IBM 1130 User Manual page 656

Section
Subsections Page
90 10
I
10
03
Figure 90.5 summarizes the overlay pattern as it
varies with the disk search.
You can see the Overlay level 1 will not hurt
performance too much. If each arm movement
takes about 1/4 second, the processing time per
card might jump from 8 to 8 1/2 seconds. Overlay
level 2, on the other hand, may cause this program
to run significantly more slowly. A typical indexed
sequential file might require 15 disk reads to find
the correct record. This would increase the time
per card from 8 seconds to 16 seconds, or half the
throughput rate. This could become even worse if
the data file being searched were large or distant
from WS, since the SOCAL area would be proportion-
ately further away from the file area.
The overlay time itself may be ignored, since
it is quite small compared with the disk arm move-
ment time.
This example illustrates two principles:
1. The disk data file must be organized so that
items may be found qUickly (see Section 85).
2. For programs involving disk search tech-
niques, as does the example, you should try dili-
gently to avoid SOCAL Overlay level 2.
The difference between level 2 and level 1 is
either 620 words (HEAD and WRITE disk) or 700
words (HEAD, WRITE, and FIND disk), but this
does not mean that you must cut 620 or 700 words
from your program to drop from level 2 to level 1.
The CLB will use level 2 if the program is too big
for level 1.
(It
may be one word too big or 700
words too big.) Every word you can cut from
the size of the program increases the probability
that the program will fit at level 1 rather than level
Number of disk Overlay level 0 Overlay level 1 Overlay level 2
READs to find
the desired Arm
Arm
Arm
record
Overlays moves Overlays moves Overlays moves
1 0 0 4 2 7 4
2 0 0
4
2
9
6
3 0 0 4 2 11 8
4
0
0 4 2 13 10
5 0 0 4 2 15 12
10 0 0 4 2 25 22
15 0 0 4 2 35 32
20 0 0 4 2 45 42
Figure 90.5. Single-drive 1130 system
2. For this reason, you should strive to keep your
programs as small as possible. Several means of
doing this are discussed in the next subsection.
Also, Section 70 gives many FOHTRAN core saving
tips.
Note that the above analysis applies to single
disk drive 1130 systems; the addition of a second
disk drive would eliminate all the overlay-caused
arm movements -- assuming of course, that you
have placed your data file on one disk and Working
Storage on another.