Process Synchronization Basics

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Questions and Answers

What is the primary goal of process synchronization?

To allow processes to access shared data simultaneously

To increase the execution speed of processes

To ensure that no two processes access the same shared resources (correct)

To reduce the number of processes in the system

What defines a Cooperative Process?

It requires no inter-process communication

It is only executed on multi-core processors

It can execute independently of other processes

Its execution can affect other processes (correct)

What can result from concurrent access to shared data without proper synchronization?

Race conditions leading to data inconsistency (correct)

Increased data throughput

Effective use of multiple processing units

Effective resource sharing between processes

In the Producer-Consumer problem, what role does the Producer play?

Generates data and places it in a buffer Signup and view all the answers

Which of the following is NOT a common solution to the critical-section problem?

Throwing exceptions Signup and view all the answers

What is a consequence of a race condition?

Unpredictable outcomes due to the timing of process execution Signup and view all the answers

Which mechanism can be used to prevent race conditions in process synchronization?

Using synchronization hardware like mutex locks Signup and view all the answers

What defines the critical-section problem?

Preventing data inconsistency when multiple processes access shared data Signup and view all the answers

What condition must be satisfied for a producer to add a new buffer?

counter must be less than BUFFER_SIZE Signup and view all the answers

In the context of the consumer process, what action is taken when the counter is 0?

The consumer waits until there is a buffer available Signup and view all the answers

Which of the following corresponds to the critical section problem?

Restricting processes from executing their critical sections concurrently Signup and view all the answers

Which of the following is an implication of race conditions?

Counter values might not reflect accurate state due to interleaving Signup and view all the answers

What is the first requirement for solving the critical-section problem?

Mutual exclusion must be ensured Signup and view all the answers

What does 'counter++' demonstrate in the context of race conditions?

It illustrates the potential for value inconsistency during concurrent execution Signup and view all the answers

How is the producer's index adjusted after producing a new buffer?

It is incremented and wrapped at BUFFER_SIZE Signup and view all the answers

What is the role of the 'exit section' in a critical section protocol?

It defines how a process can give up control of the critical section Signup and view all the answers

What does the read_count variable represent in the readers-writers problem?

The number of readers currently accessing the data Signup and view all the answers

What condition triggers the first reader to wait for rw_mutex in the reader process?

When the read_count is equal to 1 Signup and view all the answers

What issue can arise from the algorithm presented in the dining-philosophers problem?

Potential deadlock if all philosophers pick one chopstick simultaneously Signup and view all the answers

In the context of semaphore problems, which of the following reflects an incorrect use of semaphore operations?

wait(mutex) followed by wait(mutex) Signup and view all the answers

What must a philosopher do before starting to eat in the dining-philosophers problem?

Wait for one chopstick and then wait for the second Signup and view all the answers

What describes a situation where two or more processes wait indefinitely for an event that can only be caused by one of them?

Deadlock Signup and view all the answers

In the context of semaphores, which condition allows a process to be added to a semaphore queue?

When S->value is less than zero Signup and view all the answers

What is the main issue with starvation in process synchronization?

A process may never be given CPU time Signup and view all the answers

Which classical synchronization problem involves multiple readers accessing shared data at the same time?

Readers-Writers Problem Signup and view all the answers

What does the 'bounded-buffer problem' specifically deal with?

Fixed-size buffer with producer and consumer processes Signup and view all the answers

In the first variation of the readers-writers problem, what is the consequence of giving priority to readers?

Writers may starve as readers monopolize access Signup and view all the answers

What variable must be initialized to 1 in the bounded-buffer problem to manage access?

mutex Signup and view all the answers

What problem arises when a lower-priority process holds a lock needed by a higher-priority process?

Priority inversion Signup and view all the answers

What does the progress condition ensure in a critical section?

No process will wait indefinitely to enter. Signup and view all the answers

Which of the following best describes bounded waiting?

The request must be granted after a finite number of entries. Signup and view all the answers

In Peterson's solution, which variables indicate process interest and turn?

Boolean flag[i], int turn Signup and view all the answers

What limitation does Peterson's solution have with regard to its application?

It can only synchronize two processes. Signup and view all the answers

What is a critical defect of Peterson's solution on modern hardware?

Processors can reorder operations without dependencies. Signup and view all the answers

What role does the 'turn' variable play in Peterson's algorithm?

It indicates which process can enter the critical section next. Signup and view all the answers

What happens if a process sets its flag to true in Peterson's algorithm?

It signifies the process is ready to enter the critical section. Signup and view all the answers

Which of the following is NOT a characteristic of Peterson's solution?

It is a hardware-based solution. Signup and view all the answers

What is the primary function of atomic hardware instructions in synchronization?

To perform operations without interruption Signup and view all the answers

Which method effectively protects critical regions in a multiprocessor system?

Using locks Signup and view all the answers

How does the Test_and_Set instruction work?

It tests a variable and sets it to true while returning the original value Signup and view all the answers

In the provided solution using Test_and_Set, what does a process do if the lock is already enabled?

It waits continuously in a while loop until the lock is released Signup and view all the answers

What characteristic defines the CompareAndSwap instruction?

It only updates the lock variable if it matches an expected value Signup and view all the answers

What does the boolean variable 'lock' represent in the lock mechanism?

Whether the critical section is currently locked Signup and view all the answers

Why is disabling interrupts considered inefficient on multiprocessor systems?

It prevents all processes from operating simultaneously Signup and view all the answers

What is a potential drawback of using locks for synchronization?

They can lead to unnecessary blocking of processes Signup and view all the answers

Study Notes

Operating Systems - Process Synchronization

Process synchronization is the task of coordinating the execution of processes to prevent race conditions and ensure consistent access to shared data and resources.
Processes can be classified as independent or cooperative, depending on whether one process's execution affects another.
Concurrent access to shared data can lead to data inconsistency.
Data consistency requires orderly execution mechanisms for cooperating processes.

Objectives

Introduce the critical-section problem and its software and hardware solutions to ensure the consistency of shared data.
Examine classical process synchronization problems.
Explore tools used to solve process synchronization problems.

Background

Processes can run concurrently or in parallel.
CPU schedulers switch between processes allowing concurrent execution. Multi-core processors allow parallel execution.
Concurrent access to shared data may result in data inconsistency.
Mechanisms ensure the orderly execution of cooperating processes.

Background - Process Synchronization

Process synchronization is the coordination of process execution to prevent race conditions. Race conditions occur when the outcome of a program depends on the unpredictable order in which processes access and manipulate shared data.
Processes "race" to access or modify shared data.

Producer-Consumer Problem

A classic multi-process synchronization problem.
Producers generate data, put it into a buffer, and continuously generate more data.
Consumers consume data from the buffer.
Producers and consumers share the same memory buffer with a fixed size (bounded).

Producer-Consumer Problem - Solutions

Introduce a counter to keep track of full buffers.
The counter is initialized to 0. Producers increment the counter after producing a new buffer. Consumers decrement the counter after consuming a buffer.
Code examples for producer and consumer processes are provided, demonstrating the expected outcome when incrementing and decrementing the counter.

Race Condition

A race condition arises when multiple processes try to access and modify shared resources concurrently.
The outcome of the program depends on the specific order in which processes execute.
This can lead to incorrect shared data values.

Critical Section

A critical section is a part of a program where a process accesses shared resources.
Only one process can be inside its critical section at a time per resource.

Critical Section Problem - Solution

Mutual Exclusion: Only one process can be in its critical section at any given time.
Progress: If no process is in its critical section and other processes wish to enter, the selection of the next process must not be postponed indefinitely.
Bounded Waiting: A bound must exist on the number of times that other processes are allowed to enter their critical sections after a process has made a request, and before that request is granted.

Peterson's Solution

A software-based solution to the critical-section problem limited to two processes.
Uses shared Boolean flags and a turn variable to give each process a fair chance to enter the critical section.

Synchronization Hardware

Uniprocessors can disable interrupts to prevent preemption, but this can be inefficient on multiprocessors.
Modern machines use atomic instructions—instructions that are non-interruptible—like test_and_set and compare_and_swap to provide locks for critical sections.

Mutex Locks

A simple synchronization mechanism to address the critical section problem using a shared boolean variable representing the lock status.
The acquire() operation ensures exclusive access to shared data, while the release() operation returns the lock.
Typically implemented with hardware-supported atomic operations.

Semaphore

An integer variable that can be modified by atomic wait() and signal() operations.
Wait() decrements the semaphore value and blocks the process if the value is negative.
Signal() increments the semaphore value.
Can solve various synchronization problems.

Semaphore Implementation

Issues with busy waiting in implementations (consuming CPU cycles while waiting for resource availability).
Implementing a waiting queue to allow blocked processes to be swapped out of the CPU, reducing the need for busy-waiting.

Semaphore - Problems

Deadlock: Two or more processes are waiting indefinitely for an event.
Starvation: A process may never be selected for service.
Priority inversion: Scheduling problem where a lower-priority process holds a lock needed by a higher-priority process.

Classical Problems of Synchronization

Bounded-Buffer Problem
Readers and Writers Problem
Dining Philosophers Problem

Bounded-Buffer Problem

A problem with n buffers, each holding one item.
Semaphores mutex (initialized to 1), full (initialized to 0), and empty (initialized to n) coordinate producers and consumers.
Producer threads wait() on empty and add to the buffer, then signal() full.
Consumer threads wait() on full and remove from the buffer, then signal() empty.

Readers-Writers Problem

Data is shared among multiple readers and writers.
Multiple readers can read concurrently, but only one writer can write at a time.
Several variations exist prioritizing reads or writes.

Dining Philosophers Problem

Five philosophers seat around a table with chopsticks.
Each philosopher needs two chopsticks to eat.
An algorithm ensures that no philosopher is starved from accessing the resources.

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Description

Test your knowledge on the fundamental concepts of process synchronization, including cooperative processes, the producer-consumer problem, and the critical-section problem. This quiz explores the consequences of race conditions and various solutions for ensuring proper synchronization.