Why is logarithm slower in Rust than in Java?

Александр Меньшиков :

If I run these benchmarks in Rust:

#[bench]
fn bench_rnd(b: &mut Bencher) {
    let mut rng = rand::weak_rng();
    b.iter(|| rng.gen_range::<f64>(2.0, 100.0));
}

#[bench]
fn bench_ln(b: &mut Bencher) {
    let mut rng = rand::weak_rng();
    b.iter(|| rng.gen_range::<f64>(2.0, 100.0).ln());
}

The result is:

test tests::bench_ln             ... bench:        121 ns/iter (+/- 2)
test tests::bench_rnd            ... bench:          6 ns/iter (+/- 0)

121-6 = 115 ns per ln call.

But the same benchmark in Java:

@State(Scope.Benchmark)
public static class Rnd {
    final double x = ThreadLocalRandom.current().nextDouble(2, 100);
}

@Benchmark
public double testLog(Rnd rnd) {
    return Math.log(rnd.x);
}

Gives me:

Benchmark    Mode Cnt  Score  Error Units
Main.testLog avgt  20 31,555 ± 0,234 ns/op

The log is ~3.7 times slower (115/31) in Rust than in Java.

When I test the hypotenuse implementation (hypot), the implementation in Rust is 15.8 times faster than in Java.

Have I written bad benchmarks or it is a performance issue?

Responses to questions asked in comments:

"," is a decimal separator in my country.
I run Rust's benchmark using cargo bench which always runs in release mode.
The Java benchmark framework (JMH) creates a new object for every call, even though it's a static class and a final variable. If I add a random creation in the tested method, I get 43 ns/op.

Александр Меньшиков :

The answer was given by @kennytm:

export RUSTFLAGS='-Ctarget-cpu=native'

Fixes the problem. After that, the results are:

test tests::bench_ln              ... bench:          43 ns/iter (+/- 3)
test tests::bench_rnd             ... bench:           5 ns/iter (+/- 0)

I think 38 (± 3) is close enough to 31.555 (± 0.234).

Guess you like

Origin http://10.200.1.11:23101/article/api/json?id=430304&siteId=1