Understanding the onSpinWait() Method in the Thread Class - Java 9

What is the onSpinWait() method in the Thread class of Java 9 and how is it used?

// Usage of onSpinWait() in a busy-wait loop
public class SpinWaitExample {
    public void spinUntilCondition() {
        while (!conditionMet()) {
            // Utilizes onSpinWait for optimal CPU usage
            Thread.onSpinWait();
        }
    }
    private boolean conditionMet() {
        // logic to check condition
    }
}

The onSpinWait() method is a performance optimization utility introduced in Java 9, aimed at improving the efficiency of busy-wait loops in multi-threaded environments.

// Example usage of onSpinWait in a multi-threaded scenario
public class Example {
    private final AtomicBoolean condition = new AtomicBoolean(false);
    public void spin() {
        while (!condition.get()) {
            Thread.onSpinWait(); // Hint to JVM for optimized waiting
        }
    }
    public void setCondition() {
        condition.set(true); // Change condition
    }
}

Causes

• Efficient Waiting Mechanism: It reduces CPU utilization when threads are waiting for a condition to change without blocking.
• Improved Performance: It provides hints to the JVM that the current thread is in a spin-wait state, allowing for better management of processor resources.

Solutions

• Use in Conjunction with Busy-Wait Loops: Implement onSpinWait() in loops where threads continuously check a condition, minimizing CPU usage while waiting.
• Combine with Atomic Variables: It is often used with atomic variables to provide a responsive and low-latency waiting mechanism.

Mistake: Not using onSpinWait() properly; for example, using it in place of proper locking mechanisms.

Solution: Ensure that onSpinWait() is only used in scenarios where spinning is acceptable and won't lead to excessive CPU usage.

Mistake: Assuming onSpinWait() is a substitute for Thread.sleep() or yield().

Solution: Recognize that onSpinWait() is designed for waiting in a spin-wait context, whereas Thread.sleep() and yield() are blocking mechanisms.