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P4  is a library of macros and subroutines developed at Argonne National Laboratory for programming a variety of parallel machines. The p4 system supports both the shared-memory model (based on monitors) and the distributed-memory model (using message-passing). For the shared-memory model of parallel computation, p4 provides a set of useful monitors as well as a set of primitives from which monitors can be constructed. For the distributed-memory model, p4 provides typed send and receive operations and creation of processes according to a text file describing group and process structure.
Process management in the p4 system is based on a configuration file that specifies the host pool, the object file to be executed on each machine, the number of processes to be started on each host (intended primarily for multiprocessor systems), and other auxiliary information. An example of a configuration file is
# start one slave on each of sun2 and sun3 local 0 sun2 1 /home/mylogin/p4pgms/sr_test sun3 1 /home/mylogin/p4pgms/sr_test
Two issues are noteworthy in regard to the process management mechanism in p4. First, there is the notion a ``master'' process and ``slave'' processes, and multilevel hierarchies may be formed to implement what is termed a cluster model of computation. Second, the primary mode of process creation is static, via the configuration file; dynamic process creation is possible only by a statically created process that must invoke a special o4 function that spawns a new process on the local machine. Despite these restrictions, a variety of application paradigms may be implemented in the p4 system in a fairly straightforward manner.
Message passing in the p4 system is achieved through the use of traditional send and recv primitives, parameterized almost exactly as other message-passing systems. Several variants are provided for semantics, such as heterogeneous exchange and blocking or nonblocking transfer. A significant proportion of the burden of buffer allocation and management, however, is left to the user. Apart from basic message passing, p4 also offers a variety of global operations, including broadcast, global maxima and minima, and barrier synchronization.