Serial collector
Mainly for single-cpu machine.
Algorithm:
It use a single thread to handle heap, and perform stop-the-world pause during any gc. Just see it as toy.
This is default for client-class machine (32bit jvm on windows or single-cpu machine).
Parallel collector
Algorithm:
It uses multiple gc threads to handle heap, and perform stop-the-world pause during any gc.
<= Java 8, this is default for server-class machine (multi-cpu unix-like machine or any 64bit jvm).
CMS collector
It’s designed to eliminate the long pause associated with the full gc of parallel & serial collector.
Algorithm:
It use 1 or more gc threads to scan the old generation periodically, and discard unused objects, the pause is very short, but use more cpu time.
Warning: since Java 14, it’s removed.
G1 collector
It’s low pause / server style gc, mainly for large heap (> 4Gb).
Algorithm:
- Similar as CMS, it use multiple background gc threads to scan & clear heap.
- It divide old generation into parts, it could clean old generation by copy from 1 part to another.
Thus it’s less possible to get fragmentation.
Since Java 9, this is default for server-class machine (multi-cpu unix-like machine or any 64bit jvm).
Why use G1 as default?
The main reason is to reduce the gc pause time, though the overall throughput might be reduced.