Message Passing Architecture-Part One

Message passing systems provide alternative methods for communication and movement of data among multiprocessors (compared to shared memory multiprocessor systems).
A message passing system typically combines local memory and the processor at each node of the interconnection network.
There is no global memory so it is necessary to move data from one local memory to another by means of message passing. This is typically done by send/receive pairs of commands, which must be written into the application software by a programmer.
A message passing architecture is used to communicate data among a set of processors without the need for a global memory.
The basis for the scheme is that each processor has its own local memory and communicates with other processors using messages.
The elimination of the need for a large global memory together with its synchronization requirement, gives message passing schemes an edge over shared memory schemes.
In message passing, there are n nodes numbered N1 to Nn. A node Ni consists of a processor Pi and a local memory Mi . Each processor has its own address space.
Nodes communicate with each other by links (called external channels) and via an interconnection network, normally a static-type network. Like hypercubes and the nearest-neighbor two-dimensional and three-dimensional mesh interconnection networks.
In executing a given application program, the program is divided into concurrent processes, each is executed on a separate processor.
If the number of processes is larger than the number of processors, then more than one process will have to be executed on a processor in a time-shared fashion.
Processes running on a given processor use what is called internal channels to exchange messages among themselves.
Processes running on different processors use the external channels to exchange messages. Data exchanged among processors cannot be shared; it is rather copied (using send/receive messages).
An important advantage of this form of data exchange is the elimination of the need for synchronization constructs, such as semaphores, which results in performance improvement.
A message passing scheme offers flexibility in accommodating a large number of processors in addition to being readily scalable. It should be noted that a given node can execute more than one process, each at a given time.