Define the real channel-to-channel adapter in the VM/370 system
generation procedure by using the RDEVICE macro instruction with a
device type of CTCA (DEVTYPE=CTCA). The virtual machine (VM3) must have
this device assigned to it before the IPL. Make this assignment by using
the DEDICATE statement in the virtual machine's VM/370 directory entry,
such as:
DEDICATE 280 380 where: 280 is the of the channel-to-channel adapter as generated
in the os system, and 380 is the address of the real
channel-to-channel adapter as specified in the VE/370 system procedure.
If no DEDICATE statement for the channel-to-channel adapter appears
in the virtual machine's directory entry, a resource operator with
privilege class B must attach the real channel-to-channel adapter to the
virtual machine.
Note: For further information on the virtual channel-to-channel adapter,
refer to the description of the COUPLE, DEFINE, and DETACH commands in
the ]ser§. Performance Guidelines When run in a virtual machine, the performance characteristics of an
operating system are difficult to predict. This unpredictability is a
rEsult of the complex interaction of many factors that affect
performance. These factors can be broadly classified into three groups: • Configuration factors • Workload factors • VM/370 performance factors
Performance of any virtual machine may be improved by the choice of
hardware configuration, operating system workload, and VM/370 performance options. While a specific virtual machine's performance may
not equal that of the same operating system running stand-alone on the
same System/370, in some situations the total throughput obtained in the
virtual machine environment can be equal to, or better than, that
obtained on a real machine.
These hardware configuration factors influence the performance of an
operating system in a virtual machine: • The System/370 model used. • The amount of real storage available. • The speed, capacity, and number of paging devices. • The degree of channel and control unit contention, as well as arm
contention, affecting each paging device. • Whether virtual machine assist or VM/370 extended control-program is installed on the hardware and enabled. • Interference between system paging devices and devices for processing
a user's I/O requests.
Section 1. General Considerations 33

When discussing these performance factors, this discussion assumes that
the reader is familiar with the need to design an optimal configuration
fer a specific workload and operating system. When moving a specific workload and operating system to the virtual
machine environment, an installation should plan for an increased need in such hardware requirements as real storage, DASD space, and processor
size. While VM/370's overhead for dispatching, scheduling, and paging may be relatively small, its overhead for simulating privileged
instructions may be considerable. When not operating under VM/370, an operating system runs directly on
its own hardware (native mode) and manages its resources through the use
ef privileged instructions (such as SVC and LPSi) issued in supervisor
state. When executing in a virtual machine, V"/370 dispatches the
operating system in problem state, and any privileged instructions
issued by the virtual machine cause a real privileged instruction
exception interruption. This interruption transfers control to V"/370 to simulate the instruction. The amount of work done by VM/370 in
analyzing and handling a virtual machine-initiated interruption depends upon the tYFe and complexity of the interruption. Thus, any reduction
in the number of privileged instructions issued by a virtual machine's cperating system reduces the amount of extra work VM/370 must do to
support that operating system. Virtual machine assist support has been specifically designed to
reduce VM/370's overhead associated with simulating privileged
instructicns. It is the single, most effective method for reducing
privileged instruction simulation. Any installation that is going to
run a production opera ting system under V"/370 should consider virtual
machine assist as a prerequisite for improving performance. Other steps
for improving performance (such as using specialized performance
functions) are of secondary importance compared to using virtual
machine assist. VM/370 extended control-program support (ECPS: is a hardware
assist functien that provides support over and above that provided by virtual machine assist support. It improves performance beyond
that attained by virtual machine assist support by reducing VM/370's real superviscr state time needed to support virtual machines. ECPS: VM/370 is available only on certain System/370 models as listed in These worklcad factors influence the performance of an operating system
in a virtual machine: • The type of operating system being used. • The total number of virtual machines running under VM/370. • The type of work each virtual machine is doing, especially the amount
of I/O processing required. By measuring and evaluating the effect of these workload factors on a specific configuration, an installation can understand their effect on performance and know which steps to take to improve performance. 34 VM/37C Operating Systems In Q Virtual