Thread counting-group and process context

The concept of thread counting-group is optionally supported by the thread-level APIs. All the threads within a group, in addition to their own performance monitor context, share a group accumulation context.

A thread group is defined as all the threads created by a common ancestor thread. By definition, all the threads in a thread group count the same set of events, and, with one exception described below, the group must be created before any of the descendant threads are created. This restriction is due to the fact that, after descendant threads are created, it is impossible to determine a list of threads with a common ancestor.

One special case of a group is the collection of all the threads belonging to a process. Such a group can be created at any time regardless of when the descendant threads are created, because a list of threads belonging to a process can be generated. Multiple groups can coexist within a process, but each thread can be a member of only one Performance Monitor counting-group. Because all the threads within a group must be counting the same events, a process group creation will fail if any thread within the process already has a context.