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Inter-Task Synchronization and Communication (IPC) Based on Shared Memory
Published in Gedare Bloom, Joel Sherrill, Tingting Hu, Ivan Cibrario Bertolotti, Real-Time Systems Development with RTEMS and Multicore Processors, 2020
Gedare Bloom, Joel Sherrill, Tingting Hu, Ivan Cibrario Bertolotti
Semaphores provide a simple and convenient way of enforcing mutual exclusion among an arbitrary number of tasks that want to have access to a certain shared object. As shown in Figure 7.4, in this case the semaphore is used in a standardized way, according to the following steps: A distinct semaphore ( in the figure) is associated with each shared object to be protected. Only one semaphore is needed, regardless of how complex the object is internally. The initial value of a mutual exclusion semaphore is invariably 1.All critical regions that make access to a certain shared object are surrounded by the primitives and on the corresponding semaphore . In other words, these primitives are used as brackets around each critical region and constitute its entry and exit code, respectively.
LabVIEW Features
Published in Rick Bitter, Taqi Mohiuddin, Matt Nawrocki, LabVIEW™ Advanced Programming Techniques, 2017
Rick Bitter, Taqi Mohiuddin, Matt Nawrocki
The Synchronization palette contains five categories of VIs: semaphores, occurrences, notifications, rendezvous, and queues. The semaphore is also known as a “mutex.” The purpose of a semaphore is to control access to shared resources. One example would be a section of code that was responsible for adding or removing items from an array of data. If this section of code is in the program in multiple locations, there needs to be a way to ensure that one section of the code is not adding data in front of data that is being removed. The use of semaphores would prevent this from being an issue. The semaphore is initially created. During the creation, the number of tasks is set. The default number of simultaneous accesses is one. The next step is to place the Acquire Semaphore VI before the code that you want to protect. The Boolean output of the VI can be used to drive a case structure. A “true” output will indicate that the semaphore was not acquired. This should result in the specified code not being executed. If the semaphore is acquired, the size of the semaphore is reduced, preventing another semaphore from executing until the Release Semaphore VI is executed. When the release is executed, the next available semaphore is cleared to execute.
Task Interactions and Blocking
Published in Ivan Cibrario Bertolotti, Gabriele Manduchi, Real-Time Embedded Systems, 2017
Ivan Cibrario Bertolotti, Gabriele Manduchi
Even if the priority inheritance protocol just described enforces an upper bound on the number and the duration of blocks a high-priority task τH can encounter, it has several shortcomings: In the worst case, if τH tries to acquire n mutual exclusion semaphores that have been locked by n lower-priority tasks, it will be blocked for the duration of n critical regions. This is called chained blocking.The priority inheritance protocol does not prevent deadlock from occurring. Deadlock must be avoided by some other means, for example, by imposing a total order on semaphore accesses, as discussed in Chapter 4.
Application of social game context to teaching mutual exclusion
Published in Automatika, 2018
Miroslav Popović, Klemo Vladimir, Marin Šilić
To ease the difficulty in teaching and learning multithreaded programming, various visualization tools have been proposed. Visualization proposed by Malnati et al [14] is based on recording traces of thread execution. Thread creation events and events of acquiring and releasing locks are recorded and afterwards visualized in the form of a sequence diagram which can help students to better understand the behavior of a multithreaded program. Similar visualization tool proposed by Carr et al [15] provides visualization support for more synchronization primitives, including semaphores, monitors, and channels. In addition to displaying sequence of executions of synchronization primitives, it also provides special views for visualizing suspended threads for each synchronization mechanism and displaying content of messages exchanged over channels. Manickam and Aravind [16] use animation to demonstrate execution of seven mutual exclusion algorithms which meet given criteria of being simple and elegant. State of execution can be observed in each step of the execution. ConEE [17] is a visualization tool and code validator that checks the code for deadlocks and race conditions. The tool validates programs written in a simple language that supports operations with semaphore, mutex, and barrier. ConEE is limited to short programs with a small number of threads, its objective is to help students grasp the complexity of writing concurrent programs. Synchron-ITS system [18] presents a valuable interactive tutoring tool, but is limited in providing a development environment for synchronization of worker tasks. Instead, its functionality focuses on demonstrating the main concepts of shared memory and synchronization visually and interactively.