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Silberschatz, Galvin and Gagne ©2013
Operating System Concepts – 9th Edition
Chapter 5: Process
Scheduling

6.2 Silberschatz, Galvin and Gagne ©2013
Chapter 5: Process Scheduling
Basic Concepts
Scheduling Criteria
Scheduling Algorithms
Thread Scheduling
Multiple-Processor Scheduling
Real-Time CPU Scheduling
Operating Systems Examples
Algorithm Evaluation

Objectives
To introduce CPU scheduling, which is the basis for multiprogrammed operating
systems
To describe various CPU-scheduling algorithms
To discuss evaluation criteria for selecting a CPU-scheduling algorithm for a
particular system
To examine the scheduling algorithms of several operating systems

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 followed by I/O burst
CPU burst distribution is of main concern

Histogram of CPU-burst Times
Typically large number of short CPU bursts and a small number of long CPU bursts
An I/O-bound program typically has many short CPU bursts.
A CPU-bound program might have a few long CPU bursts.

CPU Scheduler
Short-term 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
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
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
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)

Scheduling Algorithm 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
Consider one CPU-bound and many I/O-bound processes
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
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

Example of SJF
ProcessArr l Time Burst Time
P1 0.0 6
P2 2.0 8
P3 4.0 7
P4 5.0 3
SJF scheduling chart
Average waiting time =
(3 + 16 + 9 + 0) / 4 = 7
P4
P3
P1
3 16
0 9
P2
24

Determining Length of Next CPU Burst
Can only estimate the length – should be similar to the previous one
Then pick process with shortest predicted next CPU burst
Can be done by using the length of previous CPU bursts, using exponential
averaging
Commonly, α set to ½
Preemptive version called shortest-remaining-time-first
:
Define
4.
1
0
,
3.
burst
CPU
next
the
for
value
predicted
2.
burst
CPU
of
length
actual
1.
≤
≤
=
=
+
α
α
τ 1
n
th
n n
t
( ) .
1
1 n
n
n
t τ
α
α
τ −
+
=
=

Prediction of the Length of the
Next CPU Burst

Examples of Exponential Averaging
α =0
τn+1 = τn
Recent history does not count
α =1
τn+1 = α tn
Only the actual last CPU burst counts
If we expand the formula, we get:
τn+1 = α tn+(1 - α)α tn -1 + …
+(1 - α )j α tn -j + …
+(1 - α )n +1 τ0
Since both α and (1 - α) are less than or equal to 1, each successive term has
less weight than its predecessor

Example of Shortest-remaining-time-first
Now we add the concepts of varying arrival times and preemption to the analysis
ProcessA arri Arrival TimeT Burst Time
P1 0 8
P2 1 4
P3 2 9
P4 3 5
Preemptive SJF Gantt Chart
Average waiting time = [(10-1)+(1-1)+(17-2)+5-3)]/4 = 26/4 = 6.5 msec
P1
P1
P2
1 17
0 10
P3
26
5
P4

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 priority scheduling where 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

Example of Priority Scheduling
ProcessA arri Burst TimeT Priority
P1 10 3
P2 1 1
P3 2 4
P4 1 5
P5 5 2
Priority scheduling Gantt Chart
Average waiting time =
8.2 msec
P2 P3
P5
1 18
0 16
P4
19
6
P1

Round Robin (RR)
Each process gets a small unit of CPU time (time quantum q), 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.
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

Example of RR with Time Quantum = 4
Process Burst Time
P1 24
P2 3
P3 3
The Gantt chart is:
Typically, higher average turnaround than SJF, but better response
q should be large compared to context switch time
q usually 10ms to 100ms, context switch 10 usec
P1 P2 P3 P1 P1 P1 P1 P1
0 4 7 10 14 18 22 26 30

Time Quantum and Context Switch Time

Turnaround Time Varies With
The Time Quantum
80% of CPU bursts should
be shorter than q

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

Multiple-Processor Scheduling
CPU scheduling more complex when multiple CPUs are available
Homogeneous processors within a multiprocessor
Asymmetric multiprocessing – only one processor accesses the system data
structures, alleviating the need for data sharing
Symmetric multiprocessing (SMP) – each processor is self-scheduling, all
processes in common ready queue, or each has its own private queue of ready
processes
Currently, most common
Processor affinity – process has affinity for processor on which it is currently running
soft affinity
hard affinity
Variations including processor sets

NUMA and CPU Scheduling
Note that memory-placement algorithms can also consider affinity

Multiple-Processor Scheduling – Load Balancing
If SMP, need to keep all CPUs loaded for efficiency
Load balancing attempts to keep workload evenly distributed
Push migration – periodic task checks load on each processor,
and if found pushes task from overloaded CPU to other CPUs
Pull migration – idle processors pulls waiting task from busy
processor

Multicore Processors
Recent trend to place multiple processor cores on same physical
chip
Faster and consumes less power
Multiple threads per core also growing
Takes advantage of memory stall to make progress on another
thread while memory retrieve happens

Multithreaded Multicore System

Real-Time CPU Scheduling
Can present obvious challenges
Soft real-time systems – no
guarantee as to when critical real-time
process will be scheduled
Hard real-time systems – task must
be serviced by its deadline
Two types of latencies affect
performance
1. Interrupt latency – time from arrival
of interrupt to start of routine that
services interrupt
2. Dispatch latency – time for
schedule to take current process
off CPU and switch to another

Real-Time CPU Scheduling (Cont.)
Conflict phase of dispatch
latency:
1. Preemption of any
process running in
kernel mode
2. Release by low-priority
process of resources
needed by high-priority
processes

Priority-based Scheduling
For real-time scheduling, scheduler must support preemptive, priority-based
scheduling
But only guarantees soft real-time
For hard real-time must also provide ability to meet deadlines
Processes have new characteristics: periodic ones require CPU at constant intervals
Has processing time t, deadline d, period p
0 ≤ t ≤ d ≤ p
Rate of periodic task is 1/p

Rate Monotonic Scheduling
A priority is assigned based on the inverse of its period
Shorter periods = higher priority;
Longer periods = lower priority
P1 is assigned a higher priority than P2.

Missed Deadlines with
Rate Monotonic Scheduling

Earliest Deadline First Scheduling (EDF)
Priorities are assigned according to deadlines:
the earlier the deadline, the higher the priority;
the later the deadline, the lower the priority

Operating System Examples
Linux scheduling
Windows scheduling
Read 6.7.1 and 6.7.2

Linux Scheduling Through Version 2.5
Prior to kernel version 2.5, ran variation of standard UNIX scheduling algorithm
Version 2.5 moved to constant order O(1) scheduling time
Preemptive, priority based
Two priority ranges: time-sharing and real-time
Real-time range from 0 to 99 and nice value from 100 to 140
Map into global priority with numerically lower values indicating higher priority
Higher priority gets larger q
Task run-able as long as time left in time slice (active)
If no time left (expired), not run-able until all other tasks use their slices
All run-able tasks tracked in per-CPU runqueue data structure
Two priority arrays (active, expired)
Tasks indexed by priority
When no more active, arrays are exchanged
Worked well, but poor response times for interactive processes

Linux Scheduling (Cont.)
Real-time scheduling according to POSIX.1b
Real-time tasks have static priorities
Real-time plus normal map into global priority scheme
Nice value of -20 maps to global priority 100
Nice value of +19 maps to priority 139

Linux Scheduling in Version 2.6.23 +
Completely Fair Scheduler (CFS)
Scheduling classes
Each has specific priority
Scheduler picks highest priority task in highest scheduling class
Rather than quantum based on fixed time allotments, based on proportion of CPU
time
2 scheduling classes included, others can be added
1. default
2. real-time
Quantum calculated based on nice value from -20 to +19
Lower value is higher priority
Calculates target latency – interval of time during which task should run at least once
Target latency can increase if say number of active tasks increases
CFS scheduler maintains per task virtual run time in variable vruntime
Associated with decay factor based on priority of task – lower priority is higher decay
rate
Normal default priority yields virtual run time = actual run time
To decide next task to run, scheduler picks task with lowest virtual run time

CFS Performance

Windows Scheduling
Windows uses priority-based preemptive scheduling
Highest-priority thread runs next
Dispatcher is scheduler
Thread runs until (1) blocks, (2) uses time slice, (3) preempted by higher-priority
thread
Real-time threads can preempt non-real-time
32-level priority scheme
Variable class is 1-15, real-time class is 16-31
Priority 0 is memory-management thread
Queue for each priority
If no run-able thread, runs idle thread

Windows Priority Classes
Win32 API identifies several priority classes to which a process can belong
REALTIME_PRIORITY_CLASS, HIGH_PRIORITY_CLASS,
ABOVE_NORMAL_PRIORITY_CLASS,NORMAL_PRIORITY_CLASS,
BELOW_NORMAL_PRIORITY_CLASS, IDLE_PRIORITY_CLASS
All are variable except REALTIME
A thread within a given priority class has a relative priority
TIME_CRITICAL, HIGHEST, ABOVE_NORMAL, NORMAL, BELOW_NORMAL, LOWEST, IDLE
Priority class and relative priority combine to give numeric priority
Base priority is NORMAL within the class
If quantum expires, priority lowered, but never below base
If wait occurs, priority boosted depending on what was waited for
Foreground window given 3x priority boost
Windows 7 added user-mode scheduling (UMS)
Applications create and manage threads independent of kernel
For large number of threads, much more efficient
UMS schedulers come from programming language libraries like C++ Concurrent Runtime
(ConcRT) framework

Windows Priorities

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