6. DATA SOURCES › 6.4 Expanded Storage Concepts › 6.4.2 Expanded Storage Control

6.4.2 Expanded Storage Control

The Real Storage Manager (RSM) handles the transfer of pages
between central and expanded storage.  The RSM determines
both whether a page will be written to expanded storage, and
when a page in expanded storage should be migrated to
auxiliary storage.  It bases these decisions on four values:

1.  Criteria age

    The installation-specified time value that is used by the
    RSM to determine whether a page should be transferred
    from central to expanded storage.  Criteria ages are
    specified for various categories of pages and workloads
    in the SYS1.PARMLIB member IEAOPTxx.  IBM supplies
    default values that are reasonable for most normal
    processing environments.

2.  System-high unreferenced interval count (UIC)

    The time since the least recently referenced page in
    central storage was referenced.  This value is therefore
    an inverse measure of central storage contention (a low
    UIC indicates a high demand for central storage).  It is
    not possible to explicitly define a minimum value for the
    UIC, but it is influenced by several parameters in the
    IEAOPTxx member of SYS1.PARMLIB.

3.  Migration age

    The time that a page has remained unreferenced in
    expanded storage.  It is an inverse measure of expanded
    storage contention (a low migration age indicates a high
    demand for expanded storage).  In periods of contention,
    the RSM moves pages in expanded storage that have
    remained unreferenced for this period of time into
    central storage, and the auxiliary storage manager (ASM)
    then pages them out to auxiliary storage.  This process
    is referred to as page migration.

4.  Think time

    The average delay between the time when the system is
    ready for new input from a terminal user and the time
    when the terminal user actually provides such input by
    pressing the "enter" key or equivalent.  This value is
    used primarily to control logical swapping, but is also
    used in terminal input and output wait swapping to
    expanded storage.

The first value, criteria age, is specified for individual
types of pages and address space by the installation (or
allowed to default to IBM-specified values).  The last three
are measured by the system control program and will vary with
the nature and volume of work being performed and with system
configuration.  All are specified or measured in seconds.

In paging situations, the RSM sends changed and unchanged
stolen pages, virtual fetch pages, and page-out requested
pages to expanded storage when the migration age is greater
than the criteria age.

In swapping situations, the RSM normally sends both swap-out
trim and working set pages to expanded storage when the
migration age is greater than the criteria age for the type
of address space in question.  However, TSO terminal wait,
steal, and pageout users' working sets are sent to expanded
storage when the sum of migration age and UIC is greater than
the sum of criteria age plus think time.

When a page is required in central storage, the RSM first
checks to see if the page is in expanded storage.  If it is
not, a page fault is generated and the page is retrieved
asynchronously from auxiliary storage by the ASM with a
normal page-in operation.  If the page is in expanded
storage, however, a page fault is not generated and the RSM
immediately transfers the page into central storage.  This
transfer happens synchronously with program execution.  It is
important to emphasize that a page fault is not generated in
this case, and the expanded-to-central transfer is not
considered a page-in.