6. DATA SOURCES › 6.4 CPU Time Measurements › 6.4.4 Variability in CPU Time › 6.4.4.4 Explicit Queueing Effects

6.4.4.4 Explicit Queueing Effects

Variation in the contention for other system resources can
have a strong effect on measured CPU time.

Queue lengths themselves can cause variability.  CPU time
increases with the number of control blocks in a list to be
scanned.  The increased time is non-captured if the list
scanner is the dispatcher.  Whether captured time is charged
to a TCB or an SRB will depend on the list, the requested
function, and even the operating system level.

This effect is pervasive:  the most important performance
impacting queues in MVS include ASCB chains run by the
dispatcher, QCBs used by ENQ/DEQ (and RESERVE in a shared
DASD environment), the FQEs used by GETMAIN/FREEMAIN to
manage virtual storage, the TQEs built by TASK and REAL
STIMER macros, and the WQEs and RQEs used to store console
messages and replies.  Of course, each active software
subsystem (JESx, IMS, CICS, etc.) has its own set of
performance-impacting queues, which may have to be taken into
account.

ENQ, RESERVE, and DEQ each require more processing time for a
resource in contention.  The request block must be queued up
if it is not available at request time.  The system must wake
the requestor when the resource becomes available.  DEQ time
increases if other tasks have subsequently requested the
resource that the current task is releasing.

CPU time varies depending on whether or not ECBs have been
posted prior to issuance of the WAIT macro instruction.

If a job is run on a processor complex with more than one
processor in a tightly coupled configuration, CPU time may
vary due to lock contention and the resulting "spins"
encountered while serializing certain supervisor services.

CPU time for WTO, WTOR, and WTL processing will vary,
depending on the time required to find a free WQE and/or RQE,
and possibly on whether a GETMAIN is necessary to build a new
element.  If the WTO or reply elements are at their limit,
additional time is required for enqueuing.