The basic favored execution option means that the virtual machine so
designated is not to be dropped from the active (in queue) subset by the
scheduler, unless it becomes nonexecutable. When the virtual machine is
executable, it is to be placed in the dispatchable list at its normal
priority position. However, any active virtual machine represents
either an explicit or implicit commitment of main storage. An explicit
storage commitment can be specified by either the virtual=real option or
the reserved page frames option. An implicit commitment exists if
neither of these options is specified, and the scheduler recomputes the
virtual machine's projected work-set at what it would normally have been
at queue-drop time. Multiple virtual machines can have the basic
favored execution option set. However, if their combined main storage
requirements exceed; the system's capacity, performance can suffer
because of thrashing.
If the favored task is highly compute bound and must compete for the
processor with many other tasks of the same type, an installation should
define the processor allocation to be made. In this case, the favored
execution percentage option can be selected for one virtual machine.
This option specifies that the selected virtual machine, in addition to
remaining in queue, is guaranteed a specified minimum percentage of the
total processor time if it can use it. To select the favored execution
option, specify the FAVORED operand on the class A, B, or F SET command.
After the option is invoked, VM/310 provides processor time for the
selected virtual machine as follows:
1. The in-queue time slice is multiplied by the specified percentage
to arrive at the virtual machine's guaranteed processor tiae. 2. The favored virtual machine, when it is executable, is always
placed at the top of the dispatchable list until it has obtained
its guaranteed processor time.
3. If the virtual machine obtains its guaranteed processor time before
the end of its in-queue time slice, it is placed in the
dispatchable list according to its calculated dispatching priority.
4. In either case (2 or 3), at the end of the in-queue time slice the
guarantee is recomputed as in step 1 and the process is repeated.
For a description of the SET command, see the Whether or not a percentage is specified, a virtual machine with the
favored execution option active is kept in the dispatching queues except
under the following conditions: • Entering CP console function mode • Loading a disabled PSW • Loading an enabled PSW with no active I/O in process • Logging on or off When the virtual machine becomes executable again, it is put back on the
executable list in Q1. If dropped from Q1, the virtual machine is
placed directly in Q2 and remains there even though it may exhaust its
allotted amount of processor usage. Virtual machines with this option
are thus considered for dispatching more frequently than other virtual
machines.
Note, however, that these options can impact the response time of
interactive users and that only one favored percentage user is allowed
at any given time.
94 IBM VM/370 System Programmer's Guide

PRIORITY The VM/370 operator can assign specific priority values to different
virtual machines. In so dOing, the virtual machine with a higher priority is considered for dispatching before a virtual machine with a
lower priority. User priorities are set by the following class A
command:
SET PRIORITY userid nn
where userid is the user's identification and nn is an integer value
from 1 to 99. The value of nn affects the user's dispatching priority
in relation to other users in the system. The priority value (nn) is
one of the factors considered in VM/370's dispatching algorithm.
Generally, the lower the value of nn, the more favorable the user's
position in relation to other users in VM/370's dispatch queues. RESERVED PAGE FRAMES VM/370 uses chained lists of available and pageable Fages. Pages for
users are assigned from the available list, which is replenished frem
the pageable list. Pages that are temporarily locked in real storage are not available
or pageable. The reserved page function gives a particular virtual
machine an essentially "private" set of pages. The pages are not
locked; they can be swaFped, but only for the specified virtual machine. Paging proceeds using demand paging with a "reference bit" algorithm to
select the best page for swapping. The number of reserved page frames
for the virtual machine is specified as a maximum. The page selection
algorithm selects an available page frame for a reserved user and marks
that page frame "reserved" if the maximum specified for the user has not
been reached. If an available reserved page frame is encountered for the
reserved user selection, it is used whether or not the maximum has been
reached.
The maximum number of reserved page frames is specified by a class A
command of the following format:
SET RESERVE userid xxx
where xxx is the maximum number required. If the page selection
algorithm cannot locate an available page for other users because they
are all reserved, the algorithm forces the use of reserved pages. This
function can be specified in only one virtual machine at anyone time. Note: xxx should never approach the total available pages, since CP
overhead is substantially increased in this situation, and excessive
paging activity is likely to occur in other virtual machines. VIRTUAL=REAL For this option, the VM/370 nucleus must be reorganized to provide an
area in real storage large enough to contain the entire virtual=real machine. In the virtual machine, each page from page 1 to the end is in
its true real storage location; only its page zero is relocated. The Part 2. Control Program (CP) 95