Measuring time in Java

Question

So I was trying to measure the time two different algorithm implementations took to accomplish a given task, and here is the result:

i    alg1  alg2
4   0.002   0.0
5   0.001   0.0
6   0.003   0.002
7   0.023   0.01
8   0.055   0.041
9   0.056   0.0
10  0.208   0.101
11  1.767   0.694
12  18.581  7.784

being i just some input parameter.

I've measured the performance of the algorithms making use of the following (naive) function:

private double getDuration() {
    return (double)(System.currentTimeMillis() - startTime) / (double)1000;
}

What would be the preferable way of getting more real results (other than 0.0, which obviously isn't true!) than using System.currentTimeMillis()? I know I could just run the algorithms over and over again and sum their results, but I have this gut feeling that there is probably some more robust way of measuring passed time in Java (both real, user and sys, if possible!).

Thanks

Answer 1

For basic timing, you can use Guava's Stopwatch class (or just grab its source code if you don't want to pull in the whole Guava library). For a more complete benchmarking solution look at Caliper by the same team.

Both of these are based on System.nanoTime(), which you should prefer over System.currentTimeMillis() for measuring elapsed time. The basic reason why is that System.currentTimeMillis() is a "clock" (which tries to return wall-time) whereas System.nanoTime() is a "timer" (which tries to return time since some arbitrary point).

You want a clock when you're trying to figure out when a single event happened, so you can line it up with your watch or the clock on your wall (or the clock in some other computer). But it's not appropriate for measuring the elapsed time between two events on the same system, since the computer will occasionally adjust its notion of how its own internal clock corresponds to wall-time. For instance, if you do

long timeA = System.currentTimeMillis();
doStuff();
long timeB = System.currentTimeMillis();
System.out.println("Elapsed time: " + (timeB - timeA));

it's possible to get a negative result if NTP adjusts backwards while doStuff() is executing. System.nanoTime(), being a timer instead of a clock, should ignore that adjustment thus avoid this problem.

(Note that all of the above is conceptual; unfortunately things can get messy at the implementation level. But this doesn't change the recommendation: System.nanoTime() is supposed to be the best timer you can get on your platform, and System.currentMilliseconds() is supposed to be the best clock you can get.)