All questions of CPU Scheduling for Computer Science Engineering (CSE) Exam

Shortest Job First Scheduling (SJF)
Shortest Job First Scheduling (SJF) is a scheduling algorithm that selects the process with the smallest execution time to execute next. Here, we will discuss the statements given in the question and determine which one is false.

Statement Analysis:

S1: It causes minimum average waiting time
SJF scheduling indeed minimizes the average waiting time because it prioritizes shorter jobs over longer ones. By executing the shortest job first, the waiting time for all processes is reduced, leading to a lower average waiting time. Therefore, this statement is true.

S2: It can cause starvation
Starvation occurs when a process is unable to progress because it is constantly being delayed by other processes with shorter execution times. In SJF scheduling, if shorter jobs constantly keep arriving, longer jobs might suffer from starvation as they may not get a chance to execute. Hence, this statement is also true.

Conclusion:
Based on the analysis of both statements:
- Statement S1: True (SJF minimizes average waiting time)
- Statement S2: True (SJF can cause starvation)

Final Verdict:
The false statement was not present in the given options. Therefore, the correct answer is option 'D' - Neither S1 nor S2, as both statements about SJF scheduling are true.

Deadlock Prevention and Deadlock Avoidance Schemes



Deadlock prevention and deadlock avoidance are two different strategies used to handle the problem of deadlock in operating systems. While both aim to prevent or avoid deadlocks, they have different approaches and requirements. Let's examine each option and determine which one is not true.



a) In deadlock prevention, the request for resources is always granted if the resulting state is safe


This statement is not true. Deadlock prevention aims to prevent the occurrence of deadlocks by ensuring that the system is always in a safe state. In this approach, requests for resources may be denied even if the resulting state is safe. The goal is to avoid the possibility of future deadlocks by ensuring that the system always remains in a safe state.



b) In deadlock avoidance, the request for resources is always granted if the resulting state is safe


This statement is true. Deadlock avoidance, also known as bankers algorithm, uses a careful resource allocation strategy to ensure that the system remains in a safe state. If a resource request can be granted without leading to a deadlock, it is allowed. This approach requires knowledge of resource requirements a priori and performs a resource allocation check before granting the request.



c) Deadlock avoidance requires knowledge of resource requirements a priori


This statement is true. Deadlock avoidance requires knowledge of resource requirements a priori, meaning that the system needs to know in advance the maximum number of resources that a process may need during its execution. This information is used to determine if a resource request can be granted without leading to a deadlock. The system uses algorithms like the bankers algorithm to allocate resources in a way that avoids deadlocks.



d) Deadlock prevention is more restrictive than deadlock avoidance


This statement is true. Deadlock prevention is more restrictive than deadlock avoidance. Deadlock prevention aims to prevent the occurrence of deadlocks by denying resource requests if granting them may lead to a deadlock. This approach can be overly restrictive and may result in resource underutilization. On the other hand, deadlock avoidance allows resource requests to be granted if they do not lead to a deadlock, resulting in better resource utilization.



In conclusion, option 'a' is not true because in deadlock prevention, the request for resources is not always granted even if the resulting state is safe.

Dispatcher module in Operating System

The dispatcher module is a component of the operating system that is responsible for giving control of the CPU to the process selected by the short-term scheduler. It is a critical component of the operating system that manages the execution of processes and ensures that the CPU is utilized efficiently.

The dispatcher module performs the following functions:

1. Context Switching: It saves the context of the currently executing process and restores the context of the selected process.

2. Process State Transition: It changes the state of the process from running to ready, waiting, or terminated based on the events.

3. Resource Allocation: It allocates system resources like memory, I/O devices, etc. to the selected process.

4. Process Synchronization: It ensures that the processes do not interfere with each other and use the shared resources efficiently.

5. Interrupt Handling: It handles the interrupts generated by the hardware devices and the software.

The dispatcher module works closely with the scheduler modules of the operating system to ensure that the processes are executed efficiently. The scheduler module selects the process to be executed from the ready queue, and the dispatcher module gives control of the CPU to the selected process.

In summary, the dispatcher module is a critical component of the operating system that manages the execution of processes and ensures that the CPU is utilized efficiently. It works closely with the scheduler module to select the process to be executed and gives control of the CPU to the selected process.

Let three process be P0, P1 and P2 with arrival times 0, 2 and 6 respectively and CPU burst times 10, 20 and 30 respectively. At time 0, P0 is the only available process so it runs. At time 2, P1 arrives, but P0 has the shortest remaining time, so it continues. At time 6, P2 arrives, but P0 has the shortest remaining time, so it continues. At time 10, P1 is scheduled as it is the shortest remaining time process. At time 30, P2 is scheduled. Only two context switches are needed. P0 to P1 and P1 to P2.

A critical section is a specific section of a program where shared resources are accessed and modified by multiple threads or processes. It is important to ensure that only one thread or process can access the critical section at a time to avoid data inconsistencies or conflicts.

Shared Resources:
In a multi-threaded or multi-process environment, multiple threads or processes may need to access certain resources, such as variables, files, or devices. These resources are typically shared among the threads or processes. However, when multiple threads or processes try to access and modify the shared resources simultaneously, it can lead to data inconsistencies or conflicts.

Need for Synchronization:
To avoid such issues, synchronization mechanisms are used to control the access to shared resources. A critical section is a part of the program where shared resources are accessed, and it needs to be synchronized to ensure that only one thread or process can access it at a time.

Avoiding Data Inconsistencies:
By ensuring that only one thread or process can access the critical section at a time, we can prevent data inconsistencies. For example, if two threads try to increment a shared variable simultaneously, they may both read the variable's value, increment it, and write it back, resulting in a lost update. By synchronizing the critical section, we can ensure that only one thread increments the variable at a time, avoiding data inconsistencies.

Using Semaphores:
One way to implement synchronization in a critical section is by using semaphores. Semaphores are variables that can be used for signaling and controlling access to shared resources. In many cases, a pair of semaphore operations, usually referred to as P and V, are used to control access to the critical section.

The P operation, also known as wait or acquire, is used to acquire the semaphore and enter the critical section. If the semaphore value is greater than zero, it is decremented, and the thread or process can proceed to access the critical section. If the semaphore value is zero, the thread or process is blocked until the semaphore becomes available.

The V operation, also known as signal or release, is used to release the semaphore and exit the critical section. It increments the semaphore value, allowing another thread or process to acquire it and enter the critical section.

Conclusion:
In summary, a critical section is a program segment where shared resources are accessed and modified. It is important to synchronize the access to the critical section to avoid data inconsistencies or conflicts. Semaphore operations, such as P and V, can be used to control access to the critical section and ensure that only one thread or process can access it at a time.

Explanation:
- Initial Situation:
- Processes X, Y, and Z need to acquire semaphores a, b, c, and d in a deadlock-free manner to avoid any synchronization issues.
- All semaphores are initialized to one.
- Order of P operations:
- X: P(b)P(a)P(c)
- X first acquires semaphore b, then a, and finally c.
- Y: P(b)P(c)P(d)
- Y first acquires semaphore b, then c, and finally d.
- Z: P(a)P(c)P(d)
- Z first acquires semaphore a, then c, and finally d.
- Explanation of the chosen order (Option B):
- This order ensures that each process can acquire the necessary semaphores without causing a deadlock.
- X acquires b before a to prevent a deadlock with Y.
- Y acquires b before c to avoid a deadlock with X.
- Z acquires a first to prevent a deadlock with X and then acquires c and d.
- Conclusion:
- The order of invoking P operations in Option B (X: P(b)P(a)P(c), Y: P(b)P(c)P(d), Z: P(a)P(c)P(d)) represents a deadlock-free sequence for the processes.