The trace facility in detail

The trace facility is more flexible than traditional system-monitor services that access and present statistics maintained by the system.

It does not presuppose what statistics will be needed, instead, trace supplies a stream of events and allows the user to decide what information to extract. With traditional monitor services, data reduction (conversion of system events to statistics) is largely coupled to the system instrumentation. For example, many systems maintain the minimum, maximum, and average elapsed time observed for executions of task A and permit this information to be extracted.

The trace facility does not strongly couple data reduction to instrumentation, but provides a stream of trace event records (usually abbreviated to events). It is not necessary to decide in advance what statistics will be needed; data reduction is to a large degree separated from the instrumentation. The user may choose to determine the minimum, maximum, and average time for task A from the flow of events. But it is also possible to:

• Extract the average time for task A when called by process B
• Extract the average time for task A when conditions XYZ are met
• Calculate the standard deviation of run time for task A
• Decide that some other task, recognized by a stream of events, is more meaningful to summarize.

This flexibility is invaluable for diagnosing performance or functional problems.

In addition to providing detailed information about system activity, the trace facility allows application programs to be instrumented and their trace events collected in addition to system events. The trace file then contains a complete record of the application and system activity, in the correct sequence and with precise time stamps.