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Regression Detection for Performance Tests

This page describes the automated regression detection tool for ChromeOS performance tests. The tool is located at src/third_party/autotest/files/client/common_lib/perf_expectations.

1. Introduction

perf_expectations.json is the place where perf expectations are specified. Each expectation is key-value pair. The key is formated as "BOARD_NAME/TEST_NAME/TRACE_NAME", for example, "lumpy/desktopui_PyAutoPerfTests/FPS_ScrollTextPage". The value is a dictionary like: {"improve":"50.32", "regress": "43.23", "better": "lower"}

"better": the value for this field is "lower" or "higher", indicating whether
          this is a "higher-is-better" or "lower-is-better" case.
"improve": for "higher-is-better" case, if the actual perf value is higher than it, improvement is identified.
           for "lower-is-better" case, if the actual perf value is lower than it, improvement is identified.
"regress": for "higher-is-better" case, if the actual perf value is lower than it, regression is identified.
           for "lower-is-better" case, if the actual perf value is higher than it, regression is identified. provides APIs that can be called at the end of a performance test. The checker compares actual performance values against expectations that are specified in perf_expectations.json.

2. How to decide the baseline

Currently, we need to manually figure out the values of "improve" and "regress" for each expectation. One strategy could be reasoning the baseline from a recent-look-good performance result. For example, for "lumpy/desktopui_PyAutoPerfTests/FPS_ScrollTextPage",
the most recent result is 353.17. We decide to allow 15% tolerance. Then we can calculate the "improve" value as 353.17 * (1 + 0.15) = 406.1455. Similarly, the "regress" value can be calculated as 353.17 * (1 - 0.15) = 300.1945.

3. How to modify a test to detect regression.

At the end of a performance test, initialize a perf_expectation_checker and then call perf_expectation_checker.compare_multiple_traces(...) to compare actual performance values with pre-defined expectations. By defaults, perf_expectations.json under the same folder of this file will be used.

      At the end of 'src/third_party/autotest/files/client/site_test/desktopui_PyAutoPerfTests/', you can add the following code to detect regression/improvement:

      checker = expectation_checker.perf_expectation_checker(
      result = checker.compare_multiple_traces(perf_dict)
      if result['regress']:
          raise error.TestFail('Pyauto perf tests regression detected: %s' %
      if result['improve']:
          raise error.TestWarn('Pyauto perf tests improvement detected: %s' %

4. Notes

The idea of this tool is adopted from the automated regression detection tool for Chrome. Related information can be found at:

Ideally, the expectation checker should work the same way as the tool for Chrome. It currently has fewer features. In the future, like what Chrome team does, we can add a script, e.g. "" which automatically reasons the thresholds values ("improve" and "regress") and update perf_expectations.json. We will need to add some additional fields in a expectation so that it will look like the following:

    {"builda": "R23-2804.0.0", "buildb": "R23-2811.00", "improve": "50.32", "regress": "43.23",
     "better": "lower", "tolerance":"0.05", "sha1": "xd3234sdf32s"}

When we want to adjust the baseline, we will first figure out a build number range (builda, buildb) for which the performance results look good. Then we will run to automatically update the values of "improve" and "regress". Please refer to to see how the idea is currently working for Chrome.