Question

Possible Duplicate:
In Java, what is the best way to determine the size of an object?

I found this piece of code on the net:

static Runtime runtime = Runtime.getRuntime();
...
long start, end;
Object obj;
runtime.gc();
start = runtime.freememory();
obj = new Object(); // Or whatever you want to look at
end =  runtime.freememory();
System.out.println("That took " + (start-end) + " 
bytes.");

But it is not very reliable, any idea on how to do it better?

Answer 1

The JVM allocates memory in chunks so it can be performed across threads concurrently. This called the Thread-Local Allocation Buffer.

TLAB – Thread-local allocation buffer. Used to allocate heap space quickly without synchronization. Compiled code has a “fast path” of a few instructions which tries to bump a high-water mark in the current thread’s TLAB, successfully allocating an object if the bumped mark falls before a TLAB-specific limit address.

If you turn this off with -XX:-UseTLAB you will get more accurate memory usage information.

public static void main(String... args) {
    for (int i = 0; i < 10; i++) {
        long used1 = memoryUsed();
        new Object();
        long used2 = memoryUsed();
        System.out.println(used2 - used1);
    }
}

public static long memoryUsed() {
    return Runtime.getRuntime().totalMemory() - Runtime.getRuntime().freeMemory();
}

prints with default options

0
0
0
0
0
0
0
0
0
0

with -XX:-UseTLAB

16
16
16
16
16
16
16
16
16
16