• Read the VM/370 system symbol 38 • Dynamically update the VM/370 directory. 3C • Generate accounting cards for the user. 4C • Save the 3704/3705 Communications controller program in 50 page image format. • Specify that the 3270's PA2 key is to be used for either: 54
- Simulating an external interruption to a virtual machine,
such as VS lPL. - Clearing the output display of a 3270 screen. • Display the number of characters, up to the size of the 58
output area, on a display screen in one write operation. • Edit error messages. 5C • Determine the virtual machine storage size. 60 • Find, load, or purge a named segment. 64
--or--
Release discontiguous storage. • Invoke the virtual machine communication facility. 68 • Load a specific 3800 named system into a virtual 74
machine.
--or-- Save a storage image in a specific 3800 named
system. • Communicate between a virtual machine and VM/370 for MSS support. • Update fields in an online directory entry without
recompiling the directory. PAGE WAITS 78
84
If, during its execution, an OS- or DOS-created task or program must
wait for a VM/370 service such as a virtual storage page, VM/370 marks
the virtual machine nondispatchable even though other partitions or
tasks in that virtual machine may be ready and available for processing.
Those other tasks in the virtual machine cannot be dispatched by the
operating system until the VM/370 page wait is satisfied. Thus, the
highest priority program of the virtual machine gets almost all of the
Frocessor time allocated to that virtual machine, if it can use the time. Therefore, programs running in the other partitions experience
significant degradation. When multiFrogramming systems must be executed in a virtual machine,
.ake the partitions or regions as large as practical and execute all
jobs V=R. Also, consider using the VM/370 virtual =real option, reserved
page frames, or locked pages. When using the VM/370 virtual=real
oFticn, it eliminates paging for one virtual machine, but this may adversely affect the paging performance of other virtual machines. The
reserved Fage frames option tends to keep the most active pages in
storage, and the locked pages option locks the specified pages in
storage. Section 1. General Considerations 11

If the region size is made too large, certain programs, such as
the as sort/merge program, do not run efficiently. I/O WAITS On a real machine, when a task is for an I/O operation to ccmFlete, the lower priority tasks are g1ven use of the processor. Under VM/370, the I/O operations of a particular virtual machine are
overlapped with the processor execution of that and other virtual
machines. Consequently, lower priority tasks created by OS and DOS are
given the processor resource less frequently when executing in a virtual
machine than when executing in a real machine.
To set the priority of a virtual machine, the VM/370 system operator
can issue the CP command SET PRIORITY. A low priority value gives the
virtual machine a higher priority, and this priority ensures that VM/370 dispatches the virtual machine for execution more frequently than other
virtual machines.
To ensure that the lower priority DOS or OS tasks have a chance to
execute, installations can use the favored execution option. This
option reduces the effect of a variable system load on the favored
virtual machine. It allows an installation to modify the normal VM/370 scheduling algorithms and forces VM/370 to devote more of its processor
time to a given virtual machine. The option causes VM/370 to keep the sFecified virtual machine in the active queue, unless it becomes
nonexecutable. To obtain this option for a specific virtual machine,
the system operator must issue the CP command SET FAVORED. To guarantee that a certain amount of processor time is made
available to a virtual machine, installations can use the favored
execution oFtion with the percentage value specified in the SET FAVORED co •• and.
For more details about these performance options, refer to the SPOOLING Most multiprogramming operating systems, such as DOS/VS and OS/VS, have
their own sFooling subsystems (such as POWER, PCWER/VS, HASP, or JES). Because VM/370 also provides its own spooling, double spooling can
occur. Thus, should an installation: • Use only the operating system's spooling subsystem? • Use only VM/370's spooling? • Use double spooling?
If an installation has a significant amount of printing or punching
to do, it may appear that one of the spooling subsystems should be
eliminated. This 1S not necessarily true. In fact, if the multiFrogramming operating system's spooling subsystem blocks its output
(as does VS1), the most efficient spooling arrangement is usually to let
both VM/370 and VS1 spool.
12 IBM VMi370 Operating Systems in a Virtual Machine