Hi folks,
while reading the awesome
https://shipilev.net/blog/2014/nanotrusting-nanotime/ I have some questions
on the "Building Performance Models" part.
Specifically, when you want to compare 2 operations (ie A,B) and you want
to emulate the same behaviour of a real application you need to amortize
the cost of such operations:
in JMH this is achieved with BlackHole.consumeCPU(int tokens), but any
microbenchmark tool (if not on the JVM) could/should provide something
similar.
Said that, now the code to measure is not just A or B but is composed by 2
operations: amortization; A() (or B);
For JMH that means:
@Benchmark
int a() {
BlackHole.consumeCPU(tokens);
//suppose that rawA() return an int and rawA() is the original call re A
return rawA();
}
in JMH is up to the tool to avoid Dead Code Elimination when you return a
value from a benchmarked method.
The point of the article seems to be that given that "performance is not
composable" if you want to compare A and B cost (with amortization) you
cannot create a third benchmark:
@Benchmark
int amortization() {
BlackHole.consumeCPU(tokens);
}
And use the benchmark results (eg throughput of calls) to be subtracted
from the results of a() (or b()) to compare A and B costs.
I don't understand the meaning of "performance is not composable" and I
would appreciate your opinion on that, given that many people
of this list have experience with benchmarking.
Thanks,
Franz
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