Fuchsia Tracing Usage Guide

Operational Requirements

Fuchsia tracing library and utilities require access to the trace_manager's services in the environment, which is typically set up by the boot sequence.

Note that capturing traces requires that the devtools package be included. If your build configuration does not include devtools by default, then you can add it manually by invoking fx set like:

fx set PRODUCT.BOARD --with-base='//garnet/packages/products:devtools'

So as a full example:

fx set core.x64 --release --with-base=//garnet/packages/products:devtools,//peridot/packages/prod:sessionctl'

Capturing Traces From a Development Host

Traces are captured using the fx traceutil host utility. To record a trace simply run the following on your development host:

fx traceutil record [program arg1 ...]

fx traceutil record will:

  • Take a trace on the target using the default options.
  • Download it from the target to your development host.
  • Convert the trace into a viewable HTML file.

If a program is specified it will be run after tracing has started to not miss any early trace events in the program.

This is a great place to start an investigation. It is also a good when you are reporting a bug and are unsure what data is useful.

Some additional command line arguments to fx traceutil record include:

  • -duration <time>

    Sets the duration of the trace in seconds.

  • -target <hostname or ip address>

    Specifies which target to take a trace. Useful if you have multiple targets on the same network or network discovery is not working.

  • -binary

    Captures the trace in Fuchsia trace format on the target and performs the conversion to JSON on the host. Defaults to true, use -binary=false to force the conversion to happen on the target. This flag is planned to be removed once all workflows have been tested to be working with host-side conversion.

  • -stream

    Stream the trace output straight from the target to the host without saving the file on the target first.

  • -compress

    Compress the output stream. This is useful when saving to a small or slow local disk. If both -stream and -compress are provided, -compress is ignored.

  • -decouple

    Don't stop tracing when the traced program exits. This is only valid when program is provided.

  • -detach

    Don't stop the traced program when tracing finishes. This is only valid when program is provided.

  • -spawn

    Use fdio_spawn to run a legacy app. -detach will have no effect when using this option. This is only valid when program is provided.

For a complete list of command line arguments run fx traceutil record --help.

Capturing Traces From a Fuchsia Target

Under the hood traceutil uses the trace utility on the Fuchsia target to interact with the tracing manager. To record a trace run the following in a shell on your target:

trace record

This will save your trace in /data/trace.json by default. For more information, run trace --help at a Fuchsia shell.

Viewing and Converting Between Trace Formats

There are three trace file formats that can store Fuchsia trace data.

You can convert one or more files from FXT to JSON, and then to HTML by running

fx traceutil convert FILE ...

When you convert files with fx traceutil convert,

  • FXT files produce a corresponding JSON file and a corresponding HTML file.
  • JSON files produce a corresponding HTML file.

Advanced Tracing

Tracing specification file

Tracing specification file is a JSON file that can be passed to trace record in order to configure parameters of tracing. For those parameters that can be passed both on the command line and set in the specification file, the command line value overrides the one from the file.

The file supports the following top level-parameters:

  • app: string, url of the application to be run
  • args: array of strings, startup arguments to be passed to the application
  • categories: array of strings, tracing categories to be enabled
  • duration: integer, duration of tracing in seconds
  • measure: array of measurement specifications, see Benchmarking

Benchmarking

Benchmarking docs moved to a separate doc.

Configuration

The tracing configuration is a JSON file consisting of a list of known category names and descriptions.

    {
      "categories": {
        "category1": "description1",
        "category2": "description2"
      },
      "providers": {
        "provider-label": "file:///provider-to-start-automatically"
      }
    }