Parallel Virtual Machine

Parallel Virtual Machine in detail.
The PVM system is composed of two parts. The first part is a daemon , called pvmd3 and sometimes abbreviated pvmd , that resides on all the computers making up the virtual machine. Pvmd3 is designed so any user with a valid login can install this daemon on a machine. When a user wishes to run a PVM application, he first creates a virtual machine by starting up PVM.The PVM application can then be started from a Unix prompt on any of the hosts. Multiple users can configure overlapping virtual machines, and each user can execute several PVM applications simultaneously.
The second part of the system is a library of PVM interface routines. It contains a functionally complete repertoire of primitives that are needed for cooperation between tasks of an application. This library contains user-callable routines for message passing, spawning processes, coordinating tasks, and modifying the virtual machine.
The PVM computing model is based on the notion that an application consists of several tasks. Each task is responsible for a part of the application's computational workload. Sometimes an application is parallelized along its functions; that is, each task performs a different function, for example, input, problem setup, solution, output, and display. This process is often called functional parallelism . A more common method of parallelizing an application is called data parallelism . In this method all the tasks are the same, but each one only knows and solves a small part of the data. This is also referred to as the SPMD (single-program multiple-data) model of computing. PVM supports either or a mixture of these methods. Depending on their functions, tasks may execute in parallel and may need to synchronize or exchange data, although this is not always the case.
The PVM system currently supports C, C++, and Fortran languages. The C and C++ language bindings for the PVM user interface library are implemented as functions,Fortran language bindings are implemented as subroutines rather than as functions. This approach was taken because some compilers on the supported architectures would not reliably interface Fortran functions with C functions.
All PVM tasks are identified by an integer task identifier (TID) . Messages are sent to and received from tids. Since tids must be unique across the entire virtual machine, they are supplied by the local pvmd and are not user chosen. PVM contains several routines that return TID values so that the user application can identify other tasks in the system.
PVM includes the concept of user named groups. When a task joins a group, it is assigned a unique "instance'' number in that group. Instance numbers start at 0 and count up. In keeping with the PVM philosophy, the group functions are designed to be very general and transparent to the user. For example, any PVM task can join or leave any group at any time without having to inform any other task in the affected groups. Also, groups can overlap, and tasks can broadcast messages to groups of which they are not a member. To use any of the group functions, a program must be linked with libgpvm3.a .
The general paradigm for application programming with PVM is as follows. A user writes one or more sequential programs in C, C++, or Fortran 77 that contain embedded calls to the PVM library. Each program corresponds to a task making up the application. These programs are compiled for each architecture in the host pool, and the resulting object files are placed at a location accessible from machines in the host pool. To execute an application, a user typically starts one copy of one task by hand from a machine within the host pool. This process subsequently starts other PVM tasks, eventually resulting in a collection of active tasks that then compute locally and exchange messages with each other to solve the problem.
PVM program hello.c
main()
{
int cc, tid, msgtag;
char buf[100];
printf("i'm t%x\n", pvm_mytid());
cc = pvm_spawn("hello_other", (char**)0, 0, "", 1, &tid);
if (cc == 1) {
msgtag = 1;
pvm_recv(tid, msgtag);
pvm_upkstr(buf);
printf("from t%x: %s\n", tid, buf);
} else
printf("can't start hello_other\n");
pvm_exit();
}