Fall 2000M.B. Ibáñez Lecture 11 Uniprocessor Scheduling

Fall 2000M.B. Ibáñez Types of Scheduling Long-term –performed when new process is created Medium-term –Swap-in, swap-out processes to/from main memory Short-term –which ready process to execute next

Fall 2000M.B. Ibáñez Short-term Scheduling Criteria User-oriented Response Time –Elapsed time between the submission of a request until there is output. Turnaround time –How long it takes to execute a process

Fall 2000M.B. Ibáñez Short-term Scheduling Criteria System-oriented CPU utilization –We want to keep the CPU as busy as possible Throughput –Maximize the number of processes completed per unit time

Fall 2000M.B. Ibáñez Decision Mode Nonpreemptive –Once a process is in the running state, it will continue until it terminates or blocks itself for I/O Preemptive –Currently running process may be interrupted and moved to the Ready state by the operating system –Allows for better service since any one process cannot monopolize the processor for very long

Fall 2000M.B. Ibáñez First-Come, First Served Scheduling As each process becomes ready, it joins the ready queue When the current process ceases to execute, the oldest process in the Ready queue is selected

Fall 2000M.B. Ibáñez FCFS example

Fall 2000M.B. Ibáñez FCFC evaluation Performs much better for long processes than short ones The turnaround time for short processes can be high It tends to favor processor-bound processes over I/O-bound processes It is not an attractive alternative on its own for single- processor system. However, it is often combined with a priority scheme to provide an effective scheduling.

Fall 2000M.B. Ibáñez Shortest-job-first scheduling -nonpreemptive- The process with the shortest expected processing time is selected next

Fall 2000M.B. Ibáñez Shortest-job-first scheduling -nonpreemptive- comments It needs to know or at least estimate the required processing time of each process A risk with this policy is the possibility of starvation for longer processes, as long as there is a steady supply of shorter processes

Fall 2000M.B. Ibáñez Shortest-job-first scheduling -preemptive- The scheduler always chooses the process that has the shortest expected remaining processing time

Fall 2000M.B. Ibáñez Shortest-job-first scheduling -preemptive- Comments There is a risk of starvation of longer processes Short jobs are given immediate preference to a running longer job