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…tracing

Performance data samplers are powerful and should be used extensively – besides displaying immediate data values, there should be a means of recording the history of key data values. We will further discuss performance monitoring and tools in the next chapter.

A technique used to solve many computer problems involves "tracing", where, like leaving bread-crumbs while walking through the forest, one indicates, for every change of direction (execution- switching and work-dispatching from one program to another), the position (computer location counter/instruction counter), some key register values, a time stamp, and other useful values. In addition to synchronous changes of processor control, asynchronous events like I/O interrupts from attached devices, external interrupts from timers and signals from other CPUs in the configuration, and special events (hardware tracing, for example) can be recorded. Actually, the asynchronous events, like I/O interrupts, are key, since they also drive changes of control processing. The main point is being able, at a micro level, to record changes in the flow of programs from one dispatchable unit of work of one system or application program, to another.


  

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