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5.2 Silberschatz, Galvin and Gagne ©2005
Operating System Concepts – 7th Edition, Feb 2, 2005
Chapter 5: CPU Scheduling
 Basic Concepts
 Scheduling Criteria
 Scheduling Algorithms
 Multiple-Processor Scheduling
 Real-Time Scheduling
 Thread Scheduling
 Operating Systems Examples
 Java Thread Scheduling
 Algorithm Evaluation

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

Alternating Sequence of CPU And I/O Bursts

Histogram of CPU-burst Times

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

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
 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

Optimization Criteria
 Max CPU utilization
 Max throughput
 Min turnaround time
 Min waiting time
 Min response time

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 is:
 Waiting time for P1 = 0; P2 = 24; P3 = 27
 Average waiting time: (0 + 24 + 27)/3 = 17
P1 P2 P3
24 27 30
0

FCFS Scheduling (Cont.)
Suppose that the processes arrive in the order
P2 , P3 , P1
 The Gantt chart for the schedule is:
 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
P1
P3
P2
6
3 30
0

Shortest-Job-First (SJF) 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

Process Arrival Time Burst Time
P1 0.0 7
P2 2.0 4
P3 4.0 1
P4 5.0 4
 SJF (non-preemptive)
 Average waiting time = (0 + 6 + 3 + 7)/4 = 4
Example of Non-Preemptive SJF
P1 P3 P2
7
3 16
0
P4
8 12

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)
 Average waiting time = (9 + 1 + 0 +2)/4 = 3
P1 P3
P2
4
2 11
0
P4
5 7
P2 P1
16

Determining Length of Next CPU Burst
 Can only estimate the length
 Can be done by using the length of previous CPU bursts, using
exponential averaging
:
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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
 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 (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

Example of RR with Time Quantum = 20
Process Burst Time
P1 53
P2 17
P3 68
P4 24
 The Gantt chart is:
 Typically, higher average turnaround than SJF, but better response
P1 P2 P3 P4 P1 P3 P4 P1 P3 P3
0 20 37 57 77 97 117 121 134 154 162

Time Quantum and Context Switch Time

Turnaround Time Varies With The Time Quantum

Multilevel Queue
 Ready queue is partitioned into separate queues:
foreground (interactive)
background (batch)
 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.

Multilevel Feedback Queues

Multiple-Processor Scheduling
 CPU scheduling more complex when multiple CPUs are
available
 Homogeneous processors within a multiprocessor
 Load sharing
 Asymmetric multiprocessing – only one processor
accesses the system data structures, alleviating the need
for data sharing

Real-Time Scheduling
 Hard real-time systems – required to complete a
critical task within a guaranteed amount of time
 Soft real-time computing – requires that critical
processes receive priority over less fortunate ones

Thread Scheduling
 Local Scheduling – How the threads library decides which
thread to put onto an available LWP
 Global Scheduling – How the kernel decides which kernel
thread to run next

Pthread Scheduling API
#include <pthread.h>
#include <stdio.h>
#define NUM THREADS 5
int main(int argc, char *argv[])
{
int i;
pthread t tid[NUM THREADS];
pthread attr t attr;
/* get the default attributes */
pthread attr init(&attr);
/* set the scheduling algorithm to PROCESS or SYSTEM */
pthread attr setscope(&attr, PTHREAD SCOPE SYSTEM);
/* set the scheduling policy - FIFO, RT, or OTHER */
pthread attr setschedpolicy(&attr, SCHED OTHER);
/* create the threads */
for (i = 0; i < NUM THREADS; i++)
pthread create(&tid[i],&attr,runner,NULL);

Pthread Scheduling API
/* now join on each thread */
for (i = 0; i < NUM THREADS; i++)
pthread join(tid[i], NULL);
}
/* Each thread will begin control in this function */
void *runner(void *param)
{
printf("I am a threadn");
pthread exit(0);
}

Operating System Examples
 Solaris scheduling
 Windows XP scheduling
 Linux scheduling

Solaris 2 Scheduling

Solaris Dispatch Table

Windows XP Priorities

Linux Scheduling
 Two algorithms: time-sharing and real-time
 Time-sharing
 Prioritized credit-based – process with most credits is
scheduled next
 Credit subtracted when timer interrupt occurs
 When credit = 0, another process chosen
 When all processes have credit = 0, recrediting occurs
 Based on factors including priority and history
 Real-time
 Soft real-time
 Posix.1b compliant – two classes
 FCFS and RR
 Highest priority process always runs first

The Relationship Between Priorities and Time-slice length

List of Tasks Indexed According to Prorities

Algorithm Evaluation
 Deterministic modeling – takes a particular
predetermined workload and defines the performance of
each algorithm for that workload
 Queueing models
 Implementation

5.15

Java Thread Scheduling
 JVM Uses a Preemptive, Priority-Based Scheduling Algorithm
 FIFO Queue is Used if There Are Multiple Threads With the Same
Priority

Java Thread Scheduling (cont)
JVM Schedules a Thread to Run When:
1. The Currently Running Thread Exits the Runnable State
2. A Higher Priority Thread Enters the Runnable State
* Note – the JVM Does Not Specify Whether Threads are Time-Sliced
or Not

Time-Slicing
Since the JVM Doesn’t Ensure Time-Slicing, the yield() Method
May Be Used:
while (true) {
// perform CPU-intensive task
. . .
Thread.yield();
}
This Yields Control to Another Thread of Equal Priority

Thread Priorities
Priority Comment
Thread.MIN_PRIORITY Minimum Thread Priority
Thread.MAX_PRIORITY Maximum Thread Priority
Thread.NORM_PRIORITY Default Thread Priority
Priorities May Be Set Using setPriority() method:
setPriority(Thread.NORM_PRIORITY + 2);

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