Ensuring JMH benchmark method gets compiled - jvm

Question

I have a benchmark with a #Param controlling the input size, such as:
#State(Scope.Benchmark)
class MyBenchmark {
#Param({"100", "10000", "1000000"})
public int ARRAY_SIZE;
public int[] INPUT;
#Setup
public void setUp() {
INPUT = new int[ARRAY_SIZE];
// ...
}
#Benchmark
public void compute() {
// ...
}
}
When the input is large, compute doesn't get invoked enough times to trigger compilation during warmup. Since I would like to measure peak performance, I want to ensure the method is invoked enough during warmup to be compiled.
Is there a good way to do this? I could set a higher number of warmup iterations, but this would apply to all input sizes when it's really only necessary for the large ones which don't get compiled.

Answer 1

Instead of increasing the number of iterations, you can try making compilation happen earlier.
Since JDK 9, there is -XX:CompileThresholdScaling JVM option to adjust frequencies and thresholds of the compilation policy. The lower is the value - the earlier compilation starts.
E.g. -XX:CompileThresholdScaling=0.05 will make thresholds 20x lower.
The option can be applied globally or on a per-method basis. For example, to apply it only to compute benchmark, add the following annotation:
#Benchmark
#Fork(jvmArgsAppend = {"-XX:CompileCommand=CompileThresholdScaling,*MyBenchmark*::compute*,0.05"})
public void compute() {
// ...
}
Alternatively, you may extract larger parameter values into another #State object and create a separate benchmark method with different #Warmup options:
#State(Scope.Benchmark)
public static class SmallParams {
#Param({"100", "10000"})
int size;
}
#State(Scope.Benchmark)
public static class LargeParams {
#Param({"1000000"})
int size;
}
#Benchmark
#Warmup(iterations = 5)
public void computeSmall(SmallParams params) {
//
}
#Benchmark
#Warmup(iterations = 50)
public void computeLarge(LargeParams params) {
//
}
Or just run the benchmark twice, overriding parameters in the command line:
-p size=100,10000 -wi 5
-p size=1000000 -wi 50