Programming For Performance; Reducing Core Storage - IBM 1130 User Manual

Section
Subsections Page
90 20
I
30 01
Programming for Performance
You have seen in the preceding examples that system
performance is very closely related to the size of a
program.
In
general, the larger the program, the
more slowly it will run. This degradation is not
evidenced in a gradual way; because of the SOCAL
and LOCAL system, it will show up in sudden jumps
or drops in throughput rates. Suppose you have an
1131 Model 2B (SK) and the familiar "typical"
program. With no overlays you have about 4500
words for your program; with Overlay 1, about
4920 words; with Overlay 2, about 5620 words.
Assuming these figures to be exact, this means that:
If
your program size is It will:
1 -- 4500 words Fit with no overlays
4501 -- 4920 words Require Overlay 1
4921 -- 5620 words Require Overlay 2
5621 words or more Not fit in core stor-
age without further
work
If you add 1 word to a 4920-word program, it will
suddenly require Overlay 2 and may take twice as
long to execute. (It may also take no longer than
before -- this depends on the
pr~gram.)
Conversely, if you have a program at level 2,
it may take anywhere from one word to 700 words to
make it drop to level 1.
If
it was 4921 before, it
will take only one word; if it was 5620, it will take
7:'0 words.
Reducing Core Storage Requirements
To make a long story short, every word counts.
You should always keep this fact in mind and strive
to write efficient programs. Section .70 gives many
core saving tips; Section 65 also gives some ideas
for improving the SOCAL system. Repeating the
FOR TRAN tips (the details are given in Section
70. 50.20):
1. Use the DATA statement wherever possible.
2. Keep FORMAT statements compact.
3. Take square roots and raise numbers to
powers in the most efficient manner.
4. Code efficient I/O statements.
5. Avoid long subroutine argument lists.
6. Don't include unneeded I/O devices on the
*IOCS card.
7. Avoid arithmetic with constant subscripts.
S. Remove the TRACE from production status
programs. The trace package requires about 140
words of core storage.
In
addition, it requires that
Data Switch 15 be interrogated every time you
"execute" an equal sign, IF statement, or computed
GO TO. This requires 150 to 200 microseconds
each time; some programs may do this tens of
thousands of times in the course of one run.