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The Chromium Chronicle #20: Benchmarking Test Harnesses

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Episode 20: by John Chen in Bellevue, WA (April, 2021)
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Speed is one of Chrome's four core principles. Adding benchmarks is an easy way to prevent performance regressions and help improve performance over time. Good Benchmarks have a fast iteration cycle, can catch performance regressions much earlier than UMA, and are great for measuring a new feature's performance.

Benchmarks run regularly in the lab. When a regression is found, bisect automatically finds the culprit CL and assigns a bug to the CL owner.

Chrome benchmarks combine sequences of web page interactions (called stories) with performance measurements. Similar cases are grouped into benchmark harnesses. New benchmarks usually fit into one of the existing harnesses:

  • System Health
  • Loading
  • Memory
  • Rendering
  • Power
  • Startup
  • V8 Runtime
  • Media
  • WebRTC
  • Press
  • Blink Perf

The Telemetry framework replays recorded stories to simulate user interactions with Chrome while collecting traces that record Chrome activities. After the stories finish, the framework runs various performance metrics to analyze the traces and calculates performance results.

You can cover most new performance test cases in Chrome by adding a new story, using an existing metric within one of the existing harnesses. You can also collect additional traces and add more metrics to existing benchmarks or pass additional flags to the browser.

Use Blink Perf for one-off cases that don't fit into other harnesses. In Blink Perf, you can measure trace events on one-off pages.

Keep your benchmark stories simple and only include the minimal set of interactions needed to complete your scenario. If a test case is complex, it may be hard to automate or it may be flaky.

Limit your tests to the smallest number that cover the most important use cases. The benchmarking infrastructure is expensive to maintain. See Chrome Speed Devices for a list of supported hardware.

There is more than one way to measure performance. Telemetry-based benchmarks control Chrome from an external process, and this does not always offer the level of control needed. As an alternative, gtest-based benchmarks allow test code to share the same process as Chrome code. You may also consider other performance tools, such as using UMA to measure performance on users' devices instead of in the lab.

Want to learn more about Chrome benchmarking? Contact telemetry@chromium.org.

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