Java - Measuring Method Execution Time

Question

I am trying to measure the complexity of an algorithm using a timer to measure the execution time, whilst changing the size of the input array.

The code I have at the moment is rather simple:

public void start() {
    start = System.nanoTime();
}

public long stop() {
    long time = System.nanoTime() - start;
    start = 0;
    return time;
}

It appears to work fine, up until the size of the array becomes very large, and what I expect to be an O(n) complexity algorithm turns out appearing to be O(n^2). I believe that this is due to the threading on the CPU, with other processes cutting in for more time during the runs with larger values for n.

Basically, I want to measure how much time my process has been running for, rather than how long it has been since I invoked the algorithm. Is there an easy way to do this in Java?

Thomas Uhrig · Accepted Answer · 2012-05-05 16:42:13Z

3

Measuring execution time is a really interesting, but also complicated topic. To do it right in Java, you have to know a little bit about how the JVM works. Here is a good article from developerWorks about benchmarking and measuring. Read it, it will help you a lot.

The author also provides a small framework for doing benchmarks. You can use this framework. It will give you exaclty what you needs - the CPU consuming time, instead of just two time stamps from before and after. The framework will also handle the JVM warm-up and will keep track of just-in-time-compilings.

You can also use a performance monitor like this one for Eclipse. The problem by such a performance monitor is, that it doesn't perform a benchmark. It just tracks the time, memory and such things, that your application currently uses. But that's not a real measurement - it's just a snapshot at a specific time.

answered May 5, 2012 at 16:42

Thomas Uhrig

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Luke Over a year ago

Thanks, that article is really helpful.

Louis Wasserman · Accepted Answer · 2012-05-05 16:40:28Z

1

Benchmarking in Java is a hard problem, not least because the JIT can have weird effects as your method gets more and more heavily optimized. Consider using a purpose-built tool like Caliper. Examples of how to use it and to measure performance on different input sizes are here.

answered May 5, 2012 at 16:40

Louis Wasserman

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Comments

Neil Coffey · Accepted Answer · 2012-05-05 16:54:23Z

If you want the actual CPU time of the current thread (or indeed, any arbitrary thread) rather than the wall clock time then you can get this via ThreadMXBean. Basically, do this at the start:

ThreadMXBean thx = ManagementFactory.getThreadMXBean();
thx.setThreadCpuTimeEnabled(true);

Then, whenever you want to get the elapsed CPU time for the current thread:

long cpuTime = thx.getCurrentThreadCpuTime();

You'll see that ThreadMXBean has calls to get CPU time and other info for arbitrary threads too.

Other comments about the complexities of timing also apply. The timing of the individual invocation of a piece of code can depend among other things on the state of the CPU and on what the JIT compiler decides to do at that particular moment. The overall scalability behaviour of an algorithm is generally a trend that emerges across a number of invocations and you will always need to be prepared for some "outliers" in your timings.

Also, remember that just because a particular timing is expressed in nanoseconds (or indeed milliseconds) does not mean that the timing actually has that granularity.

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