What is a thread?
• A thread is a flow of execution through the process code, with its own program counter, system registers and stack.
• A thread is also called a light weight process.
• Threads provide a way to improve application performance through parallelism.
• Each thread belongs 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.
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 user level threads, 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 begins with a single thread and begins 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 kernel level, 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 programmed to be multi-threaded. All of the threads within an application are supported within a single 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.
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 blocked.
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.
• A thread is a flow of execution through the process code, with its own program counter, system registers and stack.
• A thread is also called a light weight process.
• Threads provide a way to improve application performance through parallelism.
• Each thread belongs 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.
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 user level threads, 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 begins with a single thread and begins 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 kernel level, 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 programmed to be multi-threaded. All of the threads within an application are supported within a single 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.
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 blocked.
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.
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