Logical swapping

To use central storage more effectively and reduce processor and channel subsystem overhead, the SRM logical swap function attempts to prevent the automatic physical swapping of address spaces. Unlike a physically swapped address space, where the LSQA, fixed frames, and recently referenced frames are placed on auxiliary storage, SRM keeps the frames that belong to a logically swapped address space in central storage.

Address spaces swapped for wait states (for example, TSO/E terminal waits) are eligible to be logically swapped out whenever the think time associated with the address space is less than the system threshold value. SRM adjusts this threshold value according to the demand for central storage. SRM uses the unreferenced interval count (UIC) to measure this demand for central storage. As the demand for central storage increases, SRM reduces the system threshold value; as the demand decreases, SRM increases the system threshold value.

The system threshold value for think time fluctuates between low and high boundary values. The installation can change these boundary values in the IEAOPTxx parmlib member. The installation can also set threshold values for the UIC; setting these threshold values affects how SRM measures the demand for central storage.

SRM logically swaps address spaces if the real frames they own are not required to immediately replenish the supply of available real frames. Any address space subject to a swap out can become logically swapped as long as there is enough room in the system. Address spaces with pending request swaps or those marked as swaps due to storage shortages are the only address spaces that are physically swapped immediately.

Large address spaces that have been selected to be swapped to replenish central storage are trimmed before they are swapped. The trimming is done in stages and only to the degree necessary for the address space to be swapped. In some cases, it might be necessary to trim pages that have been recently referenced in order to reduce the address space to a swappable size.

SRM's logical swapping function periodically checks all logically swapped out address spaces to determine how long they've been logically swapped out. SRM physically swaps out those address spaces that have been logically swapped out for a period greater than the system threshold value for think time only when it is necessary to replenish the supply of available frames.