The G1 Garbage Collector (G1GC) is a garbage collector introduced in Java 7 update 4. It is designed to improve the overall garbage collection performance and reduce GC pause times in large heap sizes. The G1GC is a server-style garbage collector, which means that it is designed to be used in large heap sizes that are common in server applications.
The G1GC divides the heap into multiple regions, each of which is a contiguous range of memory. The size of these regions is configurable, but the default is 1MB. During normal operation, the G1GC continuously collects the least used regions that have the most garbage, and compacts them to free up memory. This makes it possible to perform garbage collection concurrently with the running application, and reduces the overall GC pause times.
The G1GC uses a generational garbage collection algorithm, similar to other garbage collectors in the JVM. It divides the heap into young and old generations. Objects that are recently created are placed in the young generation, while long-lived objects are placed in the old generation. The G1GC performs garbage collection on both the young and old generations, but the focus is on the old generation, as it is more likely to contain long-lived objects.
One of the key advantages of the G1GC is its ability to dynamically adjust the amount of time spent on garbage collection based on the application’s behavior. The G1GC uses a technique called “pause prediction” to estimate the amount of time required to collect the heap, and adjusts the garbage collection behavior accordingly. This allows the G1GC to provide consistent garbage collection performance and minimize GC pause times.
Overall, the G1 Garbage Collector is a modern garbage collector that offers improved performance and reduced GC pause times for large heap sizes. However, it is important to note that the G1GC may not be the best choice for all applications. For example, applications with small heap sizes may not see significant performance benefits from using the G1GC, and may instead benefit from using other garbage collectors in the JVM.