Operating Systems CPU Scheduling

Agenda for Today What is Scheduler and its types Short-term scheduler Dispatcher Reasons for invoking scheduler Optimization criteria FCFS, SJF, SRTF, RR, Multi level Queues With Examples

CPU Scheduling  Scheduling processes in the ready queue  Short-term scheduler  Different types of schedulers

Life of a Process

Histogram of CPU- burst Times

CPU Scheduler  Short-term scheduler  Selects a process from among the processes in the ready queue  Invokes the dispatcher to have the CPU allocated to the selected process

Dispatcher  Dispatcher 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 start (or restart) it

Dispatcher  Dispatch latency – time it takes for the dispatcher to stop one process and start another running.  Typically, a few microseconds

CPU Scheduler  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

CPU Scheduler  Scheduling under 1 and 4 is nonpreemptive.  All other scheduling is preemptive.

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

Scheduling Criteria  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)

Optimization Criteria  Maximize CPU utilization  Maximize throughput  Minimize turnaround time  Minimize waiting time  Minimize response time

FCFS Scheduling  The process that enters the ready queue first is scheduled first, regardless of the size of its next CPU burst  Example: ProcessBurst Time P 1 24 P 2 3 P 3 3  Suppose that processes arrive into the system in the order: P 1, P 2, P 3

FCFS Scheduling  Processes are served in the order: P 1, P 2, P 3  The Gantt Chart for the schedule is:  Waiting times P 1 = 0; P 2 = 24; P 3 = 27  Average waiting time: ( )/3 = 17 P1P1 P2P2 P3P

 Suppose that processes arrive in the order: P 2, P 3, P 1.  The Gantt chart for the schedule is:  Waiting time for P 1 = 6; P 2 = 0 ; P 3 = 3  Average waiting time: ( )/3 = 3  Convoy effect short process behind long process P1P1 P3P3 P2P FCFS Scheduling

Shortest-Job-First (SJF) Scheduling  Process with the shortest CPU burst is scheduled first.  Non-preemptive – once CPU given to a process it cannot be preempted until completes its CPU burst.

Shortest-Job-First (SJF) Scheduling  Preemptive – if a new process arrives with CPU burst length less than remaining time of current executing process, preempt it—Shortest- Remaining-Time-First (SRTF).  SJF is optimal non-preemptive scheduling algorithm – gives minimum average waiting time for a given set of processes.

Non-Preemptive SJF  Process Arrival Time Burst Time P P P P  Gantt chart  Average waiting time = ( )/4 = 4 P1P1 P3P3 P2P P4P4 12

Preemptive SJF  Process Arrival TimeBurst Time P P P P  Gantt chart  Average waiting time = ( )/4 = 3 P3P3 P2P P4P4 57 P2P2 P1P1 16 P1P1

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  Non-preemptive

Priority Scheduling  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.

Round Robin (RR)  Each process gets a small unit of CPU time, called time slice or quantum, which is usually milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready queue.

Round Robin (RR)  If there are n processes in the ready queue, the time quantum is q, and context switch time is t cs, then no process waits more than (n-1)(q+t cs ) time units  Used in time-sharing systems where response time is an important performance criteria

Performance  q large  FCFS  q small  q must be large with respect to context switch, otherwise overhead is too high. Round Robin (RR)

Round Robin Example ProcessBurst Time P 1 53 — 33 — 13 P 2 17 P 3 68 — 48 — 28 — 8 P 4 24 — 4  The Gantt chart with quantum 20 is: P1P1 P2P2 P3P3 P4P4 P1P1 P3P3 P4P4 P1P1 P3P3 P3P

Round Robin Example ProcessTurnaround TimeWaiting Time P – 53 = 81 P – 17 = 20 P – 68 = 94 P – 24 = 97  Average waiting time = 73  Average waiting time for SJF = 38  Typically, higher average turnaround than SJF, but better response.

Quantum vs Context Switch Process Time = 10 Quantum Context Switches

Multilevel Queues  Ready queue is partitioned into separate queues: - foreground (interactive) - background (batch)  Each queue has its own priority and scheduling algorithm: - foreground – RR - background – FCFS

Multilevel Queues  Scheduling must be done across queues.  Fixed priority scheduling; i.e., serve all from foreground then from background.  Time slice – each queue gets a certain percentage of CPU time, e.g., 80% to foreground in RR and 20% to background in FCFS

Multilevel Queues

Multilevel Feedback Queues  A process can move between the various queues; aging can be implemented this way.  Multilevel-feedback-queue scheduler defined by the following parameters:  Number of queues  Scheduling algorithms for each queue

Multilevel Feedback Queues  Method used to determine when to upgrade a process  Method used to determine when to demote a process  Method used to determine which queue a process will enter when that process needs service

Multilevel Feedback Queues