PDP Chapter 8: Programming Shared Address Space Platforms
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Questions and Answers

What is the primary purpose of OpenMP directives?

  • To translate code between different programming languages
  • To optimize memory allocation in parallel computing
  • To manage concurrency and synchronization without explicit setup (correct)
  • To create software libraries for data analysis

    • Which statement correctly describes the use of the 'num_threads' clause in OpenMP?

  • It is used to specify a condition under which threading occurs.
  • It determines the maximum number of threads that can be created.
  • It specifies the exact number of threads to be created. (correct)
  • It indicates the optimal number of threads for execution.

    • What does the 'private' clause do in an OpenMP directive?

  • It creates copies of specified variables accessible only to each thread. (correct)
  • It indicates that variables are shared among threads.
  • It sets initial values for variables before entering the parallel region.
  • It restricts variable access to the main thread only.

    • How does the 'firstprivate' clause differ from the 'private' clause?

    <p>It initializes variables to their original values before the parallel directive is executed.</p> Signup and view all the answers

    When using the 'if' clause in an OpenMP directive, what is its primary function?

    <p>To conditionally execute the parallel construct based on a scalar expression.</p> Signup and view all the answers

    What happens if the 'is_parallel' variable is set to zero in the given OpenMP example?

    <p>The parallel directive will be ignored, and execution remains serial.</p> Signup and view all the answers

    In an OpenMP context, what role does the master thread play when a parallel directive is executed?

    <p>It becomes the primary thread and manages all other subsequent threads.</p> Signup and view all the answers

    What is the purpose of the counter in a barrier implementation?

    <p>To hold the threads until all have reached the barrier.</p> Signup and view all the answers

    How does the mylib_rwlock_unlock function determine whether to signal pending writers?

    <p>By checking if there are no remaining readers.</p> Signup and view all the answers

    In the mylib_rwlock_wlock function, what condition causes the thread to wait?

    <p>If there are active writers and readers.</p> Signup and view all the answers

    What primary function does the mylib_init_barrier function serve?

    <p>To initialize the count of the barrier to zero.</p> Signup and view all the answers

    In a read-write lock mechanism, what is the state of the writer variable when a write lock is active?

    <p>It is incremented by one.</p> Signup and view all the answers

    What happens when the last thread reaches the barrier?

    <p>It wakes up all other threads waiting at the barrier.</p> Signup and view all the answers

    What must occur before a thread exits the mylib_rwlock_wlock function?

    <p>The writer variable must be incremented.</p> Signup and view all the answers

    In the barrier structure, what is the purpose of the ok_to_proceed condition variable?

    <p>To signal when a thread can continue past the barrier.</p> Signup and view all the answers

    What is the runtime complexity of a linear barrier for n threads?

    <p>O(n)</p> Signup and view all the answers

    In the context of barriers, what is a log barrier?

    <p>A barrier that organizes threads using multiple condition variables.</p> Signup and view all the answers

    Which statement about OpenMP is correct?

    <p>OpenMP is a directive-based API applicable to FORTRAN, C, and C++.</p> Signup and view all the answers

    What should never be relied upon for exchanging data between threads?

    <p>Liveness of data from scheduling assumptions</p> Signup and view all the answers

    How can the performance of a barrier implementation be improved?

    <p>By organizing multiple barrier variables in a tree formation.</p> Signup and view all the answers

    Which of the following statements regarding the use of OpenMP directives is true?

    <p>OpenMP can be used to manage shared memory in parallel programming.</p> Signup and view all the answers

    What is a crucial tip for designing asynchronous programs?

    <p>Avoid relying on scheduling for synchronization.</p> Signup and view all the answers

    What does the reduction clause in OpenMP primarily target?

    <p>Aggregating values across multiple threads.</p> Signup and view all the answers

    Study Notes

    Programming Shared Address Space Platforms

    • This topic covers parallel and distributed processing.
    • It focuses on programming models for concurrency and synchronization.
    • Programming models provide support for expressing concurrency and synchronization.

    Topic Overview

    • Thread Basics
    • The POSIX Thread API
    • Synchronization Primitives in Pthreads
    • Composite Synchronization Constructs
    • OpenMP: a Standard for Directive-Based Parallel Programming

    Overview of Programming Models

    • Programming models support expressing concurrency and synchronization.
    • Process-based models treat data associated with a process as private by default.
    • Lightweight processes (threads) assume memory is global.
    • Directive-based models extend the threaded model, facilitating thread creation and synchronization.

    Thread Basics

    • A thread is a single stream of control in a program.
    • All memory in a logical thread model is globally accessible to all threads.
    • Function call stacks are local to a thread for liveness reasons.
    • A flat model (all memory global) can lead to poor performance in physically distributed machines.
    • Threads provide software portability, support for latency hiding, scheduling, and load balancing.

    The POSIX Thread API (Pthreads)

    • Pthreads is a standard thread API.
    • The underlying concepts are largely independent of the API.
    • Pthreads can be used in conjunction with other thread APIs.

    Thread Basics: Creation and Termination

    • Pthreads offers functions for concurrency specification.
    • pthread_create creates a thread running a specific function.
    • pthread_join waits for a thread to complete execution.

    Multithreaded Calculate PI

    • This example calculates Pi using multiple threads.
    • It demonstrates creating, managing, and joining threads for parallel execution.
    • Multithreading improves performance for computationally intensive tasks. Performance is dependent on the number of threads, sample points, and underlying architecture.

    Critical Section

    • Incoherent results can arise when multiple threads access the same data item without proper synchronization.
    • A critical section is a code segment that should be executed by only one thread at a time.
    • Mutual exclusion mechanisms prevent race conditions within critical sections.

    Critical Section (Continued)

    • Threads must observe shared data writes in a defined order.
    • Rules enforce consistency among reads and writes to shared locations by different threads.

    Synchronized Programming model

    • A synchronized program orders all accesses to shared data by synchronization operations.
    • Data references are ordered via synchronization (a read and write pair) when thread access is separated by a synchronization operation pair.

    Synchronized Programming model (Continued)

    • Lock and unlock operations guarantee sequential access to shared data.
    • Mutual exclusion ensures only one thread can update (or access) the variable at a time.

    Synchronization

    • Synchronization is a software technique to manage thread access to shared memory items and code sections.
    • It requires synchronization elements (like mutexes) and operations acting on these elements (e.g., lock, unlock).

    Synchronization (Continued)

    • Synchronization routines (like lock and unlock) are user-level software routines.
    • Lock: grants access for only one thread.
    • Unlock: releases access allowing other threads to acquire.

    Synchronization: Atomic Primitives

    • Atomic primitives guarantee that the memory location change is performed without interruption by other threads.
    • This helps in safely updating shared memory.

    Mutual Exclusion

    • Critical sections require mutual exclusion to ensure safety.
    • Pthreads offers pthread_mutex_lock, pthread_mutex_unlock, and pthread_mutex_init functions to manage mutex locks.
    • pthread_mutex_trylock is often more efficient than a lock on busy systems.

    Producer-Consumer Using Locks

    • The producer-consumer scenario has constraints related to shared buffer access.
    • The producer must not overwrite the buffer if the consumer hasn't processed the previous task.
    • Consumers wait for a task in the shared buffer to be present.

    Producer-Consumer Using Locks (Continued)

    • Individual consumers pick up tasks sequentially in the producer/consumer queue.
    • Multiple producers and consumers must be managed.
    • Mechanisms to ensure access are needed.

    Producer-Consumer Using Locks (Continued)

    • Illustration of producer/consumer interaction scenarios.

    OpenMP: a Standard for Directive-Based Parallel Programming

    • OpenMP is a directive-based API for shared memory programming.
    • It simplifies concurrency management and handles synchronization.

    OpenMP Programming Model

    • OpenMP directives are based on #pragma compiler directives.
    • A directive consists of a directive name and a clause list.
    • OpenMP programs execute serially until the parallel directive.
    • The main thread becomes the master thread of a thread group.

    OpenMP Programming Model (Continued)

    • The clause list dictates conditional parallelization, thread numbers, and data handling.
    • if, num_threads, private, firstprivate, shared, and default clauses are key elements of the clause list.

    Reduction Clause in OpenMP

    • Multiple local copies of a variable are combined into a single copy (e.g., using +,-,*, /, &,| operations) when threads finish execution.
    • The variables in a reduction list are implicitly private.

    OpenMP Programming: Example; Producer/Consumer

    • Illustrative example of using OpenMP for a producer/consumer model, demonstrating variable management (shared and private) as well as data access.

    Specifying Concurrent Tasks in OpenMP

    • for and sections directives specify iterative and non-iterative parallel task assignments.

    Specifying Concurrent Tasks in OpenMP (Continued)

    • The for directive splits iteration space amongst threads.
    • schedule, nowait, and ordered are useful clauses within the for directive.
      • nowait is commonly used when for-loops execute concurrently without waiting for the completion of the loop iterations from other thread(s).
    • sections are used for non-iterative tasks.

    OpenMP Directives: Examples

    Numerous examples show how to use for, sections, parallel, critical, and other directives.

    OpenMP Library Functions

    • OpenMP provides functions to control thread execution, such as omp_set_num_threads.
    • These functions manage low-level operations, such as thread creation and management, under OpenMP.

    OpenMP Environment Variables

    • OMP variables guide OpenMP execution (e.g., OMP_NUM_THREADS to set the number of threads).
    • These variables control how the OpenMP program behaves and, for example, how execution time varies according to the number of threads.

    Producer-Consumer Using Locks (Code Examples)

    Illustrative code examples of producer/consumer implementations that are implemented using synchronization constructs.

    Condition Variables for Synchronization

    • Condition variables allow a thread to wait for specific conditions to hold.
    • Threads wait for the condition to become true before continuing.

    Condition Variables for Synchronization (Continued)

    • Threads synchronize to wait on the condition.

    Condition Variables for Synchronization (Continued)

    • The condition variable pthread_cond_wait, pthread_cond_signal, pthread_cond_broadcast are commonly employed.

    Barriers

    • Barriers synchronize all threads participating in a barrier construct before continuing execution.
    • Counter-based implementations track the number of threads arriving at the barrier.
    • The final thread reaching the barrier signals/notifies other waiting threads to proceed.

    Barriers (Continued)

    • Barrier concepts enable synchronization.
    • Linear, and log barrier implementation techniques and their performance evaluations.

    Overheads of Locking

    • Locks introduce serialization and can impede performance for critical sections.
    • Use pthread_mutex_trylock for efficiency gains, as avoiding lock queues in busy systems can reduce idling.

    Tips for Designing Asynchronous Programs

    • Explicit programming techniques minimize reliance on scheduling during data exchanges.
    • Using group synchronization aids in avoiding false sharing and data contention issues.
    • Pthreads and OpenMP tools offer support for explicit thread programming.

    Assigning Iterations to Threads

    OpenMP's schedule class (static, dynamic, guided, or runtime) guides the assignment of iterations to threads Different scheduling classes influence how workload is distributed amongst threads. Different scheduling approaches yield different performance.

    Parallel For Loops

    • For loops often use nowait (avoid implicit synchronization) for more concurrent execution (and thus improved performance for loops that use no inter-thread synchronization). This means threads may perform the tasks without waiting for each other after a particular loop iteration is done.

    Parallel For Loops: Example

    • Example illustrating OpenMP parallel for and similar directives.

    The sections Directive

    • Assign tasks non-iteratively to multiple threads.
    • The section directives segment the code and enable parallel execution of blocks amongst threads.

    Nesting parallel Directives

    • OMP_NESTED environment variable allows omp parallel directives within other omp parallel clauses, creating nested parallel regions.
    • Enhances concurrency.

    Synchronization Constructs in OpenMP

    • OpenMP provides barrier, single, master, critical, and ordered directives to manage complex synchronization tasks in threaded applications.

    OpenMP Library Functions

    • omp_set_num_threads, omp_get_num_threads, omp_get_max_threads, omp_get_thread_num, and omp_in_parallel control thread execution or retrieve details.
    • These functions generally manage the low-level details of thread operations.

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    Programming Shared Address Space Platforms PDF

    Description

    Test your knowledge on OpenMP directives, their clauses, and the behavior of functions related to parallel programming. This quiz covers essential concepts such as 'num_threads', 'private', and read-write locks in OpenMP. Enhance your understanding of parallel computing with these targeted questions.

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