2. PERFORMANCE REPORTING ANALYSIS › 2.2 Real Storage Management Analysis › 2.2.4 MVS Concepts › 2.2.4.3 MVS Real Storage Control Mechanisms › 2.2.4.3.3 Page Steal Algorithm

2.2.4.3.3 Page Steal Algorithm

The page replacement process in MVS maintains a queue of free
pages to use in satisfying requests for new pages.  This
queue, called the Available Frame Queue (AFQ), is replenished
by stealing pages from the address spaces in real memory.
Page stealing is performed based on various values of the UIC
of each page.  Thus the UIC is used to define the working set
of each address space.  The changes described in this section
were made in response to various operational problems that
were discovered with continued field experience with MVS.

These changes were always made with two primary objectives in
mind:

    o  To achieve a balance holding enough page frames on the
       AFQ.

    o  To distribute the burden of paging across the address
       spaces in real memory.

In MVS/XA and MVS/ESA systems, the page steal process takes
four pages from each address space or common area before
making a second pass to steal more pages.

The page stealing process is invoked when the available frame
queue falls below a threshold and steals enough pages to
maintain the queue at a target level.  When the page stealing
process is invoked, the RFR takes all pages whose UIC equals
the system UIC.  No differentiation is made between address
spaces.

This algorithm corrects problems of the previous algorithms.
It also tends to steal more pages from the types of address
spaces that were previously favored, the low CPU users.
Thus, the non-swappable online systems (e.g., CICS) that use
their pages less frequently than other address spaces
experience heavier paging rates.