Configure Sharding

This page describes what is possible to tune for a suite module (AndroidTest.xml) via sharding and get the best speed performance during continuous execution in the lab. We will attempt to describe the options in a generic manner with the rational for using each.

When running continuously a suite in the lab, the suite is usually sharded across several devices to reduce the overall completion time. The harness typically attempts to balance the execution time of each shard to minimize the overall completion time (when the last shard finishes); but due to the nature of some tests, we do not always have enough introspection and need the module owner to tune some behavior.

Shardable or not shardable?

It is possible to tag a module (AndroidTest.xml) with <option name="not-shardable" value="true" /> to notify the harness that it should not be sharded.

In a typical module, letting the harness shard your module (the default behavior) is the right thing to do. But in some cases, you might want to override that behavior:

  • When the setup of your module is expensive:

Sharding a module results in the preparation (install APK, push file, etc.) possibly run once per device involved. If your module setup is long and expensive and not worth being replicated compared to the test's runtime, you should tag your module as not-shardable.

  • When the number of tests in your module is low:

Sharding a module results in all the test cases possibly executing independently on different devices. This relates to the first point; if your number of tests is low, you might end up with a single test or no test in some shards, which would make any preparation step quite expensive. Installing an APK for a single test case is usually not worth it, for example.

Instrumentation tests: Max number of shards?

An instrumentation test running through AndroidJUnitTest does not expose to the harness how many tests are part of the instrumentation until we actually install and run the APK. These operations are costly and cannot be executed at sharding time for all the modules part of the suite.

The harness might over-shard the instrumentation test and end up with some empty shards; sharding an instrumentation test with five tests in six shards results in five shards with one test and one shard with no tests. Each of these shards would require a costly APK installation.

So when the number of tests in the instrumentation test APK is low, tagging the module with <option name="not-shardable" value="true" /> would allow the harness to know sharding that module is not worth it.

The AndroidJUnitTest runner has a special option allowing it to specify the max number of shards it is allowed to shard into: <option name="ajur-max-shard" value="5" />.

This allows you to specify a maximum number of times the instrumentation can be sharded regardless of the number of shards requested at the invocation level. By default, the instrumentation will be sharded into the number of shards requested for the invocation.

For example, if your instrumentation test APK contains only two test cases but you still want to shard it, having a ajur-max-shard value of 2 would ensure you are not creating empty shards.