[julia-users] Re: I have two data collection for mapreduce() vs. generator, which is correct/better implementation? (Julia 0.5)

[DNF]

This is about twice as fast with, with @simd:

function f2(a, p)
    @assert length(a) == length(p)
    s = 0.0
    @simd for i in eachindex(a)
        @inbounds s += abs((a[i] - p[i])/a[i])
    end
    return 100s/length(a)
end

julia> @benchmark f(a, p)
BenchmarkTools.Trial:  
  samples:          115 
  evals/sample:     1 
  time tolerance:   5.00% 
  memory tolerance: 1.00% 
  memory estimate:  144.00 bytes 
  allocs estimate:  7 
  minimum time:     41.96 ms (0.00% GC) 
  median time:      42.53 ms (0.00% GC) 
  mean time:        43.49 ms (0.00% GC) 
  maximum time:     52.82 ms (0.00% GC) 

julia> @benchmark f2(a, p) 
BenchmarkTools.Trial:  
  samples:          224 
  evals/sample:     1 
  time tolerance:   5.00% 
  memory tolerance: 1.00% 
  memory estimate:  0.00 bytes 
  allocs estimate:  0 
  minimum time:     21.08 ms (0.00% GC) 
  median time:      21.86 ms (0.00% GC) 
  mean time:        22.38 ms (0.00% GC) 
  maximum time:     27.30 ms (0.00% GC)


Weirdly, they give slightly different answers:

julia> f(a, p)
781.4987197415827 

julia> f2(a, p) 
781.498719741497


I would like to know why that happens.

On Friday, October 7, 2016 at 10:29:20 AM UTC+2, Martin Florek wrote:
>
> Thanks Andrew for answer. 
> I also have experience that eachindex() is slightly faster. In Performance 
> tips I found macros e.g. @simd. Do you have any experience with them?
>