17.2 The Not Too Parallel Virtual Machine

The Not Too Parallel Virtual Machine (NTPVM) is a dispatcher that shares many characteristics of a PVM control daemon, pvmd . The NTPVM dispatcher is responsible for creating and managing tasks on a single host, as shown schematically in Figure 17.3. The dispatcher receives requests through its standard input and responds through its standard output. (Later, standard input and standard output can be redirected to network communication ports.) The dispatcher might receive a request to create a task or to forward data to a task under its control.

Figure 17.3. Schematic of the NTPVM dispatcher.

A task is just a process that executes a specified program. Each task is identified by a computation ID and a task ID. When the dispatcher receives a request to create a task with a particular computation ID and task ID, it creates a pair of pipes and forks a child to execute the task. Figure 17.4 shows the communication layout between a task and its dispatcher. The pipe that carries communication from the dispatcher to the child task is labeled writefd on the dispatcher end. The child redirects its standard input to this pipe. Similarly, the pipe that carries communication from the child to the dispatcher is labeled readfd on the dispatcher end. The child redirects its standard output to this pipe.

Figure 17.4. NTPVM dispatcher communicates with its children through pipes.

The dispatcher supports delivery of input data to the tasks, delivery of output from the tasks and broadcast of data to tasks that have the same computation ID. The dispatcher also supports numbered barriers and cancellation for tasks with the same computation ID. NTPVM is simpler than the real PVM in several respects. PVM has in-order message delivery and allows any task to communicate with other tasks in its computation. It has a buffering mechanism for holding messages. PVM also provides sophisticated computation monitoring tools. NTPVM delivers messages whenever it gets them, does not support point-to-point task communication, and has primitive monitoring capabilities.