Pageable link pack area (PLPA/Extended PLPA)

This area contains SVC routines, access methods, and other read-only system programs along with any read-only reenterable user programs selected by an installation that can be shared among users of the system. Any module in the pageable link pack area will be treated by the system as though it came from an APF-authorized library. Ensure that you have properly protected SYS1.LPALIB and any library named in LPALSTxx or on an LPA statement in PROGxx to avoid system security and integrity exposures, just as you would protect any APF-authorized library.

It is desirable to place all frequently used refreshable SYS1.LINKLIB and SYS1.CMDLIB modules in the PLPA because of the following advantages:

If possible, PLPA is backed by central storage above 16 megabytes; central storage below 16 megabytes is then available for other uses.
The length of time that a page occupies central storage depends on its frequency of use. If the page is not used over a period of time, the system will reuse (steal) the central storage frame that the page occupies.
The most frequently used PLPA modules in a time period will tend to remain in central storage.
PLPA paged-in modules avoid program fetch overhead.
Two or more programs that need the same PLPA module share the common PLPA code, thus reducing the demand for central storage.
The main cost of unused PLPA modules is paging space, because only auxiliary storage is involved when modules are not being used.
All modules in the PLPA are treated as refreshable, and are not paged-out. This action reduces the overall paging rate compared with modules in other libraries.

See Placing modules in the system search order for programs for an alternative suggestion on the placement of some PLPA and SYS1.CMDLIB modules. Any installation may also specify that some reenterable modules from the LNKLST concatenation, SYS1.SVCLIB, and/or the LPALST concatenation be placed in a fixed extension to the link pack area (FLPA) to further improve performance (see The fixed link pack area (FLPA)).

Modules loaded into the PLPA are packed within page boundaries. Modules larger than 4K begin on a page boundary with smaller modules filling out. PLPA can be used more efficiently through use of the LPA packing list (IEAPAKxx). IEAPAKxx allows an installation to pack groups of related modules together, which can sharply reduce page faults. The total size of modules within a group should not exceed 4K, and the residence mode (RMODE) of the modules in a group should be the same. For more information about IEAPAKxx, see z/OS MVS Initialization and Tuning Reference .