CPU Scheduling Algorithms

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Questions and Answers

What is a key feature of Round Robin scheduling?

Non-preemptive nature

Prioritizes processes based on arrival time

Assigns a fixed time slice to processes (correct)

Minimizes average turnaround time

Which statement is true regarding FCFS scheduling?

It is efficient for processes of varying lengths.

It always minimizes average waiting time.

It is a preemptive scheduling algorithm.

Once a process starts, it runs to completion. (correct)

What is a potential issue with Shortest Job First (SJF) scheduling?

Requires complex implementation.

Increases average turnaround time.

Can lead to starvation of longer processes. (correct)

Always guarantees a fair allocation of CPU time.

In priority scheduling, what can happen to low-priority processes?

They may experience starvation. Signup and view all the answers

Which characteristic distinguishes multilevel queue scheduling?

It divides processes into several queues with different algorithms. Signup and view all the answers

What happens to a process in Round Robin scheduling if it does not complete within its time quantum?

It moves to the back of the queue for further execution. Signup and view all the answers

What is a significant drawback of using small time quanta in Round Robin scheduling?

Increased average turnaround time. Signup and view all the answers

Which of the following best describes a counting semaphore?

A synchronization mechanism that operates on non-negative integers. Signup and view all the answers

What is the primary function of the Signal (V) operation in semaphore operations?

To increment the semaphore value and potentially release a blocked process. Signup and view all the answers

Which scenario is most likely to benefit from the application of a binary semaphore?

Enforcing mutual exclusion for a single shared resource. Signup and view all the answers

How does Round Robin scheduling contribute to the prevention of starvation of processes?

By ensuring every process receives equal time slices cyclically. Signup and view all the answers

What is the consequence of high overhead due to frequent context switching in Round Robin scheduling?

Potential decrease in overall system efficiency. Signup and view all the answers

Which issue is commonly associated with the use of semaphores?

Priority inversion leading to resource allocation delays. Signup and view all the answers

What happens if a process attempts to execute the Wait (P) operation on a semaphore and the semaphore value is zero?

The process is placed in a waiting state until the semaphore value is increased. Signup and view all the answers

Study Notes

CPU Scheduling

1. Round Robin Scheduling

Description: Time-sharing scheduling algorithm that assigns each process a fixed time slice (quantum).
Key Features:
- Preemptive: interrupts processes after a set time.
- Fairness: ensures all processes receive CPU time.
- Turnaround Time: can increase average turnaround time if the time slice is too large or too small.

2. FCFS Scheduling (First-Come, First-Served)

Description: Simple scheduling algorithm where processes are executed in the order of their arrival.
Key Features:
- Non-preemptive: once a process starts executing, it runs to completion.
- Easy to implement, but can lead to the "Convoy Effect" where short processes wait for long ones.
- Average waiting time can be high, especially with longer jobs.

3. SJF Scheduling (Shortest Job First)

Description: Prioritizes processes with the shortest execution time.
Key Features:
- Can be preemptive or non-preemptive.
- Minimizes average waiting time and turnaround time.
- Starvation issue: longer processes may be waiting indefinitely if short processes keep arriving.

4. Priority Scheduling

Description: Processes are assigned priority levels; the CPU is allocated to the highest priority process.
Key Features:
- May be preemptive or non-preemptive.
- Can lead to starvation for low-priority processes.
- Priorities can be defined statically or dynamically.

5. Multilevel Queue Scheduling

Description: Processes are divided into several queues, each with its own scheduling algorithm.
Key Features:
- Different queues may use different scheduling techniques (e.g., Round Robin, FCFS).
- Prioritizes queues: high-priority queues are served first.
- Suitable for systems with varying process types and requirements.

6. Semaphore

Description: A synchronization primitive used to manage access to shared resources.
Types:
- Binary Semaphore: can take values 0 or 1, used for mutual exclusion.
- Counting Semaphore: can take non-negative integer values, used for managing a limited number of resources.
Usage: Helps to prevent race conditions in concurrent processes.

7. Synchronization

Description: Mechanisms to ensure correct sequencing of process execution.
Key Concepts:
- Mutex (Mutual Exclusion): allows only one process to access a resource at a time.
- Condition Variables: enable processes to wait until a certain condition occurs.
- Deadlock: a situation where two or more processes are unable to proceed because each is waiting for the other to release a resource.
Techniques: Includes locks, semaphores, message passing, and monitors to ensure processes operate correctly in a concurrent environment.

CPU Scheduling

Round Robin Scheduling is a time-sharing scheduling algorithm where each process is allocated a fixed time slice (quantum). It is a preemptive algorithm, meaning that a running process can be interrupted after its time slice expires. This approach helps ensure fairness, as all processes receive CPU time, although it can lead to increased average turnaround time depending on the size of the time slice.
FCFS (First-Come, First-Served) Scheduling is a straightforward non-preemptive algorithm where processes are executed in the sequence they arrive. Once a process starts, it continues until completion. Although easy to implement, it can lead to the "Convoy Effect," where short processes are delayed by long ones.
SJF (Shortest Job First) Scheduling prioritizes processes with the shortest execution time. It can be either preemptive or non-preemptive. This approach minimizes average waiting time and turnaround time. However, it suffers from a potential starvation issue where longer processes may be indefinitely delayed due to the continued arrival of shorter jobs.
Priority Scheduling assigns priority levels to processes and allocates CPU time to those with the highest priority levels. It can be preemptive or non-preemptive. This method can lead to starvation as low-priority processes may never get CPU time. Priority levels can be either statically defined or dynamically adjusted.
Multilevel Queue Scheduling divides processes into multiple queues based on their characteristics (e.g., I/O bound, CPU bound). Each queue has its own scheduling algorithm. Processes in higher priority queues are served first. This approach is suitable for systems with varying workloads and process types.

Synchronization

Semaphores are synchronization primitives used to control access to shared resources. They help prevent race conditions, which occur when multiple processes try to access a shared resource simultaneously.
- Binary Semaphores are used for mutual exclusion and only take values of 0 or 1.
- Counting Semaphores manage limited resources and can take non-negative integer values.
Synchronization refers to the mechanisms ensuring the correct order of process execution in a concurrent environment.
- Mutex (Mutual Exclusion) is a synchronization technique that guarantees only one process can access a critical resource at a time.
- Condition Variables allow processes to wait until a specific condition is met before proceeding.
- Deadlock happens when two or more processes are stuck waiting for each other to release a resource, causing them to be unable to proceed.
- Common synchronization techniques include locks, semaphores, message passing, and monitors.

Round Robin Scheduling

Definition: A CPU scheduling algorithm that distributes time slices (also known as "quanta") to processes in a cyclical order.
Key Features:
- Fairness: All processes receive the same share of CPU time.
- Preemptive: Processes can be interrupted and sent back to the ready queue.
Mechanism:
- Processes are kept in a circular queue.
- Processes execute for a specific time quantum and are then moved to the back of the queue if not finished.
Time Quantum:
- A crucial parameter: too large can lead to poor responsiveness, too small can cause excessive context switching.
Benefits:
- Simple and easy to implement.
- Prevents processes from being starved.
- Well-suited for time-sharing systems.
Drawbacks:
- High overhead due to frequent context switching with small time quanta.
- The average turnaround time might be longer compared to other algorithms.

Semaphore

Definition: A synchronization primitive that acts as a gatekeeper to regulate access to a shared resource in concurrent programming.
Types:
- Binary Semaphore: Holds only two values (0 and 1), primarily employed for mutual exclusion (ensuring only one process is accessing the resource at a time).
- Counting Semaphore: Can hold any non-negative integer, used to manage a pool of resources by keeping track of available resources.
Operations:
- Wait (P or down operation): Decreases the semaphore value, blocking the process if the value is less than or equal to zero.
- Signal (V or up operation): Increases the semaphore value, releasing a blocked process if it becomes positive.
Use Cases:
- Managing shared resources like printers or shared memory.
- Coordination among multiple processes or threads to avoid race conditions.
Benefits:
- Helps prevent deadlock situations.
- Provides a mechanism for process communication and synchronization.
Drawbacks:
- It can lead to priority inversion (a higher priority process waiting for a lower priority process).
- Complexity in reasoning about program behavior due to the potential for race conditions if not handled carefully.

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Description

Test your knowledge on various CPU scheduling algorithms such as Round Robin, FCFS, and SJF. Understand their key features, advantages, and disadvantages. This quiz will help reinforce the concepts related to process management in operating systems.