Estimating, checking, and setting DSALIM

The DSALIM system initialization parameter specifies the upper limit of the total amount of storage within which CICS® can allocate the individual dynamic storage areas (DSAs) that reside in 24-bit storage (below 16 MB, also known as below the line). If your installation is constrained for 24-bit storage, set a value for the DSALIM parameter equivalent to the sum of the CDSA and UDSA. If you have sufficient virtual storage to allow a greater value for DSALIM, you can use the formulas given here.

About this task

Accurate sizing of the DSALIM value is not critical. It is better to specify a DSALIM value that is slightly greater than your expected requirements rather than slightly smaller. You can tune the DSALIM parameter to a smaller value after you obtain data from your running system.

Make sure that you understand the requirements for MVS™ storage in 24-bit storage outside the CICS DSAs, to avoid other subsystem problems. For more information about the operating system components that use MVS storage, see MVS storage below 2 GB.

The minimum DSALIM value is 2 MB and the default value is 5 MB. The maximum DSALIM value is 16 MB. The extent size for the CDSA, RDSA, and SDSA is in 256 KB increments. If transaction isolation is active, the extent size for the UDSA is 1 MB and each UDSA extent must be aligned on a megabyte boundary. If transaction isolation is not active, the allocation is in 256 KB extents.

CICS allocates 24-bit kernel stack storage when it is required. Tasks obtain 4 KB extension stack segments whenever they require 24-bit stack storage. CICS preallocates a reserve pool of 24-bit extension stack segments that tasks can use if no other 24-bit stack storage is available. For more information about kernel storage, see CICS kernel storage.

Procedure

  • To check the DSALIM value that currently applies to a running CICS region, use one of the following methods:
    • CICS Explorer®: Global Dynamic Storage Areas view
    • CICSPlex® SM: Dynamic storage areas - CICSDSA view
    • CEMT: CEMT INQUIRE DSAS or CEMT INQUIRE SYSTEM
    • CICS SPI: INQUIRE SYSTEM
    Links to information about these resources are at the end of this topic.
  • To estimate a suitable DSALIM value, use the following steps:
    1. If you have sufficient virtual storage to specify a generous DSALIM value, use the following formula to estimate a value:

      CDSA + UDSA + SDSA + RDSA

      Round up the value of each component in your calculation to a 256 KB boundary.

    2. If your current installation DSALIM value is larger than necessary, use the following formula to estimate a DSALIM value:

      Peak CDSA Used + Peak UDSA Used + Peak SDSA Used + Peak RDSA Used

      Round up the value of each component in your calculation to a 256 KB boundary.

  • To change the DSALIM value for the CICS region, use one of the following methods while CICS is running:
    • CICS Explorer: Global Dynamic Storage Areas view
    • CICSPlex SM: Dynamic storage areas - CICSDSA view
    • CEMT: CEMT SET DSAS or CEMT SET SYSTEM
    • CICS SPI: SET SYSTEM
    Links to information about these resources are at the end of this topic.

Results

If there are no extents free in the CICS DSAs in 24-bit storage, CICS cannot implement a reduction of DSALIM. The storage manager applies MVS FREEMAIN requests to extents as they become available until the new DSALIM value is reached.

A short-on-storage condition can occur when you reduce DSALIM.