gtpc3m0mConcepts and Structures

TPF System Loosely Coupled Multiprocessing

Loosely coupled multiprocessing within the TPF environment refers to a set of central processing complexes (CPCs) each with its own private main storage, and all of which share a set of module CUs to access a shared database. This form of multiprocessing is represented, primarily, by the database support and the underlying I/O services offered by the TPF system.

When several CPCs attempt to simultaneously modify the same module record, an XLF restricts record updating to only one CPC at a time.

For such a mechanism to be effective, the performance overhead to determine if more than one CPC wants to update the record must be minimized and the probability of multiple CPCs wanting to simultaneously update the same record should be low. Both of these conditions are met in the TPF environment.

Observe, as with tightly coupled multiprocessing, loosely coupled multiprocessing comes at some performance expense. Because modules are shared, module CUs or coupling facilities (CFs) must pay the performance expense of setting and checking lock indicators. A CPC must occasionally wait for a record that is currently locked. However, multiprogramming is employed during these waiting periods. Message processing rates in a loosely coupled environment of n CPCs are never equal to n times the message processing rate of a single CPC using module CUs that have neither the locking facility nor the corresponding system software that controls the locks.