6. DATA SOURCES › 6.2 Data Collector and Processing Considerations › 6.2.2 Time of Day (TOD) Clock Synchronization

6.2.2 Time of Day (TOD) Clock Synchronization

In a JES2 multi-access spool (MAS) or JES3 environment, all
CPUs in the complex must have their time-of-day clock
values synchronized.  When this is not done, jobs may appear
to have begun or even finished execution before they were
read in.  Meaningful turnaround statistics require CPU clock
synchronization.

For example, a JES3 complex has four processors defined as
SYS1, SYS2, SYS3, and SYS4.  SYS1 is the Global Processor and
handles all JES functions.  All jobs are read in on SYS1 and
receive a reader time stamp from that machine.  Once the jobs
are read in and undergo JCL conversion, they may be routed to
execute on any of the four processors.  If SYS1's clock shows
8:00:00 a.m.  at the same instant that SYS2's clock shows
7:45:00 a.m., then the SMF records for jobs executing on SYS2
will show that they began execution up to 15 minutes before
they were read in.

The above example makes batch turnaround times look great,
but the reality is that non-synchronization of the CPU clocks
makes time stamp analysis useless when job activities span
processors.

In a JES3 environment where a single processor handles all
spool activity, a good rule of thumb is to set that
processors's clock to a value and immediately set the local
processors clocks to the same value plus an additional second
or two.

In an environment where different processors share JES
responsibilities, try to set the clocks so that ALL
processors are exactly synchronized.

The JES2 SYNCTOL parameter and AUTOMATE/MVS are two methods
available to facilitate TOD clock synchronization.  The IBM
9037 Sysplex Timer is a tabletop unit that synchronizes the
time-of-day (TOD) clocks among multiple processors.