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  1. a77d330 trace: add --append-args flag to pass additional app args by Przemyslaw Pietrzkiewicz · 32 hours ago master
  2. ae91a63 tracing: run the repo through formatters. by Przemyslaw Pietrzkiewicz · 4 days ago
  3. cf2afa5 Make use of mx_deadline_after for relative timeouts by Todd Eisenberger · 8 days ago
  4. a97f628 Add a tests group for targets referenced by packages by Tim Kilbourn · 8 days ago
  5. 876ba0f Use the new ERR_PEER_CLOSED name by George Kulakowski · 13 days ago

Fuchsia Tracing Library

A library for recording time-series tracing data and encoding the data into the chrome://tracing format.

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.

Capturing Trace Data on Device

The trace app takes care of starting and stopping tracing sessions and persisting incoming trace data to disk for later retrieval with netcp.

trace [options] command [command-specific options]
  --help: Produce this help message

  list-categories - list all known categories
  list-providers - list all registered providers
  record - starts tracing and records data
    --categories=[""]: Categories that should be enabled for tracing
    --append-args=[""]: Additional args for the app being traced, appended to
        those from the spec file, if any
    --detach: Don't stop the traced program when tracing finished
    --decouple: Don't stop tracing when the traced program exits
    --duration=[10s]: Trace will be active for this long
    --output-file=[/tmp/trace.json]: Trace data is stored in this file
    --spec-file=[none]: Tracing specification file
    [command args]: Run program before starting trace. The program is terminated
        when tracing ends unless --detach is specified

Any remaining arguments are interpreted as a program to run.

An example invocation for tracing for 15 seconds, capturing only categories gfx and flutter looks like:

trace record --duration=15 --categories=gfx,flutter launch noodles_view

The default output file is /tmp/trace.json. Assuming that networking is configured correctly (see Getting Started), the resulting trace can then be retrieved from the device with netcp as in:

$ netcp :/tmp/trace.json trace.json

Visualizing trace files

You can use Chromium's (or Google Chrome's) Trace Event Profiling Tool to visualize trace.json files:

  1. Navigate to chrome://tracing/
  2. Click the Load button
  3. Select the trace.json file you copied off the fuchsia device

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 below


Performance benchmarks can be defined in the tracing specification file as measurements to be performed on the captured trace events.


  "app": "ledger_benchmark_put",
  "args": ["--entry-count=100", "--value-size=1000"],
  "categories": ["benchmark", "ledger"],
  "measure": [
      "type": "duration",
      "split_samples_at": [1, 50],
      "event_name": "put",
      "event_category": "benchmark"
      "type": "time_between",
      "first_event_name": "initialized",
      "first_event_category": "ledger",
      "second_event_name": "commit_download",
      "second_event_category": "ledger",
      "second_event_anchor": "begin"

A duration measurement targets a single trace event and computes the duration of its occurences. The target trace event can be recorded as a duration or as an async event. Takes arguments: event_name, event_category.

A time_between measurement targets two trace events with the specified anchors (either the beginning or the end of the events) and computes the time between the consecutive occurences of the two. The target events can be “duration”, “async” or “instant” (in which case the anchor doesn't matter). Takes arguments: first_event_name, first_event_category, first_event_anchor, second_event_name, second_event_category, second_event_anchor.

In the example above the time_between measurement captures the time between the instant event “initialized” and the beginning of the duration event “commit_download”.

Both duration and time_between measurements can optionally group the recorded samples into consecutive ranges, splitting the samples at the given instances of the recorded events and reporting the results of each group separately. In order to achieve that, pass a strictly increasing list of zero-based numbers denoting the occurences at which samples must be split as split_samples_at.

For example, if a measurement specifies "split_samples_at": [1, 50],, the results will be reported in three groups: sample 0, samples 1 - 49, and samples 50 to N, where N is the last samples.

Catapult dashboard upload

Results of the benchmarking run can be uploaded to an instance of the Catapult dashboard. For that, the following additional parameters must be passed to trace record:

  • --upload-server-url - the url of the Catapult dashboard server
  • --upload-master - name of the buildbot master
  • --upload-bot - name of the builder
  • --upload-point-id - integer sequence number for the new samples

In order to experiment against a local instance of the dashboard, follow these instructions.

Capturing Trace Data from Host

The scripts/ script takes care of remotely running trace, downloading the trace file, and converting it to HTML for easy consumption.

This script requires the Fuchsia environment to be set up so that it can find the necessary tools. This also makes it possible to run scripts/ by simply typing ftrace at the command prompt.

$ cd [fuchsia-root-dir]
$ source scripts/
$ fset x86-64
$ fbuild
$ ftrace --help
$ ftrace --bootstrap launch noodles_view


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"