System-related swaps

Swaps due to storage shortages: Two types of shortages cause swaps: auxiliary storage shortages and pageable frame shortages. If the number of available auxiliary storage slots is low, SRM will swap out the address space that is acquiring auxiliary storage at the fastest rate. For a shortage of pageable frames, if the number of fixed frames is very high, SRM will swap out the address space that acquired the greatest number of fixed frames. This process continues until the number of available slots rises above a fixed target, or until the number of fixed frames falls below a fixed target.
Swaps to improve central storage usage: The system will swap out an address space when the system determines that the current mix of address spaces is not best utilizing central storage. The system swaps out address spaces to create a positive effect on system paging and swap costs.
Swap out an address space to make room for an address space: The system will swap in an address space when the system determines that it has been out longer than its recommendation value would dictate. See Working set management for information about the recommendation value.
Swaps due to wait states: In certain cases, such as a batch job going into a long wait state (LONG option specified on the WAIT SVC, an STIMER wait specification of greater than or equal to 0.5 seconds, an ENQ for a resource held by a swapped out user), the address space will itself signal SRM to be swapped out in order to release storage for the use of other address spaces. Another example would be a time sharing user's address space that is waiting for input from the terminal after a transaction has completed processing. SRM also detects address spaces in a wait state. That is, address spaces in central storage that are not executable for a fixed interval will be swapped. (See Logical swapping.)
Request Swap: The system may request that an address space be swapped out. For example, the CONFIG STOR, OFFLINE command requests the swap out of address spaces that occupy frames in the storage unit to be taken offline.
Transition Swap: A transition swap occurs when the status of an address space changes from swappable to nonswappable. For example, the system performs a transition swap out before a nonswappable program or V=R step gets control. This special swap prevents the job step from improperly using reconfigurable storage.