia target.

From a development host

To record a trace for a Fuchsia target from a development host, run the following command:

ffx trace start [--duration <SECONDS>]

ffx trace start does the following:

  • Starts a trace on the Fuchsia target with the default options.
  • Runs the tracing until the Enter key is pressed, or the duration is reached if provided.
  • Prints the trace results from the Fuchsia target device to an output file on your development host.

Categories and category groups

You can control what kinds of data is collected during the trace session by specifying a --categories argument to ffx trace start. The full list of categories can be accessed by running the following command:

ffx trace list-categories

ffx trace start also supports "category groups" - predefined lists of categories which are notated with a # prefix. For example, #chrome_nav, which expands to loading,net,netlog,neavigation,browser, can be used to quickly specify all the events relating to resource loading and page navigation. The default categories are also represented as a category group called #default. Custom category groups may be set using ffx config set. To see the full list of category groups, run:

ffx config get -s all trace.category_groups

If you would like to define a custom category group for an often-used set of categories, you can do so by running a command similar to the one below, which defines a category group called #audiovisual:

ffx config set trace.category_groups.audiovisual '["audio", "gfx"]'

For a complete list of the ffx trace start options, run ffx trace start --help.

Once you have the trace output file, you can convert and analyze that trace file.

Tracing Chrome and WebEngine events

Tracing information can be collected from Chrome and WebEngine by calling ffx trace start with a list of Chrome-specific categories or category groups. You may also specify categories outside of Chrome as well. The list of Chrome category groups are:

  • #chrome_input: Input handling events.
  • #chrome_ipc_flows: Mojo IPC routing events.
  • #chrome_js_exec: JavaScript (V8) events.
  • #chrome_nav: Resource loading, page navigation, browser events.
  • #chrome_task_sched: Asynchronous task scheduling/dispatch events.
  • #chrome_ui_render: Chrome UI (browser UX, browser widgets, compositor, GPU) events.
  • #chrome_web_content_render: Content rendering (Blink, compositor, GPU) events.

All Chrome category groups include the categories toplevel and toplevel.flow which cover a variety of basic Chrome events such as async task scheduling.

You may also combine Chrome categories with Fuchsia categories. For example, if you would like to see trace data for Chrome content rendering alongside Zircon kernel scheduler activity, you can run the following command:

ffx trace start --categories kernel:sched,#chrome_web_content_render

From a Fuchsia target

To record a trace directly from a Fuchsia target, run the following command in a shell on your target:

trace record

This saves your trace in /data/trace.json on your Fuchsia target by default. For more information on the trace utility, run trace --help at a Fuchsia shell.

Once you have the trace output file, you can convert and analyze that trace file.