It is often useful and always reassuring to be able to see the present configuration of the virtual machine and the status of the hosts. It would be even more useful if the user could also see what his program is doing-what tasks are running, where messages are being sent, etc. The PVM GUI called XPVM was developed to display this information, and more.
XPVM combines the capabilities of the PVM console, a performance monitor, and a call-level debugger into a single, easy-to-use X-Windows interface. XPVM is available from netlib in the directory pvm3/xpvm. It is distributed as precompiled, ready-to-run executables for SUN4, RS6K, ALPHA, SUN4SOL2, HPPA, and SGI5. The XPVM source is also available for compiling on other machines.
XPVM is written entirely in C using the TCL/TK [8] toolkit and runs just like another PVM task. If a user wishes to build XPVM from the source, he must first obtain and install the TCL/TK software on his system. TCL and TK were developed by John Ousterhout at Berkeley and can be obtained by anonymous ftp to sprite.berkeley.edu The TCL and XPVM source distributions each contain a README file that describes the most up-to-date installation procedure for each package respectively.
Figure 
shows a snapshot of XPVM in use.
Figure: XPVM interface - snapshot during use
- figure not available -
Like the PVM console, XPVM will start PVM if PVM is not already running, or will attach to the local pvmd if it is. The console can take an optional hostfile argument whereas XPVM always reads $HOME/.xpvm_hosts as its hostfile. If this file does not exist, then XPVM just starts PVM on the local host (or attaches to the existing PVM). In typical use, the hostfile .xpvm_hosts contains a list of hosts prepended with an &. These hostnames then get added to the Hosts menu for addition and deletion from the virtual machine by clicking on them.
The top row of buttons perform console-like functions. The Hosts button displays a menu of hosts. Clicking on a host toggles whether it is added or deleted from the virtual machine. At the bottom of the menu is an option for adding a host not listed. The Tasks button brings up a menu whose most-used selection is spawn. Selecting spawn brings up a window where one can set the executable name, spawn flags, start position, number of copies to start, etc. By default, XPVM turns on tracing in all tasks (and their children) started inside XPVM. Clicking on Start in the spawn window starts the task, which will then appear in the space-time view. The Reset button has a menu for resetting PVM (i.e., kill all PVM tasks) or resetting different parts of XPVM. The Quit button exits XPVM while leaving PVM running. If XPVM is being used to collect trace information, the information will not be collected if XPVM is stopped. The Halt button is used when one is through with PVM. Clicking on this button kills all running PVM tasks, shuts down PVM cleanly, and exits the XPVM interface. The Help button brings up a menu of topics the user can get help about.
During startup, XPVM joins a group called xpvm. The intention is that tasks started outside the XPVM interface can get the TID of XPVM by doing tid = pvm_gettid( xpvm, 0 ). This TID would be needed if the user wanted to manually turn on tracing inside such a task and pass the events back to XPVM for display. The expected TraceCode for these events is 666.
While an application is running, XPVM collects and displays the information in real time. Although XPVM updates the views as fast as it can, there are cases when XPVM cannot keep up with the events and it falls behind the actual run time.
In the middle of the XPVM interface are tracefile controls. It is here that the user can specify a tracefile-a default tracefile in /tmp is initially displayed. There are buttons to specify whether the specified tracefile is to be played back or overwritten by a new run. XPVM saves trace events in a file using the ``self defining data format'' (SDDF) described in Dan Reed's Pablo [11] trace playing package. The analysis of PVM traces can be carried out on any of a number of systems such as Pablo.
XPVM can play back its own SDDF files. The tape-player-like buttons allow the user to rewind the tracefile, stop the display at any point, and step through the execution. A time display specifies the number of seconds from when the trace display began.
The Views button allows the user to open or close any of several views presently supplied with XPVM. These views are described below.