IBM System/370 145 Manual page 90

indirect data address lists (when necessary), and short-term fix
pages that will be referenced during I/O initiation, execution, and
interruption handling. Channel program translation and page fixing
are performed prior to the initiation of each I/O operation with a
channel program that contains virtual storage addresses. Channel
status word retranslation and page unfixing are performed at the
completion of these I/O operations. The amount of CPU time this
function requires per data set is affected by the number of I/O
requests (EXCP macros) issued, the number of CCW's in the channel
programs started, the number of pages that must be fixed, and
whether or not indirect data address lists have to
be
constructed.
Studies have shown that a large portion of the total CPU time
specifically required to support a virtual storage environment is
used to perform channel program translation and page fixing.
• CPU time is required to process page faults and for the execution of
other control program code that is specifically required to support
a virtual storage environment. CPU time is required for such things
as servicing additional program interruptions, managing and
allocating real and external page storage, maintaining tables used
by DAT hardware, and testing for paged or nonpaged mode of program
operation, for example.
• I/O time is required for paging operations. The amount of paging
I/O time required is related to the number of page faults that occur
and the speed of the paging I/O device(s) used. In OS/VS2
environments, the total I/O time required for paging includes some
I/O time that is also required in 'OS MVT environments to load
transient control program routines.
• Direct access storage is required for external page storage. The
amount required depends on the amount of virtual storage that is to
be supported and the way in which the particular operating system
organizes and manages external page storage. (See the programming
systems supplements for external page requirements by device type.)
• The amount of real storage required by the resident (fixed) control
program is increased by the amount of real storage needed for
additional routines and code that are included specifically to
support a demand paged virtual storage environment. The
significance of this increase will be greater in systems with
smaller real storage sizes.
The effect this additional use of hardware resources has on the
performance of a given system configuration depends on the resource
requirements of the job stream and the current utilization of system
resources. To the degree that the additional required CPU and I/O time
can be overlapped with existing CPU and I/O time that currently is
unoverlapped, system throughput is not affected. System throughput will
be affected by the increase in CPU and I/O time that cannot be overlapped.
When a virtual storage operating system is used with an existing
system configuration, for example, and the same job stream is processed,
performance is affected by the use of any new performance enhancements
the operating system provides as well as
by
an increase in resource
utilization that is required to support a virtual storage environment.
Figure 15.15.1 conceptually illustrates possible system performance
when a virtual storage operating system is installed on an existing'
Model 145 configuration. A sample throughput obtained using a
nonvirtual storage operating system is shown in panel 1. (It is not
meant to represent any specific Model
145
throughput.) Panels 2 and
~
illustrate the conditions under which existing performancec~.l1 be
maintained, and the last two illustrate the conditions under whiCh
existing performance can be improved.
80 A Guide to the IBM System/310 Model 145