On the Optimization of Performance of Time-Sharing Systems by Simulation

A simulation model of a time-sharing system
with a finite noncontiguous store and an infinite 
auxiliary store is used to study the variation of system
parameters such as store size, number of jobs 
allowed to execute simultaneously, job-scheduling algorithm,
etc.  The effects of these variations on 
a measure of system performance is used to ascertain which
of the parameters controllable by the job-scheduling 
algorithm, including the scheduling itself, require optimization,
and which of the parameters not normally 
controllable by the scheduling algorithm have a marked
effect on system performance.  System performance 
is based upon the mean cost of delay to all jobs processed.
 It is shown that significant improvements 
in the measure of system performance can be obtained by
using variable time-slice techniques and by selecting 
the optimum round-robin cycle time.  It appears that these
features would benefit from optimization whereas 
other parameters controllable by the scheduling algorithm
affect system performance in a predictable 
manner and would not benefit from optimization.  Features
not normally under the control of the scheduling 
algorithm can also have a marked effect on the measure
of performance; in particular, supervisor overheads, 
the size of the store, and the speed of the CPU.  A comparison
is made between the results of the simulation 
model and two analytical equations for quantum-oriented
nonpreemptive time-sharing systems.  The comparison 
is found to be very favorable.

CACM June, 1972

Blatny, J.
Clark, S. R.
Rourke, T. A.

time-sharing, simulation studies, optimization,
measure of performance, scheduling algorithms

3.80 4.30 4.32

CA720601 JB January 31, 1978  9:19 AM

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