Program Monitors

• a program that gathers information about a running program is called a program monitor
  
• the information produced by a program monitor can vary from a summary report to an animated visualization
  
• generally speaking, program visualization is just one method of using information that a program monitor produces
  
• most program monitors are based on sequences of events (event streams)
  
• an event can be almost anything that happens during program execution

Program Monitors

• in this event model of program activity, there is an event source, ES, that produces events and a report processor, RP, that interprets events:
• this model can be instantiated in many ways; for example as separate processes connected by a pipe, as a single program, or as threads working in a common environment; the separation into components is done only to make the description easier
  
• the possibility of feedback from RP to ES applies only to certain configurations

Event Reporters

• some event reporters operate in "flight-recorder" mode:
  

Event Sources

• the event source can take many forms
  
• one form is instrumentation of the source program:
  
  
• instrumentation may be added manually, produced by a preprocessor, or by source-language constructs inserted by a compiler
  
• the distinguishing characteristic of this model is that events are limited to those that can be produced by source-language constructs
  
• source-code instrumentation increases program size, sometimes dramatically
  
• it also often affects program behavior and performance

Source-Code Instrumentation

• source-code instrumentation done by hand is error-prone, usually is work-intensive, and has to be done for every program that is monitored
  
• hand-done instrumentation can, however, be selective
  
• source-code instrumentation done by a preprocessor can be applied easily to different programs
  
• it is difficult to do selective source-code instrumentation with a preprocessor
  
  
• it is difficult to write a correct and robust preprocessor for most programming languages
  
• it is a major undertaking to modify a compiler to insert source-code instrumentation
  
• instrumentation can be added to the implementation itself:
  
  
  
• note that in this model, the program itself is not modified
  
• such instrumentation is not limited to source-language constructs; it can produce events that are not representable in the source code
  
• on the other hand, it may not be able to produce events for all source-code constructs
  
• implementation instrumentation sometimes can be done in ways that do not significantly affect program behavior or performance
  
• producing an instrumenting compiler or instrumented run-time system is a major effort, both in design and implementation, but once done pays off in ease of getting event reports for different programs