IBM System/370 145 Manual page 96

there is a higher p.robabili ty that CPU processing will be held up
waiting for a paging operation to complete. As the CPU enters the wait
state more frequently to wait for paging I/O and less paging I/O is
overlapped, the paging factor causes performance to degrade more
rapidly.
The actual virtual-to-real storage ratio at the time active paging
begins in Figures 15.15.2 and 15.15.3 is a variable and depends on the
way in which virtual storage is used, that is, active-to-passive page
ratio of concurrently executing tasks.
Number of
page faults
per second
~=1
R
Figure 15.15.2.
1 - - - -
Passive paging - - - - -
I
I
1
1
1
Virtual-to-real storage ratio
Maximum
paging
capabilit;--------"/
Task
I
deactivation /
Active
-paging
I
I
General effect on page faults of increasing the ratio
of the virtual storage used to the real storage
present in the system.
Figure 15.15.4 i.llustrates the way in which the paging factor only
can affect system performance in a given configuration, based on the
active-to-passive page ratio. If the ratio of active to passive pages
for executing tasks is relatively high most of the time, as shown in
curve 1, the virtual-to-real storage ratio at the point at which active
paging begins will be relatively low. Performance drops very rapidly in
this case as more virtual storage is used. This happens because the
increased paging processing (I/O and CPU time) cannot be overlapped with
other processing. This situation may apply to an installation initially
when a switch from a nonvirtual storage to a virtual storage environment
is made and more virtual storage is used, since existing programs were
structured for optimum performance in a given partition or region size
rather than for optimum performance in a virtual storage environment.
86 A Guide to the IBM System/310 Model 145