2. PERFORMANCE REPORTING ANALYSIS › 2.5 Virtual Storage Management Analysis › 2.5.4 Virtual Storage Concepts in MVS/XA › 2.5.4.2 Initialization and Management of Virtual Storage

2.5.4.2 Initialization and Management of Virtual Storage

Some of these areas of virtual storage are established at the
time MVS is initialized and are not subject to any subsequent
dynamic management by VSM.

At the time of IPL (Initial Program Load), the IPL control
module creates the necessary virtual environment for other
programs to be loaded from SYS1.NUCLEUS.  The IPL control
module and the IRIMs (IPL Resource Initialization Modules)
that it subsequently loads into memory load the nucleus,
including the PSA and the nucleus map, build the page frame
table above the nucleus map, allocate minimal SQA and ESQA
areas, and allocate LSQA for the master scheduler.  Control
is then passed to the NIP (Nucleus Initialization Program) to
complete the initialization of various system resources,
including the system virtual storage areas.

The NIP, together with some of the RIMs (Resource
Initialization Modules) which it invokes, expands SQA and
ESQA to match the parameters found in SYS1.PARMLIB.  It then
creates or resets PLPA and EPLPA (depending on whether the
person doing the IPL requested a cold start or a warm start).
Next, the modules to be placed in FLPA, EFLPA, MLPA, and
EMLPA are loaded.  This is followed by the allocation of CSA
and ECSA according to the parameters in SYS1.PARMLIB.  At
this time the NIP has essentially finished establishing the
systemwide areas of virtual storage.  It then creates various
system address spaces, the first of which is the Master
Scheduler, to which it finally passes control.

All of these areas which are established by these
initialization processes have their sizes set during
initialization.  The establishment of their overall sizes
determines the sizes of the private and extended private
areas, since they are the portions of virtual memory below
and above the 16MB line that are left over after all the
system areas have been designated.

Only four of these system areas are subject to continuing
dynamic management by VSM:  the SQA, ESQA, CSA, and ECSA
areas.  The dynamics of virtual memory usage within these
areas and within certain critical address spaces in the
Private Area can become a major concern, since exhausting
available virtual memory in these areas can lead to
performance degradation, abnormal termination of these
critical address spaces, or, under extreme conditions, the
abnormal termination of the entire system.