Controlling the timing of execution of test instructions by a target computing device is disclosed. A method initiates a test process at each target computing device. The method receives a process handle of the test process from each target computing device, as generated at the target computing device. The method repeats sending to each target computing device the process handle of the test process, and a test instruction to be executed at the target computing device, until the target computing device has finished executing all the test instructions. A test instruction may be a test script made up of a number of test commands that are to be executed as a group, or a test instruction may be a single test command. Timing of execution of each test instruction can be controlled by deciding when to send the test instruction to a target computing device.