COURSE NAME :OPERATING SYSTEM
COURSE CODE : 381CCS-3
CHAPTER 3 : PROCESSES, THREADS
REFERENCE DETAIL FOR CHAPTER 3
Topic Text Reference Chapter No. Page No
Processes ,
Threads
“Operating System
Concepts”, 10th Edition,
Abraham SilberSchatz,
Peter Baer Galvin,
Greg Gagne, Wiley, 2018
Chapter 3 105-122
Chapter 4
160-162
166-168
188-194
2.
2
Process
Concepts
TheProcess
Process
State
PCB
Threads
Process
Scheduling
Scheduling
Queues
CPU
Scheduling
Context
Switch
Operating on
Process
Process Creation
Process
Termination
CHAPTER 3 : PROCESSES, THREADS
3.
3.1 PROCESS CONCEPTS
3
•Program becomes process when an executable file is loaded into
memory
Execution of program started via GUI mouse clicks, command line entry,
etc.
An operating system executes a variety of programs that run as a process.
3.1.1 THE PROCESS
Process – a program in execution; process execution must progress in
sequential fashion. No parallel execution of instructions of a single process
Multiple parts
• The program code, also called text section
• Current activity including program counter, processor registers
• Stack containing temporary data
Function parameters, return addresses, local variables
• Data section containing global variables
• Heap containing memory dynamically allocated during run time
Figure 3.1 Layout of
a process in memory
Program is passive entity stored on disk (executable file); process is active
• Consider multiple users executing the same
4.
3.1 PROCESS CONCEPTS(CONT…)
4
3.1.2PROCESS STATE
As a process executes, it changes state
• New: The process is being created
• Running: Instructions are being executed
• Waiting: The process is waiting for some event to occur
• Ready:The process is waiting to be assigned to a
processor
• Terminated: The process has finished execution
Figure 3.2 Diagram of process state
5.
3.1 PROCESS CONCEPTS(CONT…)
5
3.PROCESS CONTROL BLOCK
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
3.1.4 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
Figure 3.3 Process
control block (PCB).
6.
6
Goal --Maximize CPU use, quickly switch processes onto CPU core
Process scheduler: selects among available processes for next execution on
CPU
Degree of multiprogramming: The number of processes currently in memory
An I/O-bound process : spends more of its time doing I/O than computations.
A CPU-bound process, spends more of its time doing computations.
3.2.1 SCHEDULING QUEUES
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
3.2 PROCESS SCHEDULING
Figure 3.4 The ready queue and wait queues.
7.
3.2.1 SCHEDULING QUEUES(CONT…)
Queueing Diagram: It is a common representation of process.
• The circles represent the resources that serve the queues,
• The arrows indicate the flow of processes in the system.
A new process is initially put in the ready queue, It waits there until it is
selected for execution, or dispatched.
Once the process is allocated a CPU core and is executing, one of several
events
could occur
The process could issue an I/O request and then be placed in an I/O wait
queue.
The process could create a new child process and then be placed in a wait
queue while it awaits the child’s termination.
The process could be removed forcibly from the core, as a result of an
interrupt or
having its time slice expire, and be put back in the ready queue.
7
Figure 3.5 Queueing-diagram representation of process scheduling
8.
8
3.2.2 CPU SCHEDULING
The role of the CPU scheduler is to select from among the processes that are in
the ready queue and allocate a CPU core to one of them.
The CPU scheduler must select a new process for the CPU frequently.
Processes can be described as either:
• I/O-bound process – spends more time doing I/O than computations, many
short CPU bursts
• CPU-bound process – spends more time doing computations; few very long
CPU bursts
Swapping : remove a process from memory (and from active contention for the
CPU) Later, the process can be reintroduced into memory, and its execution can
be continued where it left off.
a process can be “swapped out” from memory to disk, where its current
status is saved, and later “swapped in” from disk back to memory, where
its status is restored.
Swapping is typically only necessary when memory has been
overcommitted and must be freed up.
reduce the degree of multiprogramming.
9.
3.2.3 CONTEXT SWITCH
A context switch occurs when the CPU switches from one process to another.
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
9
Figure 3.6 Diagram showing context switch from process to process.
10.
10
3.3.1 PROCESS CREATION
Parent process create children processes, which, in turn create other processes,
forming
a tree of processes. 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
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
3.3 OPERATION ON PROCESSES
Figure 3.7 Process
creation using the
fork() system call.
11.
11
3.3.2 PROCESS DELETION
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
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
3.3 OPERATION ON PROCESSES (CONT…)
12.
12
A threadis a basic unit of CPU utilization; it comprises a thread ID, a
program counter (PC), a register set, and a stack.
It shares with other threads belonging to the same process its code
section, data section, and other operating-system resources, such as
open files and signals.
A traditional process has a single thread of control.
If a process has multiple threads of control, it can perform more than
one task at
a time.
3.4 THREAD
Figure 3.8 Single-threaded and multithreaded processes
13.
13
MOTIVATION
Most modernapplications are multithreaded
Threads run within application
Multiple tasks with the application can be implemented by separate
threads
• Update display
• Fetch data
• Spell checking
• Answer a network request
Process creation is heavy-weight
Thread creation is light-weight
Can simplify code, increase
efficiency
Kernels are generally
multithreaded
BENEFITS
Responsiveness – may allow continued execution if part of process is
blocked, especially important for user interfaces
Resource Sharing – threads share resources of process, easier than
shared
memory or message passing
Economy – cheaper than process creation, thread switching lower
overhead than context switching
Scalability – process can take advantage of multicore architectures
3.4.1 OVERVIEW
Figure 3.9 Multithreaded
server
architecture
14.
14
User threadsare supported above the kernel and are managed without
kernel support
Three primary thread libraries:
• POSIX Pthreads
• Windows threads
• Java threads
Kernel threads are supported and managed directly by the operating
system
Examples – virtually all general -purpose operating systems, including:
• Windows
• Linux
• Mac OS X
• iOS
• Android
3.4.2 MULTI THREADING MODEL
Figure 3.10 User and kernel threads.
15.
15
Each user-levelthread maps to kernel thread
Creating a user-level thread creates a kernel
thread
More concurrency than many-to-one
Number of threads per process sometimes
restricted due to overhead
Examples
• Windows
• Linux
A) Many-to-one Model
Many user-level threads mapped to single kernel thread
One thread blocking causes all to block
Multiple threads may not run in parallel on muticore system because
only one may be in kernel at a time
Few systems currently use this model
Examples:
• Solaris Green Threads
• GNU Portable Threads
Figure 3.11 Many-to-one Model
B) One-to-one Model
3.4.2 MULTI THREADING MODEL
Figure 3.12 One-to-one Model
16.
C) Many-to-Many Model
Allows many user level threads to be mapped to many kernel threads
Allows the operating system to create a sufficient number of kernel
threads
Windows with the ThreadFiber package
Otherwise not very common
D) Two-Level Model
Similar to M:M, except that it allows a user thread to be bound to kernel
thread
16
Figure 3.14 Two-Level Model
3.4.2 MULTI THREADING MODEL(CONT…)
Figure 3.13 Many-to-Many Model
17.
17
A) Semantics offork() and exec() system calls
If one thread in a program calls fork(), does the new process
duplicate all threads, or is the new process single-threaded?
Some UNIX systems have chosen to have two versions of fork(), one
that duplicates all threads and another that duplicates only the
thread that invoked the fork() system call.
exec() usually works as normal – replace the running process
including all threads
B) Signal handling
Signals are used to notify a process that a particular event has
occurred.
A signal handler is used to process signals
• Signal is generated by particular event
• Signal is delivered to a process
• Signal is handled by one of two signal handlers: default, user-
defined
Where should a signal be delivered for multi-threaded?
• Deliver the signal to the thread to which the signal
applies
• Deliver the signal to every thread in the process
• Deliver the signal to certain threads in the process
• Assign a specific thread to receive all signals for the
3.4.3 THREADING ISSUES
18.
18
C) Thread Cancellation
Terminating a thread before it has finished
Thread to be canceled is target thread
Two general approaches:
• Asynchronous cancellation terminates the target thread
immediately
• Deferred cancellation allows the target thread to periodically check
if it should be cancelled
D) Thread- Local Storage
Thread-local storage (TLS) allows each thread to have its own copy of
data
Useful when you do not have control over the thread creation process
(i.e., when using a thread pool)
Different from local variables
• Local variables visible only during single function invocation
• TLS visible across function invocations
Similar to static data
• TLS is unique to each thread
3.4.3 THREADING ISSUES(CONT…)
19.
19
E) Scheduler Activation
Both M:M and Two-level models require communication to
maintain the appropriate number of kernel threads allocated to
the application
Typically use an intermediate data structure between user and
kernel threads –
lightweight process (LWP)
• Appears to be a virtual processor on which
process can schedule user thread to run
• Each LWP attached to kernel thread
• How many LWPs to create?
Scheduler activations provide upcalls –
a communication mechanism from the
kernel to the upcall handler in the thread
library
This communication allows an application to
maintain
the correct number kernel threads
3.4.3 THREADING ISSUES(CONT…)
Figure 3.15 Light weight Process
