Show resident memory behavior after releasing memory in two flavors: direct and heap

memory-test.java

package io.logz.netty;

import java.lang.management.ManagementFactory;
import java.nio.ByteBuffer;
import java.util.ArrayList;
import java.util.List;

public class MemoryMain {
    public static void main(String[] args) {
        System.out.println("hello there - process id "+getCurrentProcessId());
        List<ByteBuffer> arrays = new ArrayList<>();
        new Thread(() -> {
            for (int i = 0; i < 1000*4; i++) {
                ByteBuffer bb = ByteBuffer.allocateDirect(1000*1000);
//                ByteBuffer bb = ByteBuffer.allocate(1000*1000);
                for (int j = 0; j < bb.capacity(); j++) {
                    bb.put((byte) ('a' + (j % 20)));
                }
                arrays.add(bb);
            }
            System.out.println("Heap: "+getHeapUsage() +"mb");
            System.out.println("Total Memory: "+Runtime.getRuntime().totalMemory() / (1024*1024)+"mb");
            System.out.println("Done allocating buffers. Waiting 10 seconds");
            if (waitAbit(10)) return;

            arrays.clear();
            System.gc();
            System.out.println("Cleared buffers. Waiting 10 seconds");
            if (waitAbit(10)) return;
            System.out.println("Heap: "+getHeapUsage() +"mb");
            System.out.println("Total Memory: "+Runtime.getRuntime().totalMemory() / (1024*1024)+"mb");
            System.out.println("Just waiting anothe 20sec");
            if (waitAbit(20)) return;
        }).start();

        if (waitAbit(180)) return;
    }

    private static boolean waitAbit(int seconds) {
        try {
            Thread.sleep(1000*seconds);
        } catch (InterruptedException e) {
            System.out.printf("Interrupted!");
            return true;
        }
        return false;
    }

    public static int getCurrentProcessId() {
        return Integer.valueOf(ManagementFactory.getRuntimeMXBean().getName().split("@")[0]);
    }

    public static long getHeapUsage() {
        return ManagementFactory.getMemoryMXBean().getHeapMemoryUsage().getUsed() / (1024*1024);
    }
}

After GC, if allocate() used, resident is still high although heap is at 40mb. When using allocateDirect() resident is back to 40mb after GC. So JVM is reluctant to give back free memory to the operating system.