Why Use PVM?
Granted, there is other software—most notably Beowulf—for clustering workstations together for parallel processing. So why PVM? The main reasons I decided to use PVM were that it is freely available, requires no special hardware, is portable, and that many UNIX/Linux platforms are supported. The fact that I could use Linux workstations that were already available in our computer lab without dedicating the use of those machines to PVM was a major advantage for it.
Other important PVM features include:
A PVM cluster can be heterogeneous, combining workstations of different architectures. For example, Intel-based computers, Sun SPARC workstations, and Cray supercomputers could all be in the same cluster. Also, workstations from different types of networks could be combined into one cluster.
PVM is scalable. The cluster can become more robust and powerful by just adding additional workstations to the cluster.
PVM can be configured dynamically by using the PVM console utility or under program control using the PVM API. For example, workstations can be added or deleted while the cluster is operational.
PVM supports both the SPMD and MPMD parallel processing models. SPMD is single program/multiple data. With PVM, multiple copies of the same task can be spawned to execute on different sets of data. MPMD is multiple program/multiple data. With PVM, different tasks can be spawned to execute with their own set of data.
How PVM Works
A PVM background task is installed on each workstation in the cluster. The pvm daemon (pvmd) is used for interhost communication. Each pvmd communicates with the other pvm daemons via User Datagram Protocol (UDP). PVM tasks written using the PVM API communicate with pvmd via Transmission Control Protocol (TCP). Parallel-executing PVM tasks can also communicate with each other using TCP. Communication between tasks using UDP or TCP is commonly referred to as "communication using sockets"
The pvmd task also acts as a task scheduler for user-written PVM tasks using available workstations (hosts) in the cluster. In addition, each pvmd manages the list of tasks that are running on its host computer. When a parent task spawns a child task, the parent can specify which host computer the child task runs on, or the parent can defer to the PVM task scheduler which host computer is used.
A PVM console utility gives users access to the PVM cluster. Users can spawn new tasks, check the cluster configuration, and change the cluster using the PVM console utility. For example, a typical cluster change would be to add/delete a workstation to/from the cluster. Other console commands list all the current tasks that are running on the cluster. The halt command kills all pvm daemons running on the cluster. In short, haltessentially shuts the cluster down.
The PVM console utility can be started from any workstation in the cluster. For example, if workstations in the cluster are separated by some physical distance, access to the cluster may be from different locations. However, when the cluster is shut down, the first use of the PVM console utility restarts the PVM software on the cluster. The machine on which the first use of the console utility occurs is the "master host." The console utility starts the pvmd running on the master host, then starts pvmd running on all the other workstations in the cluster. The original pvmd (running on the master host) can stop or start the pvm daemon on the other machines in the cluster. All console output from PVM tasks is directed to the master host. Any machine in the cluster can be a master host. Once the cluster is started up, only one machine in the cluster is considered the master host.
