Substitution of a faster resource
operations, by building small temporary rather than using an I/O device.
(virtual storage) for I/O data sets in virtual storage
Frequently, there can be a performance gain when CP paging is
substituted for virtual machine I/O operations. The performance of an
operating system such as as can be improved by specifying as resident as
many frequently used as functions (transient sutroutines. ISlft and so forth) as are possible. In this way, paging I/O is substituted
for virtual machine-initiated I/O. In this case, the only work to be
done by CP is to place into real storage the page that contains the
desired routine or data.
Three CP performance options are available to reduce the CP overhead
associated with virtual machine I/O instructions or other privileged
instructions used by the virtual machine's I/O Supervisor:
1. The virtual=real option removes the need
reference translation and paging before
specific virtual machine.
for CP to perform storage
each I/O operation for a
2. The virtual machine assist feature reduces the real supervisor
state used by VM/370. For a detailed description of the
feature, see "Virtual Machine Assist Feature" later in this
section. For a list of processors on which the feature is
available, see the !11LJ1Q g.!anning gnd Guide,.
3. VM/370 Extended Control-Program Support (ECPS) further reduces the
real supervisor state tiae used by Vftj370. For a detailed
description of ECPS, see "Vft/370 Extended Control-Program support (ECPS)" later in this section. For a list of processors on which ECPS is available, see the VM/31Q g!!g Assignment and use of these
Performance Options." options is discussed in "Vft/370 Paging Considerations When virtual machines refer to virtual storage addresses that
currently in real storage, they cause a paging exception
associated CP paging activity.
are not
and the
The addressing characteristics of programs executing in virtual
storage have a significant effect on the number of page exceptions
experienced by that virtual aachine. Routines that have widely
scattered storage reference tend to increase the paging load of a
particular virtual machine. When possible, modules of code that are
dependent upon each other should be located in the same page. Reference
tables, constants, and literals should also be located near the routines
that use them. Exception or error routines that are infrequently used
should not be placed within main routines, but located elsewhere. When an available page of virtual storage contains only reenterable
code, paging activity can be reduced, since the page, although referred
to, is never changed, and thus does not cause a write operation to the
paging device. The first copy of that page is written on the paging
device when that frame is needed for some other more active page. Only inactive pages that have changed must be paged out. 90 IBM VM/310 System Programmer's Guide
page of GC20-1807-7 As Updated April 1, 1981 by TNL GN25-0829 Virtual machines that reduce their paging activity by controlling their of addressable space improye resource management for that
virtual machine, the VM/370 system, and all other virtual machines. The
total paqing load that must be handled by CP is reduced, and m{)re time
is available for productive virtual machine use.
Additional dynamic paging storage may be gained by controlling free
storage allocation. The amount of free storage allocated at V8/370 initialization time can be controlled by the installation. When the System is being generated, the FREE operand of the SYSCOR macro
statement may be used to specify the number of free storage pages to be
allocated at system load time.
If, at IFL time, the amount of storage that these pages represent is
greater than 25 percent of the VM/370 storage size (not including the V=R area, if any), a default number of pages is used. The default value
is 3 pages for the first 256K bytes of storage plus i page for each
additional 64K bytes (not including the V=R size, if any).
The SYSCOR macro definition can be found in VM/37Q-flanninq- Guigg. CP pcovides three performance options, locked pages, reserved page
frames, and a virtual=real area, to reduce the paging requirements of
virtual machines. Generally, these facilities require some dedication
of real storaqe to the chosen virtual machine and, therefore, improve
its performance at the expense of other virtual machines.
LOCKED PAGES OPTION The LOCK command, which is available to the system operator (with
privilege class A), can be used to permanently fix or lock specific
pages of virtual storage into real storage. In so doing, all paging IIO for these page frames is eliminated. Since this facility reduces total real storage resources (real page
frames) that are available to support other virtual machines, only
frequently used pages should be locked into real storage. Since page
zero (the first 4096 bytes) of a virtual machine storage is referred to
and changed frequently (for example, whenever a virtual machine
interrupt occurs or when a CSW is stored), it should be the first page
of a particular virtual machine that an installation considers locking.
The virtual machine interrupt handler paqes might also be considered
good candidates for locking. Other pages to be locked depend upon the work being done by the
particular virtual machine and its usage of virtual storage.
The normal CP paging mechanism selects unreferenced page frames in
real storaqe for replacement by active pages. Page frames belonging to
inactive virtual machines will all eventually be selected and paged out
if the real storage frames are needed to support active virtual machine
pages. When virtual machine activity is initiated on an infrequent or
irregular basis, such as from a remote terminal in a teleprocessing
inquiry s.ystem, some or all of its virtual storage may have been paged
out before the time the virtual machine must begin processing. Some pages will then have to be paged in so that the virtual machine can
respond to the teleprocessing request compared with running the same
teleprocessing program on a real machine. This paging activity may cause
an increase in the time required to respond to the request compared with Part 2. Control Program (CP) 91
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