Collector Combinations: Serial, Parallel, Concurrent, G1, Z

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∟Collector Combinations: Serial, Parallel, Concurrent, G1, Z

This section describes different garbage collector combinations supported by JVM 12 HotSpot: Serial, Parallel, Concurrent, G1 and Z

In previous sections, we discussed garbage collectors for Young generation and Tenured generation independently:

4 garbage collectors for the Young generation: Serial, Parallel Scavenge (PS), Parallel New (ParNew), and Garbage First (G1).
4 garbage collectors for the Tenured generation: Serial, Parallel Old (ParOldGen), Concurrent Mark Sweep (CMS), and Garbage First (G1).

A single generation collector, called Z (or ZGC) The Z collector has been introduced since JVM 11 and it is still available only on Linux operating systems as an experimental option. The Z (or ZGC) Collector can be invoked by the "-XX:+UseZGC" option.

Now let's look dependencies and compatibilities of garbage collectors. As of JVM 17, HotSpot JVM can support the following 5 combinations of garbage collectors:

GC Combination           Minor GC          Major GC            Thread
--------------           --------          -------            ------

Serial                   Serial            Serial             1
-XX:+UseSerialGC         Stop-the-world    Stop-the-world

Parallel                 Parallel Scavenge Parallel Old
-XX:+UseParallelGC       Mostly Parallel   Mostly Parallel    Multi
-XX:+UseParallelOldGC    Stop-the-world    Stop-the-world

Concurrent               Parallel New      CMS
-XX:+UseConcMarkSweepGC  Mostly Parallel   Mostly Parallel    Multi
                         Stop-the-world    Mostly concurrent

Concurrent Old only      Serial            CMS
-XX:+UseConcMarkSweepGC  Mostly Parallel   Mostly Parallel    Multi
-XX:-UseParNewGC         Stop-the-world    Mostly concurrent

Garbage First (G1)       Garbage First     Garbage First      Multi
-XX:+UseG1GC             Mostly Parallel   Mostly Parallel
                         Stop-the-world    Mostly concurrent

Z - 1 Generation                     Z                        Multi
-XX:+UseZGC                          Mostly Parallel
                                     Mostly concurrent

Note that:

When using "-XX:+UseParallelGC" to invoke Parallel Scavenge for the Young generation, Parallel Old is automatically selected for the Tenured generation.
When using "-XX:+UseParallelOldGC" to invoke Parallel Old for the Tenured generation, Parallel Scavenge is automatically selected for the Young generation.
When using "-XX:+UseConcMarkSweepGC" to invoke CMS for the Tenured generation, Parallel New is automatically selected for the Young generation.
When using "-XX:+UseConcMarkSweepGC" and "-XX:-UseParNewGC" together, you are forcing the JVM to use Serial to the Young generation. This combination is actually deprecated and will be disabled in the future.
Other combinations are not compatible and disabled.