The size specified for the coupling facility list structure in
the CFRM policy should be large enough to allow for the recommended
number of list headers and list entries, as described below. Specifying
more than this minimum amount of space allows MVS™ to queue
more signals on a list simultaneously and improves signaling performance.
- It is recommended that the coupling facility list structure be
large enough to have at least one signal queued simultaneously on
every list header used as a signaling path.
The number of data
elements needed for these list entries depends on the size of the
signals that are sent. For example, for a 61K byte signal (the largest
signal supported by the MVS signaling service), 16 data elements
would be needed. Since the entry to element ratio is 1:1, 16 list
entries would also be allocated. Thus, to allow for one 61K byte signal
to be queued simultaneously over every signaling path, the list structure
must be large enough to accommodate 16 list entries per path.
- It is also desirable for the list structure to be large enough
to accommodate the peak number of signals simultaneously queued over
each list header for a signaling path.
Otherwise, MVS is forced
to queue the signal on the sending system. This delays signal delivery
and increases the amount of real storage needed for message buffers.
- MVS also needs list entries to manage the coupling
facility list structure. The number and size of these list entries
varies with:
- The number of list headers in the list structure,
- The maximum number of systems that can connect to the list structure,
- The release of MVS that those systems are running.
In OS/390®, it is recommended that the list structure
be large enough to provide one list entry per connected system, plus
eight additional list entries for control data. Each list entry requires
only one data element.
If fewer than the recommended number of list entries are available, MVS might
continue to use the coupling facility list structure for signaling,
although possibly with some degradation.