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

nucleus is relocated and placed beyond the virtual=real machine in real
storage. This option is discussed in more detail in "VM/370 Performance nn";"ft"" " Since the entire address space required by the virtual machine is
locked, these page frames are not available for use by other virtual
machines except when the virtual=real machine is not on. This
option often increases the paging activity for other virtual machine
users, and in some cases for VM/370. (Paging activity on the system may
increase substantially, since all other virtual machine storage
requirements must be managed with fewer remaining real page frames.)
The virtual=real option may be desirable or mandatory in certain
situations. The virtual=real option is desirable when running a virtual
machine operating system (like DOS/VS or OS/VS) that performs paging of
its own because the possibility of double paging is eliminated. The
option must be used to allow programs that execute self-modifying
channel programs or have a certain degree of hardware timing
dependencies to run under VM/370. VM/370 Performance Options VM/370 provides a number of options an installation may use to improve
the performance of virtual machines and VM/370. Several options improve
the performance of installation specified virtual machines; other
options improve the performance of all virtual machines and VM/370. The
options, described in the following discussion are: I • Favored execution I • User priority I • Reserved page frames I • Virtual=real I to.! Affinity I • Virtual machine assist I • Extended Control-Program Support
Specifying a performance option may mean making a performance
trade-off; improving the performance of one virtual machine at the
expense of VM/370 and other virtual machines. For example, after an
operator specifies favored execution for a virtual machine, that virtual
machine receives more processor time than other virtual machines= Therefore, before specifying any performance option, identify the
option's performance trade-offs and assess their impact on system
performance. FAVORED EXECUTION The favored execution options allow an installation to modify the normal
scheduling algorithms and force the system to devote more of its
processor resources to a given virtual machine than would ordinarily be
the case. The options provided are: • The basic favored execution option • The favored execution percentage option
Part 2. Control Program (CP) 93