Performance Options VM/370 offers the following performance options to enhance the operating
efficiency of one or aore virtual machines. This enhancement is
nor. ally at the expense of the performance of other virtual machines. In certain instances, enhancements to a specific virtual machine (or
aultiple virtual machines) are detrimental to the operating efficiency
of other virtual The performance options are: • Virtual machine assist • VM/370 Extended Control-Program Support • Reserved page frames • Favored execution • Virtual=real • Locked pages • Priority • Affinity in an attached processor environment You can dynaMically assign or withdraw any of these options except
for the virtual=real option (it can only be dynamically withdrawn) fro.
virtual machine users during system operations. You can assign to one
virtual machine a "mix" of performance options, or the options may be
assigned to separate virtual machines. Efficient use of these options
depends upon the characteristics of the program applications; the nUMber, size, and complexity of the virtual machines being run; and the
size, I/O configuration, and CPU model of the real computer being used.
In addition to using these performance options, you can also use the SET PRIORITY command to change a virtual machine's execution priority
and, thereby, change its performance. For example, you could assign the following mix of One virtual machine: Reserved Pages=10 Favored Execution=80% Virtual Machine Assist
Another virtual machine: Virtual=Real priority=1
A third virtual machine:
Locked Pages=2 The favored execution (with percentage value specified), VIRT=REAL, and the reserved page option are unique in that each option cannot be
applied to multiple virtual machines at the same time. However, no.such
restrictions apply to the locked pages, virtual machine assist, VM/370 Extended Control-Program Support, priority, or favored execution
(without the percentage value) options; they can be applied' to multiple virtual machines. RESERVED PAGE FRAMES OPTION The CP paging routine uses chained lists of available and pageable pages
to control real storage use. CP assigns pages for users from the
available list, which it replenishes from the pageable list.
Section 2. Initiating VM/370 Operation 27

Pages that are temporarily locked in real storage are not available
or pageable. Paging proceeds using demand paging to select the best
page for swapping. The reserved page option gives a particular virtual machine an essentially private set of pages. The pages are not locked;
they can be swapped, but normally only for the virtual machine for which
they are reserved. You specify the number of reserved pages for the virtual machine as a maxi.a._ ehen the page selection routine selects an available page for a reserved user, it marks that page reserved if
the maximum specified for the user has not previously been reached. If CP encounters an available (idle) reserved page for the reserved user
during page selection, it is used whether or not the maximum bas been
reached. Only an operator with privilege
reserved pages. Issue the CP SET format: SET RESERVE userid xx class A can specify the RESERVE co •• and in the number of
following
userid is the name of the V"/310 user to whose virtual machine you
are to assign the reserved page option. xx is the maximum number of pages you assign.
If the CP page selection routine cannot locate an available page for
other users because they are all reserved, the routine uses one of the
reserved pages as a last resort. FAYORED EXECUTION OPTION The favored execution option guarantees that a particular virtual machine gets up to a fixed percentage of processor time. Favored in
this sense means that CP provides execution time up to the specified
percentage to that virtual machine, that virtual machine can
use that much processor time. At regular time intervals, the CP dispatcher checks the processor time used by the favored virtual machine. If the guaranteed percentage is exceeded, the machine gets its normal priority for the remainder of the time interval. If the
percentage used is less than the guaranteed value, the favored virtual machine has the highest execution priority until it reaches that
threshold. You, as the system oFerator with privilege class A, specify
the percentage of processor time assured by the favored option in the CP SET command. The format of the command is: r , SET FAVORED userid 'xx I 10FFI L userid is the name of the V"/310 user whose virtual machine is to
be assigned the favored option xx is any percentage value from 1 to 99 OFF operand stops favored operation.
28 VH/370 Operator's Guide