Using busy rotation as wait strategy in Java

In the dynamic world of Java programming, achieving optimal performance is often a central goal for developers. This is an area where different waiting strategies, including busy rotation, can play a key role. This article aims to understand in detail busy rotation as a wait strategy in Java, its importance and how to utilize it effectively.

Understand the waiting strategy

In concurrent programming, the wait policy determines how threads should wait when no work is available. Different wait strategies can greatly affect the performance of concurrent applications. A common approach is blocking, where a thread gives up its processor time and goes to sleep until new work becomes available. However, waking up a sleeping thread can take a significant amount of time, causing performance degradation in latency-sensitive applications. This is where Busy Spin comes into play.

Busy rotation explanation

Busy spin is a wait strategy in which a thread continuously loops (or "spins") until work becomes available. This strategy avoids the delay caused by thread wake-up, but at the cost of higher CPU utilization. Busy spinning is best suited for low-latency, high-performance applications where work wait time is typically less than the cost of putting a thread to sleep and waking it up

In Java you can create a simple busy spin loop as shown below -

while (!workAvailable()) {
   Thread.onSpinWait();
}

Here, workAvailable() is a method to check whether the thread has work. If there is no work available, the thread will call Thread.onSpinWait(), which indicates to the JVM that it is in a busy spin loop.

Thread.onSpinWait() method in Java

Java 9 introduced the Thread.onSpinWait() method, which is specifically designed for busy spins. When a thread calls this method, it signals to the JVM that it is in a busy spin loop. This allows the JVM and underlying hardware to optimize the execution of the spin loop, for example, by reducing power consumption or the priority of the spin thread.

When to use busy rotation

Although busy spinning has the potential to reduce latency, it should be used with caution as it may result in high CPU usage. It's best suited for high-performance applications where low latency is critical and the wait time for work is typically short. Examples might include financial trading systems or high-performance gaming servers.

Conversely, for applications where low CPU usage is more important than low latency, or where threads frequently need to wait a long time to do their work, other waiting strategies (such as yielding or blocking) may be more appropriate.

in conclusion

Mastering different wait strategies, including busy spinning, is critical to writing efficient concurrent Java applications. Busy spinning can significantly reduce latency in some scenarios, but it needs to be used with caution due to high CPU usage. The Thread.onSpinWait() method introduced in Java 9 provides a valuable tool for implementing and optimizing busy spin loops in Java.

Keep in mind that the choice of wait strategy should always depend on the specific requirements of the application. Understanding the trade-offs of different wait strategies enables you to make informed decisions and create high-performance, scalable, and efficient applications using Java.

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