Starvation Demonstration with Thread Priorities

This code snippet demonstrates thread starvation, where a low-priority thread is continuously denied access to a shared resource by higher-priority threads.

The Code

This code creates two threads: `highPriorityThread` and `lowPriorityThread`. `highPriorityThread` is assigned the maximum priority, while `lowPriorityThread` is assigned the minimum priority. Both threads repeatedly attempt to acquire a lock on the `resource` object and print a message. Because the `highPriorityThread` is given priority by the OS scheduler, it will likely acquire the lock much more frequently than the `lowPriorityThread`, potentially leading to starvation.

public class StarvationExample {

    private static final Object resource = new Object();
    private static volatile boolean running = true;

    public static void main(String[] args) throws InterruptedException {
        Thread highPriorityThread = new Thread(() -> {
            while (running) {
                synchronized (resource) {
                    System.out.println("High priority thread is running");
                    try { Thread.sleep(1); } catch (InterruptedException e) {}
                }
            }
            System.out.println("High priority thread finished");
        });
        highPriorityThread.setPriority(Thread.MAX_PRIORITY);

        Thread lowPriorityThread = new Thread(() -> {
            while (running) {
                synchronized (resource) {
                    System.out.println("Low priority thread is running");
                    try { Thread.sleep(1); } catch (InterruptedException e) {}
                }
            }
             System.out.println("Low priority thread finished");
        });
        lowPriorityThread.setPriority(Thread.MIN_PRIORITY);

        highPriorityThread.start();
        lowPriorityThread.start();

        Thread.sleep(5000); // Let the threads run for 5 seconds
        running = false; // Signal the threads to stop
    }
}

Concepts Behind the Snippet

This example demonstrates how thread priorities can lead to starvation. When multiple threads compete for a resource, the scheduler typically gives preference to higher-priority threads. If higher-priority threads continuously acquire the resource, lower-priority threads may be unable to access it, resulting in starvation. Thread priorities are not guaranteed to be strictly enforced by all operating systems and JVM implementations, so the effect might vary.

Explanation of Starvation

The `highPriorityThread` will likely dominate the `resource` lock, preventing the `lowPriorityThread` from executing its synchronized block frequently. As a result, you will see "High priority thread is running" printed much more often than "Low priority thread is running". In some cases, the `lowPriorityThread` might not even get a chance to execute at all during the 5 seconds the threads are allowed to run.

Real-Life Use Case Section

Starvation can occur in real-time systems where tasks have different priorities. A high-priority task might continuously preempt lower-priority tasks, preventing them from completing. This can lead to system instability if the starved tasks are essential for the system's overall functionality. Another example is in web servers. If requests are processed based on priority (e.g., premium users get higher priority), requests from regular users may be starved during periods of high load.

Best Practices to Avoid Starvation

Several strategies can be used to mitigate starvation: 1. Avoid Using Thread Priorities: Relying heavily on thread priorities can lead to unpredictable behavior and starvation. Consider using other synchronization mechanisms. 2. Fairness: Use fairness policies in locks (e.g., `ReentrantLock` with a fair ordering policy) to ensure that all threads eventually get a chance to acquire the lock. 3. Resource Allocation Strategies: Implement resource allocation strategies that guarantee all threads will eventually get access to the resources they need. 4. Aging: Gradually increase the priority of threads that have been waiting for a long time. This can help to prevent starvation by giving them a temporary boost in priority.

Interview Tip

Be prepared to discuss the difference between deadlock and starvation. Deadlock involves a complete standstill, while starvation involves a thread being perpetually denied access to a resource. Also, be able to explain how thread priorities can contribute to starvation and suggest ways to avoid it.

When to use them

Understanding starvation is essential when designing concurrent systems where fairness and responsiveness are important. You generally want to avoid starvation! Recognizing the potential for starvation helps you choose appropriate synchronization mechanisms and scheduling policies to ensure that all threads get a fair share of resources.

Alternatives

Alternatives to relying on thread priorities include: 1. Fair Locks: Use `ReentrantLock` with a fair ordering policy. This ensures that threads acquire the lock in the order they requested it. 2. Queues: Use queues to manage tasks and ensure that all tasks are eventually processed. 3. Work-Stealing: Use a work-stealing algorithm to distribute tasks among threads. This can help to balance the workload and prevent some threads from being overloaded while others are idle.

Pros

• Demonstrates the potential issues of using thread priorities.
• Highlights the concept of unfair resource allocation.

Cons

• The effects of thread priorities can vary depending on the JVM and operating system.
• The example is simplified and may not fully capture the complexity of real-world starvation scenarios.