OPERATING SYSTEM CONCEPT AND PRACTISE

THREADS A thread is a basic unit of CPU utilization; it comprises a thread ID, a program counter, a register set, and a stack. If a process has multiple threads of control, it can perform more than one task at a time. In certain situations, a single application may be required to perform several similar tasks. Threads also play a vital role in remote procedure call (RPC) systems, RPC servers are multithreaded.

Single and Multithreaded Processes

BENEFITS Responsiveness Resource Sharing Economy Utilization of MP Architectures

Responsiveness. Multithreading an interactive application may allow a program to continue running even if part of it is blocked or is performing a lengthy operation. Resource sharing: By default, threads share the memory and the resources of the process to which they belong. It allows an application to have several different threads of activity within the same address space. Economy: Allocating memory and resources for process creation is costly, it is more economical to create and context-switch threads. Utilization of multiprocessor architectures: The benefits of multithreading can be greatly increased in a multiprocessor architecture.

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. Kernel threads are supported and managed directly by the operating system. There are three ways to establish link between user threads and kernel threads Many to one One to many Many to many

Many-to-One Model

Many user-level threads mapped to single kernel thread The entire process will block if a thread makes a blocking system call Only one thread can access the kernel at a time. Green threads—a thread library available for Solaris GNU Portable threads.

One-to-One

Each user-level thread maps to kernel thread. It allows multiple threads to run in parallel on multiprocessors. Creating a user thread requires creating the corresponding kernel thread. Examples Windows NT/XP/2000 Linux Solaris 9 and later

Many-to-Many Model

Multiplexes many user-level threads to a smaller or equal number of kernel threads. When a thread performs a blocking system call, the kernel can schedule another thread for execution. Solaris prior to version 9 Windows NT/2000 with the ThreadFiber package

Two-level Model

THREAD LIBRARIES A thread library provides the programmer an API for creating and managing threads. There are two primary ways of implementing a thread library. To provide a library entirely in user space with no kernel support. Implement a kernel-level library supported directly by the operating system.

There are 3 main thread libraries in use: POSIX Pthreads, Win32, Java Pthreads 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)

WIN32 Threads Threads are created in the Win32 API using the Create Thread ()function. DWORD data type is an unsigned 32-bit integer Threads are created in the Win32 API using the CreateThread () function

Java Threads Java threads may be created by: Extending Thread class Java threads are managed by the JVM Java threads may be created by: Extending Thread class Implementing the Runnable interface

THREADING ISSUES Semantics of fork() and exec() system calls Thread cancellation Signal handling Thread pools Thread specific data Scheduler activations

Fork() and exec() Does fork() duplicate only the calling thread or all threads? Some UNIX systems have chosen to have two versions of fork() If a thread invokes the exec () system call, the program specified in the parameter to exec () will replace the entire process

Thread Cancellation Thread cancellation is the task of terminating a thread before it has completed. A thread that is to be canceled is often referred to 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

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 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

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.

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)

Scheduler Activations Both M:M and Two-level models require communication to maintain the appropriate number of kernel threads allocated to the application Scheduler activations provide upcalls - a communication mechanism from the kernel to the thread library This communication allows an application to maintain the correct number kernel threads

Operating System examples 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)

Windows XP 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