When to choose SerialGC, ParallelGC over CMS, G1 in Java?

Question

In Java 9, G1 GC is the default Garbage collector. As of now, I heard of some people preferring CMS garbage collector over G1GC as it seems to be not stable and has some nasty bugs.

What happened with ParallelGC (no buzz these days) ? Is there any use case in which we would like to prefer ParallelGC over CMS/G1 ?

Also, is there any case where SerialGC could out perform all these parallel collectors ?

Answer 1

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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).

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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).

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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.

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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).

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Why use G1 as default?

The main reason is to reduce the gc pause time, though the overall throughput might be reduced.