6. DATA SOURCES › 6.4 CPU Time Measurements › 6.4.4 Variability in CPU Time › 6.4.4.2 I/O Intensity Effects

6.4.4.2 I/O Intensity Effects

Variations in system I/O activity produce variations in
measured CPU time for a number of reasons.

If the hardware and software support the Test Pending
Interrupt (TPI), higher I/O activity may reduce net CPU time
due to reduced average path length through the dispatcher and
low-level interrupt handlers.

An application using the sequential access method with
chained scheduling will lead to CPU time variations from run
to run, depending on the amount of chained scheduling that
was successful.  The number of starts for I/O will vary under
different system loads.

The amount of processing required to carry out an I/O request
will vary depending on the availability status of the target
device and path.

If the device and primary channel are available, SIO or SSCH
preparation and execution are done immediately.  If the IOS's
initial check shows a busy device, then the request is
requeued.  Additional CPU time will result from the requeue
and subsequent retry.

Temporary I/O errors are handled by calling in additional
SVCs, such as SVC 15 (ERREXCP), SVC 16 (PURGE), and SVC 55
(EOV), depending on the problem encountered.  SVC execution
may be charged against the task in control at interrupt time
or against the TCB that owns the I/O, depending on the
circumstances.