The mean turnaround time is (4a + 3b + 2c + d)/4. The first job finishes at time a, the second finishes at time a + b, and so on. Consider the case of four jobs, with run times of a, b, c, and d, respectively. the turnaround times are now 4, 8, 12, and 20 minutes for an average of 11 minutes. Now let us consider running these four jobs using shortest job first, as shown in Fig. (b) Running them in shortest job first order. (a) Running four jobs in the original order. By running them in that order, the turnaround time for A is 8 minutes, for B is 12 minutes, for C is 16 minutes, and for D is 20 minutes for an average of 14 minutes.Īn example of shortest job first scheduling. Here we find four jobs A, B, C, and D with run times of 8, 4, 4, and 4 minutes, respectively. When several equally important jobs are sitting in the input queue waiting to be started, the scheduler picks the shortest job first. In an insurance company, for example, people can predict quite accurately how long it will take to run a batch of 1000 claims, since similar work is done every day. Now let us look at another nonpreemptive batch algorithm that assumes the run times are known in advance. If they arrive in the order of P3, P2, P1Ĭonsider the following processes arrive at time 0, 1, 2, 3 respectivelyĪverage response time = (0+0+99+99)/4 = 49.5Īverage turnaround time = (1+100+100+199)/4=100 If they arrive in the order of P1, P2, P3Īverage turnaround time = (24+27+30)/3=27Ĭase ii. With a scheduling algorithm that preempted the compute-bound process every 10msec, the I/O-bound processes would finish in 10 sec instead of 1000 sec, and without slowing down the compute-bound process very much.Ĭonsider the following processes arrive at time 0ĬPU Burst/Service/Running Time (in ms) 24 3 3Ĭase i. The net result is that each I/O-bound process gets to read 1 block per second and will take 1000 sec to finish. ![]() When the compute-bound process gets its disk block, it runs for another 1 sec, followed by all the I/O-bound processes in quick succession. All the 1/0 processes now run and start disk reads. The compute-bound process runs for 1 sec, then it reads a disk block. Suppose that there is one compute-bound process that runs for 1 sec at a time and many I/O-bound processes that use little CPU time but each have to perform 1000 disk reads to complete. Unfortunately, first-come first-served also has a powerful disadvantage. What could be simpler to understand and implement? Adding a new job or unblocked process just requires attaching it to the end of the queue. Picking a process to run just requires removing one from the front of the queue. With this algorithm, a single linked list keeps track of all ready processes. It is also fair in the same sense that allocating scarce sports or concert tickets to people who are willing to stand on line starting at 2 A.M. The great strength of this algorithm is that it is easy to understand and equally easy to program. When a blocked process becomes ready, like a newly arrived job, it is put on the end of the queue. When the running process blocks, the first process on the queue is run next. As other jobs come in, they are put onto the end of the queue. It is not interrupted because it has run too long. When the first job enters the system from the outside in the morning, it is started immediately and allowed to run as long as it wants to. Basically, there is a single queue of ready processes. With this algorithm, processes are assigned the CPU in the order they request it. Probably the simplest of all scheduling algorithms is nonpreemptive first come first-served. Multilevel Feedback Queue Scheduling algorithm.Highest Response Ratio Next (HRRN) algorithm.Shortest Remaining time (SRT) algorithm.Shortest Job First Scheduling (SJF) algorithm.First-come, first-served scheduling (FCFS) algorithm.List of scheduling algorithms are as follows: For scheduling arrival time and service time are also will play a role. Scheduling algorithms decide which of the processes in the ready queue is to be allocated to the CPU is basis on the type of scheduling policy and whether that policy is either preemptive or non-preemptive. For these scheduling algorithms assume only a single processor is present. The main objective of short-term scheduling is to allocate processor time in such a way as to optimize one or more aspects of system behavior. Scheduling algorithms or scheduling policies are mainly used for short-term scheduling. ![]() The goal of any scheduling algorithm is to fulfill a number of criteria: no task must be starved of resources – all tasks must get their chance at CPU time. A scheduling algorithm is the algorithm which dictates how much CPU time is allocated to Processes and Threads.
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