Operating System - Multi-Threading
What is Thread?
A thread is a flow of execution through the process code, with
its own program counter that keeps track of which instruction to execute next,
system registers which hold its current working variables, and a stack which
contains the execution history.
A thread shares with its peer threads few information like code
segment, data segment and open files. When one thread alters a code segment
memory item, all other threads see that.
A thread is also called a lightweight 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.
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. The following figure shows the
working of a single-threaded and a multithreaded process.
Difference between
Process and Thread
|
S.N.
|
Process
|
Thread
|
|
1
|
Process is heavy weight or resource intensive.
|
Thread is light weight, taking lesser resources than a
process.
|
|
2
|
Process switching needs interaction with operating system.
|
Thread switching does not need to interact with operating
system.
|
|
3
|
In multiple processing environments, each process executes the
same code but has its own memory and file resources.
|
All threads can share same set of open files, child processes.
|
|
4
|
If one process is blocked, then no other process can execute
until the first process is unblocked.
|
While one thread is blocked and waiting, a second thread in
the same task can run.
|
|
5
|
Multiple processes without using threads use more resources.
|
Multiple threaded processes use fewer resources.
|
|
6
|
In multiple processes each process operates independently of
the others.
|
One thread can read, write or change another thread's data.
|
Advantages of Thread
- Threads
minimize the context switching time.
- Use of threads
provides concurrency within a process.
- Efficient
communication.
- It is more
economical to create and context switch threads.
- Threads allow
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, the 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 starts with a single 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.
- Multithreaded
application cannot take advantage of multiprocessing.
Kernel Level Threads
In this case, thread management is 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
multithreaded. 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 be multithreaded.
Disadvantages
- Kernel threads
are generally slower to create and manage than the user threads.
- Transfer of
control from one thread to another within the same process requires a mode
switch to the Kernel.
Multithreading 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. Multithreading models are three types
- Many to many
relationship.
- Many to one
relationship.
- One to one
relationship.
Many to Many Model
The many-to-many model multiplexes any number of user threads
onto an equal or smaller number of kernel threads.
The following diagram shows the many-to-many threading model
where 6 user level threads are multiplexing with 6 kernel level threads. In
this model, developers can create as many user threads as necessary and the
corresponding Kernel threads can run in parallel on a multiprocessor machine.
This model provides the best accuracy on concurrency and when a thread performs
a blocking system call, the kernel can schedule another thread for execution.
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 by the thread
library. 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 the system does not support them, then the
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 allows another thread to run when a thread makes a blocking
system call. It supports multiple threads 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
|
Kernel-Level Thread
|
|
1
|
User-level threads are faster to create and manage.
|
Kernel-level threads are slower to create and manage.
|
|
2
|
Implementation is by a thread library at the user level.
|
Operating system supports creation of Kernel threads.
|
|
3
|
User-level thread is generic and can run on any operating
system.
|
Kernel-level thread is specific to the operating system.
|
|
4
|
Multi-threaded applications cannot take advantage of
multiprocessing.
|
Kernel routines themselves can be multithreaded.
|
