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

Tc relate these measurement values to system workload for a specific
configuratien, an installation must define its workload. The definition
cf workload varies with the environment:
Interactive time-sharing
system
Batch system
Arrival rate of transactions and the CPU time
and working set size required for each
transaction.
Job throughput and resource requirements (CPU time, region or partition size, and number of EXCPs issued) for each job.
By using these workload definitions, an installation can measure its
workload under as follows: users under OFerating system under VM/370 Number of active users User I/O requests executed When both an operating system and users run under the
system, workload measurement depends upon which type of
dominates the VM/370 system.
same environment
To measure workload performance in a specific configuration, an
installation can use the Field Developed program VM/370 Performance/Mcnitor Analysis program (5798-CPX). This program plots a
number of important system variables (such as CPU usage, various
centention measurements, and paging rates) against workload measurement
fer both the and operating system workloads under VM/370. For a
sFecific configuration, it allows an installation to relate processor
usage, storage usage, and resource contention to the total system
workload in beth interactive and batch production environments.
By using this analysis program, an installation can eventually
determine its optimum processor model, storage size, and l/C
configuraticn fer a specific workload. Thus, an installation may
determine that it needs to do such things as: redistribute data sets to
reduce arm centention, add control units and channels to reduce l/C
centention, and add paging devices to reduce interference between system
and user I/O Frocessing.
These specialized software techniques influence the performance
of an operating system in a virtual machine: • Whether VH/VS handshaking is used. • The tYFe and number of VM/370 performance options in use by one or more virtual machines. • Whether the VM/370 System Extensions Program Product
used. (5748-XE1) is VM/VS handshaking (described earlier in this section
under the teFic "VM/VS Handshaking") permits instructions issued by an
oFerating system in a virtual machine to be processed directly by the
Frocessor. It also permits VM/370 to simulate privileged instructions. Section 1. General Considerations 35