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

April 1, 1981
running the teleprocessing program on
time is variable, depending upon the
must occur.
a real mac hine.
number of paging
Further response
operations that
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 1s 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 attached processor mode, 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 will receive the message DMKCPV165I PAGE (hexloc) NOT LOCKED, SHARED PAGE for each of the shared pages within the range. RESERVED PAGE FRAMES OPTION A more flexible approach than locked pages is the reserved 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 line. Pages will not be locked into these frames. They can be
paged out, but only for other active pages of the same virtual machine.
when a temporarilv inactive virtual machine having this option is
reactivated, 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. Althouqh 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 "VK/370 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 teleprocessing applications
where response times must be kept as short as possible. VIRTUAL=REAL OPTION The VM/370 virtual=real option CP paging for the selected
virtual machine. All pages of virtual machine storage, except page
zero, are locked in the real storaqe locations they would use on a real
computer. CP controls real page zero, but the remainder of the CP 92 IBM VM/370 system Programmer's Guide