Iteration with CartesianIndices(array) is slow when the dimension of the array is large

I’ve been playing around with CartesianIndices. It works beautifully for the most part, but I’ve noticed that it becomes slower compared to linear indexing, when the dimension of the array is large. Here is a minimal example using Julia 1.2.0.

using BenchmarkTools

#input is 5-dimensional array
dim_5 = rand(Float64, (27*ones(Int64,5))...)
#input is 15-dimensional array
dim_15 = rand(Float64,(3*ones(Int64, 15))...)
#the number of elements is 3^15 = 14,348,907 for both of them

function cartesiansum(array)
    sum = zero(eltype(array))
    for index in CartesianIndices(array)
        sum += array[index]
    return sum

function linearsum(array)
    sum = zero(eltype(array))
    for index in eachindex(array)
        sum += array[index]
    return sum

println("Cartesian sum with dimension = 5")
@btime cartesiansum(dim_5)
println("Linear sum with dimension = 5")
@btime linearsum(dim_5)
println("Cartesian sum with dimension = 15")
@btime cartesiansum(dim_15)
println("Linear sum with dimension = 15")
@btime linearsum(dim_15)

Note that I made the number of elements to be the same between dim_5, dim_15. Here is the output.

Cartesian sum with dimension = 5
  84.114 ms (1 allocation: 16 bytes)
Linear sum with dimension = 5
  24.007 ms (1 allocation: 16 bytes)
Cartesian sum with dimension = 15
  407.303 ms (1 allocation: 16 bytes)
Linear sum with dimension = 15
  25.045 ms (1 allocation: 16 bytes)

As you can see, there is a significant slowdown when I use CartesianIndices for dimension 15 array.

Is there any workaround around this. or are we advised not to use CartesianIndices for large dimension arrays?

Don’t use CartesianIndices full stop, unless absolutely unavoidable. Consider whether you really need large dimensional arrays at all. Indexing into Array always converts into linear indices in order to compute addresses. This involves a little bit of offset computation (integer addition and multiplication).

Just look at @code_native getindex(myarray, first(CartesianIndices(myarray))) versus getindex(myarray, first(eachindex(myarray))) and weep (and the code looks ok, it’s just a consequence of what you ask your computer to do).

That makes sense. I’ll take a look at those codes. Thanks!