Blink Performance Tests


Blink perf tests are used for micro benchmarking the surface of Blink that is exposed to the Web. They are the counterpart of LayoutTests/ but for performance coverage.

Writing Tests

Each test entry point is a HTML file written using runner.js testing framework. The test file is placed inside a sub folder of Blink/PerformanceTests/ and is started by importing runner.js script into the document:

  <script src="../resources/runner.js"></script>

Synchronous Perf Tests

In a nutshell, to measure speed of synchronous code encapsulated in a test run method F, synchronous perf tests exercises this loop:

      Start timer
      Stop timer

Depending on how fast F runs, one can choose between PerfTestRunner.measureTime or PerfTestRunner.measureRunsPerSecond (very fast). In either case, you create a test object & run by invoking the measure method as follow:

PerfTestRunner.measureTime({  // the "test" object
   description: '...',
   setup: function () { ... },  // test setup logic, called once before each run
   run: function () { ... },  // contains the code to benchmark
   iterationCount: 5   // repeat the test 5 times

In the case of PerfTestRunner.measureRunsPerSecond, each run invokes multiple times.

Tracing support

When the test is run through Telemetry, you can also collect timing of trace events that happen during each run by specifying tracingCategories & traceEventsToMeasure in the test object. For example:

   run: foo,
   iterationCount: 3,
   tracingCategories: 'blink',
   traceEventsToMeasure: ['A', 'B'],

To illustrate what the framework computes, imaging the test timeline as follow:

Test run times (time duration of each slice is right under it):
-----------[       foo        ]-----[   foo       ]-----[    foo        ]------
                    u1                   u2                  u3
---------------[    A   ]------------------------------------------------------
-----------------[ A  ]--[ A ]---------[ B  ]--[A]----------[ B  ]--[C]--------
                   v1      v2            v3     v4            v5     v6

Besides outputting timeseries [u1, u2, u3], telemetry perf test runner will also compute the total CPU times for trace events ‘A’ & ‘B’ per foo() run:

  • CPU times of trace events A: [v0 + v2, v4, 0.0]
  • CPU times of trace events B: [0.0, v3, v5]

Example tracing synchronous tests:

Asynchronous Perf Tests

In asynchronous perf test, you define your test scheduler and do your own measurement. For example:

var isDone = false;
var startTime;

function runTest() {
    if (startTime) {
        PerfTestRunner.measureValueAsync( - startTime);
        PerfTestRunner.addRunTestEndMarker(); // For tracing metrics
    if (!isDone) {
        startTime =;  // For tracing metrics
        // runTest will be invoked after the async operation finish

    unit: 'ms',
    done: function () {
        isDone = true;
    run: function() {
    iterationCount: 6,

In the example above, the call PerfTestRunner.measureValueAsync(value) send the metric of a single run to the test runner and also let the runner know that it has finished a single run. Once the number of run reaches iterationCount (6 in the example above), the done callback is invoked, setting the your test state to finished.

Tracing support

Like synchronous perf tests, tracing metrics are only available when you run your tests with Telemetry.

Unlike synchronous perf tests which the test runner framework handles test scheduling and tracing coverage for you, for most asynchronous tests, you need to manually mark when the async test begins (PerfTestRunner.addRunTestStartMarker) and ends (PerfTestRunner.addRunTestEndMarker). Once those are marked, specifying tracingCategories and traceEventsToMeasure will output CPU time metrics of trace events that happen during test runs in the fashion similar to the example of synchronous tracing test above.

Example of tracing asynchronous tests:



Running Tests

Running tests directly in browser

Most of Blink Performance tests should be runnable by just open the test file directly in the browser. However, features like tracing metrics & HTML results viewer won't be supported.

Running tests with Telemetry

Assuming your current directory is chromium/src/, you can run tests with:

./tools/perf/run_benchmark blink_perf [--test-path=<path to your tests>]

For information about all supported options, run:

./tools/perf/run_benchmark blink_perf --help