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Watch your HotSpot compiler go

Don't start profiling your Java application too early

By Vladimir Roubtsov, JavaWorld.com, 04/11/03

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April 11, 2003

QIn "My Kingdom for a Good Timer!," what is the purpose of this part of the sample code:

// JIT/hotspot warmup:
for (int i = 0; i < 3000; ++ i)
        {
            timer.start ();
            timer.stop ();
            timer.getDuration ();
            timer.reset ();
        }


A The short answer is the above code snippet "warms up" the JVM with respect to the ITimer implementation. Let's investigate what I mean by that in more detail.

It's easier to illustrate my point with a specially crafted code snippet:

public class Main
{
    public static void main (String [] args)
    {
        // Create an ITimer using the Factory class:
        final ITimer timer = TimerFactory.newTimer ();
        
        for (int repeat = 0; repeat < 25; ++ repeat)
        {
            timer.start ();
            sum (100);
            timer.stop ();
            
            System.out.println (repeat + ": " + timer.getDuration ());
            timer.reset ();
        }
    }
    
    public static int sum (int n)
    {
        if (n <= 1)
            return 1;
        else
            return n + sum (n - 1);
    }
} // End of class


As you can see, I use the ITimer high-resolution API once again to measure the performance of a test method called sum(), which is a very naïve implementation of the sum of numbers 1+2+...+n (an arithmetic progression). It is implemented recursively by design. You should never use such an implementation inside a real application. However, it suits my discussion precisely because of its high method call overhead.

The above code snippet invokes sum(100) 25 times and logs the cost of every call. The results from running in Java 2 Platform, Standard Edition (J2SE) 1.4.1 for Win32 are quite interesting: 

0: 0.08888
1: 0.04755
2: 0.03448
3: 0.03462
4: 0.03432
5: 0.03437
6: 0.03440
7: 0.03460
8: 0.03441
9: 0.03445
10: 0.0344
11: 0.0343
12: 0.0343
13: 0.0345
14: 0.8865
15: 0.0444

16: 0.0107
17: 0.0103
18: 0.0102
19: 0.0102
20: 0.0102
21: 0.0103
22: 0.0101
23: 0.0103
24: 0.0103


Notice that something strange happens around the 15th repeat: the method slows down for a while and then speeds up by a factor of 3.5. "It's just a random garbage collection (GC) delay," I hear some seasoned Java programmers mutter. Except it is not: a sporadic GC run would not explain why sum() gets faster afterwards and stays that way (you can see that by making the upper limit for the repeat loop variable much larger than 25). Furthermore, you can repeat this experiment numerous times, and the phenomenon will always happen at about the same point in the program's execution.

What is going on?

The riddle has a simple explanation. A call to sum(n) implies n total calls to the method; n-1 of them are recursive. After 15 calls to sum(100), the method body executes a grand total of 1,500 times. As it happens, my code executes in the client HotSpot JVM by default, and 1,500 is the default method invocation threshold for when a method is considered for compilation into native code. Execution slows down while HotSpot spends extra CPU cycles to compile sum() into native code and then speeds up because the JVM switches to the method's natively compiled, and thus noticeably faster, version.

For further proof you can add the -XX:+PrintCompilation HotSpot option (see Resources) to your JVM command line and watch the HotSpot engine tell you when it compiles various methods:

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