2. PERFORMANCE REPORTING ANALYSIS › 2.5 Virtual Storage Management Analysis › 2.5.2 Usage Guidelines › 2.5.2.1 Horizontal Versus Vertical Address Expansion

2.5.2.1 Horizontal Versus Vertical Address Expansion

HORIZONTAL EXPANSION OF VIRTUAL ADDRESSING

A major historical theme in the design of MVS has been what
can be viewed as a horizontal expansion of the virtual
storage space.

In order to provide virtual storage constraint relief (VSCR),
applications which compete for virtual storage might be run
on physically separate processors.  For instance, IMS, which
tends to require a large Common System Area (CSA) and CICS,
which needs a large private region (PVT), might be run on
separate processors.  This separation of a large CSA and a
large Private Area onto separate processors would constitute
expansion of these two workloads into two separate virtual
address ranges in the two processors.

The increased capability to spread the virtual addressing
range horizontally within a single MVS system has been
implemented in MVS/370 through the use of cross-memory
address spaces.  Such horizontal expansion has permitted
major functions to be isolated in their own address spaces,
thus effectively expanding the virtual addressing range
available beyond the 16MB range of MVS/370 and the 2 GB range
of MVS/XA by allowing access by one address space into
another one, thus logically including the address space
identification (ASID) as a high-level portion of the address.

For the MVS/XA operating system itself, this horizontal
expansion has been implemented via supplemental address
spaces:

    o  PCAUTH
    o  TRACE
    o  GRS
    o  CATALOG
    o  CONSOLE
    o  ALLOCAS
    o  LLA
    o  SMF

JES3, with its use of an auxiliary address space is an
example of a subsystem making use of cross-memory services to
provide this same sort of horizontal expansion of virtual
storage.

This trend is being continued with the advent of PR/SM and
MVS/ESA.
VERTICAL EXPANSION OF VIRTUAL ADDRESSING

The advent of MVS/370 provided a vertical expansion of the
virtual storage addressing range, since it removed the former
restriction that all private regions share one 16MB address
range.  The subsequent expansion of this range to 2 GB with
the arrival of MVS/XA continued this trend.  However,
programs cannot take advantage of the virtual addresses above
16 MB without at least some modification and reassembly.
Thus, since many of the larger users of virtual storage are
subsystems like JES3, VTAM, or IMS, it is often necessary to
upgrade to a new release of such a product to obtain needed
VSCR.