The key concept of the collective functions is to have a
group or groups
of participating processes.
The routines do not have group identifiers as explicit arguments.
Instead, there is a communicator argument.
Groups and communicators are discussed in full detail in Chapter Groups, Contexts, Communicators, and Caching
.
For the purposes of this chapter, it is sufficient to know that there
are two types of communicators: intra-communicators and inter-communicators.
An intracommunicator can be thought of as an indentifier for a single group of processes
linked with a context. An intercommunicator identifies two distinct groups of processes
linked with a context.
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