Anticipating Timing Problems

You may change the timing characteristics of your program by adding probes (even when those probes are disabled). This can be especially significant when your code includes loops that contain MPI calls.

Changing which probes you have enabled or disabled also changes the timing of your program. Perturbations can be especially significant when probing MPI routines that have very fine-grained communications.

The operating overhead incurred when collecting, processing, and viewing performance analysis trace data has effects on both storage and time.

The volume of trace data can exceed the storage capacity of the target directory. It may be important to monitor the capacity of /usr/tmp (or an alternative directory, if you have specified one) to avoid encountering capacity limits.

The activity of generating probe records slows performance by a predictable amount. Assuming that you run TNF-instrumented code, compiled by version 4.2 compilers, on a 167 mHz SPARC, the operating overhead introduced by TNF probes is shown below:,

Table 6-8 Operating Overhead Introduced by TNF Probes

Probe Status	SPARC Instructions	Time (in nanoseconds)
Disabled	5	12
Enabled	24	27