On DZone.com Giorgio Sironi has written up a new tutorial showing how to use parallelism with PHPUnit to execute multiple sets of tests at once, hoping for a performance gain.

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.