Spurious Wakeups in Java: Reality or Myth?

The concept of spurious wakeups in Java synchronization has been a subject of discussion for quite some time. While the potential for such behavior exists, the question remains: Do they actually occur in practice?

Linux's Wakeup Mechanism

According to the Wikipedia article on spurious wakeups, the Linux implementation of the pthread_cond_wait() function utilizes the futex system call. When a process receives a signal, it may return abruptly with EINTR, causing its blocking system calls to terminate early.

Futex and Spurious Wakeups

This race condition arises because pthread_cond_wait() cannot resume the waiting thread if it has missed a real wakeup while executing outside the futex system call. As a result, a POSIX signal can trigger a spurious wakeup.

Example in Java

The provided Java program demonstrates the concept:

public class Spurious {
    public static void main(String[] args) {
        Lock lock = new ReentrantLock();
        Condition cond = lock.newCondition();
        lock.lock();
        try {
            try {
                cond.await();
                System.out.println("Spurious wakeup!");
            } catch (InterruptedException ex) {
                System.out.println("Just a regular interrupt.");
            }
        } finally {
            lock.unlock();
        }
    }
}

Provoking Spurious Wakeups

To induce a spurious wakeup in this Java program, a signal can be sent to the process while it is waiting on the condition. This can be achieved on Linux using a command like:

kill -s SIGUSR1 

Performance Benefits

While spurious wakeups can be considered an annoyance in some scenarios, their occurrence is generally rare in modern operating systems. They do, however, play a role in the performance optimization of the system by preventing unnecessary busy waiting and reducing overhead when dealing with multiple threads waiting on the same condition variable.