249. Introducing G1

The G1 Garbage Collector is probably the most mature, maintained, and improved GC in Java. It was introduced in JDK 7 update 4, and from JDK 9, it became the default GC. This GC sustains high throughput and low latency (a few hundred milliseconds), being known for its balanced performance.

Internally, G1 splits the heap into equally small chunks (max size of 32 MB), which are independent of each other and can be allocated dynamically to Eden, Survivor, or Tenured spaces. Each such chunk is called the G1 heap region. So, G1 is a region-based GC.

Figure 12.15: G1 splits the memory heap into equal small chunks

This architecture has a significant number of advantages. Probably, the most important one is represented by the fact that the Old generation can be cleaned up efficiently by cleaning it up in parts that sustain low latency.

For a heap size smaller than 4 GB, G1 will create regions of 1 MB. For heaps between 4 and 8 GB, G1 will create regions...