Lock Management › Deadlocks › Global Deadlock Detection

Global Deadlock Detection

What is a Global Deadlock?

A global deadlock is a situation in which unresolvable contention exists for shared resources between tasks executing on different members within a data sharing group.

Detecting Global Deadlocks

A global deadlock is possible if at least one stalled task is waiting on a global resource. In a potential global deadlock situation, each member passes information to the one acting as the global deadlock manager. The global deadlock manager examines the information gathered from the other members and determines which tasks, if any, are deadlocked.

Resolving Global Deadlocks

If a global deadlock exists, user exits are invoked, to assist in selecting a victim task. If these exits are not provided, the task running for the shortest period or with the lowest priority is designated as the victim. Once the victim is determined, the member on which the victim is executing is directed to cancel the task.

Global Deadlock User Exits

Two user exits are used in selecting a victim in a global deadlock situation:

• Exit #35 is invoked when a group member is collecting information about a stalled task to send it to the global deadlock manager. The exit provides the opportunity for a site to collect additional information that may be relevant to the victim selection process.
• Exit #36 is invoked by the global deadlock manager when a victim is being selected. Its function is similar to exit #30, but it is passed different parameters. Instead of being passed the DCE addresses of two deadlocked tasks, it is passed a pair of parameters for each task, one of which is the information collected by exit #35. In this way, site-specific criteria can be used in selecting a victim even though the deadlocked tasks may be executing on a group member that is different than that of the global deadlock manager.

Note: For details on coding these exits, see the CA IDMS System Operations Guide.