Default for XX:MaxDirectMemorySize

From sun.misc.VM, it's Runtime.getRuntime.maxMemory(), that's what is configured with -Xmx. E. g. if you don't configure -XX:MaxDirectMemorySize and do configure -Xmx5g, the "default" MaxDirectMemorySize will also be 5 Gb, and the total heap+direct memory usage of the app may grow up to 5 + 5 = 10 Gb.

From http://www.docjar.com/html/api/sun/misc/VM.java.html

i see:

 163       // A user-settable upper limit on the maximum amount of allocatable direct
 164       // buffer memory.  This value may be changed during VM initialization if
 165       // "java" is launched with "-XX:MaxDirectMemorySize=<size>".
 166       //
 167       // The initial value of this field is arbitrary; during JRE initialization
 168       // it will be reset to the value specified on the command line, if any,
 169       // otherwise to Runtime.getRuntime.maxDirectMemory().
 170       //
 171       private static long directMemory = 64 * 1024 * 1024;

so it appears to default to 64 megs.

For JDK8:

The 64MB are set arbitrarily initially, ...

(From: https://github.com/frohoff/jdk8u-dev-jdk/blob/master/src/share/classes/sun/misc/VM.java#L186 )

    // A user-settable upper limit on the maximum amount of allocatable direct
    // buffer memory.  This value may be changed during VM initialization if
    // "java" is launched with "-XX:MaxDirectMemorySize=<size>".
    //
    // The initial value of this field is arbitrary; during JRE initialization
    // it will be reset to the value specified on the command line, if any,
    // otherwise to Runtime.getRuntime().maxMemory().
    //
    private static long directMemory = 64 * 1024 * 1024;

... but then the directMemory is set to maxMemory() ~= Heapsize here (if the maxDirectMemorySize-Parameter is not set):

(from: https://github.com/frohoff/jdk8u-dev-jdk/blob/master/src/share/classes/sun/misc/VM.java#L286 )

  // Set the maximum amount of direct memory.  This value is controlled
  // by the vm option -XX:MaxDirectMemorySize=<size>.
  // The maximum amount of allocatable direct buffer memory (in bytes)
  // from the system property sun.nio.MaxDirectMemorySize set by the VM.
  // The system property will be removed.
  String s = (String)props.remove("sun.nio.MaxDirectMemorySize");
  if (s != null) {
      if (s.equals("-1")) {
         // -XX:MaxDirectMemorySize not given, take default
          directMemory = Runtime.getRuntime().maxMemory();
      } else {
         long l = Long.parseLong(s);
          if (l > -1)
              directMemory = l;
      }
  }

The test seems to support this claim, "test.java.nio.Buffer.LimitDirectMemory.java":

(from https://github.com/frohoff/jdk8u-dev-jdk/blob/da0da73ab82ed714dc5be94acd2f0d00fbdfe2e9/test/java/nio/Buffer/LimitDirectMemory.java#L74)

 if (size.equals("DEFAULT"))
            return (int)Runtime.getRuntime().maxMemory();