BELOWHEAP
BELOWHEAP controls the allocation of library heap storage that must be located below the 16 MB line. The heap that is controlled by BELOWHEAP is intended for items such as control blocks used for I/O.
- Non-CICS® default
- BELOWHEAP=((8K,4K,FREE),OVR)
- CICS default
- BELOWHEAP=((4K,4080,FREE),OVR)
- 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 16 MB 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 that is allocated to BELOWHEAP increments is released when the last of the storage is freed. FREE is the default.
- KEEP
- Specifies that storage that is allocated to BELOWHEAP increments is not released when the last of the storage is freed.
- OVR
- Specifies that the option can be overridden. OVR is the default.
- NONOVR
- Specifies that the option cannot be overridden.
CICS considerations
The default increment size under CICS is 4080 bytes, rather than 4096 bytes, to accommodate the 16-bytes 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).
z/OS® UNIX considerations
The BELOWHEAP option applies to the enclave.
Usage notes
Both the initial size and the increment size are rounded to the nearest multiple of 8 bytes. If you specify BELOWHEAP(0), the initial BELOWHEAP is obtained on the first use and will be the increment size.
Performance considerations
BELOWHEAP improves performance when you specify values that minimize the number of times that the operating system allocates storage. The RPTSTG runtime option generates a report of storage your application uses while it is running. You can use its numbers to help determine what values to specify.
For more information
- See z/OS Language Environment Programming Guide for more information about Language Environment® heap storage.
- See RPTSTG for more information about the RPTSTG runtime option.
- For more information about tuning your application with storage report numbers, see z/OS Language Environment Programming Guide.