Programming Considerations New applicaticn programs should be designed to operate efficiently in a
paging environment. Whenever possible, use VM/370 paging instead of DOS/VS or OS/VS paging. That is, make the DOS/VS partitions and OS/VS regions virtual=real (V=R) and large enough to contain the largest jobs.
Eliminate all overlays and, if possible, combine into one larger job any
aultistep jobs that use temporary DASD storage. PAGING FACTORS Installations should be aware that the following factors affect the
performance of a virtual machine: • The frequency of real interruptions that occur • The frequency and type of privileged instructions executed • Whether the virtual machine assist or VM/370 extended control-program
support hardware is on the machine and enabled by both the system
operator and by the user • The frequency of START I/O (SIO) instructions • Locality of reference for paging activity within virtual storage
These factors are in addition to those described under the topic "Performance Guidelines" in this section. REDUCING PAGING ACTIVITY When a virtual machine refers to virtual storage addresses that are not
in real storage, a page fault (and paging activity) occurs. Routines
that have widely scattered storage references tend to increase the
paging load caused by this virtual machine. When possible, modules dependent upon each other, as well as the
related reference tables, constants, and literals, should be located in
the same 4K page. Infrequently used routines, such as those that handle
unusual error conditions, should not be placed near main routines. To
minimize paging, reentrant coding techniques should be used whenever
possible. ABNORMAL TERMINATIONS IN A VIRTUAL MACHINE Whenever pcssible with a virtual storage operating system, use its
dumping procedure instead of VM/370's. The CP dump program does not
print out second level storage pages (that is, V=V regions or partitions
of OS/iS and DOS/VS machines) in the correct sequence. Pages that
happen to be stored on the OS/VS or DOS/VS paging disk are not printed
at all. Also, several special formatting dump programs are available to
help a user trace through DOS/VS and OS/VS control blocks. For more
debugging infcrmation, refer to the Section 1. General Considerations 5

REDUCING A VIRTUAL MACHINE'S I/O OPERATIONS The number of SIO instructions executed by a virtual machine may be
substantially reduced by: • Using 4K byte blocking factors for I/O areas • Preallocating the DASD space for as or OS/VS work data sets • Using virtual storage instead of DASD work files for smaller temporary files • Building temporary files in virtual storage and letting page
out the data (if needed) • Omitting virtual printers, punches, and readers from each partition
or region in a virtual machine because records for these devices are
unblocked • Using the virtual operating system's spooling subystem (such as POWER/VS or JES) because these spooling subsystems use large I/O areas and long chains of CCWs VIRTUAL MACHINE OPTIONS VM/370 provides several optional services to virtual machines.
these options either in the OPTION control statement of the
directory program or, for many options, in the CP SET command. Specify The BMX (virtual block multiplexer) option allows an operating system running in a virtual machine to overlap multiple SIO requests on a specified channel path. The selector channel mode is the normal (and
default) channel mode for virtual machines. When the option is
given control, it applies to all channels in the virtual machine, except
to channel 0 and channels that have a channel-to-channel adapter (CTCA).
This option can be specified regardless of whether block multipleXer channels are attached to the computer. The CP DEFINE command can
redefine the channel mode for a virtual machine.
The ECMODE option allows the virtual machine to use the complete set of
virtual System/310 control registers and the dynamic address translation
feature of the System/310. Programming simulation and hardware features
are combined to allow use of all the available features in the hardware.
This option is required both for DOS/VS, OS/VS1, and OS/VS2 virtual machines and when executing VM/310 under It is also required
when executing the generalized trace facility (G7F) under as/MVT. 6 IBM VM/370 Operating Systems in a Virtual Machine