Pageable frame stealing

Pageable frame stealing is the process of taking an assigned central storage frame away from an address space to make it available for other purposes, such as to satisfy a page fault or swap in an address space.

When there is a demand for pageable frames, SRM will steal those frames that have gone unreferenced for a long time and return them to the system. The unreferenced interval count (UIC) represents the time in seconds for a complete steal cycle. A complete steal cycle is the time the stealing routine needs to check all frames in the system. When there is a demand for storage, the stealing routine:

tests the reference bit of a frame
decides whether to steal the frame
schedules the page-out.

When there is no demand for storage, no stealing occurs.

The UIC algorithm forecasts the UIC, based on the current stealing rate. The UIC can vary between 0 and 65535 and gets calculated every second. When there is no demand for storage in the system (no stealing occurs) the system has a UIC of 65535. If there is a very high demand for storage in the system, the system has a UIC close to 0.

Stealing takes place strictly on a demand basis, that is, there is no periodic stealing of long-unreferenced frames. A complete steal cycle can take days.

SRM modifies the stealing process for address spaces that it is managing and for address spaces that are storage critical. For these address spaces, SRM attempts to enforce the address space's real storage target that was set when SRM decided that the address space was to be managed.