Two marks Questions with Answers

Two Marks Questions with Answers

Q.1 Define process. What is the information maintained in a PCB ?

Ans. A process is simply a program in execution. i.e. an instance of a program. execution. PCB maintains pointer, state, process number, CPU register, PC, memory allocation etc.

Q.2 Define task control block.

Ans. TCB is also called PCB.

Q.3 What is PCB? Specify the information maintained in it.     AU CSE/IT: Dec.-12

Ans. Each process is represented in the operating system by a process control block. PCB contains information like process state, program counter, CPU register, accounting information etc.

Q.4 What is independent process ?

Ans. Independent process cannot affect or be affected by the execution of another process.

Q.5 Name and draw five different process states with proper definition.    AU: Dec.-17

Ans. Process states are new, running, waiting, ready and terminated. Fig. 2.16.1 shows process state diagram.

Q.6 Define context switching.

Ans. Switching the CPU to another process requires saving the state of the old process and loading the saved state for the new process. This task is known as context switch.

Q.7 What are the reasons for terminating execution of child process ?

Ans. Parent may terminate execution of children processes via abort system call for a variety of reasons, such as:

1.Child has exceeded allocated resources.

2. Task assigned to child is no longer required.

3. Parent is exiting and the operating system does not allow a child to continue if its parent terminates.

Q.8 What is ready queue ?

Ans. The processes that are residing in main memory and are ready and waiting to execute are kept on a list called the ready queue.

Q.9 List out the data fields associated with process control blocks.

Ans. Data fields associated with process control block is CPU registers, PC, process state, memory management information, input-output status information etc.

Q.10 What are the properties of communication link ?

Ans. Properties of communication link

1. Links are established automatically.

2. A link is associated with exactly one pair of communicating processes.

3. Between each pair there exists exactly one link.

4. The link may be unidirectional, but is usually bidirectional.

Q.11 What is socket ?

Ans. A socket is defined as an endpoint for communication.

Q.12 What is non-preemptive scheduling ?

Ans.Non-preemptive scheduling ensures that a process relinquishes control of the CPU only when it finishes with its current CPU burst.

Q.13 Differentiate preemptive and non-preemptive scheduling.

Ans. :

Q.14 What do you mean by short term scheduler ?

Ans. Short term scheduler, also known as a dispatcher executes most frequently, and makes the finest-grained decision of which process should execute next. This scheduler is invoked whenever an event occurs.

Q.15 Which are the criteria used for CPU scheduling ?

Ans.Criteria used for CPU scheduling are CPU utilization, throughput, turnaround time, waiting time, response time.

Q.16 Explain why two level scheduling is commonly used.

Ans. It provides the hybrid solution to the problem of providing good system utilization and good user service simultaneously.code

Q.17 Why is it important for the scheduler to distinguish I/O-bound programs from CPU-bound programs?

Ans. I/O-bound programs have the property of performing only a small amount of computation before performing IO. Such programs typically do not use up their entire CPU quantum. CPU-bound programs, on the other hand, use their entire quantum without performing any blocking IO operations. Consequently, one could make better use of the computer's resources by giving higher priority to I/O-bound programs and allow them to execute ahead of the CPU-bound programs.

Q.18 What is response time?

Ans. Response time is the amount of time it takes from when a request was, submitted until the first response is produced, not output.

Q.19 Define waiting time.

Ans. Amount of time a process has been waiting in the ready queue.

Q.20 Define scheduling algorithm ?

Ans. In multiprogramming systems, whenever two simultaneously in the ready state, a choice has to be made which process to run next. The part of the OS that makes the choice is called the scheduler and the algorithm it uses is called the scheduling algorithm.

Q.21 Define the term dispatch latency.

Time it takes for the dispatcher to stop one process and start another running.

Q.22 What is preemptive priority method?

Ans. A preemptive priority will preempt the CPU if the newly arrived process is higher than the priority of the currently running process.

Q.23 What is medium term scheduling ?

Ans. Medium-term scheduling used especially with time-sharing systems as an intermediate scheduling level. A swapping scheme is implemented to remove partially run programs from memory and reinstate them later to continue where they left off.

Q.24 What is preemptive scheduling ?

Ans. Preemptive scheduling can preempt a process which is utilizing the CPU in between its execution and give the CPU to another process.

Q.25 What is the difference between long-term scheduling and short-term scheduling ?

Ans. Long term scheduling adds jobs to the ready queue from the job queue. Short term scheduling dispatches jobs from the ready queue to the running state.

Q.26 List out any four scheduling criteria.

Ans. Response time, throughput, waiting time and turn around time.

Q.27 Define the term 'Dispatch latency'.

Ans. Dispatch latency: Time it takes for the dispatcher to stop one process and start another running. It is the amount of time required for the scheduler to stop one process and start another.

Q.28 Distinguish between CPU-bounded and I/O bounded processes.

Ans.

Q.29 Define priority inversion problem.

Ans. The higher priority process would be waiting for the low priority one to finish. This situation is known as priority inversion problem.

Q.30 What advantage is there in having different time-quantum sizes on different levels of a multilevel queuing system?

Ans.Processes that need more frequent servicing, for instance, interactive processes such as editors, can be in a queue with a small time quantum. Processes switch no need for frequent servicing can be in a queue with a larger quantum, requiring fewer context switches to complete the processing, and thus making more efficient use of the computer.

Q.31 How does real-time scheduling differs from normal scheduling ?

Ans. Normal scheduling provides no guarantee on when a critical process will be scheduled; it guarantees only that the process will be given preference over non-critical processes. Real-time systems have stricter requirements. A task must be serviced by its deadline; service after the deadline has expired is the same as no service at all.

Q.32 What is Shortest-Remaining-Time-First (SRTF) ?

Ans. If a new process arrives with CPU burst length less than remaining time of process, current executing preempt. This scheme is known as the

Shortest-Remaining-Time-First.

Q.33 What is round robin CPU scheduling ?

Ans. Each process gets a small unit of CPU time (time quantum). After this time has elapsed, the process is preempted and added to the end of the ready queue.

Q.34 What is meant by starvation in operating system?

Ans. Starvation is a resource management problem where a process does not get the resources (CPU) it needs for a long time because the resources are being allocated to other processes.

Q.35 What is an aging?

Ans. Aging is a technique to avoid starvation in a scheduling system. It works by adding an aging factor to the priority of each request. The aging factor must increase the requests priority as time passes and must ensure that a request will eventually be the highest priority request

Q.36 How to solve starvation problem in priority CPU scheduling ?

Ans. Aging - as time progresses increase the priority of the process, so eventually the process will become the highest priority and will gain the CPU. i.e., the more time is spending a process in ready queue waiting, its priority becomes higher and higher.

Q.37 What is convoy effect?

Ans. A convoy effect happens when a set of processes need to use a resource for a short time, and one process holds the resource for a long time, blocking all of the other processes. Essentially it causes poor utilization of the other resources in the system.

Q.38 How can starvation / indefinite blocking of processes be avoided in priority scheduling ?

Ans. A solution to the problem of indefinite blockage of processes is aging. Aging is a technique of gradually increasing the priority of processes that wait in the system for a long time.

Q.39"Priority inversion is a condition that occurs in real time systems where a low priority process is starved because higher priority processes have gained hold of the CPU" - Comment on this statement.        AU CSE: May-17 

Ans. A low priority thread always starts on a shadow version of the shared resource, the original resource remains unchanged. When a high-priority thread needs a resource engaged by a low -priority thread, the low priority thread is preempted, the original resource is restored and the high priority thread is allowed to use the original

Q.40 Provide two programming examples in which multithreading provides better. performance than a single-threaded solution.

Ans. A Web server that services each request in a separate thread.

A parallelized application such as matrix multiplication where different parts of the matrix may be worked on in parallel.

An interactive GUI program such as a debugger where a thread is used to monitor user input, another thread represents the running application, and a show third thread monitors performance.

Q.41 State what does a thread share with peer threads.

Ans. Thread share the memory and resource of the process.

Q.42 Define a thread. State the major advantage of threads.

Ans. A thread is a flow of execution through the process's code with its own program counter, system registers and stack.

2. Efficient communication.

Advantages: 1. Minimize context switching time.

Q.43 Can a multithreaded solution using multiple user-level threads achieve better performance on a multiprocessor system than on a single processor system ?

 Ans. A multithreaded system comprising of multiple user-level threads cannot make use of the different processors in a multiprocessor system simultaneously. The operating system sees only a single process and will not schedule the different threads of the process on separate processors. Consequently, there is no performance benefit associated with executing multiple user-level threads on a multiprocessor system.

Q.44 What are the differences between user-level threads and kernel-level threads ? (Refer section 2.8.3)

Q.45 What are the benefits of multithreads?

Ans. Benefits of multithreading is responsiveness, resource sharing, economy and utilization of multiprocessor architecture.

Q.46 Why a thread is called as light weight process ?

Ans. Thread is light weight taking lesser resources than a process. It is called light weight process to emphasize the fact that a thread is like a process but is more efficient and uses fewer resources and they also share the address space.

Q.47 Name one situation where threaded programming is normally used ?

Ans. Threaded programming would be used when a program should satisfy multiple tasks at the same time. A good example for this would be a program running with

GUI.

Q.48 Describe the actions taken by a thread library to context switch between user-level threads.

Ans. Context switching between user threads is quite similar to switching between Kernel threads, although it is dependent on the threads library and how it maps user threads to kernel threads. In general, context switching between user threads involves taking a user thread of its LWP and replacing it with another thread. This act typically involves saving and restoring the state of the registers.

Q.49 What is a thread pool ?

Ans. A thread pool is a collection of worker threads that efficiently execute asynchronous callbacks on behalf of the application. The thread pool is primarily used to reduce the number of application threads and provide management of the worker threads.

Q.50 What is deferred cancellation in thread ?

Ans. The target thread periodically checks whether it should terminate, allowing it an opportunity to terminate itself in an orderly fashion. With deferred cancellation, one thread indicates that a target thread is to be cancelled, but cancellation occurs only after the target thread has checked a flag to determine if it should be cancelled or not. This allows a thread to check whether it should be cancelled at a point when it can be cancelled safely.

Q.51 What is Pthread ?

Ans. Pthreads refers to the POSIX standard 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. 

Q.52 What is thread cancellation ?

Ans.Under normal circumstances, a thread terminates when it exits normally, either by returning from its thread function or by calling pthread_exit.However, it is possible for a thread to request that another thread terminate. This is called canceling a thread.

Q.53 List the benefit of multithreading.

Ans.

Benefit of multithreading:

It takes less time to create thread than a new process.

It takes less time to terminate thread than process.

Q.54 Under what circumstances is user level threads is better than the kernel level threads?

Ans. User-level threads are much faster to switch between, as there is no context switch; further, a problem-domain-dependent algorithm can be used to schedule among them. CPU-bound tasks with interdependent computations, or a task that will switch among threads often, might best be handled by user-level threads

Q.55 What resources are required to create threads?

Ans. Thread is smaller than a process, so thread creation typically uses fewer resources than process creation.

Creating either a user or kernel thread involves allocating a small data structure to hold a register set, stack, and priority.

Q.56 Differentiate single threaded and multi-threaded processes.

Ans.Single-threading is the processing of one command at a time. When one thread is paused, the system waits until this thread is resumed. In Multithreaded processes, threads can be distributed over a series of processors to scale. When one thread is paused due to some reason, other threads run as normal.

Q.57 Give an programming example in which multitreading does not provide better performance than single threaded solution.

Ans. Any kind of sequential program is not a good candidate to be threaded. An example of this is a program that calculates an individual tax return. 

Another example is a "shell" program such as the C-shell or Korn shell. Such a program must closely monitor its own working space such as open files, environment variables, and current working directory.

Q.58 Define mutual exclusion.

Ans. If a collection of processes share a resource or collection of resources, then often mutual exclusion is required to prevent interference and ensure consistency when accessing the resources.

Q.59 How the lock variable can be used to introduce mutual exclusion?

Ans. We consider a single, shared, (lock) variable, initially 0. When a process wants to enter in its critical section, it first tests the lock. If lock is 0, the process first sets it to 1 and then enters the critical section. If the lock is already 1, the process just waits until (lock) variable becomes 0. Thus, a 0 means that no process in its critical section, and 1 means hold your horses - some process is in its critical section.

Q.60 What is the hardware feature provided in order to perform mutual exclusion operation indivisibly ?

Ans. Hardware features can make any programming task easier and improve system efficiency. It provide special hardware instructions that allow user to test and modify the content of a word.

Q.61 Discuss why implementing synchronization primitives by disabling interrupts is gnome not appropriate in a single processor system if the synchronization primitives be are to be used in user level programs.

Ans. If a user level program is given the ability to disable interrupts, then it can disable the timer interrupt and prevent context switching from taking place, thereby allowing it to use the processor without letting other processes execute.

Q.62 What is race condition ?

Ans. A race condition is a situation where two or more processes access shared data concurrently and final value of shared data depends on timing.

Q.63 Define entry section and exit section.

Ans. Each process must request permission to enter its critical section. The section of the code implementing this request is the entry section. The critical section is followed by an exit section. The remaining code is the remainder section.

Q.64 Elucidate mutex locks with its procedure.

Ans. Mutex lock is software tools to solve the critical-section problem. A mutex lock has a boolean variable available whose value indicates if the lock is available or not. If the lock is available, a call to acquire() succeeds, and the lock is then considered unavailable

Q.65 Name any two file system objects that are neither files nor directories and what the advantage of doing so is.

Ans. Semaphore and monitors the system objects. Advantage is to avoid critical section problem.

Q.66 What is binary semaphore ?

Ans. Binary semaphore is a semaphore with an integer value that can range only between 0 and 1.

Q.67 What is semaphore? Mention its importance in operating systems.

Ans. Semaphore is an integer variable. It is a synchronization tool used to solve critical section problem. The various hardware based solutions to the critical section problem are complicated for application programmers to use.

Q.68 What is the meaning of the term busy waiting ?

Ans. Busy waiting means a process waits by executing a tight loop to check the status/value of a variable.

Q.69 What is bounding waiting ?

Ans. After a process made a request to enter its critical section and before it is granted the permission to enter, there exists a bound on the number of times that other processes are allowed to enter.

Q.70 Why can't you use a test and set instruction in place of a binary semaphore ?

Ans. A binary semaphore requires either a busy wait or a blocking wait, semantics not provided directly in the Test and Set. The advantage of a binary semaphore is that it does not require an arbitrary length queue of processes waiting on the semaphore.

Q.71 What is concept behind strong semaphore and spinlock ?

Ans.Semaphore can be implemented in user applications and in the kernel. The process that has been blocked the longest is released from the queue first is called a strong semaphore..

Using simple lock variable, process synchronization problem is not solved. To avoid this, spinlock is used. A lock that uses busy waiting is called a spin lock.

Q.72 What is bounded buffer problem ?

Ans. The bounded buffer producers and consumers assume that there are fixed buffer sizes i.e. a finite numbers of slots are available. To suspend the producers when the buffer is full, to suspend the consumers when the buffer is empty, and to make sure that only one process at a time manipulates a buffer so there are no race conditions or lost updates.

Q.73 State the assumption behind the bounded buffer producer consumer problem.

Ans. Assumption: It is assume that the pool consists of 'n' buffers, each capable of holding one item. The mutex semaphore provides mutual exclusion for accesses to the buffer pool and is initialized to the value 1.

Q.74 What is a critical section and what requirements must a solution to the critical section problem satisfy ?

Ans. Consider a system consisting of several processes, each having a segment of code called a critical section, in which the process may be changing common variables, updating tables, etc. The important feature of the system is that when one process is executing its critical section, no other process is to be allowed to execute its critical section. Execution of the critical section is mutually exclusive in time.

A solution to the critical section problem must satisfy the following three requirements: 1. Mutual exclusion 2. Progress 3. Bounded waiting

Q.75 Define 'monitor'. What does it consist of?  AU: CSE/IT: Dec.-11

Ans. Monitor is a highly structured programming language construct. It consists of private variables and private procedures that can only be used within a monitor. 

Q.76 Explain the use of monitors.

Ans. Use of monitors:

a) It provides a mutual exclusion facility. b) A monitor support synchronization by the use of condition variables. c) Shared data structure can be protected by placing it in a monitor.

Q.77 Give the queueing diagram representation of process schedulling. 

Ans. Refer Fig. 2.2.2.

Q.78 What are kernal threads.  AU: May-22

Ans.Kernel threads are handled by the operating system directly and the thread management is done by the kernel. The context information for the process as well as the process threads is all managed by the kernel.