Loading a disabled PSW Loading an enabled PSW with no active 110 in process
Logging on or off.
When the virtual machine becomes executable again, it is put back on the dispatch
list in Q 1. If dropped from Q 1, the virtual machine is placed directly in the Q2 dis­
patch list. If the percentage option of the SET FAVORED command is specified,
the deadline priority is calculated at queue drop time by:
current time-of-day + length of allowed processor in-queue time slice
favored percentage
User Priority
For example, if the processor in-queue time slice is 1 second, and the specified per­
centage is 10 percent, then the value added to the current time-of-day is 10 sec­
onds. The virtual machine should receive one processor time slice (1 second) once
every 10 seconds.
Note, however, that these options can impact the response times of other virtual
machines. To provide a virtual machine with both options, basic and percentage,
both forms of the command for that virtual machine must be issued. The percent­
age form of the SET FAVORED command can be used to specify any number of
logged-on virtual machines.
Although the SET FAVORED command prevents specifying more than 100
% for
a particular virtual machine, nothing is done to prevent allocating more than 1 00% to a number of virtual machines. Where more than 1 00% has been allocated, the
favored virtual machines compete for the available resources on a pro-rata basis.
An individual virtual machine's allocation is, roughly, proportional to the percent­
age allocated to it, divided by the total percentage allocated to all virtual machines.
The effect of allocating more than 1000/0 of the system on interactive (Ql)
responses is unpredictable.
Note: The percentage of the processor time actually received by the favored usc normally remains relatively close to the percentage specified in the command.
However, it is not an absolute value, and varies depending on the total load on the
system and the type of load on the system. If, for example, there are multiple vir­
tual machines on the runlist that are compute bound (That is, are not queue
dropped before the end of their in-queue time slice), then the favored user may not
receive its requested percentage of the total processor time.
The VM/SP operator can assign specific priority values to different virtual
machines. In so doing, the virtual machine with a higher priority is allocated a larg­
er share of the system resources 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 the calcu­
lation of the deadline priority. The deadline priority is the basis on which all virtual
machines in the system are ordered on both the eligible list and the dispatch list.
The deadline priority calculation is based on the assumption that the average or
normal (default) user priority is 64.
Performance Guidelines 31

Reserved Page Frames
Virtual=Real VM/SP uses chained lists of available and pageable pages. Pages for users are
assigned from the available list, which is replenished from the page able 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 swapped, but only for the spec­
ified 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 algo­
rithm 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 paRe 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.
For this option, the VM/SP 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 virtual machine is still run in dynamic address trans­
lation mode, but since the virtual page address is the same as the real page address,
no CCW translation is required. Since CCW translation is not performed, no
check is made to ensure that I/O data transfer does not occur into page zero or any
page beyond the end of the virtual=real machine's storage.
For information about generating a virtual=real system, see the VM / SP Installa­
tion Guide.
Figure 3 on page 33 is an example of a real storage layout with the virtual=real
option. The V =R area is 128K and real storage is 512K.
32 VM/SP System Programmer's Guide