I am currently working on my Bachelors Thesis on the advantages/disadvantages of GraalVM Native Image compared to a Jar running on the JVM.
During one of the Tests i am calling a certain Method which allocates and populates an Array of size 10^6. After that the function loops the array and performs arithmetic operations (It is a variant of the ackley function). The usual runtime of this method was between 3 and 4 seconds, but sometimes the method would complete after just 50 ms (when running as either the Native Image or as the jar file running on the JVM).
Since the array is populated by using the Math.Random() function I dont think it is due to caching and the Native Image rules out JIT compilation as the source for these outliers.
The endpoint looks like this, where dtno is the Data Transfer Object containing the "range" variable:
@PostMapping(path="/ackley")
public static @ResponseBody long calculateackley (@RequestBody dtno d) {
long start = System.nanoTime();
double res = ackley(d.range);
long end = System.nanoTime();
System.out.println("Ackley funtion took: "+res);
return end - start;
}The ackley function looks like this:
public static long ackley(int range){
long start = System.nanoTime();
if(range!=0){
double[] a = new double[range];
int counter = 0;
for(int i=-range/2;i<range/2;i++){
a[counter++] = Math.random()*range*i;
}
double sum1 = 0.0;
double sum2 = 0.0;
for (int i = 0 ; i < a.length ; i ++) {
sum1 += a[i]*a[i];
sum2 += (Math.cos(2*Math.PI*a[i]));
}
double result = -20.0*Math.exp(-0.2*Math.sqrt(sum1 / ((double )a.length))) + 20
- Math.exp(sum2 /((double)a.length)) + Math.exp(1.0);
}
long end = System.nanoTime();
return end - start;
}As already mentioned the range variable in the test was 10^6. What I was also suspecting is that, since the result and sum are never actually used to calculate the return value, the programm decides to skip everything between for loop and the decleration of "end".
In the graph from the JMeter test you can see that these fast execution times where all during ramp up and the very end of the testrun.
In the summary Report you can see the huge deviation from the average runtime.
Performance Test Summary Report
If anyone could give me a hint or a good source, where I could find a hint as to what is going on, I would be very thankfull.