What is the default Full GC interval in Java 8

Question

We noticed a strange behavior on our production server (JavaEE, Wildfly 10). The Java VM do once per hour Full GC although there is still enough memory available.

The VM start parameters are:

-server
-Xms4g
-Xmx8g

The following parameters are not activated:

-Dsun.rmi.dgc.client.gcInterval=3600000
-Dsun.rmi.dgc.server.gcInterval=3600000

So what is the default Full GC interval in Java 8 and how can I configure it to be dynamic?

Answer 1

You say that

-Dsun.rmi.dgc.client.gcInterval=3600000
-Dsun.rmi.dgc.server.gcInterval=3600000

are not "activated". However, those are the default values for those properties; i.e. the values that you get if you DON'T specify values for the properties.

Reference: Oracle's sun.rmi Properties documentation.

So what is the default Full GC interval in Java 8

See above for the DCG triggered full GCs.

Apart from that, a full GC happens when requested (by a System.gc() call) or when the JVM decides it is necessary. There is no fixed interval if you are not using RMI.

... and how can I configure it to be dynamic?

If you mean, how can you stop RMI from triggering a full GC every hour:

• One way is to specify (much) larger values for those properties.

• Another way is to launch the JVM with the -XX:+DisableExplicitGC flag. Note: this will disable all calls to System.gc() not just the calls that RMI's DGC makes. See also: Impact of setting -XX:+DisableExplicitGC when NIO direct buffers are used for some of the problems that this may cause.

• A third way is to use -XX:+ExplicitGCInvokesConcurrent flag. This reduces the impact of calls to System.gc(), but it will only work with GCs that support concurrent collection; e.g. CMS, G1, ZGC or Shenandoah.

  You can potentially tune other things to reduce the concurrent collectors "stop the world" pauses, but that is beyond the scope of this question.

Answer 2

Another solution would be to set the option

 -XX:+ExplicitGCInvokesConcurrent 

in combination with

 -XX:+UseG1GC

This way System.gc() will not be disabled but less harmful by making the GC work in concurrent mode instead of causing "stop the world" situations. I used this to successfully reduce my GC pauses in WildFly from 10 seconds to 500ms.