Busy Waiting in Multithreading in Java

Using the multithreading approach known as "busy waiting," a thread keeps waiting for a condition to be satisfied without giving up CPU control. Because the thread actively uses CPU cycles as it waits, this strategy may result in poor CPU utilization.

A thread in Java may experience busy waiting if it keeps looping until a condition is met. Due to its resource inefficiency, it is often seen as an inferior technique when compared to alternative synchronization mechanisms, even if it is occasionally essential.

Busy Waiting

When a thread is busy waiting, it keeps checking a condition in a loop without adding any value. Until the condition is met, the thread keeps running and uses CPU resources. In contrast, with more effective waiting systems, a thread is either stopped or put to sleep until it receives explicit notification.

Output:

 
Waiting thread started...
Condition changed to true.
Condition met. Exiting waiting loop.   

In this instance, waitingThread continuously monitors the conditionMet variable's value to carry out busy waiting. WaitingThread breaks out of the loop when conditionChangerThread sets conditionMet to true.

Features of Busy Waiting

1. CPU-intensive: When the loop continues for a long time, the waiting thread actively uses CPU resources, which might result in excessive CPU utilization and decreased system performance.
2. Easy to Implement: All that's needed to implement busy waiting is a loop that continually checks the condition.
3. Low Latency Reaction: When the situation changes, the waiting thread may respond very instantly since it is continuously monitoring it.

Disadvantages of Busy Waiting

Busy waiting is often seen as inefficient because of a number of disadvantages, even if it can provide minimal latency in reaction to a change in condition:

1. High CPU Usage: Because the waiting thread is continuously operating, busy waiting might result in high CPU usage. It may impact the effectiveness of other programs and procedures.
2. Inefficient Use of Resources: The thread is not producing any valuable work while it is waiting. CPU cycles that may be put to better use elsewhere are wasted as a result.
3. Scalability Problems: Resource waste accumulates in a multi-threaded environment when several threads are occupied waiting, resulting in poor scalability and perhaps crashing the system.

Alternatives to Busy Waiting

To avoid the inefficiencies of busy waiting, Java provides several synchronization mechanisms that allow threads to wait more efficiently.

1. Using wait() and notify() Methods

As part of Java's Object class, the wait() and notify() methods allow a thread to wait and to be notified when a condition changes. When we call the wait() method, a thread releases any locks it has and enters a waiting state until another thread calls the notifyAll() or notify() method on the same object.

2. Using java.util.concurrent Package

Java's java.util.concurrent package provides various constructs to manage synchronization without busy waiting:

• CountDownLatch: It permits one or more threads to wait for a sequence of actions carried out by other threads to finish.
• CyclicBarrier: It enables a group of threads to wait for one another to get to a shared barrier.
• Semaphore: Controls access to a resource by managing a set of permissions.
• Lock and Condition: A more versatile and potent synchronization tool than wait/notify and synchronized.

When to Use Busy Waiting?

Busy waiting, however ineffective, may be justified under some of the following situations:

Low Latency Requirement: When there is very little waiting time and a thread has to respond quickly to a condition change.

1. Hardware Interaction: In order to communicate directly with hardware in some low-level programs, busy waiting may be necessary.
2. Spinlocks: A spinlock is a kind of busy waiting that is intended to safeguard crucial areas where there should be very little waiting time.

Conclusion

In Java applications, busy waiting is a straightforward but ineffective method of thread synchronization that can result in excessive CPU use and subpar performance. Java has more effective synchronization techniques to assist in preventing the problems associated with busy waiting, such as locks, conditions, and wait() or notify() methods.