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OS Process Scheduling Q & A #3
Question: What is Shortest Remaining Time, SRT scheduling?
Answer: Shortest Remaining Time, SRT is a preemptive scheduling. In SRT, the process with smallest runtime to complete (i.e remaining time) is scheduled to run next, including new arrivals. In SRT, a running process may be preempted by new process with shorter estimated run time. It keeps track of the elapsed service time of the running process and handles occasional preemption.
Consider the following 4 jobs
| Job | Arrival Time | Burst Time |
|---|---|---|
| 1 | 0 | 7 |
| 2 | 1 | 4 |
| 3 | 3 | 9 |
| 4 | 4 | 5 |
The schedule of the SRT is as follows:
Job1 is started at time 0, being only job in queue. Job 2 arrives at time 1. The remaining time for job 1 is larger (6 time units) than the time required by job2 (4 time units), so job 1 is preempted and job2 is scheduled. The average turnaround time for the above is:
| Job | Turn Around Time |
|---|---|
| 1 | 16-0 = 16 |
| 2 | 5-1 = 4 |
| 3 | 25-3 = 22 |
| 4 | 10-4 = 6 |
| 48 |
The average turnaround time is 48/4=12 time unit.
Advantage
Average turnaround time is less.
Disadvantage
Sometime a running process which is being almost completed is preempted because a new job with very small runtime arrives. It is not really worth doing.