IBM System/370 145 Manual page 89

approach must
be
taken in planning for and in evaluating system
performance in a virtual storage~ environment.
Many of the same factors that affect system performance in.a DOS/VS
r
OS/VS1
r
or OS/VS2 environment are the same as those that apply to OOS
Version 4
r
OS
MFTr
or OS MVT, respectively. First, the system
configuration must include the hardware resources (CPU speed, channels,
I/O devices, real storage) required for the control program and job mix.
This subsection identifies the system resources specifically required to
support a virtual storage environment. Second, the system should be
designed to balance resource usage to achieve optimum throughput, and to
use applicable performance and control program design options the
particular operating system offers, taking into account the
characteristics of the installation job stream.
The performance of a system iln a virtual storage environment is also
affected by certain new factors :that do not apply to systems without
virtual storage support. This subsection ident:i,.fies these new factors,
explains how they generally
affe~ct
system performance, and indicates
steps that can be taken to incre;i:lse and maximi·ze system performance when
a virtual storage operating syst4em is used.
This discussion applies to DOl3/VS, OS/VS1, and OS/VS2
r
and is
restricted to performance factors that are common to the virtual storage
environments they support. The ~~irtual storage operating systems also
offer new performance-oriented enhancements that are not related to the
implementation of virtual storagte. These unique performance features
are discussed in the optional progra.mming systems supplements ..
The performance information ill this subsection is designed to present
concepts and considerations for
Ci
virtual storage environment. Figures
and graphs are used for illustra1:.ive purposes. They do not represent
any particular installation or
m4i~asured
results. Their purpose is to
illustrate the interrelated factors of multiprogramming performance in a
virtual storage environment. Th4i~ performance information presented is
conceptual. It is based on the experience and judgment of IBM
individuals with performance kno~'ledge, and on performance measurements
made during development of OS/VS11 and OS/VS2. Therefore, i t may not
apply to all installations.
SYSTEM RESOURCES REQUIRED TO
SUP1~RT
A VIRTUAL STORAGE ENVIRONMENT
In order to support a demand paged virtual storage environment using
System/310, in which programs arE~ operating in paged mode, additional
system resources are used by the IBM-supplied virtual storage operating
systems, as follows:
• Dynamic address translation hardware requires CPU time to perform
virtual storage to real storalge address translation. The amount of
time required is determined by the System/370 model and the number
of times the full table-lookUlp translation procedure must be
performed. The Model 145, fOir example, has a translation lookaside
buffer that is designed to reduce use of the full table-lookup
translation procedure. The CPU time required is also affected by
program structure (which is d.i~cussed later). A small amount of
additional CPU time is also required to pretest certain instructions
that reference storage
r
as discussed under "Instruction
Nullification" in Section 15:10. Studies have shown that a
relatively small percentage of the total CPU time specifically
required to support a virtual storage environment is devoted to
address translation by DAT hardware.
• CPU time is required to translate the virtual storage addresses in
channel programs (CCW lists) into real storage addresses, build
A Guide to the IBM System/370 Model 145 79