Background: My team works on a Java program which accepts a text as input, processes it and displays the result. Each run of the program is stateless and independent of other runs. We noticed that when running the processing method in a loop on the same text, then consistently, the first run takes the longest time to complete, and the next runs take less and less time, until the execution time stabilizes after a few dozen runs on a value much lower than the initial running time.
I tried to test if this is a general phenomenon for any program, and tried running the following code, which has a nested loop, and measured how long the inner loop took:
String s = "abcefghijklmnopqrstuvwxyz";
int TIMES = 10;
int INNER_TIMES = 1000000;
long count = 0;
for (int i = 0; i < TIMES; i++) {
long start = System.currentTimeMillis();
for (int j = 0; j < INNER_TIMES; j++) {
List<String> list = new ArrayList<>();
list.add(s);
}
count++;
long end = System.currentTimeMillis();
double time = (end - start) / 1000.0;
System.out.println(count + ": " + time + " seconds");
}
The results were similar to what we had noticed in our program:
1: 0.036 seconds
2: 0.018 seconds
3: 0.016 seconds
4: 0.009 seconds
5: 0.01 seconds
6: 0.009 seconds
7: 0.02 seconds
8: 0.014 seconds
9: 0.009 seconds
10: 0.01 seconds
I tried running this many times, and the results were pretty much consistent. The first run of the inner loop always takes around 0.035-0.036 seconds, and from the 4th run and onwards (even when increasing TIMES to 1000) it takes around 0.008-0.01 seconds (with some exceptions).
Both my team leader and I were stumped about why this happens. Is this a known phenomenon in Java? In software in general? Why does it seem like the software needs to "warm up" before it reaches optimal performance?
