LazySet Operations in Java

LazySet is a method offered by Atomic classes in Java. The lazySet method offers significantly cheaper volatile writes for single writers. Let's first find out how lazySet method achieves this performance gain. To find out this, we need to go into the Java Memory Model (JMM). Suppose a primitive variable is shared among different CPU cores. Each core has a local cache where it can store the variable for faster access so that it doesn't have to go to the main memory every time it has to access the variable. If one thread makes a change to this variable, it won't be reflected to the other core since it has a different cache. To mitigate this problem, the Java memory model offers volatile keyword. The reads and writes on a volatile variable would by-pass the cache and go to the main memory. But this comes at a latency cost and lazySet can be used to mitigate this cost. Please note that the cache is not really bypassed, but a memory barrier is used.

The Java Atomic variables have get() and set() methods that work as reads and writes on volatile variables with the additional feature of semi-volatile write. When you change the value of an Atomic variable through the lazySet method, the value is updated in the cache but is not pushed to the main memory. It can take indefinite amount of time of the change to be pushed to the maim memory so that other threads can see the new value. Since in case of lazySet, value is not pushed to the main memory every time, writes are significantly faster. The most important use case lazySet operation is to implement asynchronous logging.