ERROR RECORDING AND ANALYSIS The operating systems commonly run
write error records to the error
virtual machine VK/370 intercepts
own error recording cylinders.
operating systems reside in this
To access the recorded data, use
information about error recording UNSUPPORTED DEVICES in virtual machines all use SVC 76 to
recording data sets. However, in a SVC 76 and records the error on its
Therefore, error records from all
one centralized error recording area.
the CKS CPEREP command. For further
and CPEREP, refer to OL!SEg Virtual machine users may be able to use I/O devices that Ve/370 does
not support. An unsupported device is a device type that is not listed
in the DEVTYPE operand of the RDEVICE macro instruction. To use an
unsupported device, a user must attach or dedicate the device to a
virtual machine. A dedicated device is one that is not shared among
users, but is used exclusively by one user. However, VM/370 supports
these dedicated devices only under these conditions: • No timing dependencies exist in the device or the program. • No dynamically modified channel programs exist in the access method,
except when OS ISAK or OS/VS TCAK Level 5 are used. • No special functions need to be provided by • None of the other CP restrictions are violated. (Refer to the VM/370 restrictions list in the • The device is generated into the VK/370 nucleus (by using the RDEVICE macro instruction with the appropriate CLASS operand). I/O devices that are part
require real device equivalents.
of a virtual machine's configuration
However, exceptions to this rule are: • Unit record devices, which VK/370 can simulate by using spooling
techniques. • Virtual 2311 disks which VK/370 can map onto 2314 or 2319 disks. One to two full 2311 units can be mapped onto a 2314 or 2319 disk in this
manner. • When 3350 disks are used in 3330-1 or 3330-11 compatibility mode.
4 IBM V3/310 operating Systems in a Virtual Machine

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