Q

DB2, CICS, CPUs & TCBs

CICS sub-tasks DB2 activities to separate CPUs. However, does this sub-tasking spread across all CPUs, or only one other CPU?

You don't have control over which central processing units (CPUs) associated with the multiple virtual storage (MVS) images that any particular TCB will run on. It's entirely up to the MVS and its scheduling algorithms as to which physical CPU a TCB will service on.

That being said, if you have multiple CPUs assigned to the MVS, it's very probable that a lot of one CPU will be used by customer information control system (CICS) because CICS tends to be busy most of the time (apart from when it sleeps because there are no dispatchable CICS tasks in the CICS region).

Using DB2 v5 and CICS TS 1.3, the TCBs used for DB2 access are chained off the QR TCB (the one which CICS does all it's work apart from VSAM Open/Close operations). In CICS TS 2.2 and DB2 V6/7, then the QR sub-tasks disappear to be replaced with L8 TCBs -- which sit along the QR TCB. This provides better performance as the QR TCB can be doing other work while DB2 is running.

This means that if the MVS image only has one CPU assigned to it, there will not be an essential performance benefit in CICS TS 2.2 with DB2 v6/7 because cycles will be consumed by MVS changing from the QR TCB to the L8 TCB. Thus, you should have at least two CPUs assigned to the MVS image to take advantage of this function.

Now, in a production environment, one of the important tuning functions is to ensure that there are enough CPUs assigned to the MVS image to properly service all workloads. CICS is not usually CPU bound (I/O is occurring) so the CPU can quite happily be servicing more than one CICS region -- but throughput can be adversely effected if there are not enough CPUs assigned to MVS.


 

This was first published in September 2002

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