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 1 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 aore 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 pages. Pages for
users are assigned from the available list, which is replenished from 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 swapped, 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 co.mand 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. !Qte: 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 '8/370 nucleus .ust 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 translation mode, but
1-32 IBM VM/370 System Logic and Problem Determination--Volume 1

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 virtua1=rea1 machine's storage. Systems that are generated system loader (DMKLDOOE). virtua1=rea1 system, see the with the virtua1=rea1 option use the
For information about generating a 2yste! Figure 12 is an
virtua1=rea1 option. example of a real storage layout with
The V=R area is 128K and real storage is 512K. Virtual Storage Addresses I I CP PAGE 0 (MODULE DMKPSA) 4K I I Virtual Page 1 I I VIRTUAL=REAL AREA
/
/ SIZE = 128K BYTES I (!inimu. size is 32K bytes.) 128K, OKt Virtual Page
4K I 132KI / REMAINDER OF CP BUCLEUS / I I I / DYNAMIC PAGING AREA
/ and I FREE STORAGE i I I I I I /
/ I I 0 I I I /
/ I I I /
/ I Real Storage Addresses OK 4K 128K
132K (DMKSLC) End of CP Nucleus (DMKCPE) 512K (End of real
storage)
Figure 12. Storage Layout in a Virtual=Real Machine the
There are several considerations for the virtua1=rea1 option that
affect overall systea operation:
1. The area of contiguous storage built for the virtua1=rea1 aachine
aust be large enough to contain the entire addressing space of the
largest virtua1=rea1 machine. The virtua1=rea1 storage size that a VM/310 systea allows is defined during system generation when the
option is selected.
2. The storage reserved for the virtua1=rea1 aachine can only be used by a virtual aachine with that option specified in the VM/310 directory. It is not available to other users for paging space, nor
for VM/310 usage until released froa virtua1=rea1 status by a
systea operator via the CP UlLOCK co.aand. Once released, VM/310 aust be loaded again before the virtua1=rea1 option can becoae
active again.
CP Introduction 1-33