1. Threads

2. Introduction
Virtually all modern operating systems provide features enabling a process to contain multiple threads of control
We will introduce many concepts associated with multithreaded computer systems
We will discuss APIs for the Pthreads, Windows, and Java thread libraries

3. Objectives
To introduce the notion of a thread—a fundamental unit of CPU utilization
To discuss the APIs for the Pthreads, Windows, and Java thread libraries
To examine issues related to multithreaded programming
To cover operating system support for threads in Windows and Linux

4. Motivation A thread is a basic unit of CPU utilization
It comprises a thread ID, a program counter, a register set, and a stack
It shares with other threads belonging to the same process
The code section
The data section
other operating-system resources, such as open files and signals
A traditional (or heavyweight) process has a single thread of control
If a process has multiple threads of control, it can perform more than one task at a time

5. Motivation

6. Motivation
An application typically is implemented as a separate process with several threads of control
Example
A web browser might have one thread to display images or text while another thread retrieves data from the network
In certain situations, a single application may be required to perform several similar tasks
A web server accepts client requests for web pages, images, sound, and so forth

7. Motivation
The process-creation method was in common use before threads became popular
Process creation is time consuming and resource intensive
It is generally more efficient to use one process that contains multiple threads

8. Benefits Responsiveness. Resource sharing. Economy. Scalability.
Multithreading an interactive application may allow a program to continue running even if part of it is blocked
Resource sharing.
Processes can only share resources through shared memory and message passing. Such techniques must be explicitly arranged by the programmer.
Threads share the memory and the resources of the process to which they belong by default
Economy.
Allocating memory and resources for process creation is costly
It is more economical to create and context-switch threads
Scalability.
The benefits of multithreading can be even greater in a multiprocessor architecture, where threads may be running in parallel on different processing cores

9. Multicore Programming
Single-CPU systems evolved into multi-CPU systems
A more recent trend is to place multiple computing cores on a single chip
Whether the cores appear across CPU chips or within CPU chips, we call these systems multicore or multiprocessor systems
Multithreaded programming provides a mechanism for more efficient use of these multiple computing cores

10. Multicore Programming
On a system with a single computing core, concurrency means that the execution of the threads will be interleaved over time
The processing core is capable of executing only one thread at a time

11. Multicore Programming
On a system with multiple cores concurrency means that the threads can run in parallel
The system can assign a separate thread to each core

12. Multicore Programming
Parallelism
The system is parallel if it can perform more than one task simultaneously
Concurrency
A concurrent system supports more than one task by allowing all the tasks to make progress
Differences
An application may process the task serially from start to end, or split the task up into subtasks which can be completed in parallel
An application may process one task at a time (sequentially) or work on multiple tasks at the same time (concurrently)

13. Amdahl’s Law
An application may have both serial (nonparallel) and parallel components
It is a formula that identifies potential performance gains with additional computing cores
If S is the serial part and N is the processing cores, the formula appears as follows:
If application is 75% parallel / 25% serial, moving from 1 to 2 cores results in speedup of 1.6 times
As N approaches infinity, speedup approaches 1 / S

14. Amdahl’s Law

15. Types of Parallelism Data parallelism
It focuses on distributing subsets of the same data across multiple computing cores performing the same operation
Example: Summing the contents of an array of size N
Single-core system
one thread would simply sum the elements [0] [N − 1].
Dual-core system
thread A, running on core 0, could sum the elements [0] [N/2 − 1] while thread B, running on core 1, could sum the elements [N/2] [N − 1]

16. Types of Parallelism Task parallelism
It involves distributing tasks (threads) across multiple computing cores
Each thread is performing a unique operation
Different threads may be operating on the same data, or they may be operating on different data
Example: Summing the contents of an array of size N
Task parallelism might involve two threads, each performing a unique statistical operation on the array of elements

17. 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 are supported above the kernel and are managed without kernel support
Kernel threads are supported and managed directly by the operating system
A relationship must exist between user threads and kernel threads
There are three common ways of establishing such a relationship
The many-to-one model
The one-to-one model
The many-to-many model

18. Many-to-One Model It maps many user-level threads to one kernel thread
Thread management is done by the thread library in user space
The entire process will block if a thread makes a blocking system call
Only one thread can access the kernel at a time
Multiple threads are unable to run in parallel on multicore systems
Examples:
Solaris Green Threads
GNU Portable Threads

19. One-to-One Model It maps each user thread to a kernel thread
It provides more concurrency by allowing another thread to run when a thread makes a blocking system call
It also allows multiple threads to run in parallel on multiprocessors
The drawback to this model is that creating a user thread requires creating the corresponding kernel thread
The overhead of creating kernel threads can burden the performance of an application
Examples
Windows
Linux
Solaris 9 and later

20. Many-to-Many Model
It multiplexes many user-level threads to a smaller or equal number of kernel threads
The number of kernel threads may be specific to either a particular application or a particular machine
When a thread performs a blocking system call, the kernel can schedule another thread for execution
Examples
Solaris prior to version 9

21. Variations
It still multiplexes many user level threads to a smaller or equal number of kernel threads
It also allows a user-level thread to be bound to a kernel thread
This variation is sometimes referred to as the two-level model
Examples
IRIX
HP-UX
Tru64 UNIX
Solaris 8 and earlier

22. Thread Libraries
A thread library provides the programmer with an API for creating and managing threads
There are two primary ways of implementing a thread library
The first approach is to provide a library entirely in user space
All code and data structures for the library exist in user space
Invoking a function in the library results in a local function call in user space and not a system call
The second approach is to implement a kernel-level library supported directly by the operating system
Code and data structures for the library exist in kernel space

23. Thread Libraries Three main thread libraries are in use today
POSIX Pthreads
It may be provided as either a user-level or a kernel-level library
Windows,
It is a kernel-level library available on Windows systems
Java
The Java thread API allows threads to be created and managed directly in Java programs
JVM is running on top of a host operating system, thus, the Java thread API is generally implemented using a thread library available on the host system

24. Pthreads
Pthreads refers to the POSIX standard (IEEE c) defining an API for thread creation and synchronization
This is a specification for thread behavior, not an implementation
Operating-system designers may implement the specification in any way they wish

25. Pthreads

26. Pthreads

27. In-Class Activity
What’s the result of the following code?

28. In-Class Activity What’s the result of the following code? Result
Waiting for thread to finish...
thread_function is running. Argument was Hello World
Thread joined, it returned Thank you for the CPU time
Message is now Bye!

29. Multiple Threads

30. Windows Threads

31. Java Threads
All Java programs comprise at least a single thread of control
Even a simple Java program consisting of only a main() method runs as a single thread in the JVM
Java threads are available on any system that provides a JVM including Windows, Linux, and Mac OS X
There are two techniques for creating threads in a Java program
One approach is to create a new class that is derived from the Thread class and to override its run() method
An alternative technique is to define a class that implements the Runnable interface

32. Java Threads
Example

33. Java Threads
Example

34. In-Class Activity
What’s the result of the following code?

35. In-Class Activity
What’s the result of the following code?
Result

36. Thread Pools
Unlimited threads could exhaust system resources, such as CPU time or memory
Create a number of threads in a pool where they await 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
Separating task to be performed from mechanics of creating task allows different strategies for running task i.e., Tasks could be scheduled to run periodically
Windows API supports thread pools:

37. Thread Pools
The number of threads in the pool can be set heuristically based on factors such as
the number of CPUs in the system,
the amount of physical memory,
the expected number of concurrent client requests
More sophisticated architectures can dynamically adjust the number of threads in the pool according to usage patterns
Such architectures provide the further benefit of having a smaller pool— thereby consuming less memory—when the load on the system is low

38. Fork() and Exec()
If one thread in a program calls fork(), does the new process duplicate all threads, or is the new process single-threaded?
Some UNIX systems have chosen to have two versions of fork()
one that duplicates all threads and
another that duplicates only the thread that invoked the fork() system call
The exec() system call typically works in the same way as described
If a thread invokes the exec() system call, the program specified in the parameter to exec() will replace the entire process—including all threads

39. Signals
A signal is used in UNIX systems to notify a process that a particular event has occurred
A signal may be received either synchronously or asynchronously
Examples of synchronous signal include illegal memory access and division by 0
Synchronous signals are delivered to the same process that performed the operation
When a signal is generated by an event external to a running process, that process receives the signal asynchronously

40. Signals All signals follow the same pattern:
A signal is generated by the occurrence of a particular event
The signal is delivered to a process
Once delivered, the signal must be handled
A signal may be handled by one of two possible handlers:
A default signal handler
A user-defined signal handler

41. Signals
Every signal has a default signal handler that the kernel runs when handling that signal
This default action can be overridden by a user-defined signal handler
Signals are handled in different ways
Some signals (such as changing the size of a window) are simply ignored
Others (such as an illegal memory access) are handled by terminating the program

42. Signals
Delivering signals is more complicated in multithreaded programs
A process may have several threads
Where should a signal be delivered?
In general, the following options exist:
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

43. kill(pid t pid, int signal)
Signals
The standard function for delivering a signal is
kill(pid t pid, int signal)
It specifies the process (pid) to which a particular signal (signal) is to be delivered
Most multithreaded versions of UNIX allow a thread to specify which signals it will accept and which it will block
An asynchronous signal may be delivered only to those threads that are not blocking it
As signals need to be handled only once, a signal is typically delivered only to the first thread found that is not blocking it

44. pthread kill(pthread t tid, int signal)
Signals
Pthreads provides the following function, which allows a signal to be delivered to a specified thread (tid):
pthread kill(pthread t tid, int signal)
Although Windows does not explicitly provide support for signals, it allows us to emulate them using asynchronous procedure calls (APCs)
The APC facility enables a user thread to specify a function that is to be called when the user thread receives notification of a particular event

45. Thread Cancellation
Thread cancellation involves terminating a thread before it has completed
A thread that is to be canceled is often referred to as the target thread
Cancellation of a target thread may occur in two different scenarios:
Asynchronous cancellation
One thread immediately terminates the target thread
Deferred cancellation
The target thread periodically checks whether it should terminate, allowing it an opportunity to terminate itself in an orderly fashion
The difficulty with cancellation occurs in situations where resources have been allocated to a canceled thread

46. Thread Cancellation
In Pthreads, thread cancellation is initiated using the pthread cancel()
The identifier of the target thread is passed as a parameter to the function
pthread t tid;
/* create the thread */
pthread create(&tid, 0, worker, NULL);
. . .
/* cancel the thread */
pthread cancel(tid);

47. Thread Cancellation
Invoking pthread cancel()indicates only a request to cancel the target thread
The actual cancellation depends on how the target thread is set up to handle the request
Pthreads supports three cancellation modes
Each mode is defined as a state and a type
Pthreads allows threads to disable or enable cancellation
Obviously, a thread cannot be canceled if cancellation is disabled.
However, cancellation requests remain pending, so the thread can later enable cancellation and respond to the request

48. Thank you for your Attention!