The BELOWHEAP runtime option controls the allocation of restricted library heap storage (below heap). Storage that is restricted must be located below the 16M line (24-bit addressable storage).

The default value for non-CICS applications is BELOWHEAP(8K,4K,FREE).

The default value for CICS® applications is BELOWHEAP(4K,4080,FREE).

Syntax

                                                     .-FREE-.   
>>-BElowheap--(--+-----------+--,--+-----------+--,--+------+--->
                 '-init_size-'     '-incr_size-'     '-KEEP-'   

>--)-----------------------------------------------------------><

init_size

Determines the minimum initial size of the below heap storage. This value can be specified as n, nK, or nM bytes of storage. The actual amount of allocated storage is rounded up to the nearest multiple of 8 bytes.

incr_size

Determines the minimum size of any subsequent increment to the area below the 16M line, and is specified in n, nK, or nM bytes of storage. This value is rounded up to the nearest multiple of 8 bytes.

FREE

Specifies that storage allocated to BELOWHEAP increments is released when the last of the storage is freed.

KEEP

Specifies that storage allocated to BELOWHEAP increments is not released when the last of the storage within that increment is freed.

CICS consideration

The default increment size is 4080 bytes, rather than 4096 bytes, to accommodate the 16-byte CICS storage check zone. Without this accommodation, an extra page of storage is allocated (only when the storage allocation is below the 16-MB line). If you choose to change the increment size, it is recommended that you adjust for the 16-byte CICS storage check zone.

z/OS® UNIX considerations

• In a multithreaded environment, the below heap is shared by all threads in the process.
• When ALL31(OFF) is in effect, Language Environment allocates thread-specific control blocks from the below heap.

Usage notes

If BELOWHEAP(0) is specified, the initial below heap segment is obtained on the first use and is based on the increment size.

Performance consideration

You can improve performance with the BELOWHEAP runtime option by specifying values that minimize the number of times the operating system allocates storage. See RPTSTG for information about how to generate a report you can use to determine the optimum values for the BELOWHEAP runtime option.

For more information

For more information about heap storage and heap storage tuning with storage report numbers, see z/OS Language Environment Programming Guide.