1.1 CPU I/O Burst Cycle

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1 PROCESS SCHEDULING ALGORITHMS As discussed earlier, in multiprogramming systems, there are many processes in the memory simultaneously. In these systems there may be one or more processors (CPUs) but the number of processes in memory is always greater than number of CPUs. Therefore, when a process requests for an I/O operation, it is removed from the CPU and placed in waiting state. The CPU is allocated to next process waiting for the CPU. Multiprogramming increases CPU utilization. It ensures that the CPU always has one job to execute. The Short Term Scheduler also known as CPU scheduler is responsible for allocation and de-allocation of the CPU to processes. The CPU Scheduler does this job by using CPU Scheduling Algorithms. 1.1 CPU I/O Burst Cycle The execution of a process consists of an alternation of CPU Burst and I/O Burst. A process starts and ends its execution with a CPU burst. CPU activity is suspended whenever a process needs to perform an I/O operation. If CPU bursts are longer than I/O bursts for a particular process then the process said to CPU bound. If I/O bursts are longer than CPU bursts for a particular process then the process said to I/O bound. 1.2 Preemptive Scheduling The scheduling is said to preemptive if the CPU can be taken away from a process even if the process doesn t want to release the CPU.

2 1.3 Non- Preemptive Scheduling The scheduling is said to non-preemptive if the CPU cannot be taken away from a process until it releases the CPU itself. We'll discuss four major scheduling algorithms here which are following 1. First Come First Serve (FCFS) Scheduling 2. Shortest-Job-First (SJF) Scheduling 3. Priority Scheduling 4. Round Robin(RR) Scheduling Arrival Time: It is the time when a process arrives in ready queue. Service Time: It is the time when a process gets the CPU and starts its execution. Execute Time: It is the time needed by a process to complete its execution. Wait Time: It is the time for which a process waits before it gets the CPU. 1.4 First Come First Serve (FCFS) 1. Jobs are executed on first come, first serve basis. 2. Removes a process from the processor only if it blocks (i.e., goes into the Wait state) or terminates 1. Easy to understand and implement. 2. Poor in performance as average wait time is high. Prof. M. Fahim, , itsfahim@hotmail.com Page 2

3 P0 0-0 = 0 P1 5-1 = 4 P2 8-2 = 6 P = 13 Average Wait Time: ( ) / 4 = Shortest Job First (SJF) 1. Best approach to minimize waiting time. 2. If a shorter process arrives then the currently executing process may be interrupted (known as preemption). 3. er should know in advance how much time process will take. Prof. M. Fahim, , itsfahim@hotmail.com Page 3

4 4. Starvation is possible, especially in busy system with many small processes. 5. This algorithm is no more in use. Starvation: It is a situation when a process waits for the CPU indefinitely. P0 3-0 = 3 P1 0-0 = 0 P = 14 P3 8-3 = 5 Average Wait Time: ( ) / 4 = Priority Scheduling 1. Each process is assigned a priority. with highest priority is to be executed first and so on. Prof. M. Fahim, , itsfahim@hotmail.com Page 4

5 2. es with same priority are executed on first come first serve basis. 3. Priority can be decided based on memory requirements, time requirements or any other resource requirement. P0 0-0 = 0 P1 3-1 = 2 P2 8-2 = 6 P = 13 Average Wait Time: ( ) / 4 = Round Robin Scheduling 1. Each process is provided a fix time to execute called quantum. Prof. M. Fahim, , itsfahim@hotmail.com Page 5

6 2. Once a process is executed for given time period, it is preempted and other process executes for given time period. 3. Context switching is used to save states of preempted processes. P0 (0-0) + (12-3) = 9 P1 (3-1) = 2 P2 (6-2) + (15-9) = 10 P3 (9-3) + (18-12) = 12 Average Wait Time: ( ) / 4 = 8.25 Prof. M. Fahim, , itsfahim@hotmail.com Page 6