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: .. I. The virtual=real option resoves 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 reduces the real supervisor state time used by V!/370. See PlanDiDg for a list of the processors on which it is available.
3. V!/370 Extended Control-Program Support further reduces the real
supervisor state time used by VM/370. See for a list of the processors on which it is
available.
Assignment and use of these options is discussed in "Preferred virtual !achines." Paging Considerations When virtual machines refer to virtual storage addresses that are not
currently in real storage, they cause a paging exception and the
associated CP paging activity. The addressing characteristics of programs executing in virtual storage have a significant effect on the number of page exceptions
experienced by that virtual machine. Routines that have widely
scattered storage tend to increase the paging load of a
particular virtual machine. When possible, 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 reenter able
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. Virtual machines that reduce !heir paging activity by controlling
their use of addressable space 1mprove resource management for that
virtual machine, the V!/370 system, and all other virtual .achines. The
total paging load that must be handled by CP is reduced, and more tiae
is available for productive virtual machine use.
Additional dynaaic paging storage may be gained by controlling free
storage allocation. The amount of free storage allocated at V!/370 initialization time can be controlled by the installation. When the Systea 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 IPt tiae, the amount of storage that these pages represent is
greater than 25 percent of the V!/370 storage size (not including the V=R area, if any), a default nuaber 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).
CP Introduction 1-27

The SYSCOR .acro definition can be found in !AL310 Planning and CP provides three performance options, locked pages, reserved page
fraaes, and a virtual=real area, to reduce the paging requirements of
virtual machines. Generally, these facilities require soae dedication
of real storage to the chosen virtual machine and, therefore, improve its performance at the expense of other virtual .achines. LOCKED PAGES OPTION The LOCK co •• and, which is available to the systea operator (with
privilege class A), can be used to permanently fix or lock specific user
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 page fraaes) that are available to support other virtual aachines, only
frequently used pages should be locked into real storage. Since page
zero (the first 4096 bytes) of a virtual .achine storage is referred to
and changed frequently (for exaaple, whenever a virtual aachine
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 aight also be considered
good candidates for locking. Other pages to pe locked depend upon the work being done by the
particular virtual aachine and its usage of virtual storage.
The normal CP paging aechanism selects unreferenced page fraaes in
real storage for replacement by active pages. Page fraaes belonging to
inactive virtual aachines will all eventually be selected and paged out
if the real storage fraaes are needed to support active virtual aachine
pages. When virtual aachine activity is initiated on an infrequent or
irregular basis, such as froa a remote terminal in a teleprocessing
inquiry system, soae or all of its virtual storage aay have been paged
out before the time the virtual machine must begin processing. Soae pages will then have to be paged in so that the virtual aachine can
respond to the teleprocessing request compared with running the saae teleprocessing prograa on a real machine. This paging activity aay cause
an increase in the tiae required to respond to the request coapared with
running the teleprocessing program on a real aachine. Further response tiae is variable, depending upon the number of paging operations that aust occur.
Locking specific pages of the virtual aachine's prograa into real
storage aay 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 reaains locked until either the user logs
off or the system operator (privilege class A) issues the UNLOCK comaand for that page. If the "locked pages" option is in effect and the user
loads his system again (via IPL) or loads another systea, 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
aachine storage, including the user's locked pages.
1-28 IBM VM/310 System Logic and Problem Determination--Voluae 1