Chapter 3: Processes
Operating System Concepts – 10th Edition Silberschatz, Galvin and Gagne ©2018
Outline
Process Concept
Process Scheduling
Operations on Processes
Interprocess Communication
IPC in Shared-Memory Systems
IPC in Message-Passing Systems
Operating System Concepts – 10th Edition 3.2 Silberschatz, Galvin and Gagne ©2018
Process Concept
An operating system executes a variety of programs that
run as a process.
Process – a program in execution;
• process execution must progress in sequential fashion.
• No parallel execution of instructions of a single process
• The status of the current activity of a process is
represented by the value of the program counter and
the contents of the processor’s registers.
The memory layout of a Process
• The program code, also called text section
• Data section containing global variables
• Stack containing temporary data
Function parameters, return addresses, local
variables
• Heap containing memory dynamically allocated during
run time
Operating System Concepts – 10th Edition 3.3 Silberschatz, Galvin and Gagne ©2018
Process Concept (Cont.)
Program is passive entity stored on disk (executable file); process is
active
• Program becomes process when an executable file is loaded into
memory
Execution of program started via GUI mouse clicks, command line
entry of its name, etc.
One program can be several processes
• Consider multiple users executing the same program
Several users may be running different copies of the mail
program,
The same user may invoke many copies of the web browser
program.
• Each of these is a separate process; and although the text
sections are equivalent, the data, heap, and stack sections vary.
Operating System Concepts – 10th Edition 3.4 Silberschatz, Galvin and Gagne ©2018
Memory Layout of a C Program
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Process State
As a process executes, it changes state
• New: The process is being created
• Ready: The process is waiting to be assigned to a
processor
• Running: Instructions are being executed
• Waiting: The process is waiting for some event to occur
• Terminated: The process has finished execution
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Process Control Block (PCB)
Information associated with each process(also
called task control block)
Process state – running, waiting, etc.
Program counter – location of instruction to
next execute
CPU registers – contents of all process-
centric registers
CPU scheduling information- priorities,
scheduling queue pointers
Memory-management information – memory
allocated to the process
Accounting information – CPU used, clock
time elapsed since start, time limits
I/O status information – I/O devices allocated
to process, list of open files
Operating System Concepts – 10th Edition 3.7 Silberschatz, Galvin and Gagne ©2018
Threads
So far, process has a single thread of execution
Consider having multiple program counters per process
• Multiple locations can execute at once
Multiple threads of control -> threads
Must then have storage for thread details, multiple
program counters in PCB
This feature is especially beneficial on multicore systems,
where multiple threads can run in parallel.
Explore in detail in Chapter 4
Operating System Concepts – 10th Edition 3.8 Silberschatz, Galvin and Gagne ©2018
Process Scheduling
Process scheduler selects among available processes for next execution on
CPU core
Goal -- Maximize CPU use, quickly switch processes onto CPU core
The degree of multiprogramming: The number of processes currently in
memory.
Maintains scheduling queues of processes
• Ready queue – set of all processes residing in main memory, ready and
waiting to execute
• Wait queues – set of processes waiting for an event (i.e., I/O)
• Processes migrate among the various queues
Operating System Concepts – 10th Edition 3.9 Silberschatz, Galvin and Gagne ©2018
Representation of Process Scheduling
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Context Switch
When CPU switches to another process, the system must
save the state of the old process and load the saved
state for the new process via a context switch
Context of a process represented in the PCB
Context-switch time is pure overhead; the system does
no useful work while switching
• The more complex the OS and the PCB the longer
the context switch
Time dependent on hardware support
• Some hardware provides multiple sets of registers per
CPU multiple contexts loaded at once
Operating System Concepts – 10th Edition 3.11 Silberschatz, Galvin and Gagne ©2018
CPU Switch From Process to Process
A context switch occurs when the CPU switches from one
process to another.
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Multitasking in Mobile Systems
Some mobile systems (e.g., early version of iOS) allow only one
process to run, others suspended
Due to screen real estate, user interface limits iOS provides for a
• Single foreground process- controlled via user interface
• Multiple background processes– in memory, running, but not
on the display, and with limits
• Limits include single, short task, receiving notification of
events, specific long-running tasks like audio playback
Android runs foreground and background, with fewer limits
• Background process uses a service to perform tasks
• Service can keep running even if background process is
suspended
• Service has no user interface, small memory use
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Operations on Processes
System must provide mechanisms for:
• Process creation
• Process termination
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Process Creation
Parent process create children processes, which, in turn create other
processes, forming a tree of processes
Generally, process identified and managed via a process identifier (pid)
Resource sharing options
• Parent and children share all resources
• Children share subset of parent’s resources
• Parent and child share no resources
Execution options
• Parent and children execute concurrently
• Parent waits until children terminate
Address space
• Child duplicate of parent
• Child has a program loaded into it
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A Tree of Processes in Linux
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Process Creation (Cont.)
UNIX examples
• fork() system call creates new process
• exec() system call used after a fork() to replace the
process’ memory space with a new program
• Parent process calls wait()waiting for the child to
terminate
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C Program Forking Separate Process
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Creating a Separate Process via Windows API
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Process Termination
Process executes last statement and then asks the operating
system to delete it using the exit() system call.
• Returns status data from child to parent (via wait())
• Process’ resources are deallocated by operating system
Parent may terminate the execution of children processes using
the abort() system call. Some reasons for doing so:
• Child has exceeded allocated resources
• Task assigned to child is no longer required
• The parent is exiting, and the operating systems does not
allow a child to continue if its parent terminates
Operating System Concepts – 10th Edition 3.20 Silberschatz, Galvin and Gagne ©2018
Process Termination
Some operating systems do not allow child to exists if its parent
has terminated. If a process terminates, then all its children
must also be terminated.
• cascading termination. All children, grandchildren, etc.,
are terminated.
• The termination is initiated by the operating system.
The parent process may wait for termination of a child process
by using the wait()system call. The call returns status
information and the pid of the terminated process
pid = wait(&status);
If no parent waiting (did not invoke wait()) process is a
zombie
If parent terminated without invoking wait(), process is an
orphan
Operating System Concepts – 10th Edition 3.21 Silberschatz, Galvin and Gagne ©2018
Android Process Importance Hierarchy
Mobile operating systems often have to terminate processes to
reclaim system resources such as memory. From most to least
important:
• Foreground process
• Visible process
• Service process
• Background process
• Empty process
Android will begin terminating processes that are least
important.
Operating System Concepts – 10th Edition 3.22 Silberschatz, Galvin and Gagne ©2018
Multiprocessor Architecture – Chrome Browser
Many web browsers ran as single process (some still do)
• If one web site causes trouble, entire browser can hang or crash
Google Chrome Browser is multiprocess with 3 different types of
processes:
• Browser process manages user interface, disk and network I/O
• Renderer process renders web pages, deals with HTML,
Javascript. A new renderer created for each website opened
Runs in sandbox restricting disk and network I/O, minimizing
effect of security exploits
• Plug-in process for each type of plug-in
Operating System Concepts – 10th Edition 3.23 Silberschatz, Galvin and Gagne ©2018
End of Chapter 3
Operating System Concepts – 10th Edition Silberschatz, Galvin and Gagne ©2018
