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Scheduling: some description of the most important problems, models and solution methods

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The simplest instance that can be considered for scheduling is a uni-processor (1 CPU) environment. CPU scheduling is an attempt to allocate and de-allocate this central resource to the processes in an optimal manner. Maximum CPU utilization and throughput; and Minimum turn-around time (the time for which a process resides in the system) are the goals to be achieved. Following are some of the very basic assumptions established to acquire the basic understanding of the process.This algorithm is considered as the simplest and most robust of all. It can be implemented using a FIFO process queue. It simply schedules the processes according to their arrival times. The basic version of this strategy does not allow process preemption during processing. The algorithm is explained through the following example with table 1.The grey portion highlights the processing time and the blue portion represents the respective waiting time of each process in a process queue. The average turnaround time is 8.6 units and the average waiting time of processes is 6 units.The duration of stay of a process is always unpredictable due to the incorporated conditional structures and random input requests. A deterministic model is not sufficient to study this algorithm. Simulation or large scale implementation with beta testing are better techniques for analysis.The FCFS environment is also designated as a prioritized environment. The arrival time is the basis of priority. Theoretically the average turnaround time seems to reduce when the priority is given to the shortest duration process first.This approach produces the minimum optimal turnaround times and waiting times theoretically. The main reason of its being impractical is the absence of advance knowledge of process durations. For the above example the average turnaround time and average waiting time with SRTF are 7 units and 3.33 units respectively. The anomalies associated with this strategy are outlined as,The main theme is to assign integers as priorities explicitly or through a mathematical model after a careful evaluation of most of the important factors. This resolves the issue of unrealistic priorities of SRTF. The process queue becomes a heap based priority queue which further resolves the problem of accommodating external priorities or system designated specialized priorities. The remedy to starvation is achieved through waiting time based increment in the priority by
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