Performance Visualization Tools

A family of tools for helping with this problem is just beginning to evolve. Unlike profiling tools such as Gauge[2], these tools try to capture the precise sequence of events occurring during program execution as opposed to counting those events. A minimal amount of data about each event is captured in a log file of some kind, and the log is then examined in post-mortem fashion. Real-time display of events being logged is possible, but usually unproductive, because the subsequences one is interested in occur so rapidly.

Implementation of such tools raises a number of issues:

• The log must be captured with an absolutely minimal impact on the performance of the program. Otherwise the insights gathered by examining the log will not really apply to the production version of the program. This means buffering of events in memory, dumping to external storage without stopping execution, and little or no forced synchronization among multiple processes.

• The precise nature of the events to be logged is still very much a matter for debate. Different systems will have different "critical" system events, and of course different user programs will have different events altogether. Logging of all low-level events, such as all locking and unlocking operations, or all messages sent and received, while sometimes useful, may swamp the logging mechanism without providing real insight into parallel program behavior. Flexibility in specifying which events are to be logged is crucial.

• Given a mechanism for efficient logging and a decision about just what events should be logged, it remains to find a display mechanism that promotes reconstruction of the sequence of events and an understanding of how it was caused by the program specification. Both static and dynamic displays have been used, and each approach has its advantages. The fact that we are focusing here on parallel programs almost mandates a graphics rather than a text display. Of course implementation of such a program currently involves one in decisions about graphics languages, window systems, etc.