2. PERFORMANCE REPORTING ANALYSIS › 2.2 Real Storage Management Analysis › 2.2.4 MVS Concepts › 2.2.4.4 Expanded Storage Concepts › 2.2.4.4.2 System Control of Expanded Storage

2.2.4.4.2 System Control of Expanded Storage

The Real Storage Manager (RSM) handles the transfer of pages
between central and expanded storage.  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 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,
    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.

Criteria age, is specified for individual types of pages and
address space by the installation (or allowed to default to
IBM-specified values).  The others 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, 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, 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, RSM first checks
to see if the page is in expanded storage.  If 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, no
page fault is generated and 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.  Indeed, RMF Version 3 does not even report page
movement from expanded to central storage.

Swapping to expanded storage differs from swapping to
auxiliary storage.  When an address space is swapped out to
expanded storage, RSM and the System Resources Manager (SRM)
divide the working set pages into primary and secondary
working set groups.  The primary group, which is managed by
RSM as a single entity, contains all LSQA and fixed pages,
plus one page from each virtual storage segment included in
the whole working set.  The secondary group comprises all
working set pages not included in the primary group.

Once the address space has been swapped out to expanded
storage, RSM recognizes the difference between the primary
and secondary working sets when it makes its decisions about
migration.  Secondary pages are migrated first, and only
after all secondary pages have been migrated will the primary
working set be migrated.

If the address space is swapped back in before it has been
migrated, the primary working set is swapped in first, at
which time the address space becomes dispatchable.  Secondary
working set pages are then treated the same as all other
pages in expanded storage.  This means that if they are
migrated, they will be sent to local page data sets rather
than swap data sets.  If you do a substantial amount of
swapping to expanded storage, this could be an argument in
favor of an auxiliary storage subsystem configuration
consisting only of local page data sets with no swap data
sets.