CPU Scheduling PDF

Module 5: CPU Scheduling Basic Concepts Scheduling Criteria Scheduling Algorithms Multiple-Processor Scheduling Real-Time Scheduling Algorithm Evaluation Operating System Concepts  Basic Concepts...

Module 5: CPU Scheduling Basic Concepts Scheduling Criteria Scheduling Algorithms Multiple-Processor Scheduling Real-Time Scheduling Algorithm Evaluation Operating System Concepts  Basic Concepts Maximum CPU utilization obtained with multiprogramming CPU–I/O Burst Cycle – Process execution consists of a cycle of CPU execution and I/O wait. CPU burst distribution Operating System Concepts  Alternating Sequence of CPU And I/O Bursts Operating System Concepts  CPU Scheduler Selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them. CPU scheduling decisions may take place when a process: 1. Switches from running to waiting state. 2. Switches from running to ready state. 3. Switches from waiting to ready. 4. Terminates. Scheduling under 1 and 4 is nonpreemptive. All other scheduling is preemptive. Operating System Concepts  Dispatcher Dispatcher module gives control of the CPU to the process selected by the short-term scheduler; this involves: – switching context – switching to user mode – jumping to the proper location in the user program to restart that program Dispatch latency – time it takes for the dispatcher to stop one process and start another running. Operating System Concepts  Scheduling Criteria CPU utilization – keep the CPU as busy as possible Throughput – # of processes that complete their execution per time unit Turnaround time – amount of time to execute a particular process Waiting time – amount of time a process has been waiting in the ready queue Response time – amount of time it takes from when a request was submitted until the first response is produced, not output (for time-sharing environment) Operating System Concepts  Optimization Criteria Max CPU utilization Max throughput Min turnaround time Min waiting time Min response time Operating System Concepts  First-Come, First-Served (FCFS) Scheduling Example: Process Burst Time P1 24 P2 3 P3 3 Suppose that the processes arrive in the order: P1 , P2 , P3 The Gantt Chart for the schedule is: P1 P2 P3 0 24 27 30 Waiting time for P1 = 0; P2 = 24; P3 = 27 Average waiting time: (0 + 24 + 27)/3 = 17 Operating System Concepts  FCFS Scheduling (Cont.) Suppose that the processes arrive in the order P2 , P3 , P1. The Gantt chart for the schedule is: P2 P3 P1 0 3 6 30 Waiting time for P1 = 6; P2 = 0; P3 = 3 Average waiting time: (6 + 0 + 3)/3 = 3 Much better than previous case. Convoy effect short process behind long process Operating System Concepts  Shortest-Job-First (SJR) Scheduling Associate with each process the length of its next CPU burst. Use these lengths to schedule the process with the shortest time. Two schemes: – nonpreemptive – once CPU given to the process it cannot be preempted until completes its CPU burst. – Preemptive – if a new process arrives with CPU burst length less than remaining time of current executing process, preempt. This scheme is know as the Shortest-Remaining-Time-First (SRTF). SJF is optimal – gives minimum average waiting time for a given set of processes. Operating System Concepts  Example of Non-Preemptive SJF Process Arrival Time Burst Time P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4 SJF (non-preemptive) P1 P3 P2 P4 0 3 7 8 12 16 Average waiting time = (0 + 6 + 3 + 7)/4 - 4 Operating System Concepts  Example of Preemptive SJF Process Arrival Time Burst Time P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4 SJF (preemptive) P1 P2 P3 P2 P4 P1 0 2 4 5 7 11 16 Average waiting time = (9 + 1 + 0 +2)/4 - 3 Operating System Concepts  Priority Scheduling A priority number (integer) is associated with each process The CPU is allocated to the process with the highest priority (smallest integer  highest priority). – Preemptive – nonpreemptive SJF is a priority scheduling where priority is the predicted next CPU burst time. Problem  Starvation – low priority processes may never execute. Solution  Aging – as time progresses increase the priority of the process. Operating System Concepts  TRY Process AT BT priority P1 0 5 2 P2 3 7 1 P3 5 4 3 P4 8 12 1 Preemptive priority Non Preemptive Priority Operating System Concepts  Compute the average waiting time using Preemptive SJF algorithm, Preemptive Priority ??? Process Arrival Time Burst Time priority P1 0 7 2 P2 3 3 1 P3 7 2 5 P4 12 6 3 P5 13 5 2 P6 17 4 0 Operating System Concepts  Round Robin (RR) Each process gets a small unit of CPU time (time quantum), usually 10-100 milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready queue. If there are n processes in the ready queue and the time quantum is q, then each process gets 1/n of the CPU time in chunks of at most q time units at once. No process waits more than (n-1)q time units. Performance – q large  FIFO – q small  q must be large with respect to context switch, otherwise overhead is too high. Operating System Concepts  Example: RR with Time Quantum = 20 Process Burst Time P1 53 P2 17 P3 68 P4 24 The Gantt chart is: P1 P2 P3 P4 P1 P3 P4 P1 P3 P3 0 20 37 57 77 97 117 121 134 154 162 Typically, higher average turnaround than SJF, but better response. Operating System Concepts How a Smaller Time Quantum Increases Context Switches Operating System Concepts  Multilevel Queues Operating System Concepts  Example of Multilevel Feedback Queue Three queues: – Q0 – time quantum 8 milliseconds – Q1 – time quantum 16 milliseconds – Q2 – FCFS Scheduling – A new job enters queue Q0 which is served FCFS. When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q1. – At Q1 job is again served FCFS and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q2. Operating System Concepts