The virtual=real option may be desirable or mandatory in certain
situations. The virtual=real option is desirable when running a virtual machine operating systea (like DOS/VS or OS/VS) that perforas 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/310. Preferred Virtual Machine Options Vft/310 provides seven functions that create a special virtual machine environment:
1. Favored execution
2. Priority
3. Reserved page frames 4. Virtual=real option !ffi!!it! 6. Virtual machine assist
1. Extended Control-Program Support
The first five functions are designed to improve the performance of a
selected virtual aachine; the last two functions improve the performance of VM/310. Although each of the first five functions could be applied
to a different virtual machine, usually they are applied to only one if optimum performance is required for that one specific virtual machine. The sixth and seventh functions can be applied to as many virtual machines as desired. 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 aachine than would ordinarily be
the case. The options provided are:
1. The basic favored execution option.
2. The favored execution percentage option.
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 iaplicit coamitment of main storage. In explicit
storage com.itment can be specified by either the virtual=real option or
the reserved page frames option. In implicit commitaent exists if
neither of these options is specified, and the scheduler recoaputes the
virtual machine's prOjected work-set at what it vould noraally 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 coapete for the
processor with aany other tasks of the same type, an installation can
define the processor allocation to be aade. In this case, the favored
execution percentage option can be selected for one virtual aachine. 1-30 IBft VM/310 System Logic and Problem Determination--Volume 1

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. The favored execution option can
only be invoked by a system operator with command privilege class A. The format of the command is as follows: r , SET FAVORED userid Inn I L userid identifies the virtual machine to receive favored execution
status.
nn is any value from 1 through 99 and specifies the percentage
of the in-aueue time slice that is guaranteed to this virtual machine. OFF specifies that the virtual machine is to be removed from favored execution status.
The percentage option of the SET FAVORED command is administered as
follows:
1. The in-queue time slice is multiplied by the specified percentage
to arrive at the virtual machine's guaranteed processor time. 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. Whether or not a percentage is specified, a virtual aachine 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. CP Introduction 1-31