Limiting the use of memory objects

While there is no practical limit to the virtual storage above the bar, practical limits exist to the real storage frames and auxiliary storage slots that back the area. To control the amount of real and auxiliary storage that an address space can use for memory objects at one time, your installation can establish an installation default MEMLIMIT that sets the total number of usable virtual pages above the bar for a single address space. You set this default in two ways:

On the MEMLIMIT parameter in the SMFPRMxx parmlib member
Through the SET SMF or SETSMF commands.

The default takes effect if a job does not specify MEMLIMIT on the JCL JOB or EXEC statement, or REGION=0 in the JCL; the MEMLIMIT specified in an IEFUSI exit routine overrides all other MEMLIMIT settings. If REGION=0 is specified in the JCL and the IEFUSI exit limits the REGION size but does not set MEMLIMIT, MEMLIMIT is defaulted to the REGION size above 16MB.

Figure 1 shows how the system chooses which MEMLIMIT applies.

Figure 1. How the system chooses which MEMLIMIT applies

How the system chooses the MEMLIMIT to use

For more information, see the following publications:

For the SMFPRMxx parameters that establish the installation defaults for MEMLIMIT, see z/OS MVS Initialization and Tuning Reference.
For the SETSMF or SET SMF commands that change the MEMLIMIT dynamically and D SMF,O that displays the current MEMLIMIT, see z/OS MVS System Commands.
For the JOB and EXEC statements that can set a MEMLIMIT for a job or jobstep, see z/OS MVS JCL Reference.
For the SMF installation exit IEFUSI that can override the MEMLIMIT for a job or an address space, see z/OS MVS Installation Exits.

The system enforces the MEMLIMIT when you issue the IARV64 GETSTOR and CHANGEGUARD services. When your unconditional request for new storage (either for a new memory object or for more usable storage in an existing memory object) causes the MEMLIMIT to be exceeded, the system abends the program. IBM® recommends that programs use the COND parameter to make a conditional request and check the return code to make sure the storage is available.

If a SET SMF or SETSMF command changes the default MEMLIMIT (either the system default or the installation default) for an address space that is already created the following changes occur:

If the command raises the current default MEMLIMIT, all address spaces whose MEMLIMIT values are set through SMF run with the higher default.
If the command lowers the current default MEMLIMIT, all address spaces whose MEMLIMIT values are set through SMF keep their original (higher) system default.

You cannot use SET SMF or SETSMF command to change the MEMLIMIT value that is set through JCL or the IEFUSI installation exit.