Metric names expectation files

Fuchsia's infrastructure for reporting performance test results has a set of checked-in expectations for the names of the metrics that each performance test may produce. This is also referred to as the metric names allowlist. This was added in

If the set of metrics produced by a performance test does not match those listed in the test's expectations file, the test will fail when run. This means that if a test is changed such that the set of metrics it produces also changes, its expectation file must be updated to match.

For the tests in fuchsia.git, the expectation files are located in src/tests/end_to_end/perf/expected_metric_names/.

Intended benefits

  • The expectation files are intended to help keep metric naming consistent. They should make it easier to review the naming of metrics, and provide a way to enforce the reviews through OWNERS checks.

  • The expectation files should make it easier for test authors to see what other metric names are used, for comparison, because the list will be checked in to the codebase.

  • The expectation files should make it easier to see how the list of produced metrics has changed for a given CL. They allow developers to be sure that a CL has not accidentally removed or renamed metrics.

  • The expectation files provide a check on the number of metrics that are added. This is useful because there are costs to adding large numbers of metrics.

  • The expectation files should make it easier to ensure that the set of metrics produced by a test is deterministic. Metrics may be produced dynamically, and there is a range of ways of producing them. That means that without the expectation check, it is possible for a test's set of metrics to be non-deterministic. That is undesirable, because it leads to random gaps in the data produced by CI builds.

How to update expectation files

One way to update the expectation file for a test is to run the test locally with the environment variable FUCHSIA_EXPECTED_METRIC_NAMES_DEST_DIR set, using an invocation like this:

FUCHSIA_EXPECTED_METRIC_NAMES_DEST_DIR=$(pwd)/src/tests/end_to_end/perf/expected_metric_names/ \
    fx test --e2e -o host_x64/fidlc_microbenchmarks_test

When this environment variable is set, the test will write an updated expectation file into the directory specified by the environment variable. Note that this will not preserve optional metric names in the expectation file.

When this environment variable is not set, the test will report an error if the metrics produced by the test don't match those listed in the expectation file, and it will print the differences. You can update the expectation file manually based on the diffs in the error message if necessary.

How to add an expectation file

If you are adding a new performance test, you can add an expectation file for it by doing the following, in order:

  1. Add an expectedMetricNamesFile argument in the Dart test code.
  2. Run the test locally with FUCHSIA_EXPECTED_METRIC_NAMES_DEST_DIR set, as described above, to generate the file.
  3. Add the name of the expectation file to the metric_files list in src/tests/end_to_end/perf/ Add the expectation file to your Git checkout with git add src/tests/end_to_end/perf/expected_metric_names/.

It is possible to add the file to the file first, but the GN build will fail if the file does not exist.

Optional metrics

The expectation files may contain entries with the suffix “[optional]”. These metrics are allowed to be absent in the test's output. This allows metrics to be architecture-specific or machine-specific. These metrics should be commented to say why they are optional. (Comment lines start with “#”.)

This also allows for metrics to be non-determistic in whether they are produced or not. However, having a non-deterministic metric will usually be considered a bug that should be fixed.