MXT

The MXT system initialization parameter specifies the maximum number, in the range 10 through 2000, of user tasks that can exist in a CICS® system at the same time. The MXT value does not include CICS system tasks.

MXT={250|number}
CICS queues requests for tasks above this number but does not action (attach) them until the number of tasks attached drops below the MXT limit. See Setting the maximum task specification (MXT).

Review the size specified on the z/OS® REGION and MEMLIMIT parameters for the CICS address space. The increase in CICS use of virtual storage above 16 MB but below 2 GB (above the line) means that you probably need to increase the REGION parameter. The increase in CICS use of virtual storage above 2 GB (above the bar) means that you probably need to increase the MEMLIMIT parameter. See Setting the limits for CICS storage.

For CICS regions that operate with transaction isolation, the transaction isolation facility increases the allocation of some virtual storage above 16 MB but below 2 GB.
  • If the CICS region operates with transaction isolation, CICS allocates storage for task-lifetime storage in multiples of 1 MB for user-key tasks that run above 16 MB but below 2 GB. 1 MB is the minimum unit of storage allocation for the extended user dynamic storage area (EUDSA) when transaction isolation is active. However, although storage above 16 MB but below 2 GB is allocated in multiples of 1 MB, MVS™ paging activity affects only the storage that is used (referenced), and unused parts of the 1 MB allocation are not paged.
  • If the CICS region operates without transaction isolation, CICS allocates user-key task-lifetime storage above 16 MB but below 2 GB in multiples of 64 KB.

The subspace group facility uses more real storage, because MVS creates a page and segment table from real storage for each subspace. The CICS requirement for real storage varies depending on the transaction load at any one time. As a guideline, each task in the system requires 9 KB of real storage, and this should be multiplied by the number of concurrent tasks that can be in the system at any one time (governed by the MXT system initialization parameter).

Note: If the MAXOPENTCBS or MAXXPTCBS system initialization parameters have not been specified, then setting MXT will affect the MAXOPENTCBS and MAXXPTCBS settings.
For more information about open TCBs, see Open TCB management.
Important: Before you change the MXT system initialization parameter, review the information in Open TCB pools.