Introduction to RTOS Part 10 - Deadlock and Starvation | Digi-Key Electronics

Summary

This video explores the classic 'Dining Philosophers Problem,' an analogy for managing concurrency in computing. It highlights two key issues in multi-threaded programming: starvation and deadlock. Using the metaphor of philosophers sharing chopsticks to eat, the video delves into potential pitfalls like tasks being blocked indefinitely or never getting a chance to execute. Solutions such as hierarchical mutex acquisition and introducing an arbitrator are discussed to prevent deadlock. The video provides valuable insights for handling synchronization issues in real-time operating systems (RTOS) and offers a challenge to solve the problem in code.

Takeaways

• 😀 The dining philosopher's problem is a classic analogy for understanding concurrency issues, such as deadlock and starvation, in multi-threaded systems.
• 😀 Philosophers represent threads, chopsticks represent mutexes, and the bowl of noodles symbolizes a shared resource in the analogy.
• 😀 Starvation occurs when high-priority tasks continually hog resources, preventing lower-priority tasks from executing.
• 😀 To prevent starvation, techniques such as introducing task delays or using multi-core systems can be used to give lower-priority tasks a chance to execute.
• 😀 Aging is a technique where tasks waiting for a long time in the ready state have their priority gradually increased to prevent starvation.
• 😀 Deadlock occurs when tasks are blocked forever, waiting for each other to release resources, making progress impossible.
• 😀 A simple algorithm for eating in the philosopher’s analogy can lead to deadlock if tasks repeatedly pick up the same resources in an uncoordinated way.
• 😀 Deadlock can be prevented by ensuring that tasks do not block forever while waiting for mutexes, often by implementing timeouts.
• 😀 Timeouts are a way to catch deadlocks by releasing resources if a task cannot acquire a mutex within a given period.
• 😀 Solutions to deadlock and live lock include assigning a priority or hierarchy to mutexes, ensuring that resources are always picked up in a consistent order, and using an arbitrator to control access to shared resources.

Q & A

What is the dining philosophers problem?

The dining philosophers problem is a classic problem in computer science, particularly in the context of operating systems and multi-threading. It involves philosophers sitting at a round table with a shared resource (a bowl of noodles) and a set of chopsticks. The philosophers can only eat when they have two chopsticks. The challenge is to devise an algorithm that ensures that all philosophers get to eat without causing starvation or deadlock.

How is the dining philosophers problem related to multi-threading?

In the dining philosophers problem, each philosopher represents a task or thread, and the chopsticks represent semaphores or mutexes used to control access to shared resources. The problem is an analogy for managing multiple threads that try to access the same shared resources in a concurrent system.

What is starvation in the context of the dining philosophers problem?

Starvation occurs when some tasks (or philosophers, in this analogy) never get a chance to access the shared resource because other tasks (or philosophers with higher priorities) continuously monopolize it. In the dining philosophers problem, it would mean some philosophers never get to eat because others keep taking the chopsticks.

How can starvation be prevented in multi-threaded programs?

Starvation can be prevented by using techniques like introducing delays or yielding control to the scheduler periodically for high-priority tasks. In multi-core systems, higher-priority tasks can run on separate cores, or tasks can be run when triggered by interrupts. Additionally, aging techniques can be used to gradually increase the priority of waiting tasks.

What is deadlock, and how does it relate to the dining philosophers problem?

Deadlock occurs when tasks are unable to proceed because each is waiting on the other to release resources, creating a circular dependency. In the dining philosophers problem, deadlock happens when all philosophers pick up their left chopstick and wait for the right one, but no one can continue eating because they don’t have both chopsticks.

What is a simple algorithm to allow philosophers to eat, and why does it result in deadlock?

A simple algorithm would have each philosopher pick up the left chopstick first, then the right one, and eat. However, this can result in deadlock if all philosophers pick up their left chopstick at the same time and then wait indefinitely for the right chopstick, causing them to be stuck without making any progress.

How can deadlock be prevented in the dining philosophers problem?

One way to prevent deadlock is by assigning a hierarchy to the chopsticks. Philosophers should always pick up the chopstick with the lower number first, ensuring that a circular waiting condition does not occur. Another method is using timeouts, so if a philosopher does not get the chopstick within a certain period, it can try again later.

What is live lock, and how is it different from deadlock?

Live lock occurs when tasks are not blocked but continue to attempt the same operation over and over without making progress. In the dining philosophers problem, this happens if all philosophers time out and repeatedly try to pick up their left chopstick without ever eating, creating an infinite loop without progress. Unlike deadlock, the tasks are not stuck, but they still can't make progress.

How does the timeout technique help in preventing deadlock?

The timeout technique prevents deadlock by ensuring that a task does not block forever while waiting for a resource. If a task can't acquire a resource within the specified time, it times out, releases any resources it has, and retries. This helps avoid a situation where tasks are blocked indefinitely.

What are two solutions to the dining philosophers problem presented in the transcript?

Two solutions presented are: 1) Assigning a hierarchy to the chopsticks, where philosophers pick up the lower-numbered chopstick first to prevent circular waiting and deadlock. 2) Using an arbitrator (such as a waiter) to grant permission to philosophers to pick up their chopsticks, ensuring that only one philosopher can eat at a time. This prevents deadlock but limits parallelism.