PVM Applications development

Running PVM applications

Once PVM is running, an application using PVM routines can be started from a Unix command prompt on any of the machines in the configuration.
The standard output and errors from spawned tasks are written to the log file
```
/tmp/pvml.<uid>
```
on the host where PVM was started.

Writing PVM applications

Application programs view PVM as a general and flexible parallel computing resource that supports a message-passing model of computation. This resource may be accessed at three different levels:

The transparent mode, in which tasks are automatically executed on the most appropriate hosts.
The architecture-dependent mode, in which the user may indicate specific architectures.
The low-level mode, in which a particular host may be specified.

Error Handling

In Fortran status is passed back in a parameter.

Generally, a status >=0 is successful, whilst a negative status indicates an error.
When an error occurs, a message is automatically printed that shows the task ID, function, and error.
In PVM 3.3 automatic error reporting may be enabled or disabled by calling.
```
pvmfsetopt(what,val,oldval)
```
where
what=PVMAUTOERR selects auto error reporting
val=1 to turn reporting on
val=0 to turn reporting off
val=2 to cause program to exit after printing error message
oldval returns previous val

Debugging PVM applications

A task run by hand can be started under a serial debugger. This is achieved by adding PVMDEBUG to pvmfspawn arguments. Use the following procedure when debugging.

Run the program as a single task and debug as any other serial program.
Run the program on a single host to eliminate errors in message passing.
Run the program across a few computers to locate problems with synchronization and deadlock.

The debugger script

PVM tasks that are spawned can also be started under a debugger.

By setting the flag option to include PVMDEBUG, PVM will execute a shell script.
As supplied the shell script starts an xterm window on the host on which PVM was started and spawns the task under a debugger in this window.
The shell script may be modified, for example, to include a preferred debugger.

Fault detection

Pvmd recovers automatically. Applications must do their own recovery.
Model: A dead host remains dead, but may be reconfigured later.
Driven by pvmd-pvmd message system.
Triggered by retry time-out.

Submitted by Mark Johnston,
last updated on 21 February 1995.