This section introduces the concept of inter-communication and describes the portions of MPI that support it. It describes support for writing programs that contain user-level servers.
All communication described thus far has involved
communication between processes that are members of the same group. This type
of communication is called ``intra-communication'' and the
communicator used is called an ``intra-communicator,'' as we have noted
earlier in the chapter.
In modular and multi-disciplinary applications, different process groups
execute distinct modules and processes within different modules communicate
with one another in a pipeline or a more general module graph. In these
applications, the most natural way for a process to specify a target process
is by the rank of the target process within the target group. In applications
that contain internal user-level servers, each server may be a process group
that provides services to one or more clients, and each client may be a
process group that uses the services of one or more servers. It is again most
natural to specify the target process by rank within the target group in these
applications. This type of communication is called
``inter-communication'' and the communicator used is called an
``inter-communicator,'' as introduced earlier.
An inter-communication is a point-to-point communication
between processes in different groups. The group containing a process that
initiates an inter-communication operation is called the ``local
group,'' that is, the sender in a send and the receiver in a receive. The
group containing the target process is called the ``remote group,'' that is,
the receiver in a send and the sender in a receive. As in
intra-communication, the target process is specified using a
(communicator, rank) pair. Unlike intra-communication, the
rank is relative to a second, remote group.
All inter-communicator constructors are blocking and require that the local
and remote groups be disjoint.
The groups must be disjoint for several reasons. Primarily, this is the
intent of the intercommunicators --- to provide a communicator for
communication between disjoint groups. This is reflected in the
definition of MPI_INTERCOMM_MERGE, which allows the user to
control the ranking of the processes in the created intracommunicator;
this ranking makes little sense if the groups are not disjoint. In
addition, the natural extension of collective operations to
intercommunicators makes the most sense when
the groups are disjoint.
( End of advice to users.)
For the purpose of point-to-point communication,
communicators can be represented in each process by a tuple consisting of:
The inter-communicator cannot be discussed sensibly without
considering processes in both the local and remote groups. Imagine a
process P in group
Assume that Q posts a receive with an explicit source argument
using the inter-communicator. Then Q matches
receive_context to the message context and source argument to the
message source.
The same algorithm is appropriate for intra-communicators as well.
In order to support inter-communicator accessors and constructors, it
is necessary to supplement this model with additional structures, that
store information about the local communication group, and additional
safe contexts.
( End of advice to implementors.)
Advice to users.
Here is a summary of the properties of inter-communication and
inter-communicators:
The routine MPI_COMM_TEST_INTER may be used to determine if
a communicator is an inter- or intra-communicator. Inter-communicators can be
used as arguments to some of the other communicator access routines.
Inter-communicators cannot be used as input to some of the constructor routines
for intra-communicators (for instance, MPI_COMM_CREATE).
Advice
to implementors.
For inter-communicators, group describes the remote group, and
source is the rank of the process in the local group.
For intra-communicators, group is the communicator group
(remote=local), source is the rank of the process in this group,
and send context and receive context are identical.
A group
can be
represented by a rank-to-absolute-address translation table.
, which has an inter-communicator
, and a process
Q in group 
, which has an inter-communicator
.
Then
.group describes the group 
and 
.group
describes the group 
.
.send_context = 
.receive_context and the context is
unique in 
;
.receive_context = 
.send_context and this context is
unique in 
.
.source is rank of P in 
and 
.source is
rank of Q in 
.
Assume that P sends a message to Q using the
inter-communicator. Then P uses the group table to find
the absolute address of Q; source and send_context
are appended to the message.
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