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 MVS Real Storage Control Mechanisms

When looking at the real and virtual storage mechanisms of
MVS, it is important to remember that MVS is not a paging
system.  That is, MVS does not manage real storage by paging;
real storage management is done primarily by swapping.
Paging in MVS provides several critical improvements over the
earlier IBM systems:

o  More efficient use of real storage.  Paging completely
   eliminates the external fragmentation of the MFT and MVT
   systems.

o  Provides the application with a large virtual memory.  MVS
   largely removed issues of memory management from
   applications.

o  Improvements in data and system integrity.  A dynamic
   addressing scheme makes this much easier to accomplish.

o  Increase in the number of concurrent jobs.  The S/360 and
   S/370 architecture has a maximum of 16 storage protection
   keys.  In the MFT and MVT systems, this would be a severe
   limitation on batch throughput.  In MVS, because of the
   protection afforded by dynamic addressing, all user
   (problem state) work except for V=R jobs may use the same
   protect key.

o  Supports Swapping - Swapping in a non-paging environment
   would be much more complex because of address relocation
   issues.

Because of this design, the MVS paging algorithms are vastly
different (and more complex) than those of a system whose
primary means of real storage control is through paging.
Management of storage is divided among the three subsystems
(RSM, ASM, and VSM) described below:

REAL STORAGE MANAGER (RSM)

The Real Storage Manager (RSM) maintains the control blocks
that translate real and virtual storage address, makes the
determination of working set contents when an address space
is swapped, and performs the page replacement operations.

AUXILIARY STORAGE MANAGER (ASM)

The Auxiliary Storage Manager (ASM) manages the page and swap
data sets that comprise the virtual space.  The page data
sets provide the auxiliary storage that backs up the storage
frames.  Prior to the introduction of the extended swap
facility (provided either by the Installed User Program (IUP)
or with MVS/SP 1.3), the swap data sets contained only the
LSQA frames from swapped-out address spaces.  The private
frames were paged out.  With the extended swap processing,
the LSQA pages and the working set of the address space are
written out together to the swap data sets.  This concept is
discussed in detail in Section 2.3.

VIRTUAL STORAGE MANAGER (VSM)

The Virtual Storage Manager (VSM) provides a map of the
private area virtual storage within each address space.  VSM
interfaces with RSM to handle the dynamic allocation of
virtual memory.

STORAGE MANAGEMENT TECHNOLOGY

MVS uses a least-recently-used (LRU) page replacement
algorithm to keep a number of free pages to be used for page
allocations and swap-ins.  Through the progression of the MVS
Systems Extensions and System Product Releases, the real
storage algorithms have been refined and additional
capabilities have been added.  The remainder of this section
defines the operation of the page replace and storage control
algorithms and traces some of the historical development of
these processes.

    1 - Swapping
    2 - UIC Update Process
    3 - Page Steal Algorithm