Operating System

21.What is a thread? 

A thread otherwise called a lightweight process (LWP) is a basic unit of CPU utilization, it comprises of a thread id, a program counter, a register set and a stack. It shares with other threads belonging to the same process its code section, data section, and operating system resources such as open files and signals. 

22.What are the benefits of multithreaded programming? 

The benefits of multithreaded programming can be broken down into four major categories: 

• Responsiveness 

• Resource sharing 

• Economy 

• Utilization of multiprocessor architectures 

23.Compare user threads and kernel threads. 

User threads 

User threads are supported above the kernel and are implemented by a thread library at the user level. Thread creation & scheduling are done in the user space, without kernel intervention. Therefore they are fast to create and manage blocking system call will cause the entire process to block 

Kernel threads 

Kernel threads are supported directly by the operating system .Thread creation, scheduling and management are done by the operating system. Therefore they are slower to create & manage compared to user threads. If the thread performs a blocking system call, the kernel can schedule another thread in the application for execution 

24.What is the use of fork and exec system calls? 

Fork is a system call by which a new process is created. Exec is also a system call, which is used after a fork by one of the two processes to replace the process memory space with a new program. 

25.Define thread cancellation & target thread. 

The thread cancellation is the task of terminating a thread before it has completed. A thread that is to be cancelled is often referred to as the target thread. For example, if multiple threads are concurrently searching through a database and one thread returns the result, the remaining threads might be cancelled. 

26.What are the different ways in which a thread can be cancelled? 

Cancellation of a target thread may occur in two different scenarios: 

• Asynchronous cancellation: One thread immediately terminates the target thread is called asynchronous cancellation. 

• Deferred cancellation: The target thread can periodically check if it should terminate, allowing the target thread an opportunity to terminate itself in an orderly fashion. 

27.Define CPU scheduling. 

CPU scheduling is the process of switching the CPU among various processes. CPU scheduling is the basis of multiprogrammed operating systems. By switching the CPU among processes, the operating system can make the computer more productive. 

28.What is preemptive and nonpreemptive scheduling? 

Under non preemptive scheduling once the CPU has been allocated to a process, the  process keeps the CPU until it releases the CPU either by terminating or switching to the waiting state. Preemptive scheduling can preempt a process which is utilizing the CPU in between its execution and give the CPU to another process. 

29.What is a Dispatcher? 

The dispatcher is the module that gives control of the CPU to the process selected by the short-term scheduler. This function involves: 

• Switching context 

• Switching to user mode 

• Jumping to the proper location in the user program to restart that program. 

30.What is dispatch latency? 

The time taken by the dispatcher to stop one process and start another running is known as dispatch latency.