Using the VR window size calculation exit routine for IMS

Because of the specialized use of channel-to-channel virtual routes by the IMS™ resource lock manager (RLM), you might find it useful to modify the VTAM® virtual route window size calculation exit routine, ISTPUCWC. The RLM in one IMS system sends one message at a time to a correspondent IMS RLM, and waits for a message in response. With this level of message traffic on a virtual route, the virtual route window size algorithm does not indicate that a demand exists to increase the window size. Thus, in the absence of other message traffic flowing on the virtual route, the window size tends to stay at its minimum value. Although this situation does not impede throughput on the virtual route, it tends to increase the number of machine instructions executed by VTAM for each IMS transaction (because VTAM processes a virtual route pacing response for every, or nearly every, IMS RLM message).

Accordingly, if a channel-to-channel virtual route between two IMS systems is not also used for non-IMS message traffic, you can make the following changes to the module to reduce the number of virtual route pacing responses sent and processed by VTAM in each of the IMS hosts:

Insert code to recognize the destination subarea address, virtual route number, and transmission priority that you have defined for use between the two IMS systems.
Insert code so that when the module recognizes such a virtual route, it sets the minimum window size to a value of 6.
The maximum window size can be computed in the normal manner, but the output of that calculation must be checked against this new minimum and must not be allowed to fall below it. That is, if the maximum window size is calculated to be less than 6, the maximum should then be set to 6.

Note: You can avoid coding the VR pacing window size calculation exit routine for IMS by allocating IMS-to-IMS communication to a specific virtual route and transmission priority with a unique class of service. This enables you to use the VRPWSnn operand on the PATH definition statement to control pacing window size.

There are two possible ways to compute the maximum window size in the IBM-supplied version of the module. One is used when the destination subarea is the same as the adjacent subarea, and the other is used in all other cases. Both parts of this logic should be copied into any new section of code added to compute window sizes for IMS RLM routes. Again, the maximum window size values computed must be adjusted, if necessary, to be at least equal to the minimum window size selected.

These changes to the module are not advisable if the IMS RLM virtual route is shared with other virtual route users. The message traffic of the other users should keep the operating window size values at acceptable levels.