The Effect Of The Monitor On Performance - IBM 1130 User Manual

Section
Subsections Page
90 10
I
10 01
The Effect of the Monitor on Performance
To return to the main subject of this chapter, you
may ask, "How does all this affect performance?"
To answer this, we can construct a flowchart of a
"typical" commercial job. Let us say
it
is basi-
cally of the type:
1. Head a card.
2. Taking a key item number from the card,
look up its approximate disk location in an index
table (indexed sequential organization).
3. Head a disk record.
4. Determine whether it is the right disk record.
If it is, continue; if not, decrease the record num-
ber by 1 and go back to step 3.
5. Do some calculations based on the data
obtained from the disk and from the card.
6. Write an updated disk record.
7. Print a line of answers on the
1132
Printer.
8. Do some arithmetic (reset indicators, clear
totals, etc.) and go back to step 1.
For the purposes of this analysis you may ignore
routines that are executed only once (initialization,
final totals) or infrequently (error messages, etc.).
Figure
90.2
shows this job in the form of a rough
flowchart.
If this program is of a size that requires no
over lays, it will run at some base speed or through-
put rate. If its size is such that it must run at
SOCAL level 1, Overlay 1 (Arithmetic) and Overlay
2 (Non-disk
II
0) must be read from the disk when-
ever required. Figure
90.3
shows when these over-
lays would be required. This will lengthen the
base running time.
Each pass will require four overlays and two
disk arm moves. The arm moves are required
because the disk data file and the SOCAL overlays
are on different areas of the same disk. The time
required for these arm movements varies, depending
on several factors, but it may be considerable.
A good average might be about 250 milliseconds or
1/4
second.
If the program must run at Overlay level 2, the
picture changes considerably, as seen by Figure
90.4. If it hits the correct disk record on the first
try, it will require seven overlays and four disk
arm moves. For each additional disk read looking
for the correct record, add two overlays and two
arm moves. Running time will be further length-
ened.
Initialization
Arithmetic
_J
--.
Read Card
•
Look up key
item number
in index table
_J
--.
Read a disk
record for
an item
t
-
Check it against
Not
the item number
found
on the card
Found
t
Calculations
t
Write a new
disk record
t
Print results
t
Not
finished Finished _
Print
E
Miscellaneous
Grand
...
arithmetic
-
Totals
XIT
Figure 90.2. "Typical" commercial job -- rough flowchart