2. PERFORMANCE REPORTING ANALYSIS › 2.2 Real Storage Management Analysis › 2.2.4 MVS Concepts › 2.2.4.3 MVS Real Storage Control Mechanisms › 2.2.4.3.1 Swapping

2.2.4.3.1 Swapping

The resource use functions of the SRM attempt to optimize the
use of system resources on a systemwide basis.  It does this
through multiprogramming level adjusting, CPU load balancing,
I/O load balancing, storage load balancing, and demand or
logical swapping.

Swapping is the primary method used by the SRM to effect this
control.  Using IPS and OPT parameters and system status
information, the SRM determines which address spaces should
have access to system resources.

There are a number of reasons for swapping.  The first three
described below control domains and the address spaces within
a domain, while the next four control system performance and
throughput.

Unilateral   If the number of active address spaces for the
Swap In:     domain is less than the value specified in
             the IPS or less than the SRM deems optimal, the
             SRM swaps in address spaces for the domain.

Unilateral   If the number of active address spaces for the
Swap-Out:    domain is greater than the value specified in
             the IPS or greater than the SRM deems optimal,
             the SRM swaps out address spaces for the domain.

Exchange     All address spaces within a domain compete with
Swap:        one another for system resources.  When an
             address space has consumed its allotted
             resources and another address space in the same
             domain is waiting to be swapped in, the SRM
             swaps out the one in real storage and swaps in
             the other.

Swaps Due    If auxiliary storage or pageable frame shortages
to Storage   occur, the SRM swaps out the address space
Shortages:   that is acquiring auxiliary storage at the
             fastest rate or the address space with the
             greatest number of fixed frames.  It continues
             this operation until the shortage is corrected.

Swaps Due    In certain cases the address space signals the
to Wait      SRM that it is a candidate for swap-out.  This
States:      occurs when it can be determined that the
             address space will not become eligible for
             execution for a relatively long time.  Examples
             are an ENQ for a resource currently held by a
             swapped-out address space or an STIMER wait of
             greater than 0.5 seconds.

Request      The system may request that an address space be
Swap:        swapped out.  For example, the VARY STOR,OFFLINE
             command requests the swap-out of address spaces
             that occupy frames on the storage unit to be
             taken offline.

Transition   A swap occurs when the status of an address
Swap:        space changes from swappable to non-swappable.
             This special swap prevents the job step from
             improperly using reconfigurable storage.

The SRM adjusts the multiprogramming level by adjusting the
number of address spaces that are held in real storage for
each domain.  When the paging rate becomes excessive, the SRM
reduces it by swapping out address spaces to reduce paging.
If the rate is too low, the SRM swaps in address spaces to
increase it.

The SRM adjusts the CPU load level by swapping in eligible
address spaces if the CPU utilization is too low or swapping
out address spaces if the CPU utilization is too high.

The SRM adjusts the I/O load level by swapping in or out
address spaces to maintain the proper utilization of the
system's logical I/O channels.

The SRM adjusts the storage load level by swapping in or out
address spaces to keep real memory properly utilized.

The SRM effectively utilizes real storage and reduces channel
and processor overhead by logically swapping some address
spaces.  Rather than swapping them to auxiliary storage, they
are left in real storage whenever possible.  This is done if
the system determines that the address space will be ready to
execute in a relatively short period of time.

With the advent of RMF 4.2, four new swap reasons are
reported, which are used to manage work flow and resource
commitment.  The SRM monitors the utilization levels of
various system resources and attempts to bring these levels
to within site-specified limits by swapping address spaces
out of central storage.  In addition, the SRM may respond to
an address space experiencing a wait condition during an APPC
conversation by swapping the address space.

Swapping may be performed in order to improve the system
paging rate.  If the SRM has been set up to manage the system
paging rate, and that rate rises above the maximum acceptable
level, the SRM will cause address spaces to be swapped out in
an attempt to lower the rate.

Swapping may also be performed in order to improve central
storage usage.  If the SRM has been set up to manage central
storage, and demand for storage rises above the maximum
acceptable level, the SRM will cause address spaces to be
swapped out in an attempt to lower the demand.

Swapping may occur to make room in central storage to swap in
an address space that has been swapped out too long.  The SRM
monitors the duration of swap-outs, and if no other condition
has made it possible to swap in an address space that has
been swapped out for a long time, the SRM will make room in
central storage for it by swapping out other address spaces.

During an APPC conversation, an address space issuing an
LU6.2 call to the APPC address space can experience a wait
condition.  When this occurs, the SRM can then make a
decision whether to swap the calling address space out or
not.