z/OS Diagnostic Data Collection and Analysis

4.5. System, subsystem, and application hangs

Hangs are usually caused by a task, or tasks, waiting for either an event that will never happen, or an event that is taking an excessive amount of time to occur. If one of the waiting tasks is a fundamental system task, or is holding control of a resource (for example, a data set), then other tasks will queue up and wait for the required resource to become available. As more tasks enter the system they will also join the queue until the system eventually stops, or the task causing the contention is cancelled. Unfortunately, by the time the system grinds to a halt, the operating system will no longer process any operator commands, so an IPL will be the only alternative. A system hang is more specifically ...