THREADS of Operating System by Noman Zahid

Threads
(Operating System)
Noman Zahid (02-134171-135)
BS(CS)-6A

What is 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.

Single threaded Process and Multi-
threaded Process

Difference between Process and Threads

Advantages of Threads :
• Thread minimizes 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.

Disadvantages of Threads :
•Blocking of parent threads will stop all child
thread.
•Security.

Types of Threads :
Threads are implemented in following two
ways :
• User Level Threads (ULT)
• Kernel Level Threads (KLT)

User Level Threads (ULT) :
User level thread implement in user level
libraries, so thread switching does not need
to call operating system and to cause
interrupt to the kernel.
In fact, the kernel knows nothing about
user level threads and manages them as if
they were single-threaded process.

Advantages of ULT :
User level threads does not require modification to operating
system.
• Easy to represent and manage.
• User level thread can run on any operating system.
• User level threads are fast and efficient.

Disadvantages of ULT :
• There is a lack of coordination between threads and
operating
• system kernel.
• ULT require non-blocking system call (i.e. Multithreaded
kernel)

Kernel Level Threads (KLT) :
In this method, the kernel knows about
and manages the threads. No runtime
system is needed in this case. Operating
system kernel provides system call to
create and manage threads.

Advantages of KLT :
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 multithreaded.

Disadvantages of KLT :
Kernel thread are generally slower to create and manage than the
user threads.
• Kernel requires Thread Control Block (TCB) for each thread in
the
pool, hence complexity increases.

Difference between ULT and KLT :

Multi-Threading Models :
Some operating system provides a combined user
level thread and kernel level thread facility. Solaris
is a good example of this combined approach.
Multi-Threading models are three types.
• Many-to-One Model
• One-to-One Model
• Many-to-Many Model

Many-to-One Model /One-to-One Model
/Many-to-Many Model

THREADS of Operating System by Noman Zahid

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