Memory Management on iSeries (SAP Library - SAP Memory Management (BC-CST-MM))

Memory Management on iSeries

Memory management on iSeries is based on temporary, dynamic memory. Both single-level storage (SLS) and teraspace are used.

In the single-level storage concept main memory and auxiliary storage (for example, disk storage) are considered as a single large address space, whose linearly addressable areas can grow to a maximum of 16 MB in size.
A teraspace is temporary dynamic storage, which provides an address space of 1 terabyte in size for each process (that is, OS/400 job). A teraspace can consist of a number of separately allocated and deallocated storage areas. Between these areas, "addressability holes" can exist, that is, areas of storage that are not allocated. Although teraspace is also based on single-level storage, it is completely transparent to the processes (OS/400 jobs).

For the memory management to function optimally, sufficient free auxiliary storage space must be available.

The extended memory is located in the teraspace and is allocated when the SAP instance is started. You specify the size of the extended memory pool using system profile parameter em/initial_size_MB. Dynamic extension is not possible.

Implementing the memory on the iSeries has the following effect:

Roll area

Unlike on other platforms, user contexts are not rolled into and out of work processes by means of copy procedures. Instead, the roll area is implemented in a similar way to the extended memory. On iSeries a roll-in is achieved by setting pointers to the user roll memory.

The roll area is only used for internal system areas of the user context. The remainder (for example, internal tables) are in extended memory.

Roll area parameters ztta/roll_area and ztta/roll_first are preset and cannot be changed.

Since no data is copied during roll-in and roll-out, the corresponding system profile parameters are not used. These parameters (rdisp/ROLL_MAXFS and rdisp/ROLL_SHM) must be set to the value 0 (zero), which is the default value.

Paging

iSeries does not distinguish between the paging cache (profile parameter rdisp/PG_SHM) and the paging file (profile parameter rdisp/PG_MAXFS). The complete paging file is buffered. As a result, the settings for rdisp/PG_SHM and rdisp/PG_MAXFS are identical.

Shared memory pools

You do not have to define or manage any shared memory pools. (Shared memory pools are defined in the system profile using parameter ipc/shm_psize.) Parameter ipc/shm_psize is ignored on iSeries.