chapter 4: multithreaded programming. 4.2 silberschatz, galvin and gagne ©2005 operating system...
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Chapter 4: Multithreaded ProgrammingChapter 4: Multithreaded Programming
4.2 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
What is ThreadWhat is Thread
“Thread is a part of a program that can be executed independently from the other parts of a program”
A thread is also considered as a flow of control within a process.
A multithreaded process contains several different flows of control within the same address space.
Generally, a process contains single thread of execution
4.3 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
What is ThreadWhat is Thread
Means it can do only one task at a time.
Programs developed using languages like ‘C’ comes under such category.
But some languages like JAVA facilitate concept of multithreading.
Programs developed using such languages can have multiple threads.
They can do more than one task in parallel.
4.4 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
What is ThreadWhat is Thread
MS-DOS is single-user, single-tasking, single threading OS.
UNIX is a multi-user, multi-tasking, single threading OS.
All modern Window & Linux OS are multi-tasking, multi-threading OS.
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Single and Multithreaded ProcessesSingle and Multithreaded Processes
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Thread V/S ProcessesThread V/S Processes
Threads are known as light weight processes, while processes are known as heavy weight processes.
Threads are the actual entities scheduled for execution on the CPU. While processes are used to group resources together.
Threads contain their own program counter, CPU registers, stack and state. While processes contain its address space(code, data, stack) as well as resources like open file, child processes, devices etc.
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Thread V/S ProcessesThread V/S Processes
Multiple threads can run in parallel in single process environment. It is analogous to multiple processes than can run in parallel in a single computer.
Different threads share process address space, open files and allocated resources to a single process. While different processes can share physical memory, printers ,scanners and other resources.
Managing threads- i.e. creating, terminating, scheduling- is quite simple rather than managing processes.
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Benefits Of Using ThreadsBenefits Of Using Threads
The benefits of multithreaded programming can be broken down into four major categories.
Responsiveness :
Multithreading allows an application to perform more than one task simultaneously.
So, when some part of an application is busy in performing lengthy operating or in waiting for an input from user, the other part can continue its execution.
4.9 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Benefits Of Using ThreadsBenefits Of Using Threads
Resource Sharing :
Threads share the memory and the resources of the process to which they belong.
Due to this different threads can work efficiently within the same single process environment.
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Benefits Of Using ThreadsBenefits Of Using Threads
Economy :
Allocating memory and resources for process is costly.
Compared to this, threads share the resources of a process by default.
Also, process creating, scheduling, termination is time consuming compared to threads.
Due to this, threads are called light weight process.
4.11 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Benefits Of Using ThreadsBenefits Of Using Threads
Utilization of Multiprocessor Architectures :
In single-processor architecture, the CPU switches among various threads very quickly to provide illusion of parallelism, but in reality, only one thread is running at time.
In a multi-processor architecture, each thread can run in parallel on different processor.
Thus it provides real parallelism. This increases concurrency.
4.12 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
User ThreadsUser Threads User threads are supported above the kernel
and are implemented by a thread library at the user level.
The library provides support for thread creation, scheduling, and management with no support from the kernel.
Three primary thread libraries:
POSIX Pthreads
Win32 threads
Java threads
4.13 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
User ThreadsUser Threads
Advantages:
User-level threads does not require modification to operating systems.
Simple Representation:
Each thread is represented simply by a Process Control, registers, stack and a small control block, all stored in the user process address space.
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User ThreadsUser Threads
Advantages:
Simple Management:
This simply means that creating a thread, switching between threads and synchronization between threads can all be done without intervention of the kernel.
Fast and Efficient:
Thread switching is not much more expensive than a procedure call.
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User ThreadsUser Threads
Disadvantages:
There is a lack of coordination between threads and operating system kernel. Therefore, process as whole gets one time slice irrespective of whether process has one thread or 1000 threads within. It is up to each thread to give up control to other threads.
4.16 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
User ThreadsUser Threads
Disadvantages:
User-level threads requires non-blocking systems call i.e., a multithreaded kernel. Otherwise, entire process will blocked in the kernel, even if there are Runnable threads left in the processes. For example, if one thread causes a page fault, the process blocks.
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Kernel ThreadsKernel Threads
Kernel threads are supported directly by the Operating System.
The kernel performs thread creation, scheduling, and management in kernel space. Because thread management is done by the OS.
Examples
Windows XP/2000
Solaris
Linux
Tru64 UNIX
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Kernel ThreadsKernel Threads
Advantages:
Because kernel has full knowledge of all threads, Scheduler may decide to give more time to a process having large number of threads than process having small number of threads.
Kernel-level threads are specially good for applications that frequently block.
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Kernel ThreadsKernel Threads
Disadvantages:
The kernel-level threads are slow and inefficient. For instance, threads operations are hundreds of times slower than that of user-level threads.
Since kernel must manage and schedule threads as well as processes. It require a full thread control block (TCB) for each thread to maintain information about threads. As a result there is significant overhead and increased in kernel complexity.
4.20 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Multithreading ModelsMultithreading Models
Many systems provide support for both user and kernel threads, resulting in different multithreading models.
There are three common types of threading implementation.
Many-to-One
One-to-One
Many-to-Many
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Many-to-OneMany-to-One
Many user-level threads mapped to single kernel thread
Examples:
Solaris Green Threads
GNU Portable Threads
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Many-to-One ModelMany-to-One Model
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One-to-OneOne-to-One
Each user-level thread maps to kernel thread
Examples
Windows NT/XP/2000
Linux
Solaris 9 and later
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One-to-one ModelOne-to-one Model
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Many-to-Many ModelMany-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
Solaris prior to version 9 Windows NT/2000 with the ThreadFiber
package
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Many-to-Many ModelMany-to-Many Model
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Threading IssuesThreading Issues
Semantics of fork() and exec() system calls
Thread cancellation
Signal handling
Thread pools
Thread specific data
Scheduler activations
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Semantics of fork() and exec()Semantics of fork() and exec()
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.
If a thread invokes the exec system call the program specified in the parameter to exec will replace the entire process-including all threads.
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Thread CancellationThread Cancellation
Terminating a thread before it has finished
Two general approaches:
Asynchronous cancellation terminates the target thread immediately
Deferred cancellation allows the target thread to periodically check if it should be cancelled
4.30 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Signal HandlingSignal Handling
Signals are used in UNIX systems to notify a process that a particular event has occurred
A signal handler is used to process signals
1. Signal is generated by particular event
2. Signal is delivered to a process
3. Signal is handled
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Signal HandlingSignal Handling
Options:
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 process
4.32 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Thread PoolsThread Pools
Create a number of threads in a pool where they await work
Advantages:
Usually slightly faster to service a request with an existing thread than create a new thread
Allows the number of threads in the application (s) to be bound to the size of the pool
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Thread Specific DataThread Specific Data
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)
4.34 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
PthreadsPthreads
A POSIX standard (IEEE 1003.1c) API for thread creation and synchronization
API specifies behavior of the thread library, implementation is up to development of the library.
Pthread API provides a set of functions to create and manage thread at the user-level.
Common in UNIX operating systems (Solaris, Linux, Mac OS X)
4.35 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Windows XP ThreadsWindows XP Threads
Implements the one-to-one mapping
Each thread contains
A thread id
Register set
Separate user and kernel stacks
Private data storage area
The register set, stacks, and private storage area are known as the context of the threads
4.36 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Windows XP ThreadsWindows XP Threads
The primary data structures of a thread include:
ETHREAD (executive thread block)
The key components of the ETHREAD include a pointer to the process to which the thread belongs and the address of the routine in which thread starts controls.
4.37 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
KTHREAD (kernel thread block)
The KTHREAD includes scheduling and synchronization information for the thread.
TEB (thread environment block)
It is a user-space data structure that is accessed when the thread is running in user mode.
Windows XP ThreadsWindows XP Threads
4.38 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Linux ThreadsLinux Threads
Linux refers to them as tasks rather than threads.
Thread creation is done through clone() system call.
clone() allows a child task to share the address space of the parent task (process).
The sharing of the address space is allowed because of the representation of a process in the Linux kernel.
A unique kernel data structure exists for each process in the system.
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Java ThreadsJava Threads
Java threads are managed by the JVM
Java threads may be created by:
Extending Thread class
Implementing the Runnable interface
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Java Thread ManagementJava Thread Management
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Java Thread StatesJava Thread States
End of Chapter 4End of Chapter 4