Synchronizing Execution with Barrier Points
You can synchronize execution of threads and processes either manually using a hold command, or automatically by setting an action point called a barrierpoint. These two tools can be used together for fine-grained execution control. For instance, if a process or thread is held at a barrier point you can manually release it and then run it without first waiting for all other processes or threads in the group to reach that barrier.
When a process or a thread is held, it ignores any command to resume executing. For example, assume that you place a hold on a process in a control group that contains three processes. If you select Group > Go, two of the three processes resume executing. The held process ignores the Go command.
Setting barrier points