What is Thread?
A thread is a flow of execution through the process code, with its own prog ram counter, system registers and stack.
A thread is a flow of execution through the process code, with its own prog ram counter, system registers and stack.
A thread is also called a light weight process. Threads provide a way to improve application performance through parallelism. Threads represent a software approach to improving performance of operating system by reducing the overhead thread is equivalent to a classical process.
Process
Types of Thread
Threads are implemented in following two ways
User Level Threads -- User managed threads
Kernel Level Threads -- Operating System managed threads acting on kernel, an operating system core.
User Level Threads
In this case, application manages thread management kernel is not aware of the existence of threads. The thread
library contains code for creating and destroying threads, for passing message and data between threads, for
scheduling thread execution and for saving and restoring thread contexts. The application beg ins with a sing le
thread and beg ins running in that thread.
Advantages
Disadvantages
Kernel Level Threads
In this case, thread management done by the Kernel. There is no thread management code in the application
area. Kernel threads are supported directly by the operating system. Any application can be prog rammed to be
multi-threaded. All of the threads within an application are supported within a sing le process.
The Kernel maintains context information for the process as a whole and for individuals threads within the
process. Scheduling by the Kernel is done on a thread basis. The Kernel performs thread creation, scheduling
and management in Kernel space. Kernel threads are generally slower to create and manage than the user
threads.
Advantages
Kernel can simultaneously schedule multiple threads from the same process on multiple processes.
If one thread in a process is blocked, the Kernel can schedule another thread of the same process.
Kernel routines themselves can multi-threaded.
Disadvantages
Kernel threads are generally slower to create and manage than the user threads.
Transfer of control from one thread to another within same process requires a mode switch to the Kernel.
Multi-threading Models
Some operating system provide a combined user level thread and Kernel level thread facility. Solaris is a good
example of this combined approach. In a combined system, multiple threads within the same application can run in
parallel on multiple processors and a blocking system call need not block the entire process. Multi-threading
Models are three types
Many to Many Model
In this model, many user level threads multiplexes to the Kernel thread of smaller or equal numbers. The number
of Kernel threads may be specific to either a particular application or a particular machine.
Following diagram shows the many to many model. In this model, developers can create as many user threads as
necessary and the corresponding Kernel threads can run in parallels on a multiprocessor.
Many to One Model
Many to one model maps many user level threads to one Kernel level thread. Thread management is done in
user space. When thread makes a blocking system call, the entire process will be blocks. Only one thread can
access the Kernel at a time,so multiple threads are unable to run in parallel on multiprocessors.
If the user level thread libraries are implemented in the operating system in such a way that system does not support them then Kernel threads use the many to one relationship modes.
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One to One Model
There is one to one relationship of user level thread to the kernel level thread.This model provides more
concurrency than the many to one model. It also another thread to run when a thread makes a blocking system
call. It support multiple thread to execute in parallel on microprocessors.
Disadvantage of this model is that creating user thread requires the corresponding Kernel thread. OS/2,
windows NT and windows 2000 use one to one relationship model.
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Difference between User Level & Kernel Level Thread S.N.
User Level Threads
1 User level threads are faster to create and manage.
.
2 Implementation is by a thread library at the user level.
3 User level thread is generic and can run on any operating system.
4 Multi-threaded application cannot take advantage of multiprocessing .
1 Kernel level threads are slower to create and manage.
2 Operating system supports creation of Kernel threads.
3 Kernel level thread is specific to the operating system.
4 Kernel routines themselves can be multi-threaded.
Each thread belong s to exactly one process and no thread can exist outside a process. Each thread represents a separate flow of control.Threads have been successfully used in implementing network servers and web server. They also provide a suitable foundation for parallel execution of applications on shared memory
multiprocessors. Following figure shows the working of the single and multi-threaded processes .
Difference between Process and Thread
multiprocessors. Following figure shows the working of the single and multi-threaded processes .
Difference between Process and Thread
- Process is heavy weight or resource intensive.
- Process switching needs interaction with operating system.
- In multiple processing environments each
- process executes the same code but has its own memory and file resources.
- If one process is blocked then no other
- process can execute until the first process is unblocked.
- Multiple processes without using threads use more resources.
- In multiple processes each process operates
- independently of the others.
- Thread is light weight taking lesser resources than a process.
- Thread switching does not need to interact with operating system.
- All threads can share same set of open files, child processes.
- While one thread is blocked and waiting , second thread in the same task can run.
- Multiple threaded processes use fewer resources.
Advantages of Thread
- Thread minimize context switching time.
- Use of threads provides concurrency within a process.
- Efficient communication.
- Economy- It is more economical to create and context switch threads.
- Utilization of multiprocessor architectures to a greater scale and efficiency.
Types of Thread
Threads are implemented in following two ways
User Level Threads -- User managed threads
Kernel Level Threads -- Operating System managed threads acting on kernel, an operating system core.
User Level Threads
In this case, application manages thread management kernel is not aware of the existence of threads. The thread
library contains code for creating and destroying threads, for passing message and data between threads, for
scheduling thread execution and for saving and restoring thread contexts. The application beg ins with a sing le
thread and beg ins running in that thread.
Advantages
- Thread switching does not require Kernel mode privileges.
- User level thread can run on any operating system.
- Scheduling can be application specific in the user level thread.
- User level threads are fast to create and manage.
Disadvantages
- In a typical operating system, most system calls are blocking .
- Multi-threaded application cannot take advantage of multiprocessing .
Kernel Level Threads
In this case, thread management done by the Kernel. There is no thread management code in the application
area. Kernel threads are supported directly by the operating system. Any application can be prog rammed to be
multi-threaded. All of the threads within an application are supported within a sing le process.
The Kernel maintains context information for the process as a whole and for individuals threads within the
process. Scheduling by the Kernel is done on a thread basis. The Kernel performs thread creation, scheduling
and management in Kernel space. Kernel threads are generally slower to create and manage than the user
threads.
Advantages
Kernel can simultaneously schedule multiple threads from the same process on multiple processes.
If one thread in a process is blocked, the Kernel can schedule another thread of the same process.
Kernel routines themselves can multi-threaded.
Disadvantages
Kernel threads are generally slower to create and manage than the user threads.
Transfer of control from one thread to another within same process requires a mode switch to the Kernel.
Multi-threading Models
Some operating system provide a combined user level thread and Kernel level thread facility. Solaris is a good
example of this combined approach. In a combined system, multiple threads within the same application can run in
parallel on multiple processors and a blocking system call need not block the entire process. Multi-threading
Models are three types
- Many to many relationship.
- Many to one relationship.
- One to one relationship.
Many to Many Model
In this model, many user level threads multiplexes to the Kernel thread of smaller or equal numbers. The number
of Kernel threads may be specific to either a particular application or a particular machine.
Following diagram shows the many to many model. In this model, developers can create as many user threads as
necessary and the corresponding Kernel threads can run in parallels on a multiprocessor.
Many to One Model
Many to one model maps many user level threads to one Kernel level thread. Thread management is done in
user space. When thread makes a blocking system call, the entire process will be blocks. Only one thread can
access the Kernel at a time,so multiple threads are unable to run in parallel on multiprocessors.
If the user level thread libraries are implemented in the operating system in such a way that system does not support them then Kernel threads use the many to one relationship modes.
One to One Model
There is one to one relationship of user level thread to the kernel level thread.This model provides more
concurrency than the many to one model. It also another thread to run when a thread makes a blocking system
call. It support multiple thread to execute in parallel on microprocessors.
Disadvantage of this model is that creating user thread requires the corresponding Kernel thread. OS/2,
windows NT and windows 2000 use one to one relationship model.
Difference between User Level & Kernel Level Thread S.N.
User Level Threads
1 User level threads are faster to create and manage.
.
2 Implementation is by a thread library at the user level.
3 User level thread is generic and can run on any operating system.
4 Multi-threaded application cannot take advantage of multiprocessing .
Kernel Level Thread
1 Kernel level threads are slower to create and manage.
2 Operating system supports creation of Kernel threads.
3 Kernel level thread is specific to the operating system.
4 Kernel routines themselves can be multi-threaded.
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