Estimating and setting REGION

The z/OS® REGION parameter limits the amount of 24-bit and 31-bit storage (storage below the bar) that the CICS® address space can use. This value includes all the storage below the bar in the private area, except for a 16 KB system region in 24-bit storage, and the items in the high private area such as the LSQA.

About this task

For an explanation of the storage areas in the z/OS address space below 2 GB, see The Virtual Storage Address Space in the z/OS MVS Initialization and Tuning Guide.

You can request up to 2047 MB of storage below the bar for the CICS region, but you must ensure that you leave enough storage below the bar for MVS™ to use in the high private area. The items in the high private area are the local shared queue area (LSQA), scheduler work area (SWA), and subpools 229 and 230. These items exist in both 24-bit (below-the-line) storage and 31-bit (above-the-line) storage. The LSQA in 31-bit storage is called the extended LSQA. Some of this storage is used for control blocks, and some is used by z/OS Communications Server and other programs.

Within the value that you specify for REGION, the following types of storage are included:
  • The CICS DSAs in 24-bit storage. The storage for these DSAs is limited by the DSALIM system initialization parameter.
  • The CICS DSAs in 31-bit storage. The storage for these DSAs is limited by the EDSALIM system initialization parameter.
  • Storage used by the CICS kernel.
  • MVS storage obtained by MVS GETMAIN requests.
  • CICS dispatcher
  • CICS storage manager
  • CICS lock manager

You cannot alter the REGION value for the CICS region while CICS is running. You can specify a new value on the next start of the CICS region.

Procedure

  1. To determine the maximum value for the REGION parameter:
    1. Use RMF or another storage monitor to determine the size of your private area.
    2. Apply the following formula: 
      Maximum possible REGION = 
			Size of private area
			- Size of system region (16K)
			- (LSQA + SWA + subpools 229 and 230)
      For more information about the high private area and LSQAs, and estimates for the sizes of the items in the high private area, see High private area.
    3. For safety, do not use more than 80% or 90% of this maximum value for the REGION parameter.
      It is useful to maintain some free storage between the top of the CICS region and the bottom of the high private area.
      If the system is static or does not change much, use up to 90% of this number. If the system is dynamic, or changes frequently, 80% would be better.
  2. To estimate the value that you require for the REGION parameter to meet your storage needs, add up your estimates for the following areas of storage:
  3. For instructions to specify the REGION parameter, and information about the amount of storage that z/OS allocates in response to your request, see REGION Parameter in the z/OS MVS JCL Reference.
    You cannot alter the REGION value for a running CICS region.
    You can set REGION in the following ways:
    • You can specify the REGION parameter in the JOB statement in the CICS JCL. In this situation, each step of the job runs in the requested amount of space.
    • You can specify the REGION parameter in the EXEC statement (program execution line) for CICS. In this situation, each step runs in its own amount of space. Use the EXEC statement instead of the JOB statement if different steps need greatly different amounts of space. For example, you could use the EXEC statement if you are using extra job steps to print auxiliary trace data sets after CICS has shut down (as in the DFHIVPOL installation verification procedure).
    • The z/OS installation exit IEFUSI can limit the REGION value that you specify. For information about IEFUSI, see IEFUSI - Step Initiation Exit in z/OS MVS Installation Exits.