How much memory is my program using Java

Question

I am made a program which uses A* search algorithm to solve 8 game puzzle. I was interested in seeing how much memory is being used by my program from start to finish.

So far i have done

In the beginning of the program

static double totalMem = Runtime.getRuntime().totalMemory()/(1024*1024);
static double memoryMax = Runtime.getRuntime().maxMemory()/(1024*1024);

and at the end of the program

        timeTaken-=System.currentTimeMillis();

        System.out.println("\n\n-------Total Time Taken = "+Math.abs(timeTaken)+
                " millisec ------\n ");

        System.out.println("-------Maximum Memory  = "+memoryMax+" MB------\n ");

        System.out.println("-------Total Memory = "+totalMem
                +" MB------\n ");

        currMem = Runtime.getRuntime().freeMemory()/(1024*1024);

        System.out.println("-------Free Memory  = "+currMem+" MB------\n ");

        double memUsed = (Runtime.getRuntime().totalMemory())/(1024*1024)-currMem;

        System.out.println("-------Total Used = "+memUsed
                +" MB------\n ");

This doesn't seems to be right. When i test with different sets of data. Any sugessitions

Answer 1

It would be better to use profiler or similar software for this purpose. You can start with jvisualvm which is included in JDK or JProfiler.

Answer 2

The problem looks related to the thread-local allocation buffer.
The objects might be getting created on TLAB.
Read this: https://blogs.oracle.com/jonthecollector/entry/the_real_thing
So everytime you run the program, it will show similar values.
Run the program by switching off this feature, pass below in VM arguments:-

-XX:-UseTLAB

Answer 3

You can also use "Java Mission Control" (newer version of JVisualVM), that allows you to make snapshots of your memory. Add this parameters at startup of your programm:

-XX:+UnlockCommercialFeatures
-XX:+FlightRecorder
-Dcom.sun.management.jmxremote -Dcom.sun.management.jmxremote.port=9010 
-Dcom.sun.management.jmxremote.local.only=true
-Dcom.sun.management.jmxremote.authenticate=false 
-Dcom.sun.management.jmxremote.ssl=false

Its in the "bin" folder of your JDK 7 too, called: "jmc.exe"

Answer 4

Runtime works correctly. You could take the difference before executing the logic and after to see how much memory used. A quick sample

import java.util.ArrayList;
import java.util.List;

public class MemoryTest {

    private static final int KB = 1024;

    public static void main(String[] args) {
        long initialMemory = getUsedMemory();
        List<String> dummy = new ArrayList<String>();
        for(int i=0; i<10000; i++){
            dummy.add("Dummy " + i);
        }
        System.out.println("Total used by program = " + (getUsedMemory() - initialMemory) + " KB");
    }

    private static long getUsedMemory() {
        Runtime runtime = Runtime.getRuntime();
        return (runtime.totalMemory() - runtime.freeMemory()) / KB;
    }
}