Does java garbage collection log entry “Full GC (System)” mean some class called System.gc()?

Question

What does \"Full GC (System)\" entry in the garbage collection logs mean? That some class called System.gc() ?

My garbage collection logs has two different entry typ

Answer 1

From the source for OpenJDK I would say it is

const bool is_system_gc = gc_cause == GCCause::_java_lang_system_gc;

// This is useful for debugging but don't change the output the
// the customer sees.
const char* gc_cause_str = "Full GC";
if (is_system_gc && PrintGCDetails) {
  gc_cause_str = "Full GC (System)";
}

I created a custom version of the Runtime class to record the thread name and stack trace to a file whenever Runtime.getRuntime().gc() was called. (System.gc() calls this) I found it useful in tracking down and removing such callers.

One place this happens is in sun.misc.GC class. The RMI will ask this class to ensure a GC has been performing in the last N seconds. If there has been no GC it triggers a full GC.

This only shows as a problem if you reduce the number of minor GCs. Ironicly it can mean you get more full GCs. ;)

I don't use RMI (except perhaps JConsole) So I have it set to a week. (Think the default is an hour)

-Dsun.rmi.dgc.server.gcInterval=604800000
-Dsun.rmi.dgc.client.gcInterval=604800000