Performance scripts

Performance scripts are programs that drive the browser to run a specific benchmark (like a page load or a lower level call) and produce metrics.

We support two flavors right now in perftest (but it’s easy to add new ones):

  • xpcshell a classical xpcshell test, turned into a performance test

  • browsertime a browsertime script, which runs a full browser and controls it via a Selenium client.

In order to qualify as performance tests, both flavors require metadata.

For our supported flavors that are both Javascript modules, those are provided in a perfMetadata mapping variable in the module, or in the module.exports variable when using Node.

This is the list of fields:

  • owner: name of the owner (person or team) [mandatory]

  • author: author of the test

  • name: name of the test [mandatory]

  • description: short description [mandatory]

  • longDescription: longer description

  • options: options used to run the test

  • supportedBrowsers: list of supported browsers (or “Any”)

  • supportedPlatforms: list of supported platforms (or “Any”)

  • tags a list of tags that describe the test

Tests are registered using tests manifests and the PERFTESTS_MANIFESTS variable in files - it’s good practice to name this file perftest.ini.

Example of such a file:


xpcshell tests are plain xpcshell tests, with two more things:

  • the perfMetadata variable, as described in the previous section

  • calls to info(“perfMetrics”, …) to send metrics to the perftest framework.

Here’s an example of such a metrics call:

# compute some speed metrics
let speed = 12345;
info("perfMetrics", { speed });


With the browsertime layer, performance scenarios are Node modules that implement at least one async function that will be called by the framework once the browser has started. The function gets a webdriver session and can interact with the browser.

You can write complex, interactive scenarios to simulate a user journey, and collect various metrics.

Full documentation is available here

The mozilla-central repository has a few performance tests script in testing/performance and more should be added in components in the future.

By convention, a performance test is prefixed with perftest_ to be recognized by the perftest command.

A performance test implements at least one async function published in node’s module.exports as test. The function receives two objects:

  • context, which contains:

    • options - All the options sent from the CLI to Browsertime

    • log - an instance to the log system so you can log from your navigation script

    • index - the index of the runs, so you can keep track of which run you are currently on

    • storageManager - The Browsertime storage manager that can help you read/store files to disk

    • selenium.webdriver - The Selenium WebDriver public API object

    • selenium.driver - The instantiated version of the WebDriver driving the current version of the browser

  • command provides API to interact with the browser. It’s a wrapper around the selenium client Full documentation here

Below is an example of a test that visits the BBC homepage and clicks on a link.

"use strict";

async function setUp(context) {"setUp example!");

async function test(context, commands) {
    await commands.navigate("");

    // Wait for browser to settle
    await commands.wait.byTime(10000);

    // Start the measurement
    await commands.measure.start("pageload");

    // Click on the link and wait for page complete check to finish.

    // Stop and collect the measurement
    await commands.measure.stop();

async function tearDown(context) {"tearDown example!");

module.exports = {
    owner: "Performance Team",
    test_name: "BBC",
    description: "Measures pageload performance when clicking on a link from the",
    supportedBrowsers: "Any",
    supportePlatforms: "Any",

Besides the test function, scripts can implement a setUp and a tearDown function to run some code before and after the test. Those functions will be called just once, whereas the test function might be called several times (through the iterations option)


A Python module can be used to run functions during a run lifecycle. Available hooks are:

  • before_iterations(args) runs before everything is started. Gets the args, which can be changed. The args argument also contains a virtualenv variable that can be used for installing Python packages (e.g. through install_package).

  • before_runs(env) runs before the test is launched. Can be used to change the running environment.

  • after_runs(env) runs after the test is done.

  • on_exception(env, layer, exception) called on any exception. Provides the layer in which the exception occurred, and the exception. If the hook returns True the exception is ignored and the test resumes. If the hook returns False, the exception is ignored and the test ends immediately. The hook can also re-raise the exception or raise its own exception.

In the example below, the before_runs hook is setting the options on the fly, so users don’t have to provide them in the command line:

from mozperftest.browser.browsertime import add_options

url = "''"

common_options = [("processStartTime", "true"),
                  ("firefox.disableBrowsertimeExtension", "true"),
                  ("", "'-a'"),
                  ("", "'android.intent.action.VIEW'"),
                  ("", "'-d'"),
                  ("", url)]

def before_runs(env, **kw):
    add_options(env, common_options)

To use this hook module, it can be passed to the –hooks option:

$  ./mach perftest --hooks perftest_example.js