public long getFreeMemory() {
    if (ibmvendor) {
      return 0;
    }

    Long r = runUnixMXBeanMethod("getFreePhysicalMemorySize");
    return (r != null ? r : -1);
  }
Return the physical free memory (not the JVM one, as it's not very useful as it depends on the
GC), but the one from the OS as it allows a little bit more to guess if the machine is
overloaded or not).
hbase-common/src/main/java/org/apache/hadoop/hbase/util/JVM.java