Chapter 5: CPU Scheduling

Chapter 5: CPU Scheduling• Basic Concepts

• Scheduling Criteria

• Scheduling Algorithms

• Thread Scheduling

• Multiple-Processor Scheduling

• Operating Systems Examples

• Algorithm Evaluation

Basic Concepts• The CPU is one of the primary computer resources, its scheduling is

central to operating-system design.

• In a single-processor system:• only one process can run at a time;

• any others must wait until the CPU is free and can be rescheduled.

• Maximum CPU utilization obtained with multiprogramming• A process is executed until it must wait, typically for the completion of some

I/O request.

• Several processes are kept in memory at one time. When one process has to wait, the operating system takes the CPU away from that process and gives the CPU to another process.

• This pattern continues. Every time one process has to wait, another process can take over use of the CPU.

CPU–I/O Burst Cycle• Process execution consists of a cycle of

CPU execution and I/O wait.

• Processes alternate between these two states.

• Process execution begins with a CPU burst, followed by an I/O burst, which is followed by another CPU burst, then another I/O burst, and so on.

• The final CPU burst ends with a system request to terminate execution.

• The durations of CPU bursts have been measured.

• An I/O-bound program typically has many short CPU bursts.

• A CPU-bound program might have a few long CPU bursts.

CPU Scheduler• Selects from among the processes in ready queue, and allocates the CPU to one

of them• Queue may be ordered in various ways

• CPU scheduling decisions may take place when a process:1. Switches from running to waiting state (the result of an I/O request or an invocation of

wait for the termination of one of the child processes (non preemptive scheduling )

2. Switches from running to ready state (an Interrupt occurs) (preemptive scheduling )

3. Switches from waiting to ready (I/O Terminates)(preemptive scheduling )

4. Terminates(non preemptive scheduling )

• Under non-preemptive scheduling, once the CPU has been allocated to a process, the process keeps the CPU until it releases the CPU, either by terminating or by switching to the waiting state.

• All other scheduling is preemptive• Consider access to shared data

• Consider preemption while in kernel mode

• Consider interrupts occurring during crucial OS activities

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

Scheduling Criteria• Many criteria have been suggested for comparing CPU-scheduling algorithms.

• CPU utilization: keep the CPU as busy as possible ( rang from 0% to 100%).

• Throughput: # of processes that complete their execution per time unit

• Turnaround time: amount of time to execute a particular process (Turnaroundtime is the sum of the periods spent waiting to get into memory, waiting in theready queue, executing on the CPU, and doing I/O).

• Waiting time – amount of time a process has been waiting in the ready queue (thesum of the periods spent waiting in the ready queue).

• Response time – amount of time it takes from when a request was submitted untilthe first response is produced, not output (for time-sharing environment)

• Scheduling Requirements:• Max CPU utilization• Max throughput• Min turnaround time• Min waiting time• Min response time

Scheduling Algorithms1. First-Come, First-Served Scheduling (FCFS)

• The process that requests the CPU first is allocated the CPU first.

• Can be Implemented using a FIFO queue.

• When a process enters the ready queue, its PCB is placed on the tail of the queue.

• CPU is allocated to the process at the head of the queue.

• The running process is then removed from the queue.

• The code for FCFS scheduling is simple to write and understand.

• Disadvantages:• The average waiting time is often quite long.

• There is a convoy effect as all the other processes wait for the one big process to get off the CPU. This effect results in lower CPU and device utilization than might be possible if the shorter processes were allowed to go first.

Scheduling Algorithms1. First-Come, First-Served Scheduling (FCFS)

Scheduling Algorithms2. Shortest Job First (SJF)

• Associate with each process the length of its next CPU burst• Use these lengths to schedule the process with the shortest time

• SJF is optimal – gives minimum average waiting time for a given set of processes

• The difficulty is knowing the length of the next CPU request

• Could ask the user

• Disadvantage:

• How to know the length of the next CPU

request.

• For long-term (job) scheduling in a batch

system, we can use as the length the process

time limit that a user specifies when he

submits the job.

• Users must estimate the process time limit

accurately, since a lower value may mean

faster response.

• SJF scheduling is used frequently in long-term

scheduling.

Determining Length of Next CPU Burst• We may not know the length of the next CPU burst, but we may be able to predict

its value.

• the next CPU burst will be similar in length to the previous ones.

• Computing an approximation of the length of the next CPU burst, we can pick the

process with the shortest predicted CPU burst.

• an exponential average of the measured lengths of previous CPU bursts.

Let tnbe the length of the nth CPU burst, and let

n+1 be our predicted value for the next CPU

burst. Then, for , 0 ≤ ≤ 1, define

Examples of Exponential Averaging

• =0• n+1 = n

• Recent history does not count

• =1• n+1 = tn

• Only the actual last CPU burst counts

• If we expand the formula, we get:n+1 = tn+(1 - ) tn -1 + …

+(1 - )j tn -j + …

+(1 - )n +1 0

• Since both and (1 - ) are less than or equal to 1, each successive term has less weight than its predecessor

Example of Shortest-remaining-time-first

Priority Scheduling• A priority number (integer) is associated with each process

• Equal Priorities are served using FCFS algorithm

• The CPU is allocated to the process with the highest priority (smallest integer)

• SJF is priority scheduling, priority is the inverse of predicted next CPU burst time

• Problem Starvation – low priority processes may never execute

• Solution Aging – as time progresses increase the priority of the process

[P1(6)+P2(0)+P3(16)+P4(18)+P5(1)]/5=41/5=8.2

Round Robin (RR)• Each process gets a small unit of CPU time (time quantum q), usually 10-100ms.

• 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.

• Timer interrupts every quantum

to schedule next process

• Performance• q large FIFO• q small q must be large with respect • to context switch, otherwise • overhead is too high

Time Quantum and Context Switch Time

80% of CPU bursts should be shorter than q

Turnaround Time Varies With The Time Quantum

Multilevel Queue• Ready queue is partitioned into separate queues, eg:

• foreground (interactive)

• background (batch)

• Process permanently in a given queue

• Each queue has its own scheduling algorithm:• foreground – RR

• background – FCFS

• Scheduling must be done between the queues:• Fixed priority scheduling; (i.e., serve all from foreground then from background). Possibility

of starvation.

• Time slice – each queue gets a certain amount of CPU time which it can schedule amongst its processes; i.e., 80% to foreground in RR

• 20% to background in FCFS

Multilevel Queue Scheduling

Multilevel Feedback Queue• 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

• 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

Example of Multilevel Feedback Queue• Three queues:

• Q0 – RR with time quantum 8 milliseconds

• Q1 – RR 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