Synchronization Concepts in Programming
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Questions and Answers

What happens if the wait for full spots is neglected in the producer code?

  • The producer will be blocked indefinitely.
  • The buffer will overflow. (correct)
  • The buffer will underflow.
  • The program will deadlock.

In the context of monitors, what guarantees synchronized access to shared data?

  • All threads are allowed simultaneous access.
  • Access to the monitor is unrestricted.
  • Only one thread can access the monitor at a time. (correct)
  • The data is always publicly accessible.

Which programming languages commonly utilize monitors for synchronization?

  • C++ and JavaScript
  • Ada, Java, and C# (correct)
  • Ruby and Go
  • Python and Java

What can be a consequence of misusing semaphores in competition synchronization?

<p>Deadlocks in the program. (D)</p> Signup and view all the answers

During cooperation synchronization, who is responsible for preventing buffer underflow or overflow?

<p>The programmer. (D)</p> Signup and view all the answers

How does a monitor handle calls if it is busy?

<p>Calls are queued until the monitor is available. (C)</p> Signup and view all the answers

Which statement about the relationship between semaphores and monitors is true?

<p>Both can be used to solve the same synchronization problems. (B)</p> Signup and view all the answers

What is the primary function of a semaphore in the provided producer/consumer model?

<p>To control access to the shared buffer. (D)</p> Signup and view all the answers

What happens when the DEPOSIT operation attempts to add a value but there is no room in the buffer?

<p>The caller is placed in the queue of emptyspots. (A)</p> Signup and view all the answers

What does the FETCH operation do when there are no values in the buffer?

<p>It waits for a value to be added before proceeding. (D)</p> Signup and view all the answers

What defines a server task?

<p>It has accept clauses but no other code (B)</p> Signup and view all the answers

What is the primary function of the wait operation in semaphores?

<p>To decrement the semaphore's counter and allow access. (D)</p> Signup and view all the answers

Which statement correctly describes the release operation in semaphores?

<p>It may transfer control to a task in the semaphore's queue. (C)</p> Signup and view all the answers

Which statement accurately describes an actor task?

<p>It can send messages only if it knows the entry name of the receiver. (D)</p> Signup and view all the answers

What is a 'selective wait' in the context of tasks with multiple accept clauses?

<p>It allows selecting from nonempty entry queues nondeterministically. (C)</p> Signup and view all the answers

In the context of producer and consumer tasks, what is the role of the semaphore named fullspots?

<p>To keep track of the number of filled spots in the buffer. (D)</p> Signup and view all the answers

What is a distinguishing feature of a binary semaphore?

<p>Its counter can only have the values 0 and 1. (A)</p> Signup and view all the answers

What happens when all entry queues are empty in a task?

<p>The task waits for a message. (C)</p> Signup and view all the answers

How does a guarded accept clause function?

<p>Its guard determines if it is open or closed. (A)</p> Signup and view all the answers

What is a potential consequence of improperly implemented wait and release operations?

<p>Deadlock can occur if the task ready queue is empty. (B)</p> Signup and view all the answers

What happens to the semaphore's counter during the wait operation if it is greater than 0?

<p>The counter is decremented. (D)</p> Signup and view all the answers

What is meant by an accept clause being 'open'?

<p>It has no guard condition. (B)</p> Signup and view all the answers

In a task with multiple entry points, what ensures that each entry is addressed?

<p>Multiple accept clauses within the loop. (D)</p> Signup and view all the answers

What dictates the execution of the code following an extended accept clause?

<p>It is executed concurrently with the caller. (C)</p> Signup and view all the answers

What happens if all select clauses in a task are closed during execution?

<p>It results in a runtime error. (C)</p> Signup and view all the answers

Which statement correctly describes the purpose of an else clause in a select statement?

<p>It avoids runtime errors by repeating the loop. (D)</p> Signup and view all the answers

What does the concurrency model in Ada 95 introduce that was not present in Ada 83?

<p>Protected objects that allow shared data management. (A)</p> Signup and view all the answers

What is the main function of a protected procedure in Ada 95?

<p>To provide mutually exclusive read-write access to data. (D)</p> Signup and view all the answers

In the Buf_Task example, what occurs when the buffer is full?

<p>The Deposit operation cannot execute. (D)</p> Signup and view all the answers

What guarantees that only one accept clause in a task can be active at a time?

<p>Competition synchronization within accept clauses. (A)</p> Signup and view all the answers

Which of the following statements is true regarding the consumer task in the example?

<p>It repeatedly fetches values for processing. (B)</p> Signup and view all the answers

What does the term 'asynchronous communication' refer to in Ada 95?

<p>Communication that does not require simultaneous sender and receiver activity. (D)</p> Signup and view all the answers

What is the purpose of High-Performance Fortran (HPF)?

<p>To assist compiler optimization for multiprocessor computers. (C)</p> Signup and view all the answers

Which statement best describes the 'DISTRIBUTE' directive in HPF?

<p>It specifies how data should be divided among available processors. (A)</p> Signup and view all the answers

What are the two kinds of data distribution mentioned in the content?

<p>BLOCK and CYCLIC. (D)</p> Signup and view all the answers

In HPF, what does the ALIGN directive do?

<p>It relates the distribution of one array to another. (A)</p> Signup and view all the answers

What is the role of the FORALL statement in HPF?

<p>It allows specified statements to execute concurrently. (C)</p> Signup and view all the answers

Which of the following best describes 'logical concurrency'?

<p>Execution of concurrent units on a single processor. (C)</p> Signup and view all the answers

What methods are used for cooperation synchronization in Java?

<p>wait, notify, notifyAll (B)</p> Signup and view all the answers

What synchronization mechanisms are mentioned in the context of subprogram concurrency?

<p>Semaphores and monitors. (C)</p> Signup and view all the answers

Which statement is true regarding the wait method in Java?

<p>It must be called within a loop. (B)</p> Signup and view all the answers

Which method in C# is used to start the execution of a thread?

<p>Start (B)</p> Signup and view all the answers

What does the PROCESSORS directive in HPF specify?

<p>The number of processors available for use. (B)</p> Signup and view all the answers

What feature distinguishes C# threads from Java threads?

<p>C# can run any method in its own thread. (D)</p> Signup and view all the answers

Which C# class is specifically used for simple integer operations synchronization?

<p>Interlocked (A)</p> Signup and view all the answers

What is a unique aspect of synchronization in C# compared to Java?

<p>Synchronization in C# is more sophisticated. (C)</p> Signup and view all the answers

What does the Monitor class in C# provide for thread synchronization?

<p>Multiple advanced methods for synchronization (D)</p> Signup and view all the answers

What is the purpose of statement-level concurrency in programming?

<p>To minimize communication among multiple processors. (C)</p> Signup and view all the answers

What is the purpose of the semaphore named 'emptyspots' in the producer-consumer model?

<p>To signal that a spot is available when an item is removed (B)</p> Signup and view all the answers

What must producers do if the buffer is full?

<p>Wait until a consumer removes an item and then proceed (D)</p> Signup and view all the answers

In the context of semaphores, what operation ensures that only one producer or consumer accesses the buffer at any given time?

<p>binary semaphore (B)</p> Signup and view all the answers

What can potentially occur if wait and release operations are improperly implemented?

<p>Data corruption due to simultaneous access (D)</p> Signup and view all the answers

Which semaphore is used to indicate that a new item has been added to the buffer?

<p>fullspots (B)</p> Signup and view all the answers

What is the main purpose of cooperation synchronization in this context?

<p>To ensure producers and consumers coordinate access to the buffer (D)</p> Signup and view all the answers

What happens when a consumer tries to remove an item from an empty buffer?

<p>The consumer enters a waiting state (D)</p> Signup and view all the answers

What is the significance of atomic operations in the context of semaphore management?

<p>They ensure that semaphore operations are completed without interruption (B)</p> Signup and view all the answers

What is the purpose of the semaphore named 'access' in the producer-consumer problem?

<p>To ensure mutually exclusive access to the buffer. (D)</p> Signup and view all the answers

Which operation must a producer perform first before adding an item to the buffer?

<p>Wait on the 'emptyspots' semaphore. (C)</p> Signup and view all the answers

What happens if the 'wait(fullspots)' operation is omitted in the consumer code?

<p>The consumer will attempt to remove an item even if the buffer is empty. (C)</p> Signup and view all the answers

What kind of synchronization does the semaphore 'emptyspots' illustrate in the producer-consumer model?

<p>Cooperation synchronization. (C)</p> Signup and view all the answers

Which scenario describes competition synchronization in the context of semaphores?

<p>Producers and consumers accessing the buffer simultaneously without coordination. (B)</p> Signup and view all the answers

In the pseudocode provided, what does the operation 'release(emptyspots)' in the consumer task do?

<p>It indicates that there is now space available in the buffer. (C)</p> Signup and view all the answers

What is the primary purpose of the 'wait' operation in the semaphore mechanisms described?

<p>To block a task until the resource is available. (B)</p> Signup and view all the answers

Which of the following best describes the implication of using semaphores in the producer-consumer problem?

<p>They ensure that only one task operates on the buffer at any given time. (B)</p> Signup and view all the answers

What problem does a monitor solve in an online library system?

<p>Guarantees that only one student can check out a book at a time. (D)</p> Signup and view all the answers

In the context of a ridesharing app, what is the primary function of message passing?

<p>To enable communication between riders and drivers. (A)</p> Signup and view all the answers

How do Java threads contribute to the functionality of an e-commerce website?

<p>They manage concurrent user requests while ensuring synchronized access to shared resources. (C)</p> Signup and view all the answers

What challenge does High-Performance Fortran (HPF) address in weather forecasting models?

<p>It efficiently distributes large datasets across multiple processors. (C)</p> Signup and view all the answers

What is a key advantage of using monitors for synchronization in shared resource operations?

<p>They ensure that operations on shared resources are executed safely and without interference. (A)</p> Signup and view all the answers

In a distributed system like a ridesharing app, what is the purpose of the rendezvous mechanism?

<p>To facilitate the successful match and notification between riders and drivers. (A)</p> Signup and view all the answers

What is the main purpose of synchronized methods in Java when handling multiple user requests?

<p>To maintain accurate updates to the inventory database without overselling. (C)</p> Signup and view all the answers

How does the use of monitors in an online library prevent the issue of duplicate book returns?

<p>By synchronizing the return process and confirming the book's status before accepting a return. (A)</p> Signup and view all the answers

What is the primary advantage of using monitors in a system managing shared resources?

<p>They provide better synchronization and modularity. (A)</p> Signup and view all the answers

In the library management system, what occurs when a student attempts to borrow a book that is already borrowed?

<p>The student is made to wait until the book is available. (B)</p> Signup and view all the answers

What is the purpose of condition variables in monitor implementations?

<p>To manage thread queues and control access. (A)</p> Signup and view all the answers

What is the main responsibility of the receiver task in a message-passing system?

<p>To fetch and process messages from the queue. (D)</p> Signup and view all the answers

In a multi-threaded application handling database requests, why is database access synchronized?

<p>To prevent data corruption caused by simultaneous read/write operations. (C)</p> Signup and view all the answers

How does the FORALL construct in High-Performance Fortran (HPF) facilitate concurrency?

<p>By specifying operations that can run concurrently across distributed data. (C)</p> Signup and view all the answers

What is the function of the wait operation in the monitor context?

<p>To suspend a thread until a certain condition is met. (C)</p> Signup and view all the answers

In the context of Java threads, what does the synchronized keyword control?

<p>It restricts access to a method to one thread at a time. (D)</p> Signup and view all the answers

What happens if the message queue in a distributed system is full?

<p>Sender tasks are blocked until space is available. (A)</p> Signup and view all the answers

What is a critical aspect of managing shared data when implementing monitors?

<p>Using encapsulation to restrict direct access to shared data. (C)</p> Signup and view all the answers

What is the role of signal(messageAvailable) in the message passing example?

<p>To notify waiting sender tasks that a message has been processed. (A)</p> Signup and view all the answers

What does the 'DISTRIBUTE' directive accomplish in High-Performance Fortran?

<p>It allocates data across multiple processors. (B)</p> Signup and view all the answers

Flashcards

DEPOSIT (Semaphore)

The DEPOSIT operation checks if there's room in the buffer. If there is, it decrements 'emptyspots', inserts the value, and increments 'fullspots'. If not, it queues the caller for 'emptyspots'.

FETCH (Semaphore)

The FETCH operation checks if there are values in the buffer. If there is, it decrements 'fullspots', removes the value, and increments 'emptyspots'. If not, it queues the caller for 'fullspots'.

Semaphores: Wait Operation

The 'wait' operation on a semaphore checks its counter. If the counter is greater than 0, it decrements the counter. If the counter is 0, it queues the caller and attempts to transfer control to another ready task.

Semaphores: Release Operation

The 'release' operation on a semaphore checks if its queue is empty. If empty, it increments the counter. If not, it puts the caller in the task ready queue and transfers control to a task from the semaphore's queue.

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Producer Task (Semaphore)

The 'producer' task creates values, waits for buffer space using 'wait' on 'emptyspots', deposits the value using 'DEPOSIT', signals completion by releasing 'fullspots'.

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Consumer Task (Semaphore)

The 'consumer' task waits for a value in the buffer using 'wait' on 'fullspots', fetches the value using 'FETCH', consumes the value, and signals completion by releasing 'emptyspots'.

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Binary Semaphore

A binary semaphore is a special type of semaphore that only has two possible values: 0 and 1. It's used for controlling access to a resource (competition synchronization)

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Competition Synchronization (Semaphores)

Using semaphores for competition synchronization involves a 'binary semaphore' named 'access' to control access to a resource, allowing only one task to access at a time.

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Semaphore

A synchronization mechanism that allows processes to communicate and coordinate access to shared resources.

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Producer-Consumer Problem

A classic synchronization problem where a producer process creates data that a consumer process consumes, using a shared buffer.

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Monitor

A language construct that encapsulates shared data and synchronization operations, ensuring mutual exclusion and safe access to shared resources.

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Competition Synchronization

A mechanism that guarantees exclusive access to shared data by only allowing one process to access it at a time, preventing data corruption.

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Cooperation Synchronization

A mechanism that facilitates communication between processes, ensuring proper order and synchronization of operations.

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Monitor Advantage over Semaphores

Monitors provide a more structured and safer approach to synchronization compared to semaphores, reducing potential for errors.

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Server Task

A task with accept clauses, but no other code. It waits for messages and processes them. Think of it as a receptionist who listens for requests and handles them.

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Actor Task

A task without accept clauses. It can send messages to other tasks but cannot receive them directly.

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Asymmetric Message Passing

In message passing, the sender must know the receiver's entry name, but the receiver doesn't need to know the sender's.

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Entry Point

A specific point in a task where a message can be received.

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Selective Wait

A task with multiple accept clauses waits for a message on any of its entry queues.

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Guarded Accept Clause

An accept clause with a 'when' condition. It only accepts a message if the condition is true.

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Open Clause

A guarded accept clause whose condition is currently true. The task can receive messages on this entry point.

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Closed Clause

A guarded accept clause whose condition is currently false. The task cannot receive messages on this entry point.

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What is a select clause in Ada?

A select clause in Ada allows a task to choose among multiple communication options. This includes accepting messages from other tasks or waiting for specific events.

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How does a select clause work?

The select clause first checks the guards of each 'accept statement. If exactly one guard is 'open' (true), the corresponding accept statement will be executed. If multiple guards are open, the task will choose one non-deterministically. If all guards are closed, a runtime error occurs unless an 'else' clause is present.

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What is the purpose of an else clause?

An 'else' clause in a select statement provides a safe fallback option when none of the guarded accept statements are ready. It prevents the task from entering a runtime error state due to a lack of communication options.

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How does Ada 95 support concurrency?

Ada 95 builds upon the concurrency features of Ada 83, adding two significant enhancements: protected objects and asynchronous communication. Protected objects provide more efficient ways to access shared data, while asynchronous communication allows tasks to interact without blocking each other.

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What are protected objects?

Protected objects in Ada provide safe and structured access to shared data. They resemble abstract data types and allow access through protected procedures (mutually exclusive) and protected functions (concurrent read-only).

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What is the difference between protected procedures and functions?

Protected procedures allow only one task to access the associated protected object at a time, ensuring mutually exclusive access. Protected functions, on the other hand, allow multiple concurrent read-only accesses to the object, preventing unintended data corruption.

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What is message passing?

Message passing is a technique used in concurrency to allow tasks to communicate and exchange data. This is done by sending and receiving messages between tasks, facilitating interaction.

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How does message passing enable competition synchronization?

Message passing, particularly using 'accept' statements in select clauses, inherently provides competition synchronization. This means only one task can be active in an accept statement at a time, preventing multiple tasks from accessing shared resources simultaneously.

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Java Threads: Cooperation Synchronization

A mechanism in Java where threads communicate and coordinate their actions using the wait, notify, and notifyAll methods. These methods allow threads to wait for specific conditions to be met before proceeding.

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Java Threads: wait Method

A method called by a thread to pause its execution until notified by another thread. The wait method must be called within a loop to ensure the thread can resume execution after being notified.

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Java Threads: notify Method

A method called by a thread to signal one of the threads waiting on the same object. The notify method wakes up a single thread from its wait state.

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Java Threads: notifyAll Method

A method called by a thread to signal all the threads waiting on the same object. The notifyAll method wakes up all threads from their wait state.

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C# Threads: Creating and Starting

In C#, a thread is created by instantiating a Thread object. However, creating the thread does not start its execution. The Start method must be called to initiate the thread's concurrent execution.

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C# Threads: Synchronization: Interlocked Class

The Interlocked class in C# provides atomic methods for synchronizing access to shared integer variables, specifically for incrementing or decrementing their values.

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C# Threads: Synchronization: lock Statement

The lock statement in C# defines critical sections of code, ensuring that only one thread can execute the code within the block at a time. This protects shared resources from simultaneous access.

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C# Threads: Synchronization: Monitor Class

The Monitor class in C# provides methods for more complex synchronization scenarios, allowing threads to wait, signal, and release locks in a controlled manner.

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What is High-Performance Fortran (HPF)?

HPF is a set of extensions to Fortran that allows programmers to provide information to the compiler to optimize code for multiprocessor computers. It helps the compiler distribute data and tasks efficiently across multiple processors.

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What are the key HPF specifications?

HPF allows you to specify the number of processors, how data is distributed across processors (BLOCK or CYCLIC), and how to align data between arrays. This helps the compiler optimize data access and communication.

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What is the purpose of the 'FORALL' statement in HPF?

The 'FORALL' statement allows multiple statements to execute concurrently, taking advantage of multiple processors. It specifies a range of indices and says that the statements inside the loop can be executed in parallel.

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What are the two main types of concurrency in HPF?

Concurrency can be at the instruction, statement, or subprogram levels. 'Physical concurrency' uses multiple processors to execute different parts of the code simultaneously. 'Logical concurrency' executes concurrent tasks on a single processor by rapidly switching between them.

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What are the two primary facilities for subprogram concurrency?

Two main approaches are used to manage how different parts of a program interact while running concurrently: 'competition synchronization' and 'cooperation synchronization'.

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What kinds of mechanisms are used to achieve concurrency?

Various mechanisms support concurrency in HPF. Semaphores, monitors, rendezvous, and threads are all techniques used to coordinate the execution of tasks and protect shared data.

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Why is High-Performance Fortran important?

HPF is crucial for optimizing code to run on multiprocessor systems. By providing specific instructions on data distribution and other aspects, it helps the compiler generate code that runs faster and more efficiently.

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Empty Spots Semaphore

Tracks the number of available spots in the shared buffer.

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Full Spots Semaphore

Tracks the number of filled spots in the shared buffer.

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Access Semaphore

A binary semaphore used for mutually exclusive access to the shared buffer.

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Atomic Operations

Operations that occur as a single, indivisible unit, ensuring data integrity.

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What is a Monitor?

A language construct that groups shared data and synchronization operations, ensuring safe access to shared resources.

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What is Competition Synchronization?

A mechanism that prevents multiple tasks from accessing shared data simultaneously, ensuring data integrity.

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What is Cooperation Synchronization?

A mechanism that facilitates communication between tasks, ensuring proper order and synchronization of their operations.

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How does Message Passing provide Competition Synchronization?

When a task sends a message using 'accept' statements, it ensures only one task can be active in that statement at a time, preventing simultaneous access to shared resources.

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Wait Operation (Semaphore)

A task performing a wait operation on a semaphore checks its counter. If the counter is positive, it decrements the counter and proceeds. If the counter is zero, the task waits until the counter becomes positive.

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Release Operation (Semaphore)

When a task releases a semaphore, it increases the semaphore counter. If there are tasks waiting on this semaphore, one of them is allowed to proceed.

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Condition Variable

A mechanism within a monitor that allows threads to wait for specific conditions to be met before proceeding.

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Message Queue

A data structure that stores messages sent between tasks in a distributed system.

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Rendezvous Mechanism

A communication pattern in message passing where a sender sends a message and waits for acknowledgment, ensuring reliable delivery.

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Java Threads

Lightweight units of execution that allow multiple tasks to run concurrently within a Java program.

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Synchronized Code

Code blocks in Java that are protected from concurrent access, ensuring only one thread can execute them at a time.

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High-Performance Fortran (HPF)

An extension of Fortran that enables programmers to optimize code for execution on multiprocessor systems.

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Data Distribution

The process of allocating data across multiple processors in a distributed system for efficient parallel processing.

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Array Alignment

Organizing data elements in arrays to optimize access patterns and minimize communication overhead.

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!HPF$ DISTRIBUTE

A directive in HPF used to specify how data should be distributed across processors, for example, 'BLOCK' or 'CYCLIC'.

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FORALL

A construct in HPF that allows multiple statements to execute concurrently, taking advantage of multiple processors.

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Deadlock

A scenario where two or more threads are blocked indefinitely, waiting for each other to release resources they need.

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Starvation

A situation where a thread is consistently denied access to a resource, even though other threads are using it.

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Mutual Exclusion

A mechanism that ensures only one thread or process can access a shared resource at a time, preventing data corruption.

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Concurrency

The ability of a system to execute multiple tasks or threads seemingly simultaneously, improving performance and responsiveness.

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Study Notes

Chapter 13: Concurrency

  • Concurrency can occur at four levels: machine instruction level, high-level language statement level, unit level, and program level.
  • Instruction- and program-level concurrency issues are not covered in this chapter.
  • Multiprocessor architectures include different types:
    • Late 1950s - one general purpose processor and one or more special-purpose processors for input/output operations.
    • Early 1960s - multiple complete processors for program-level concurrency.
    • Mid-1960s - multiple partial processors for instruction-level concurrency, Single-Instruction Multiple-Data (SIMD) machines, and Multiple-Instruction Multiple-Data (MIMD) machines.
  • This chapter focuses on shared-memory MIMD machines (multiprocessors).
  • Categories of concurrency:
    • Physical concurrency - Multiple independent processors (multiple threads of control).
    • Logical concurrency - The appearance of physical concurrency by time-sharing one processor (software designed as if there were multiple threads).
    • Coroutines (quasi-concurrency) have a single thread of control.
  • Motivations for concurrency:
    • Multiprocessor computers are now widely used that allow physical concurrency.
    • Concurrent execution can be faster than non-concurrent execution, even on single-processor machines.
    • Many real-world situations require concurrent design.
    • Program applications often operate across multiple machines.
  • A task, process, or thread is a program unit that can run concurrently with other units.
  • Tasks differ from subprograms in that task execution can begin implicitly; when a program unit starts a task, it doesn't necessarily suspend; and control might not return to the caller when a task finishes.
  • Tasks typically operate together.
  • Two categories of tasks:
    • Heavyweight tasks execute in their own address space.
    • Lightweight tasks operate in the same address space, which is more efficient.
    • A disjoint task does not affect the execution of other tasks.
  • Task synchronization mechanisms control the order of task execution.
    • Cooperation synchronization: Task A awaits completion of Task B before proceeding.
    • Competition synchronization: Two or more tasks use a shared resource that they can't use simultaneously.
  • Competition synchronization is generally achieved via mutually exclusive access.
  • A scheduler program maps task execution to available processors.
  • Task execution states:
    • New - created but not yet started
    • Ready - ready to run but not currently running
    • Running
    • Blocked - running, but cannot continue
    • Dead - no longer active.
  • Liveness is a characteristic that a program unit may or may not have.
    • In sequential code, liveness means eventually completes execution.
    • In concurrent environments, a task can easily lose liveness, which leads to deadlock.
  • Design issues for concurrency:
    • Competition and cooperation synchronization.
    • Controlling task scheduling.
    • How an application influences task scheduling.
    • How and when tasks start and end execution.
    • How and when tasks are created.
  • Methods for synchronization: semaphores, monitors, message passing.
  • Semaphores (Dijkstra, 1965) are data structures with a counter and a queue for storing task descriptors.
    • Operations: wait (P) and release (V).
    • Implement shared data structure guards.
  • Cooperation synchronization with semaphores: A shared buffer with operations DEPOSIT and FETCH using emptyspots and fullspots semaphores.
    • DEPOSIT checks emptyspots, decrements if space is available, inserts value. If not, caller is put in the emptyspots queue.
    • DEPOSIT increments fullspots when finished.
    • FETCH checks fullspots, decrements if value is available, removes value. If not, caller is put in the fullspots queue.
    • FETCH increments emptyspots when finished.
  • Semaphore wait and release operations.
  • Producer-consumer tasks using semaphores.
  • Competition synchronization with semaphores involves a third semaphore (e.g., access) for mutual exclusion.
    • access counter has only 0 or 1 values.
  • Evaluation of semaphores: misuse can lead to buffer overflow or deadlock.
  • Monitors (Ada, Java, C#): encapsulate shared data and its operations to restrict access.
    • Shared data resides in the monitor.
    • Access to monitor procedures is implicitly queued if the monitor is busy.
  • Cooperation synchronization with monitors: Programmer must guarantee shared buffer does not underflow or overflow.
  • Evaluation of monitors: a better way for competition synchronization than semaphores.
  • Message passing: a general model for concurrency; Central Idea: task communication is like seeing a doctor -- most of the time, you or the doctor wait, but when both are ready, you rendezvous.
  • Message passing rendezvous:
    • A mechanism that allows a task to indicate when it is ready to accept a message.
    • A mechanism to remember who is waiting and choose the next message fairly..
  • Ada 83:
    • Tasks have specification and body parts.
    • The specification has an interface (collection of entry points).
  • Ada task body:
    • Describes the action performed during rendezvous.
    • The sending task suspends while waiting for the message acceptance.
    • Entry points in the specification have accept clauses in the body.
  • Ada message passing semantics:
    • Task executes to the top of the accept clause, waits for messages.
    • Sender is suspended during execution of accept clause.
    • Accept parameters can transfer information in either/both directions.
    • Each accept clause has an associated queue to hold waiting messages
  • Message passing server/actor tasks:
    • Server tasks have accept clauses but no additional code.
    • Actor tasks don't have accept clauses. An actor can send messages to other tasks (asymmetric).
  • Multiple entry points: tasks can have more than one entry point in both specifications and bodies using select statements.
  • Semantics of select with guarded accept clauses:
    • Checks guard for each clause, chooses open queue.
    • If all are closed, it's a runtime error.
    • Else clause for error handling.
  • Competition synchronization with message passing:
    • Mutually exclusive access to shared data is implicit in message passing.
  • Tasks with multiple entries: Using select statements to handle different entries.
  • Java threads: concurrent units are methods named run within a process called a thread.
  • C# threads: Based on Java but with key differences (e.g., methods can run in their own threads, cleaner thread termination). Thread methods Start, Join, Sleep, Abort. Methods for synchronization using Interlocked class, lock statement, and Monitor class.
  • Java thread priority: default, NORM_PRIORITY, MAX_PRIORITY, MIN_PRIORITY.
  • Java synchronization (wait, notify, notifyAll): cooperation synchronization.
  • Java’s concurrency capabilities, while simple, are effective. Not as robust as Ada’s tasks.
  • Statement-level concurrency: Mechanisms to instruct the compiler how to map program statements onto a multiprocessor to minimize communication.
  • High-Performance Fortran (HPF) extensions to Fortran: provide information about distributing data among processors and how to align arrays across processors. Includes !HPF$ PROCESSORS, !HPF$ DISTRIBUTE, and !HPF$ ALIGN directives.
  • HPF FORALL construct: Executes a list of statements concurrently to improve overall performance.

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This quiz explores essential concepts related to synchronization in programming, including monitors, semaphores, and producer-consumer models. It covers their functionalities, guarantees of synchronized access, and common programming languages that utilize these mechanisms. Test your understanding of how these synchronization tools operate and their impact on shared data access.

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