zero, but the remainder of the CP nucleus is relocated and placed beyond the virtu­
al=real machine in real storage. This option is discussed in more detail in '··'VM/SP Performance Options". 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 logged on. This option often increases the paging activ­
ity for other virtual machine users, and in some cases for VM/SP. (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 tim­
ing dependencies to run under VM/SP. VM/SP Performance Options VM/SP provides a number of options an installation may use to improve the per­
formance of virtual machines and VM/SP. Several options improve the perform­
ance of installation specified virtual machines; other options improve the
performance of all virtual machines and VM/SP. The options, described in the fol­
lowing discussion are:
Favored execution
User priority • Reserved page frames • Virtual=real
Affinity
Multiple shadow table support
Shadow table bypass
Single processor mode
Dynamic SCP transition to or from native mode
Queue drop elimination • Virtual machine assist • 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/SP 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.
The favored execution option and user priority option both alter the normal sched­
uler algorithm. The user priority option tends to take precedence over the favored
execution option even when you specify a percentage. For example, suppose virtu­
al machine A has favored execution nn % specified and has been given a low priori­ while virtual machine B has been given a higher priority. Virtual machine A
may not get the actual percentage of the CPU that was specified with the favored
option.
Performance Guidelines 29

Favored Execution
The favored execution options allow an installation to modify the normal CP dead­
line priority calculations in the fair share scheduler to 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
The basic favored execution option means that the virtual machine so designated is
to remain in the dispatch list at all times, 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; the system can do little useful work because of excessive paging.
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. This option specifies that the selected virtual machine, in addition to
remaining in queue, is requesting a specified minimum percentage (from 1 to 100 percent) of the total processor time, if it can use it. If a virtual machine requests 100 percent of the processor time, CP keeps that virtual machine at the top of the
dispatch list. This ensures that the virtual machine always has first priority when
CP dispatches a virtual machine to the processor. 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/SP 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 requested processor time.
2. The scheduler attempts to place the virtual machine, when it is executable, at
the top of the dispatchable list until it has obtained its requested processor
time.
3. If the virtual machine obtains its requested processor time before the end of its
in-queue time slice, it is placed in the dispatchable list according to its calcu­
lated dispatching priority.
4. In either case (2 or 3), at the end of the in-queue time slice the requested per­
centage is recomputed as in step 1 and the process is repeated.
For a description of the SET command, see the VM / SP Operator's Guide.
If a percentage is not specified, a virtual machine with the favored execution option
active is kept in the dispatch list except under the following conditions: • Entering CP console function mode
30 VM/SP System Programmer's Guide