Estimating, checking, and setting EDSALIM

The EDSALIM system initialization parameter specifies the upper limit of the total amount of storage within which CICS® can allocate the individual extended dynamic storage areas (EDSAs) that reside in 31-bit (above-the-line) storage. Set the value for the EDSALIM parameter as large as you can after consideration of other areas, especially MVS™ storage.

About this task

The maximum value that you can specify for EDSALIM is limited by the following factors:
  • The size that you specified for the CICS region on the z/OS® REGION parameter in the CICS job or procedure. The value of EDSALIM must be less that the value of REGION.
  • The amount of MVS storage, outside the CICS DSAs, that you require to satisfy MVS GETMAIN requests for 31-bit storage. For more information about the operating system components that use MVS storage, see MVS storage below 2 GB.
Accurate sizing of the EDSALIM value is not critical. A good approach is as follows:
  • Initially specify an EDSALIM value that is slightly greater than your expected requirements. The default setting, 800 MB, enables the CICS region to run a reasonable workload.
  • Monitor the use of each CICS DSA in the EDSA while your system is running near peak loads.
  • Tune your EDSALIM value in the running CICS system.
Try not to specify the largest possible EDSALIM value (for example, the maximum allowable region size). If you use the maximum possible limit, you might not receive any warnings about a shortage of virtual storage until the problem becomes difficult to resolve.

You can obtain information about your current EDSA use by looking at the CICS storage manager statistics. See the information about DSA sizes in the storage manager statistics, dynamic storage areas, and task subpools. Automatic DSA sizing removes the need for accurate storage estimates for individual DSAs, with CICS dynamically changing the size of DSAs as demand requires.

The minimum EDSALIM value is 48 MB, which is the minimum required to start a CICS region. The default EDSALIM value is 800 MB. The maximum EDSALIM size is 2047 MB, which is 2 GB minus 1 MB. The extent size for the ECDSA, EUDSA, ESDSA, ERDSA, and ETDSA is 1 MB.

Kernel stack storage is also allocated from the EDSA. For more information about kernel storage see CICS kernel storage.

Remember that for CICS regions that run with transaction isolation active (set by using the TRANISO system initialization parameter), the transaction isolation facility increases the allocation of some virtual storage above 16 MB.

If transaction isolation is active, CICS allocates user-key task-lifetime storage above 16 MB in multiples of 1 MB (the minimum unit of storage allocation for the EUDSA when transaction isolation is active is 1 MB). However, MVS paging activity affects only the storage that is used (referenced), and unused parts of the 1 MB allocation are not paged.

For a CICS region that runs without transaction isolation, CICS allocates user-key task-lifetime storage above 16 MB 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 according to 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).

Procedure

  • To check the EDSALIM 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
    For more information about these resources, see the links at the end of this topic.
  • To estimate a suitable EDSALIM value, use the following steps:
    1. Check the MXT value for your CICS region.
      The MXT system initialization parameter does not include CICS system tasks, and it might also be set to a value larger than necessary. The safest estimate for calculating an EDSALIM value is to assume MXT as the number of concurrent active tasks.
    2. Check the setting of the TRANISO system initialization parameter for your CICS region.
      For the EUDSA, if the TRANISO parameter is set to NO for the CICS region, allow 64 KB per concurrent active task. If the TRANISO parameter is set to YES, allow 1 MB per concurrent active task.
      If your applications use more than 64 KB per task with the TRANISO parameter set to NO, or more than 1 MB per task with the TRANISO parameter set to YES, adjust the formulas accordingly. If you adjust the formulas, use multiples of 64 KB or 1 MB.
    3. If you have sufficient virtual storage to specify a generous EDSALIM value, use one of the following formulas to estimate a value.
      Round up the value of each component in your calculation to a 1 MB boundary, the size of an extent, to allow for fragmentation and partially used extents.
      For TRANISO=NO:
      ECDSA + EUDSA + ESDSA + ERDSA + ETDSA + (64 K x MXT)
      For TRANISO=YES:
      ECDSA + EUDSA + ESDSA + ERDSA + ETDSA + (1 MB x MXT)
    4. If your current installation EDSALIM and MXT values are larger than necessary, use one of the following formulas to estimate an EDSALIM value.
      Round up the value of each component in your calculation to a 1 MB boundary, the size of an extent, to allow for fragmentation and partially used extents.
      For TRANISO=NO:
      Peak ECDSA Used + Peak EUDSA Used + Peak ESDSA Used + Peak ERDSA Used + Peak ETDSA Used - EUDSA Peak Page Storage in Task Subpools + (64 K x Peak number of tasks)
      For TRANISO=YES:
      Peak ECDSA Used + Peak EUDSA Used + Peak ESDSA Used + Peak ERDSA Used + Peak ETDSA Used - EUDSA Peak Page Storage in Task Subpools + (1 MB x Peak number of tasks)
  • To change the EDSALIM 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
    For more information about these resources, see the links at the end of this topic.

Results

If you under-specify EDSALIM, your system can go short on storage and you might not be able to issue CICS commands to increase the limit. In this situation, use the CICS Explorer or CICSPlex SM to increase the EDSALIM value.

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