Ndarray calculation is not fast, why?

help
1

I have two variables which are named x and y and their type is ArrayBase<ViewRepr<&f64>, Dim<[usize; 1]>>

calculation 1:
let z = (&x - &y).map(|v| v.abs()).sum();
calculation 2:
let mut z = 0.0; azip!((&a in &x, &b in &y) z += (a - b).abs());

I find 'calculation 2' is much faster than 'calculation 1', why?

I have tried to replace all ndarray with vec, it is faster than 'calculation 2'.

2

You need to profile the code for sure, but I guess it's because the former have 2 intermediate allocations and copies between them while the latter is zero-allocation.

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3

Obligatory: are you compiling with --release? Without optimizations things will be slow for irrelevant reasons.

Are you measuring it with bencher or criterion? A single timer.elapsed() is very skewed due to caches, CPU frequency boosts, multitasking, and compiler potentially reordering or deleting code.

4

Have you compared the assembly the compiler generated for each?

5

Do you mean z += instead of d += ?

6

yes, you are right; thank you.

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