1. Threads
• Overview
• Multithreading Models
• Thread Libraries
• Threading Issues

2. 1. Overview
• A thread is a basic unit of CPU utilization, It includes a thread ID, a program
counter, a register set, and a stack
• It shares code section, data section, and other OS resources like open files
and signals with other threads belonging to the same process
• A traditional (or heavyweight) process has a single thread of control
• Difference between traditional single-threaded & multithreaded process
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Loganathan R, CSE, HKBKCE

3. 1. Overview Contd…
1.1 Motivation
• A single application may be required to perform several similar
tasks
• Example : A web server may have several of clients concurrently accessing it
• Tradition Solution : The server run as a single process that accepts requests and
when it receives a request, creates a separate process to service that request
• Process creation is time consuming and resource intensive
• The server create a separate thread to listen for client requests,
when a request made, it create another thread to service the
request
• RPC servers are multithreaded a server receives a message, it services the message
using a separate thread, which allows the server to service several concurrent
requests.
• Example :Java's RMI systems
• OS kernels are now multithreaded, several threads operate in the
kernel, and each thread performs a specific task, such as managing
devices or interrupt handling
• Example : Linux uses a kernel thread for managing free memory
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Loganathan R, CSE, HKBKCE

4. 1. Overview Contd…
1.2 Benefits
• Responsiveness
– Multithreading allow a program to continue running even if part of it is
blocked or is performing a lengthy operation, thereby increasing
responsiveness to the user.
– For example a multithreaded web browser could allow user interaction in
one thread while an image was being loaded in another thread
• Resource Sharing
– By default, threads share the memory and the resources of the process to
which they belong
• Economy
– Allocating memory and resources for process creation is costly, more
economical to create and context-switch threads
• Utilization of Multiprocessor Architectures
– Multithreading on a multi-CPU machine increases concurrency, threads
may be running in parallel on different processors,
Loganathan R, CSE, HKBKCE 4

5. 2. Multithreading Models
• Support for threads may be provided either at the user level, for user threads,
or by the kernel, for kernel threads
• User Threads - Thread management done by user-level threads without kernel
support. User thread libraries: POSIX Pthreads , Win32 threads, Java threads
• Kernel Threads - Supported and managed directly by the OS. Examples :
Windows XP/2000, Solaris, Linux, Tru64 UNIX, Mac OS X
• The relationship between user threads and kernel threads are established in 3
ways
2.1 Many-to-One Model
• Many user-level threads mapped to single kernel thread
• Thread management is done by the thread library in user space, so it is
efficient
• Disadvantages
– The entire process will block if a thread makes a blocking system call
– Multiple threads are unable to run in parallel on multiprocessors since
only one thread can access the kernel at a time
• Examples
– Solaris Green Threads
– GNU Portable Threads
Loganathan R, CSE, HKBKCE 5

6. 2. Multithreading Models Contd…
2.2 One -to-One Model
• Each user-level thread maps to kernel thread
• It provides more concurrency i.e. allows another thread to run when a thread
makes a blocking system call
• Allows multiple threads to run in parallel on multiprocessors
• Disadvantages
• Creating a user thread requires creating the corresponding kernel thread
• Examples : Windows NT/XP/2000, Linux
Many-to-One Model One -to-One Model
Loganathan R, CSE, HKBKCE 6

7. 2. Multithreading Models Contd…
2.3 Many-to-Many Model
• Allows many user level threads to be mapped to many kernel threads
• Allows the user and OS to create a sufficient number of user and kernel threads
• When a thread performs a blocking system call, the kernel can schedule another thread
for execution
• Windows NT/2000 with the ThreadFiber package
Two-level Model :
• Similar to M:M, except that it allows a user thread to be bound to kernel thread
• Examples : IRIX, HP-UX, Tru64 UNIX, Solaris 8 and earlier
Many-to-Many Two-level
Model Model
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8. 3. Thread Libraries
• A thread library provides the programmer an API for creating and managing
threads
• Two ways of implementation
– Provide a library entirely in user space with no kernel support - code and data
structures for the library exist in user space
– A kernel-level library supported directly by the operating system - code and data
structures for the library exist in kernel space
3.1 Pthreads
• Provided as either a user or kernel-level library
• 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
• Common in UNIX operating systems (Solaris, Linux, Mac OS X)
• All Pthreads programs must include the pthread.h header file
• pthread _t t id declares the identifier t id for the thread to be created
• The pthread_attr_t attr declares the attributes for the thread
• The attributes are set in the function call pthread_attr_init(&attr)
• A separate thread is created with the pthread_creat e () function call
• pthread_join () to wait
Loganathan R, CSE, HKBKCE 8

9. 3. Thread Libraries Contd…
3.2 Windows XP Threads
• Threads are created in the Win32 API using the CreateThread() function with
thread parameters
• Parameters includes security information, the size of the stack, and a flag that
can be set to indicate if the thread is to start in a suspended state
• WaitForSingleObj ect () function for waiting
3.3 Java Threads
• Threads are the fundamental model of program execution in a Java and
managed by the JVM
• Java threads may be created by:
–Extending Thread class (derive Thread class and override run())
–Implementing the Runnable interface[public interface Runnable{public abstract void run();}]
• start () method creates the new thread then allocates memory initialize it in
JVM, and calls run() to run thread in JVM
• Join() method to wait
• Java Thread States
Loganathan R, CSE, HKBKCE 9

10. 4. Threading Issues …
4.1 The fork() and exec() System Calls
• Does fork() duplicate only the calling thread or all threads?
• Some UNIX have Both versions
• Exec() system call works in the same way (will replace the entire process)
• If exec() is called immediately after forking, duplicating only the calling thread is
appropriate
4.2 Cancellation
• Terminating a thread before it has completed
• Multiple threads are concurrently searching a database and one thread returns the
result, the remaining threads might be canceled
• A thread that is to be canceled is referred as the target thread
• Two general approaches:
– Asynchronous cancellation terminates the target thread immediately
– Deferred cancellation allows the target thread to periodically check if it should be cancelled
• Difficulty in cancellation
• Canceling a thread asynchronously may not free a necessary system-wide resource (OS
will only reclaim System resources only)
• Deferred cancellation occurs only after the target thread has checked a flag to
determine if it should be canceled or not
• Checking whether it should be canceled at a point when it can be canceled safely is
known as cancellation points in Pthreads.
Loganathan R, CSE, HKBKCE 10

11. 4. Threading Issues Contd……
4.3 Signal Handling
• Signals are used in UNIX systems to notify a process that an particular event has
occurred may be received either synchronously or asynchronously.
• Signals whether synchronous(illegal memory access & Division by 0) or
asynchronous (external event like CTL+C,& Timer expire), follow the same
pattern: 1. Signal is generated by particular event
2. Signal is delivered to a process 3.Signal is handled
• A signal handler is used to process signals
– default signal handler that is run by the kernel when to handle the signal
– user-defined signal handler that is called to override default action
• In single-threaded programs, signals are always delivered to a process
• In multithreaded programs:
– 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
• Multithreaded versions of UNIX allow a thread to specify which signals it will accept
and which it will block
• Windows does not explicitly provide support for signals, they can be emulated using
asynchronous procedure calls (APCs)
Loganathan R, CSE, HKBKCE 11

12. 4. Threading Issues Contd……
4.4 Thread Pools
• Create a number of threads at start & place in a pool where they wait for 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
• The number of threads in the pool can be set based on factors like the number of CPUs
in the system, the amount of physical memory, and the expected number of concurrent
client requests
• Win32 API provides several functions related to thread pools
4.5 Thread Specific Data
• Allows each thread to have its own copy of data
4.6 Scheduler Activations
• Both M:M and Two-level models require communication to maintain the appropriate number of kernel
threads allocated to the application to be adjusted dynamically
• An intermediate data structure between the user and kernel threads is placed and it is known as a
lightweight process, or LWP Lightweight
• To the user-thread library, the LWP appears to be a virtual processor on which the application can
LWP Process
schedule a user thread to run and each LWP attached to a kernal thread
• If a kernel thread blocks, the LWP blocks, the user-level thread attached to the LWP also blocks
• Communication between the user-thread library and the kernel is known as scheduler activation
• The kernel provides an application with a set of virtual processors (LWPs) for the application to
schedule user threads onto LWP and informs an application about certain events is known as an
upcall
• Upcalls are handled by the thread library with an upcall handler which run on a virtual processor is
responsible to switch between threads 12