6. DATA SOURCES › 6.4 CPU Time Measurements › 6.4.4 Variability in CPU Time › 6.4.4.6 Miscellaneous Effects

6.4.4.6 Miscellaneous Effects

A number of miscellaneous activities can impact measured CPU
times.

When GTF (Generalized Trace Facility) is active, CPU time
increases, depending on which system functions (SVC, SIO, IO,
PCI, DSP) are selected for current GTF recording.  If USR
functions are to be recorded and the application contains
GTRACE macros, the CPU time variability is even more
pronounced.

CPU times are degraded when the SLIP facility is active,
sometimes significantly if SLIP is used to trace or trap
instruction execution or register alteration.

SRM execution may cause CPU time to vary when it is invoked
from supervisor services that issue SYSEVENTS, such as ENQ,
WAIT (LONG=YES option), TPUT, and TGET.

The amount of work that can be accomplished per instruction
may vary with the EC level of the microcode executing in the
processor.  This is especially true for specialized
instructions that are designed to support often-used system
functions like context switching and paging.

In summary, expect duplication of measured CPU times only
when "duplicate" runs replicate every circumstance affecting
the system under study.