DFHTCLSX and DFHTCLQ2 in RDO group DFHISCT allow you to control the amount of storage used by CICS® to execute the CLS1/2 and CLQ2 transactions respectively.
These tasks execute the activities needed to acquire an APPC conversation (CLS1/2), and to resynchronize units of work for MRO and APPC connections (CLQ2). Usually there are not many tasks, and they need no control. However, if your CICS system has many connection definitions, these may be acquired simultaneously as a result of initializing the system at startup, or as a result of a SET VTAM® OPEN, or SET IRC OPEN command.
The system definitions are optional. Install resource group DFHISCT to activate them. As supplied, the MAXACTIVE parameter in the DFHTCLSX and DFHTCLQ2 is 25. This should give sufficient control to prevent the system reaching a short-on-storage situation. (Tasks CLS1 and CLS2 each require 12K of dynamic storage, and CLQ2 tasks require up to 17K). The purge threshold should not be set to a non-zero number, and the maxactive should not be set to 0. They both prevent CICS executing tasks necessary to intersystems functions.
It is not advisable to set the MAXACTIVE value too low because network delays or errors may cause one of the tasks in the TCLASS to wait and block the use of the TCLASS by succeeding transactions. Setting a low value can also extend shutdown time in a system with a large number of connections.