Garbage Collection process, heap space and GC types in Java

 

• Garbage collection process is run by a daemon thread, processing involves

• Mark: To mark objects as reachable/unreachable

• Delete/Sweep: To delete unreachable objects from heap space

• Compacting: Making allocation contagious & fragmented

• GL Logging: -verbose: gc, --XX:+PrintGCDetails, -Xloggc:gc.log (NOT much overhead, but useful if GC is suspect)

• Heap space is divided as follows

• Serial: Works with single thread, pauses the app, to be used where pause time matters less

• Concurrent: Called as CMS (Concurrent Mark Sweep) using multiple threads along with the app. To be used where there is more memory, short pause time acceptable (for mark/remark only) and the app has resources (CPU) to share with GC.

• Parallel: Uses multiple CPUs for GC, does not kick until heap is full/near-full. Pauses the app when it runs. To be used when there is less memory, less CPUs and the app can accept pauses.

• G1 (Garbage First): Partitions the heap space into equal-sized regions. It ‘marks’ all the regions and then collects in ‘mostly empty’ areas, freeing up space. To be used where there is high memory, high CPU. It is combination of concurrent & parallel

* A couple of images above are taken from one of the YouTube videos of Ranjith Ramchandran, thank you.