Chapter 4: Multithreaded Programming

4.2 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts What is Thread “Thread is a part of a program that can be executed independently from the other parts of a program” A thread is also considered as a flow of control within a process. A multithreaded process contains several different flows of control within the same address space. Generally, a process contains single thread of execution

4.3 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts What is Thread Means it can do only one task at a time. Programs developed using languages like ‘C’ comes under such category. But some languages like JAVA facilitate concept of multithreading. Programs developed using such languages can have multiple threads. They can do more than one task in parallel.

4.4 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts What is Thread MS-DOS is single-user, single-tasking, single threading OS. UNIX is a multi-user, multi-tasking, single threading OS. All modern Window & Linux OS are multi- tasking, multi-threading OS.

4.5 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Single and Multithreaded Processes

4.6 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Thread V/S Processes Threads are known as light weight processes, while processes are known as heavy weight processes. Threads are the actual entities scheduled for execution on the CPU. While processes are used to group resources together. Threads contain their own program counter, CPU registers, stack and state. While processes contain its address space(code, data, stack) as well as resources like open file, child processes, devices etc.

4.7 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Thread V/S Processes Multiple threads can run in parallel in single process environment. It is analogous to multiple processes than can run in parallel in a single computer. Different threads share process address space, open files and allocated resources to a single process. While different processes can share physical memory, printers,scanners and other resources. Managing threads- i.e. creating, terminating, scheduling- is quite simple rather than managing processes.

4.8 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Benefits Of Using Threads The benefits of multithreaded programming can be broken down into four major categories. Responsiveness :  Multithreading allows an application to perform more than one task simultaneously.  So, when some part of an application is busy in performing lengthy operating or in waiting for an input from user, the other part can continue its execution.

4.9 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Benefits Of Using Threads Resource Sharing :  Threads share the memory and the resources of the process to which they belong.  Due to this different threads can work efficiently within the same single process environment.

4.10 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Benefits Of Using Threads Economy :  Allocating memory and resources for process is costly.  Compared to this, threads share the resources of a process by default.  Also, process creating, scheduling, termination is time consuming compared to threads.  Due to this, threads are called light weight process.

4.11 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Benefits Of Using Threads Utilization of Multiprocessor Architectures :  In single-processor architecture, the CPU switches among various threads very quickly to provide illusion of parallelism, but in reality, only one thread is running at time.  In a multi-processor architecture, each thread can run in parallel on different processor.  Thus it provides real parallelism. This increases concurrency.

4.12 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts User Threads User threads are supported above the kernel and are implemented by a thread library at the user level. The library provides support for thread creation, scheduling, and management with no support from the kernel. Three primary thread libraries: POSIX Pthreads Win32 threads Java threads

4.13 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts User Threads Advantages: User-level threads does not require modification to operating systems. Simple Representation:  Each thread is represented simply by a Process Control, registers, stack and a small control block, all stored in the user process address space.

4.14 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts User Threads Advantages: Simple Management:  This simply means that creating a thread, switching between threads and synchronization between threads can all be done without intervention of the kernel. Fast and Efficient:  Thread switching is not much more expensive than a procedure call.

4.15 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts User Threads Disadvantages: There is a lack of coordination between threads and operating system kernel. Therefore, process as whole gets one time slice irrespective of whether process has one thread or 1000 threads within. It is up to each thread to give up control to other threads.

4.16 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts User Threads Disadvantages: User-level threads requires non- blocking systems call i.e., a multithreaded kernel. Otherwise, entire process will blocked in the kernel, even if there are Runnable threads left in the processes. For example, if one thread causes a page fault, the process blocks.

4.17 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Kernel Threads Kernel threads are supported directly by the Operating System. The kernel performs thread creation, scheduling, and management in kernel space. Because thread management is done by the OS. Examples Windows XP/2000 Solaris Linux Tru64 UNIX

4.18 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Kernel Threads Advantages: Because kernel has full knowledge of all threads, Scheduler may decide to give more time to a process having large number of threads than process having small number of threads. Kernel-level threads are specially good for applications that frequently block.

4.19 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Kernel Threads Disadvantages: The kernel-level threads are slow and inefficient. For instance, threads operations are hundreds of times slower than that of user-level threads. Since kernel must manage and schedule threads as well as processes. It require a full thread control block (TCB) for each thread to maintain information about threads. As a result there is significant overhead and increased in kernel complexity.

4.20 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Multithreading Models Many systems provide support for both user and kernel threads, resulting in different multithreading models. There are three common types of threading implementation. Many-to-One One-to-One Many-to-Many

4.21 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Many-to-One Many user-level threads mapped to single kernel thread Examples: Solaris Green Threads GNU Portable Threads

4.22 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Many-to-One Model

4.23 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts One-to-One Each user-level thread maps to kernel thread Examples Windows NT/XP/2000 Linux Solaris 9 and later

4.24 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts One-to-one Model

4.25 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Many-to-Many Model Allows many user level threads to be mapped to many kernel threads Allows the operating system to create a sufficient number of kernel threads Solaris prior to version 9 Windows NT/2000 with the ThreadFiber package

4.26 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Many-to-Many Model

4.27 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Threading Issues Semantics of fork() and exec() system calls Thread cancellation Signal handling Thread pools Thread specific data Scheduler activations

4.28 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Semantics of fork() and exec() 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. If a thread invokes the exec system call the program specified in the parameter to exec will replace the entire process- including all threads.

4.29 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Thread Cancellation Terminating a thread before it has finished Two general approaches: Asynchronous cancellation terminates the target thread immediately Deferred cancellation allows the target thread to periodically check if it should be cancelled

4.30 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Signal Handling Signals are used in UNIX systems to notify a process that a particular event has occurred A signal handler is used to process signals  Signal is generated by particular event  Signal is delivered to a process  Signal is handled

4.31 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Signal Handling Options: 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

4.32 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Thread Pools 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

4.33 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Thread Specific Data Allows each thread to have its own copy of data Useful when you do not have control over the thread creation process (i.e., when using a thread pool)

4.34 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Pthreads A POSIX standard (IEEE c) API for thread creation and synchronization API specifies behavior of the thread library, implementation is up to development of the library. Pthread API provides a set of functions to create and manage thread at the user- level. Common in UNIX operating systems (Solaris, Linux, Mac OS X)

4.35 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Windows XP Threads Implements the one-to-one mapping Each thread contains A thread id Register set Separate user and kernel stacks Private data storage area The register set, stacks, and private storage area are known as the context of the threads

4.36 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Windows XP Threads The primary data structures of a thread include: ETHREAD (executive thread block)  The key components of the ETHREAD include a pointer to the process to which the thread belongs and the address of the routine in which thread starts controls.

4.37 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts KTHREAD (kernel thread block)  The KTHREAD includes scheduling and synchronization information for the thread. TEB (thread environment block)  It is a user-space data structure that is accessed when the thread is running in user mode. Windows XP Threads

4.38 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Linux Threads Linux refers to them as tasks rather than threads. Thread creation is done through clone() system call. clone() allows a child task to share the address space of the parent task (process). The sharing of the address space is allowed because of the representation of a process in the Linux kernel. A unique kernel data structure exists for each process in the system.

4.39 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Java Threads Java threads are managed by the JVM Java threads may be created by: Extending Thread class Implementing the Runnable interface

4.40 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Java Thread Management

4.41 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Java Thread States

End of Chapter 4