2. PERFORMANCE REPORTING ANALYSIS › 2.2 Real Storage Management Analysis › 2.2.4 MVS Concepts › 2.2.4.2 MVS Virtual Storage Overview
2.2.4.2 MVS Virtual Storage Overview
This section describes the individual parts of the MVS
virtual memory, identifying those parts of the virtual memory
that are permanently page-fixed (that is, cannot be paged
out). The size of these areas affects your overall real
storage management strategy, because the total size of these
areas is subtracted from the page frames available to the
workload. It also affects the size of the private area (the
amount of virtual memory available to programs).
MVS/370 provided 16 MB of virtual storage to each address
space. This storage is divided into the three areas: the
common area, the private area, and the nucleus area.
In MVS/XA and later systems, 2 GB of virtual storage are
available to each address space. This storage is divided into
four areas: the common area below the 16 MB line, the
private area below the 16 MB line, the extended common area
above the 16 MB line, and the extended private area above the
16 MB line. In current systems the nucleus, which spans the
16 MB line, is conceptually part of the common/extended
common area.
In considering the virtual address space, there are both real
storage and virtual storage issues to be considered. This
section defines each of the parts of the virtual address
space, and details the real and virtual storage issues
pertinent to each part.
Real storage issues arise because portions of the virtual
address space remain fixed in real storage at all times. A
page fixed in real storage is one that remains in real
storage and will not be paged out. Using Figure 2-27 as a
reference, the fixed portions of real storage in MVS/370 are:
o The entire nucleus area.
o The system queue area (SQA) and parts of the common
storage area (CSA) from the common area.
o The local system queue area (LSQA) from the private
areas of resident address spaces.
Using Figure 2-28 as a reference, the fixed portions of real
storage are:
o The nucleus and extended nucleus area.
o The LSQA and extended LSQA from the private areas of
resident address spaces.
o The fixed link pack area (FLPA) and extended FLPA (in
MVS/370 the FLPA was an extension of the nucleus).
o The SQA and extended SQA.
Note that in both MVS/370 and current systems, LSQA is
swappable. Thus, the real storage occupied by LSQA and the
private area fixed pages are also determined by the number of
non-swappable tasks and the number of active tasks (the
system MPL).
The size of the nucleus and common areas must all be
considered, because the total virtual space is limited.
Starting with the Systems Product releases of MVS, the size
of the available private area is the remainder after the
nucleus and the common area are subtracted.
In earlier systems, the size of the common area was fixed at
8 MB. This could be changed by modifying a constant in one of
the IPL modules. In the larger MVS/370 systems today, private
areas are as small as 5 MB. In these systems, reduction of
the virtual sizes is an important issue.
MVS itself requires parts of both real and virtual storage.
In determining how much storage is actually available to
users, the basic requirements of MVS must be subtracted from
the total configuration. The real storage manager (RSM)
controls how real storage is allocated to system and user
address spaces that are swapped in for execution.
The primary MVS requirements for real storage are:
o Fixed storage for resident supervisor code and basic
system control blocks. This includes, for example, the
code for the System Resource Manager (SRM); the real,
virtual, and auxiliary storage management routines (RSM,
VSM, ASM); the dispatcher; basic systems services; and
control blocks for defining the real/virtual storage
mappings and for address space control.
o Fixed storage requirements to support user address spaces.
This includes the page and segment tables for the address
spaces, address space control blocks, and address
space-related SRM/RSM/VSM/ASM control blocks.
Figures 2-27 and 2-28 show the virtual storage layout for all
MVS-based systems. The System Storage Usage Report described
in Section 2.2.3.2 provides the capability to track most of
the different parts of virtual memory. In storage-
constrained systems, a great deal of real storage can be made
available by tailoring the system's use of real memory.
Section 2.2.4.2.1 describes each of the components of the MVS
virtual memory. As shown in Figures 2-27 and 2-28, there is
a private area (and an extended private area in later
systems) for each address space, that is, one for the Master
Scheduler, JES2/3, VTAM or TCAM, any started system task,
each batch job, and each TSO user. Each address space in
memory has fixed storage requirements: its LSQA and address
space related control blocks in SQA. When planning for the
amount of pageable storage, the fixed storage required to
support swapped-in address spaces must be considered.
Part of real storage is partitioned into a system preferred
area to support storage reconfiguration for multiprocessors.
Allocations for SQA, LSQA, and fixed page assignments
(including frames for V=R jobs) are made from the system
preferred area if possible, in order to facilitate storage
reconfiguration.
+--- ==============
| | SQA |
| |------------|
| | PLPA |
| |------------|
Common | | MLPA |
Area < |------------|
| | BLDL |
| |------------|
| | SYSGEN PSA |
| |------------|
| | CSA |
+--- |============|============|============|
| | LSQA | LSQA | LSQA |
| |------------|------------|------------|
| | SWA | SWA | SWA |
| |------------|------------|------------|
| | 229/230 | 229/230 | 229/230 |
| |============|============|============|
Private | | | | |
Area < | MASTER | address | address |
| | SCHEDULER | space | space | ...
| | PRIVATE | 1 | 2 |
| | AREA | user region| user region|
| | USER REGION| | |
| | |------------|------------|
| | | Sys. region| Sys. region|
+--- |============|============|============|
| | RMS |
| |------------|
| | ASM tables |
| |------------|
| | fixed BLDL |
| |------------|
Nucleus < | fixed LPA |
Area | |------------|
| | |
| | NUCLEUS |
| | |
| | |
+--- |============|
Figure 2-27. MVS/370 Virtual Storage Layout
2Gb +--- |================|============|============|
| | EXTENDED LSQA | EXT LSQA | EXT LSQA |
| |----------------|------------|------------|
| | EXTENDED CSA | EXT CSA | EXT CSA |
| |----------------|------------|------------|
Extended | |EXTENDED 229/230|EXT 229/230 |EXT 229/230 |
Private | |================|============|============|
Area < | MASTER |addr. space |addr. space |
| | SCHEDULER | 1 | 2 |
| | PRIVATE AREA | extended | extended |
| | EXTENDED | user region| user region|
| | USER REGION | | |
| | | | |
+--- |================|============|============|
| | EXTENDED CSA |
Extended| |----------------|
Common | | EXTENDED PLPA/ |
Area < | FLPA/MLPA |
| |----------------|
| | EXTENDED SQA |
| |----------------|
+--- |EXTENDED NUCLEUS|
16 MB +--- ==================
| | NUCLEUS |
| |----------------|
Common | | SQA |
Area < |----------------|
| |PLPA/FLPA/MLPA |
| |----------------|
| | CSA |
+--- |================|============|============|
| | LSQA | LSQA | LSQA |
| |----------------|------------|------------|
| | SWA | SWA | SWA |
| |----------------|------------|------------|
| | 229/230 | 229/230 | 229/230 |
Private | |================|============|============|
Area < | MASTER | address | address |
| | SCHEDULER | space | space |
| | PRIVATE | 1 | 2 |
| | AREA | user region| user region|
20K | | USER REGION |------------|------------|
| | | Sys. region| Sys. region|
4K +--- |================|============|============|
Common < | PSA |
0K +--- |================|
Figure 2-28. Current systems Virtual Storage Layout
The following sections explain the portions of the virtual
storage defined in Figures 2-27 and 2-28, describe the MVS
real storage algorithms, and describe a method of determining
the working set of a program:
1 - MVS Virtual Storage Layout
2 - MVS Working Set Determination
Copyright © 2014 CA.
All rights reserved.
|
|