cs 471 - lecture 5 cpu scheduling george mason university fall 2009

CS 471 - Lecture 5

CPU Scheduling

George Mason University

Fall 2009

5.2GMU – CS 571

CPU Scheduling

Basic Concepts

Scheduling Criteria Scheduling Algorithms

• First-Come-First-Served• Shortest-Job-First, Shortest-remaining-Time-First• Priority Scheduling• Round Robin • Multi-level Queue• Multi-level Feedback Queue

Real-Time CPU Scheduling

5.3GMU – CS 571

Basic Concepts

During its lifetime, a process goes through a sequence of CPU and I/O bursts.

In a multi-programmed computer system, multiple process compete for the CPU at a given time, to complete their current CPU bursts.

5.4GMU – CS 571

Basic Concepts

The CPU scheduler (a.k.a. short-term scheduler) will select one of the processes in the ready queue for execution.

The CPU scheduler algorithm may have tremendous effects on the system performance• Interactive systems• Real-time systems

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

5.5GMU – CS 571

Ready Running

Waiting

New Terminated

Event wait

Event occurs

ExitScheduler Dispatch

Timeout

Admit

When to Schedule?

Under a simple process state transition model, CPU scheduler could be invoked at five different points: 1. When a process switches from the new state to the ready state.2. When a process switches from the running state to the waiting state. 3. When a process switches from the running state to the ready state.4. When a process switches from the waiting state to the ready state.5. When a process terminates.

5.6GMU – CS 571

Non-preemptive vs. Preemptive Scheduling

Under non-preemptive scheduling, each running process keeps the CPU until it completes or it switches to the waiting (blocked) state (points 2 and 5 from previous slides).

Under preemptive scheduling, a running process may be also forced to release the CPU even though it is neither completed nor blocked. • In time-sharing systems, when the running process

reaches the end of its time quantum (slice)

• In general, whenever there is a change in the ready queue.

Tradeoffs?

5.7GMU – CS 571

Scheduling Criteria

Several criteria can be used to compare the performance of scheduling algorithms• 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 the complete output.

• Meeting the deadlines (real-time systems)

5.8GMU – CS 571

Optimization Criteria

Maximize the CPU utilization Maximize the throughput Minimize the (average) turnaround time Minimize the (average) waiting time Minimize the (average) response time Minimize variance

In the examples, we will assume• average waiting time is the performance measure• only one CPU burst (in milliseconds) per process

5.9GMU – CS 571

Single FIFO ready queue No-preemptive

• Not suitable for timesharing systems

Simple to implement and understand Average waiting time dependant on the order

processes enter the system

First-Come, First-Served (FCFS) Scheduling

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First-Come, First-Served (FCFS) Scheduling

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:

Waiting time for P1 = 0; P2 = 24; P3 = 27

Average waiting time: (0+24+27)/3 = 17

P1 P2 P3

24 27 300

5.11GMU – CS 571

FCFS Scheduling (Cont.)

Suppose that the processes arrive in the order P2 , P3 , P1

The Gantt chart for the schedule:

Waiting time for P1 = 6; P2 = 0; P3 = 3

Average waiting time: (6 + 0 + 3)/3 = 3 Problems:

• Convoy effect (short processes behind long processes)

• Non-preemptive -- not suitable for time-sharing systems

P1P3P2

63 300

5.12GMU – CS 571

Shortest-Job-First (SJF) Scheduling

Associate with each process the length of its next CPU burst. The CPU is assigned to the process with the smallest CPU burst (FCFS can be used to break ties).

Two schemes: • nonpreemptive

• preemptive – Also known as the Shortest-Remaining-Time-First (SRTF).

Non-preemptive SJF is optimal if all the processes are ready simultaneously– gives minimum average waiting time for a given set of processes.

SRTF is optimal if the processes may arrive at different times

5.13GMU – CS 571

Process Arrival Time Burst Time

P1 0.0 7

P2 2.0 4

P3 4.0 1

P4 5.0 4

SJF (non-preemptive)

At time 0, P1 is the only process, so it gets the CPU and runs to completion

Example for Non-Preemptive SJF

P1

730

5.14GMU – CS 571


P1 0.0 7

P2 2.0 4

P3 4.0 1

P4 5.0 4

Once P1 has completed the queue now holds P2, P3 and P4

P3 gets the CPU first since it is the shortest. P2 then P4 get the CPU in turn (based on arrival time)

Avg waittime = (0+8+7+12)/4 = 6.75

Example for Non-Preemptive SJF

P1

730

P2P3

8 12

P4

16

5.15GMU – CS 571

Estimating the Length of Next CPU Burst Problem with SJF: It is very difficult to know exactly

the length of the next CPU burst. Idea: Based on the observations in the recent past, we

can try to predict.

Exponential averaging: nth CPU burst = tn; the average of all past bursts n, using a weighting factor 0<=<=1, the next CPU burst is: n+1 = tn + (1- ) n.

5.16GMU – CS 571

Example for Preemptive SJF (SRTF)


P1 0.0 7

P2 2.0 4

P3 4.0 1

P4 5.0 4

Time 0 – P1 gets the CPU Ready = [(P1,7)]

Time 2 – P2 arrives – CPU has P1 with time=5, Ready = [(P2,4)] – P2 gets the CPU

Time 4 – P3 arrives – CPU has P2 with time = 2, Ready = [(P1,5),(P3,1)] – P3 gets the CPU

P1 P2 P3

2 4 5

5.17GMU – CS 571



P1 0.0 7

P2 2.0 4

P3 4.0 1

P4 5.0 4

Time 5 – P3 completes and P4 arrives - Ready = [(P1,5),(P2,2),(P4,4)] – P2 gets the CPU

Time 7 – P2 completes – Ready = [(P1,5),(P4,4)] – P4 gets the CPU

Time 11 – P4 completes, P1 gets the CPUP1 P2 P3 P2 P4

5 7 11

P1

16

5.18GMU – CS 571


P1 0.0 7

P2 2.0 4

P3 4.0 1

P4 5.0 4

SJF (preemptive)

Average waiting time = (9 + 1 + 0 +2)/4 = 3


P1 P2 P3 P2 P4

5 7 11

P1

162 4

5.19GMU – CS 571

Priority-Based 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

SJF is a priority scheme with the priority the remaining time.

5.20GMU – CS 571

Example for Priority-based Scheduling

Process Burst Time Priority

P1 10 3

P2 1 1

P3 2 4

P4 1 5

P5 5 2

P2 P5P3

1 6 16

P1 P3 P4

18 19

5.21GMU – CS 571

Priority-Based Scheduling (Cont.)

Problem: Indefinite Blocking (or Starvation) – low priority processes may never execute.

One solution: Aging – as time progresses, increase the priority of the processes that wait in the system for a long time.

Priority Assignment• Internal factors: timing constraints, memory

requirements, the ratio of average I/O burst to average CPU burst….

• External factors: Importance of the process, financial considerations, hierarchy among users…

5.22GMU – CS 571

Round Robin (RR) Scheduling

Each process gets a small unit of CPU time (time quantum). After this time has elapsed, the process is preempted and added to the end of the ready queue.

Newly-arriving processes (and processes that complete their I/O bursts) are added to the end of the ready queue

If there are n processes in the ready queue and the time quantum is q, then no process waits more than (n-1)q time units.

Performance• q large FCFS• q small Processor Sharing (The system appears to

the users as though each of the n processes has its own processor running at the (1/n)th of the speed of the real processor)

5.23GMU – CS 571

Example for Round-Robin Process Burst Time

P1 53

P2 17

P3 68

P4 24

The Gantt chart: (Time Quantum = 20)

Average wait time = (81+20+94+97)/4 = 73 Typically, higher average turnaround time

(amount of time to execute a particular process) than SJF, but better response time (amount of time it takes from when a request was submitted until the first response is produced).

P1 P2 P3 P4 P1 P3 P4 P1 P3 P3

0 20 37 57 77 97 117 121 134 154 162

5.24GMU – CS 571

Example for Round-Robin

Process Burst Time

P1 53

P2 17

P3 68

P4 24

The Gantt chart: (Time Quantum = 30)

Average wait time = (71+30+94+77)/4 = 68

When Time Quantum = 10 get average wait time = (91+40+94+77)/4 = 75.5

P1 P2 P3 P4 P1 P3 P3

0 30 47 77 101 124 154 162

5.25GMU – CS 571

Choosing a Time Quantum

The effect of quantum size on context-switching time must be carefully considered.

The time quantum must be large with respect to the context-switch time

Modern systems use quanta from 10 to 100 msec with context switch taking < 10 msec

5.26GMU – CS 571

Turnaround Time and the Time Quantum

Turnaround time also depends on the size of the time quantum

5.27GMU – CS 571

Multilevel Queue Sometimes different processes can be partitioned into groups with

different properties. Ready queue is partitioned into separate queues:

Example, a queue for foreground (interactive) and another for background (batch) processes; or:

5.28GMU – CS 571

Multilevel Queue Scheduling Each queue may have has its own scheduling

algorithm: Round Robin, FCFS, SJF…

In addition, (meta-)scheduling must be done between the queues.• Fixed priority scheduling (i.e. serve first the queue

with highest priority). Problems?

• Time slice – each queue gets a certain amount of CPU time which it can schedule amongst its processes; for example, 50% of CPU time is used by the highest priority queue, 20% of CPU time to the second queue, and so on..

• Also, need to specify which queue a process will be put to when it arrives to the system and/or when it starts a new CPU burst.

5.29GMU – CS 571

Multilevel Feedback Queue

In a multi-level queue-scheduling algorithm, processes are permanently assigned to a queue.

Idea: Allow processes to move among various queues.

Examples• If a process in a queue dedicated to interactive

processes consumes too much CPU time, it will be moved to a (lower-priority) queue.

• A process that waits too long in a lower-priority queue may be moved to a higher-priority queue.

5.30GMU – CS 571

Example of Multilevel Feedback Queue

Three queues:

• Q0 – RR - time quantum 8 milliseconds

• Q1 – RR - time quantum 16 milliseconds

• Q2 – FCFS

Qi has higher priority than Qi+1

Scheduling

• A new job enters the queue Q0. When it gains CPU, the job receives 8 milliseconds. If it does not finish in 8 milliseconds, the job is moved to the queue Q1.

• In queue Q1 the job receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to the queue Q2.

5.31GMU – CS 571


5.32GMU – CS 571


Multilevel feedback queue scheduler is 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

The scheduler can be configured to match the requirements of a specific system.

5.33GMU – CS 571

More on Scheduling

Scheduling on Symmetric Multiprocessors

• Partitioned versus Global Scheduling• Processor Affinity (some remnants of a process may

remain in one processor's state)• Load Balancing (push vs. pull)

Real OS examples (see text 5.6)• Solaris• Windows XP• Linux

Algorithm Evaluation (5.7)

5.34GMU – CS 571

Scheduling Issues in Real-Time Systems

Timeliness is crucial

Important features of real-time operating systems• Preemptive kernels

• Low latency

• Preemptive, priority-based scheduling

5.35GMU – CS 571

Non-preemptive vs. preemptive kernels

Non-preemptive kernels do not allow preemption of a process running in kernel mode• Serious drawback for real-time applications

Preemptive kernels allow preemption even in kernel mode• Insert safe preemption points in long-duration

system calls

• Or, use synchronization mechanisms (e.g. mutex locks) to protect the kernel data structures against race conditions

5.36GMU – CS 571

Minimizing Latency Event latency is the amount of time that elapses

between the occurrence of an event and the completion time of the service

5.37GMU – CS 571

Interrupt Latency Interrupt latency is the period of time from when an

interrupt arrives at the CPU to when it is serviced.

5.38GMU – CS 571

Dispatch Latency

Dispatch latency is the amount of time required for the scheduler to stop one process and start another.

5.39GMU – CS 571

Dispatch Latency 　 (Cont.)

Conflict• Preemption of process running in kernel• Release by low-priority processes resources needed

by high-priority process

5.40GMU – CS 571

Minimizing latency

Bounding interrupt and dispatch latencies is crucial for hard real-time operating systems

What if a higher-priority process needs to read or modify the kernel data structures that are currently being accessed by a low-priority process?

Additional delays that may be caused by medium-priority processes

The priority inversion problem

5.41GMU – CS 571

Hard Real-Time CPU Scheduling

Must make sure all the processes will meet their deadlines even under worst-case resource requirements

Typically requires preemptive, priority-based scheduling• How to assign priorities?

Most real-time processes are periodic in nature (i.e. require the CPU at constant intervals for a fixed time t)

5.42GMU – CS 571

Hard Real-Time CPU Scheduling

Periodic processes require the CPU at specified intervals (periods)

p is the duration of the period (the rate is 1/p) d is the relative deadline by when the process

must be serviced (in many cases, equal to p) t is the processing time 0 <= t <= d <= p

5.43GMU – CS 571

Priority Assignment

How to assign priorities to periodic real-time processes to meet all the deadlines?

If the priority assignment is such that the relative priorities of any two processes remain the same, then it is said to be a static priority assignment.

Consider two processes:

• P1 has the period p1 = 50, processing time t1 = 20


5.44GMU – CS 571

The concept of utilization The CPU utilization of a process is defined by the

ratio of its worst-case processing time (CPU burst length) to its period

The total utilization of the real-time process set can be computed as

Utot = (ti / pi)

Two processes:• P1 has the period p1 = 50, processing time t1 = 20


Utilization = 20/50 + 35/100 = .75 utilization of the CPU – can we schedule them??

5.45GMU – CS 571

Priority Assignment (Cont.)

Two processes:



Give P2 higher priority

5.46GMU – CS 571

Priority Assignment (Cont.)

Two processes:



Give P1 higher priority

5.47GMU – CS 571

Rate Monotonic Scheduling (RMS)

A static priority assignment scheme

Assign priorities inversely proportional to the period lengths

Priorities associated with a process remain fixed

RMS is optimal among all static priority assignment schemes: if it is not able to meet all the deadlines of a periodic process set, then no other static priority assignment can do it either.

• This assumes the relative deadlines are equal to the periods!

5.48GMU – CS 571

Rate Monotonic Scheduling (RMS)

The deadlines of a process set with n processes can be always met by RMS,

if Utot ≤ n (21/n - 1)

• For n = 1, the bound is 100%

• For n = 2, the bound is 82.8 %

• For large n, the bound is ln 2 (69.8 %)

5.49GMU – CS 571

Rate Monotonic Scheduling (RMS) When the utilization bound is exceeded, meeting

the deadlines cannot be guaranteed

Consider two processes:



• Utot = 0.94 > 2 (21/2 – 1 )

5.50GMU – CS 571

Earliest Deadline First (EDF)Scheduling

Priorities are assigned according to absolute deadlines: the earlier the absolute deadline, the higher the priority.

Dynamic priority assignment scheme Again, consider two processes:



EDF can achieve 100% CPU utilization while still guaranteeing all the deadlines

cs 471 - lecture 5 cpu scheduling george mason university fall 2009

Documents

process slide

running process

process switches

ready state

waiting state

average waiting time

gmu cs

sequence of cpu