Stack data structure performance

Question

I have a Stack class below.

I'm doing performance test for it.

I see strange perforamance change when increasing load:

Test results.

Elements:500000; time:18

Elements:710000; time:21

Elements:720000; time:193

Elements:750000; time:164

Elements:1000000; time:178

Elements:1500000; time:196

Elements:2000000; time:1875

How to explain such performance degradation ?

public class ListBackedStack<T> {

    private int size;
    private Entry currentEntry;

    public ListBackedStack() {
    }

    public void push(final T data) {
        Entry newEntry = new Entry(data);
        newEntry.previous = currentEntry;
        currentEntry = newEntry;
        size++;
    }

    @SuppressWarnings("unchecked")
    public T pop() {
        T data = currentEntry.data;
        currentEntry = currentEntry.previous;
        size--;
        return data;
    }

    public int size() {
        return size;
    }

    private class Entry {
        private Entry previous;
        private T data;

        private Entry(T data) {
            this.data = data;
        }

        private void setPrevious(Entry previous) {
            this.previous = previous;
        }
    }
}

public class ListStackTest {

    @Before
    public void before() {
        stackList = new ListBackedStack<Object>();

    }

    @Test
    public void testPerformanceListStack() {

        long time = currentTime();
        for (int i = 0; i < testDataSize; i++) {
            stackList.push(i);
        }
        System.out.println("[StackList]   Elements:" + testDataSize + ";  time:" + timing(time));
        stackList = null;

    }

    private long currentTime() {
        return System.currentTimeMillis();
    }

    private long timing(long time) {
        return (currentTime() - time);
    }

    private static final int testDataSize = 710000 /** Integer.MAX_VALUE */
    ;
    private ListBackedStack<Object> stackList;
}