Embedded and Real Time Systems

1. PROCESSES AND OPERATING
SYSTEMS
T.Ramprakash
AP(Sr.Gr)/ECE
Ramco Institute of Technology
Rajapalayam
1

2. Flow of syllabus
• Introduction
• Multiple tasks and multiple processes
• Multirate systems
• Preemptive real-time operating systems
• Priority based scheduling
• Interprocess communication mechanisms
• Evaluating operating system performance
• Power optimization strategies for processes
• Example Real time operating systems
– POSIX
– Windows CE
2

3. Multiple tasks and multiple processes
Process
Multiprogramming
Multitasking
Multiprocessing
Multithreading
3

4. Process
A single execution of a program is called as
Process.
If we run the same program two different times,
we have created two different processes.
Each process has its own state that includes not
only its registers but all of its memory.
In some OSs, the memory management unit is
used to keep each process in a separate address
space.
In others, particularly lightweight RTOSs, the
processes run in the same address space.
Processes that share the same address space are
often called threads 4

5. Multiprogramming
Multiprogramming is also the ability of an
operating system to execute more than one
program on a single processor machine.
More than one task/program/job/process
can reside into the main memory at one point
of time.
A computer running excel and firefox browser
simultaneously is an example of
multiprogramming.
5

6. Memory Layout for Multiprogramming System
6

7. Multitasking
7

8. Multitasking
Multitasking is the ability of an operating
system to execute more than one task
simultaneously on a single processor machine.
Though we say so but in reality no two tasks
on a single processor machine can be executed
at the same time.
Actually CPU switches from one task to the
next task so quickly that appears as if all the
tasks are executing at the same time.
8

9. Multitasking System
9

10. Multiprocessing
Multiprocessing is the ability of an operating
system to execute more than one process
simultaneously on a multi processor machine.
In this, a computer uses more than one CPU at
a time.
10

11. Multithread
Threads are the light wait processes which are independent part of a
process or program
Processes that share the same address space are often called threads
11

12. Multithread
Multithreading is the ability of an operating
system to execute the different parts of a
program called threads at the same time.
Threads are the light wait processes which
are independent part of a process or program.
In multithreading system, more than one
threads are executed parallel on a single CPU.
12

13. Threads vs Process
Thread Process
Thread is a single unit of execution and is
part of process
Process is a program in execution and
contains one or more threads.
A thread does not have its own data
memory and heap memory. It shares the
data memory and heap memory with other
threads of the same process.
Process has its own code memory, data
memory and stack memory
A thread cannot live independently; it lives
within the process
A process contains at least one thread
Threads are very inexpensive to create Processes are very expensive to create.
Involves many OS overhead
Context switching is inexpensive and fast Context switching is complex and involves
lot of OS overhead and is comparatively
slower.
If a thread expires, its stack is reclaimed by
process
If a process dies, the resources allocated to
it are reclaimed by the OS and all the
associated threads of the process also dies
13

14. Tasks and Processes
14

15. Multirate Systems
In multirate systems, certain operation must
be executed periodically and each operation is
executed at its own rate
Ex, Automobile engines, Printers, Cellphones
15

16. Multirate Systems
Timing Requirements on processes
CPU Usage Metrics
Process state and Scheduling
Running Periodic processes
16

17. Timing Requirements on processes
Each process have several different types of
timing requirements
Timing requirements strongly influence the
type of scheduling
Scheduling policy must define the timing
requirements that it uses to determine whether
a schedule is valid
17

18. Timing Requirements on processes
Two important requirements on process :
Initiation Time:
Deadline
Initiation Time:
Process goes from waiting state to ready state
Deadline
It specifies when a computation must be finished
18

19. Timing Requirements on processes
19

20. Timing Requirements on processes
20

21. Sequence of process with a high
initiation rate
Rate Requirement: it specifies how quickly
processes must be initiated
Period: It is the time between successive
executions
21

22. Jitter:
Jitter is the delay between the time when task shall be
started, and the time when the task is being started
Missing a deadline:
Variety of actions can be taken when missing a deadline
22

23. Data Dependencies among process
DAG:
A directed acyclic graph (DAG) is a directed graph that
contains no cycles
A set of processes with data dependencies is known as a
task graph
23

24. Communication among processes at
different rates
24

25. CPU usage metrics
The initiation time is the time at which a process actually
starts executing on the CPU.
The completion time is the time at which the process
finishes its work.
The most basic measure of work is the amount of CPU
time expended by a process.
The CPU time of process i is called Ci .
CPU time is not equal to the completion time minus
initiation time; several other processes may interrupt
execution. 25

26. CPU usage metrics
The simplest and most direct measure is
Utilization:
Utilization is the ratio of the CPU time that is
being used for useful computations to the total
available CPU time.
26

27. CPU usage metrics
• This ratio ranges between 0 and 1, with 1
meaning that all of the available CPU time is
being used for system purposes.
• The utilization is often expressed as a
percentage.
• If we measure the total execution time of all
processes over an interval of time t, then the
CPU utilization is
27

28. Process State and Scheduling
• The work of choosing the order of running
processes is known as scheduling
• Scheduling States
• Waiting
• Ready
• Executing
28

29. Running Periodic Process
While Loop
Multiple Timers
29

30. Pre-emptive real-time operating systems
• Preemptive real time operation system solves
the fundamental problems of cooperative
multitasking system
• A RTOS executes processes based upon timing
constraints provided by the system designer.
• The most reliable way to meet timing
constraints accurately is to build a preemptive
OS and to use priorities to control what
process runs at any given time
30

31. Preemptive real-time operating systems
• Two Important Methods
– Preemption
– Priorities
• Process and Context
• Processes and Object Oriented Design
31

32. Preemption
• Pre-emption is an alternative to the C function
call as a way to control execution
• Creating new routines that allow us to jump
from one subroutine to another at any point
in the program
32

33. Pre-emption
• The kernel is the part of the OS that
determines what process is running
Length of the timer
period is known as
Time Quantum
33

34. Context Switching
• The set of registers that defines a process is known
as context
• The switching from one process’s register set to
another is known as context switching
• The data structure that holds the state of process is
known as record
34

35. Process Priorities
• Each process is assigned with the numerical priority
• Kernel simply look at the processes and their
priorities and select the highest priority process that
is ready to run
35

36. Process and Context
• A process is known as FreeRTOS.org as a task
• Lets assume that , everything has been initialized,
the operating system is running and we are ready for
a timer interrupt
36

37. Process and Context
37

38. vPreemptiveTick
38

39. portSAVE_CONTEXT
39

40. Process and Object oriented design
• UML often refers to processes as active objects, that
is, objects that have independent threads of control.
• The class that defines an active object is known as an
active class.
• It has all the normal characteristics of a class,
including a name, attributes and operations.
• It also provides a set of signals that can be used to
communicate with the process.
40

41. Process and Object oriented design
• It is a simple collaboration diagram in which an
object is used as an interface between two processes
41

42. Priority Based Scheduling
• Round-Robin Scheduling
• Process Priorities
• Rate Monotonic Scheduling
• Earliest Deadline first scheduling
• Shared Resources
• Priority Inversion
42

43. Round-Robin Scheduling
• Round Robin is the pre-emptive process
scheduling algorithm.
• Each process is provided a fix time to execute,
it is called a quantum.
• Once a process is executed for a given time
period, it is preempted and other process
executes for a given time period.
• Context switching is used to save states of
preempted processes.
43

44. Round-Robin Scheduling
44

45. Round-Robin Scheduling
45

46. Process Priorities
• Priority scheduling is a non-preemptive
algorithm and one of the most common
scheduling algorithms in batch systems.
• Each process is assigned a priority. Process
with highest priority is to be executed first and
so on.
• Processes with same priority are executed on
first come first served basis.
• Priority can be decided based on memory
requirements, time requirements or any other
resource requirement. 46

47. Process Priorities
47

48. Rate Monotonic scheduling
• Rate-monotonic scheduling (RMS), introduced
by Liu and Layland
• It is one of the first scheduling policies
developed for real-time systems and is still
very widely used
• Rate Monotonic Scheduling (RMS) assigns task
priorities in the order of the highest task
frequencies, i.e. the shortest periodic task
gets the highest priority, then the next with
the shortest period get the second highest
priority, and so on.
48

49. Rate Monotonic scheduling
• This model uses a relatively simple model of
the system
– All processes run periodically on a single CPU.
– Context switching time is ignored.
– There are no data dependencies between
processes.
– The execution time for a process is constant.
– All deadlines are at the ends of their periods.
– The highest-priority ready process is always
selected for execution.
49

50. Rate Monotonic scheduling
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51. Rate Monotonic scheduling
• The fraction is the fraction of time that
the CPU spends executing task i.
• It is possible to show that for a set of two
tasks under RMS scheduling, the CPU
utilization U will be no greater than
2(21/2 - 1) ∼ 0.83
• In other words, the CPU will be idle at least
17% of the time 51

52. Rate Monotonic scheduling example1
Process Execution Time Period
P1 1 3
P2 1 4
P3 2 5
Schedule the process given below using Earliest Deadline First(EDF)
scheduling policy. Compute the schedule for an interval equal to the least
common multiple of the process. Assume the time starts at t=0.
52

53. Rate Monotonic scheduling example2
Process Execution Time Period
P1 2 30
P2 4 40
P3 7 120
P4 5 60
P5 1 15
Schedule the process given below using Earliest Deadline First(EDF)
scheduling policy and Rate Monotonic Scheduling
53

54. Earliest Dead line first scheduling
• Earliest Deadline First (EDF) is a dynamic
priority algorithm
• The priority of a job is inversely proportional
to its absolute deadline;
• In other words, the highest priority job is the
one with the earliest deadline;
54

55. Earliest Dead line first scheduling
• Example
Execution Time Period
T1 1 4
T2 2 6
T3 3 8
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56. Earliest Dead line first scheduling
• Observe that at time 6, even if the deadline of task
3 is very close, the scheduler decides to schedule
task 2.
• This is the main reason why T3 misses its deadline
Execution Time Period
T1 1 4
T2 2 6
T3 3 8
56

57. Earliest Dead line first scheduling
• Observe that at time 6, the problem does not
appear, as the earliest deadline job is
executed.
57

58. Shared Resources
• While dealing with shared resources, special
care must be taken
•Race Condition
•Critical Sections
•Semaphores
58

59. Shared Resources
• Race Condition
– Consider the case in which an I/O device has a flag
that must be tested and modified by a process
– Problems may arise when other processes may
also want to access the device
– If combinations of events from the two task
operate on the device in the wrong order, we may
create a critical timing race or race condition
59

60. Shared Resources
• Critical Sections
– To prevent the race condition problems, we need to
control the order in which some operations occur
– We need to be sure that a task finishes an I/O
operations before allowing another task to starts its
own operation on that I/O device
– This is achieved by enclosing sensitive sections of code
in a critical section that executes without
interruption
60

61. Shared Resources
• Semaphores
– We create a critical section using semaphores,
which are primitive provided by the OS
– The semaphore is used to guard a resource
– we start a critical section by calling a semaphore
function that does no return until the resource is
available
– When we are done with the resource we use
another semaphore function to release it
P(); //wait for semaphore
//do protected work here
V(); //release semaphore
61

62. Priority Inversion
• A low priority process blocks execution of a
higher priority process by keeping hold of its
resource. This is Priority Inversion.
• This priority inversion is dealt with Priority
Inheritance
• In priority inheritance,
– Promotes the priority of the process temporarily
– The priority of the process becomes higher than that
of any other process that may use the resource.
– Once the process is finished with the resource, its
priority is demoted to its normal value.
62

63. INTERPROCESS COMMUNICATION MECHANISMS
• Inter-process communication mechanisms are
provided by the operating system as part of
the process abstraction.
• Two ways of communication
– Blocking Communication
• The process goes into waiting state until it receives a
response
– Non Blocking Communication
• It allows the process to continue execution after
sending the communication
63

64. INTERPROCESS COMMUNICATION MECHANISMS
• Four major styles of inter-process communication
– Shared Memory
– Message passing
– Signals
– Mailboxes
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65. Shared Memory
• CPU and I/O device communicate through a
shared memory location
65

66. Message passing
• Each communicating entity has its own
message send/receive unit
• The message is stored in the senders/receivers
endpoints
66

67. Message passing
• For example, a home control system has one
microcontroller per household device – lamp,
fan, and so on.
• The device must communicate relatively
infrequently
• Their physical separation is large enough that
we would not naturally have a sharing a
central pool of memory
• Passing communication packets among the
device is a natural implementation of
communication in many 8 bit controllers
67

68. Signals
• Another form of inter-process communication
commonly used in Unix is Signal
• A signal is analogous to an interrupt, but it is
entirely a software creation
• A signal is generated by a process and
transmitted to another process by Operating
System
68

69. Mailboxes
• It is a asynchronous communication
• Mailboxes have a fixed number of bits and can
be used for small messages
• We can also implement a mailbox using P()
and V() using main memory for the mailbox
storage
• Mail box should contain two items:
– Message
– Mail ready Flag
69

70. Mailboxes
void post(message *msg)
{
P(mailbox.sem); //wait for the mailbox
copy(mailbox.data.msg);
mailbox.flag =TRUE;
V(mailbox.sem) //release the mailbox
}
70

71. Mailboxes
Boolean pickup(message *msg)
{
boolean pickup =False
P(mailbox.sem); //wait for the mailbox
pickup=mailbox.flag;
mailbox.flag =FALSE;
copy(msg.mailbox.data);
V(mailbox.sem) //release the mailbox
return (pickup)
}
71

72. Evaluating Operating System Performance
Assumption
 Context switches requires zero time
 Ignored interrupts
 Execution time of process is constant
 Ignored cache conflicts
72

73. Evaluating Operating System Performance
Context Switching Time
Interrupt Latency
Critical Section and interrupt latency
Interrupt priorities and interrupt latency
RTOS performance evaluation tools
Cache and RTOS performance
73

74. Power optimization strategies for processes
• The RTOS and system architecture can use
static and dynamic power management
mechanism
• A power management policy is a strategy for
determining when to perform certain power
management operations
• It examines the state of the system to
determine when to take actions
74

75. Power optimization strategies for processes
• Power down trade offs
• Predictive power management
• Advanced Configuration and Power Interface
75

76. Power down trade offs
• Going in to low power mode takes time
• The more that is shut off, the longer the delay
incurred during restart
– Avoiding a power down mode can cost
unnecessary power
– Powering down too soon can cause severe
performance penalties
• The best method is to power up the system
when a request is received. This works as long
as the delay in handling the request is
acceptable.
76

77. Predictive Power Management
• Here, we predict when the next request will be made and
to start the system just before that time, saving the
requestor the startup time
• We guess about the activity patterns based on a
probabilistic model of expected behavior
• Because they relay on statistics, they may not always
correctly guess the time of next activity
• They can cause two types of problems
• The requestor may have to wait for an activity period
• The system may restart itself when no activity is imminent
77

78. Predictive Power Management
• A simple predictive technique is to use fixed
times
• For example, if a system does not receive inputs
during an interval of length TON, it shuts down
• A powered down system waits for a period TOFF
before returning to the power on mode
• The choice of TON and TOFF must be determined
by experimentations
78

79. L shaped distribution
• Srivastava and Eustace found one a graphic
terminal in which they plotted the observed
idle time (TOFF) of a graphics terminal versus
the immediately preceding active time (TON)
The idle period after a long active period is
usually very short and
the length of the idle period after a short active
period is uniformly distributed
79

80. Architecture of Power managed System
Service provider whose power is being managed
Service Requestor  making request of the power managed system
Queue  hold pending requests
Power manager  sends power management commands
Service Provider
Queue
Service Requestor
Power Manager
Request
80

81. Advanced Configuration and Power
Interface (ACPI)
• The Advanced Configuration and Power Interface
(ACPI) is an open industry standard for power
management services.
• It is designed to be compatible with a wide
variety of OSs.
• It was targeted initially to PCs.
• The OS has its own power management module
that determines the policy
• Then OS uses ACPI to send the required controls
to the hardware and to observe the hardware’s
state as input to the power manager.
81

82. Advanced Configuration and Power
Interface (ACPI)
82

83. (ACPI)
• ACPI supports the following five basic global power
states:
– G3, the mechanical off state, in which the system
consumes no power.
– G2, the soft off state, which requires a full OS reboot to
restore the machine to working condition. This state has
four sub states:
• S1, a low wake-up latency state with no loss of system context;
• S2, a low wake-up latency state with a loss of CPU and system
cache state;
• S3, a low wake-up latency state in which all system state except
for main memory is lost; and
• S4, the lowest-power sleeping state, in which all devices are
turned off.
– G1, the sleeping state, in which the system appears to be
off and the time required to return to working condition is
inversely proportional to power consumption.
– G0, the working state, in which the system is fully usable.83

84. Example of RTOS
• POSIX
• Windows CE
84

85. POSIX
• Portable Operating System Interface
• POSIX.1 – Core services
• POSIX.1b – Real-time extensions
• POSIX.1c – Thread extensions
85

86. POSIX
• POSIX is a version of Unix Operating system
• It is created by a standards organization
• POSIX-complaint operation system are source
code compatible
– (i.e) An application can be complied and run without
modification on a new POSIX
• Many RTOS are POSIX compliant and it serves as
a good model for basic RTOS techniques
86

87. POSIX
• Two methods have been proposed to improve
interrupt latency
– Dual Kernel
• co-kernel for real time process and
• Standard kernel for non real time processes
– PREEMP_RT mode
• It provides priority inheritance to reduce the latency of
many kernel operations
87

88. POSIX
88

89. Processes in POSIX
• In POSIX, a new process is created by making a
copy of an existing process
• The copying process creates two different
processes both running the same code
• The complication comes in ensuring that one
process runs the code intended for the new
process while the other process continues the
work of the old process
89

90. Processes in POSIX
• A process makes a copy of itself by calling
fork() function
• It creates a new child process which is exact
copy of parent process
• The both have the same code and the same
data values with one exceptions return
value
– Parent Process: returns the process ID of the child
process
– Child process: returns 0
90

91. POSIX
fork()
childid = fork();
if (childid == 0)
{
/* Do the child process*/
}
91

92. Processes in POSIX
• It would be clumsy to have both processes have
all the code for both parent and child processes
• POSIX provides the exec facility for overloading
the code in a process
• It takes as argument the name of the file that
holds the child’s code and the array of arguments
92

93. POSIX
• POSIX supports
– Semaphores
– Shared memory mechanism
– Message Queues
95

94. POSIX semaphores
• POSIX supports counting semaphores in the
_POSIX_SEMAPHORES option
• A Counting semaphore allows more than one
process to access a resource at a time
• If a semaphore allows up to to N resources,
then it will not block until N process have
simultaneously passed the semaphore
96

95. POSIX semaphores
• Names for the semaphore start with “/”
• sem_open() – To create a semaphore
• sem_close () – To destroy a semaphore
• sem_wait() – getting a semaphore
• sem_post() – releasing a semaphore
97

96. POSIX Shared Memory
• Shared memory functions create blocks of
memory that can be used by several processes
• shm_open()
• close()
• mmap()
• munmap()
98

97. POSIX Message Queues
• POSIX supports message queues
• No need to create a queue before creating a process
• mq_open() – to create named queue
• mq_close() – to destroy named queue
• mq_send() – to transmit a message
• mq_receive() – to receive a message
• mq_maxmsg() –Maximum number of messages
• mq_msgsize() – Maximum size of a message
99

98. Windows CE
• Windows CE supports devices such as
– smartphones,
– electronic instruction, etc.,
• Windows CE is designed to run on
– multiple hardware platforms and
– instruction set architectures
100

99. Windows CE Architecture
• Win32 API
– manage access to the operation system
101

100. Windows CE Architecture
• OEM Adaption Layer (OAL)
– provides an interface to the hardware
(OEM  Original Equipment Manufacturer)
102

101. Windows CE memory space
• Windows CE provides support for virtual
memory with a flat 32 bit virtual address
space
• Memory space is divided into kernel and user
space
103

102. Windows CE memory space
• User space is divided into
– System elements and
– User elements
104

103. Windows CE threads and drivers
• Windows CE supports two kernel-level units of
execution
• Thread
– Threads are defined by executable files
– A process can run multiple threads
– All the threads of a process share the same execution
environment
– Threads in different processes run in different
execution environment
– Threads are scheduled directly by the OS
• Driver
– Drivers are defined by dynamically linked libraries
(DLL)
– A driver may be loaded in to the OS or a process
– Drivers can create threads to handle interrupts 105

104. Windows CE Scheduling
• Each thread is assigned an integer priority
• Lower valued priorities have highest priority
• 0 is the highest priority and 255 is the lowest
• Task may be scheduled using either of two
policies
– A thread can run until the end of its quantum (or)
– A thread can run until a higher priority thread is
ready to run
106

105. Windows CE Interrupts
• The Interrupt Service Handler (ISH) is a kernel
service that provides the first response to the
interrupt
• The ISH selects an Interrupt Service Routine
(ISR) to handle the interrupt
• The ISH runs in the kernel with interrupts
turned off
• The ISR in turn calls An Interrupt Service
Thread (IST) which perform most of the work
required to handle the interrupt
107

106. Windows CE
108

107. Reference
1. Marilyn Wolf, “Computers as Components -
Principles of Embedded Computing System
Design”, Third Edition, Morgan Kaufmann
Publisher (An imprint from Elsevier), 2012.
2. Wayne Wolf, “Computers as Components -
Principles of Embedded Computer System
Design”, Morgan Kaufmann, 2nd Edition,
2008.
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