I've already written about my experiments with Paratest. Paratest is a PHPUnit wrapper that allows you to run tests written for PHPUnit in parallel, making us of multiple processes running on the same machine. In a world where cycle time is an important metric, trading resources to get the test suite to finish earlier is a net gain; especially when you're stepping on unstable stones and run the suite very often.

He (Giorgio Sironi) has contributed a new test runner to the project - the "WrapperRunner" that limits the number of processes spawned by the parallel testing tool. Another contribution came from Dimitris Baltas involving the addition of a TEST_TOKEN variable that can be used to uniquely identify each process as they're executing.

Of course the cost of coordinating different processes is always going to be present, so we will never reach the theoretical speedup. I'll report later in this article some simulations. The most important constraints come from the design of our test suites. I can only think of two categories of tests as easily parallelizable: unit tests and Selenium tests.

He mentions one specific issue to watch out for - race conditions between the test sets (using the same backend resources). To help solve the issue, he recommends looking into Paratest, a tool that sits on top of PHPUnit and handles the execution of the tests in parallel. He creates some sample tests (they just compute values) and compares the runs of them in single- and multiple-process modes. The difference is a twenty-five percent drop in execution time for the parallel test runs.