[julia-users] many-way arrays cause unexpected heap allocation

[vavasis]

Regarding Julia 0.4.5, I've discovered that arrays with many subscripts 
(apparently 8 is enough to cause trouble) unexpectedly cause heap 
allocation.  See the code and timings below.  This is in regard to a finite 
element code in which many-way arrays are used to represent multi-index 
tensors.  Can anyone explain this?  

Thanks,
Steve Vavasis


julia> @time test_manyway.test2way(100000)
  0.001836 seconds (6 allocations: 272 bytes)
49837.4971725032

julia> @time test_manyway.test4way(100000)
  0.008931 seconds (7 allocations: 432 bytes)
50050.05619989492


julia> @time test_manyway.test8way(100000)
 49.707042 seconds (385.40 M allocations: 11.103 GB, 4.16% gc time)
50062.84252292006



module test_manyway

function test2way(n)
    a2 = zeros(2,2)
    x = 0.0
    for tr = 1 : n
        for j1 = 1 : 2
            for j2 = 1 : 2
                a2[j1,j2] = rand()
            end
        end
        x += a2[1,1]
    end
    x
end

function test4way(n)
    a4 = zeros(2,2,2,2)
    x = 0.0
    for tr = 1 : n
        for j1 = 1 : 2
            for j2 = 1 : 2
                for j3 = 1 : 2
                    for j4 = 1 : 2
                        a4[j1,j2,j3,j4] = rand()
                    end
                end
            end
        end
        x += a4[1,1,1,1]
    end
    x
end

function test8way(n)
    a8 = zeros(2,2,2,2,2,2,2,2)
    x = 0.0
    for tr = 1 : n
        for j1 = 1 : 2
            for j2 = 1 : 2
                for j3 = 1 : 2
                    for j4 = 1 : 2
                        for j5 = 1 : 2
                            for j6 = 1 : 2
                                for j7 = 1 : 2
                                    for j8 = 1 : 2
                                        a8[j1,j2,j3,j4,j5,j6,j7,j8] = rand()
                                    end
                                end
                            end
                        end
                    end
                end
            end
        end
        x += a8[1,1,1,1,1,1,1,1]
    end
    x
end

end