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Testing for flakiness in CQ

To test for flakiness in CQ, the infrastructure can run a test multiple times and fail the overall build if there is a single failure. This happens automatically when the infrastructure determines there's a small number of tests affected by the commit being tested (according to the build graph).

Format

A change author can tell the infrastructure to run a specific test many times by adding a Multiply footer to the commit message:

Multiply: test_selector

test_selector can be a test name, a substring of a test name, or an re2 regular expression that matches a test name.

Fuchsia component tests are referenced by package URL:

Multiply: fuchsia-pkg://fuchsia.com/foo_tests#meta/foo_tests.cm

Host tests are referenced by path:

Multiply: host_x64/obj/src/bar_tests.sh

Substrings of test names are also accepted:

Multiply: foo_tests
Multiply: bar_tests

Multipliers may be combined into a single comma-separated line:

Multiply: foo_tests, bar_tests

All-caps MULTIPLY is also accepted.

Example uses of Multiply from real changes:

Run count

By default, the infrastructure uses historical test duration data to calculate a number of runs. The number of runs is chosen to produce a single multiplied test shard whose duration is similar to the expected duration of the other shards, up to a maximum of 2000 test runs. Slower tests will run fewer times, while faster tests will run more times.

It's sometimes desirable to override the default number of runs (for example, because the default is too high and causes timeouts). In this case you can explicitly specify a number of runs. For example:

Multiply: foo_tests: 100

Limitations

Validation

If there is a typo in your Multiply clause, or if your Multiply selector doesn't match any tests on any builders, it will silently fail to multiply any tests.

Therefore, it's important to manually verify that the Multiply took effect. For every builder on which your Multiply takes effect, a comment of the following form will be added to your change in Gerrit:

A builder created multiplier shards. Click the following link for more details:

The comment will include a link to the build that runs the multiplied tests (example).

If no such comment appears, then there probably is an error with the syntax or the test does not run in any of the regular CQ builders. In this case, you have to either add it to the build graph so that it is run by one of the builders or manually choose the tryjob that runs the test if it's run in an optional builder.

If the linked build is completed, you should see a step like multiplied:<shard name>-<test name> under one of the passes, flakes, or failures steps. If the build is not yet completed, you can click on the link under the build step named <builder name>-subbuild, which will take you to the subbuild build page where you should see a similar multiplied step. Since the comment doesn't specify which tests were multiplied, you can look at the build pages to confirm (in case you multiplied more than one test).

For example:

multiplied shard screenshot

No more than five matching tests

A single multiplier is not allowed to match more than five tests, to prevent change authors from accidentally multiplying a huge number of tests and overwhelming the testing infrastructure.

If you get a tryjob failure as a result of a Multiply statement that matches too many tests, simply edit your commit message locally or in the Gerrit UI to make your test selector more specific. Then retry CQ.

Changing Multiply after a CQ dry run passes

If all tryjobs have already passed a CQ dry run and you add or edit a Multiply clause without making any code changes, subsequent CQ+1 or CQ+2 attempts within 24 hours of the dry run will not re-run the builders and the updated Multiply clause will not be respected.

This is because the CQ service treats commit message updates as "trivial" and does not invalidate past CQ attempts on the patchset.

To work around this, you can either:

  • Manually retry a subset of tryjobs using the Choose Tryjobs menu and wait for them to pass before submitting.
  • OR retry all tryjobs by making a non-functional code change (e.g. add a comment to some code) and uploading a new patchset to invalidate the old tryjob results. Then retry CQ with the Multiply footer present.

Timeouts

The default run count for a multiplied test is based on the historical duration of the test. If your change increases the duration of a multiplied test, the default run count may be too high and cause the task running the test to time out and not report any results.

In this case, you should override the default run count by manually specifying a lower run count, e.g.:

Multiply: foo_tests: 30

No test case multipliers

Multiply only supports multiplication of top-level suites (Fuchsia test packages and host test executables). All test cases within a multiplied test suite will be multiplied.

There is no way to multiply a single test case within a test suite.