Locked Pages Option
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.
Virtual machines that reduce their paging activity by controlling their use of
addressable space improve resource management for that virtual machine, the
VM/SP system, and all other virtual machines. The total paging load that must be
handled by CP is reduced, and more time is available for productive virtual
machine use.
Additional dynamic paging storage may be gained by controlling free storage allo­
cation. The amount of free storage allocated at VM/SP 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 IPL time, the amount of storage that these pages represent is greater than 25
percent of the VM/SP 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 1 page for each additional 64K bytes (not including the V =R size, if
any).
The SYSCOR macro definition can be found in VM / SP Planning Guide and Refer­
ence.
CP provides 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 storage to the chosen vir­
tual machine and, therefore, improve its performance at the expense of other virtu­
al machines.
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 I/O 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 pages might also be considered good candidates for lock­
ing. Other pages to be locked depend upon the work being done by the particular virtu­
al machine and its usage of virtual storage.
The normal CP paging mechanism selects unreferenced page frames in real storage
for replacement by active pages. Page frames belonging to inactive virtual
machines are all selected eventually 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 system, some or all of its vir-
Performance Guidelines 27

Reserved Page Frames Option
Virlual=RealOption
tual storage may have been paged out before the time the virtual machine begins
processing. Some pages then have to be paged in so that the virtual machine can
respond to the teleprocessing request compared with running the same teleprocess­
ing program on a real machine. This paging activity may cause an increase in the
time required to respond to the request compared with running the teleprocessing
program on a real machine. Further response time is variable, depending upon the
number of paging operations that must occur.
Locking specific pages of the virtual machine's program into real storage may ease
this problem, but it is not always easy nor possible to identify which specific pages
will always be required. Once a page is locked, it remains locked until either the user logs off or the system
operator (privilege class A) issues the UNLOCK command for that page. If the "locked pages" option is in effect and the user loads his system again (via IPL) or loads another system, the locked pages are refreshed and the virtual machine's
locked pages are unlocked by the system. The SYSTEM CLEAR command, when
invoked, clears virtual machine storage, including the user's locked pages.
Note: In a system generated for attached processor or multiprocessor operation, no
shared pages are locked. If the system operator attempts to lock a shared page or
an address range containing one or more shared pages, he receives the message
DMKCPV1651 PAGE (hexloc) NOT LOCKED, SHARED PAGE
for each of the shared pages within the range.
A more flexible approach than locked pages is the reserved page frames option.
This option provides a specified virtual machine with an essentially private set of
real page frames, the number of frames being designated by the system operator
when he issues the CP SET RESERVE command. Pages are not locked into these
frames. They can be paged out, but only for other active pages of the same virtual
machine. When a temporarily inactive virtual machine having this option is reacti­
vated, these page frames are immediately available. If the program code or data
required to satisfy the request was in real storage at the time the virtual machine
became inactive, no paging activity is required for the virtual machine to respond.
This option is usually more efficient than locked pages in that the pages that remain
in real storage are those pages with the greatest amount of activity at that moment,
as determined automatically by the system. Although multiple virtual machines
may use the LOCK option, only one virtual machine at a time may have the
reserved page frames option active. Assignment of this option is discussed further
in "VM/SP Performance Options". The reserved page frames option provides performance that is generally consistent
from run to run with regard to paging activity. This can be especially valuable for
production-oriented virtual machines with critical schedules, or those running tele­
processing applications where response times must be kept as short as possible.
The VM/SP virtual=real option eliminates CP paging for the selected virtual
machine. All pages of virtual machine storage, except page zero, are locked in the
real storage locations they would use on a real computer. CP controls real page
28 VM/SP System Programmer's Guide