parallel-processing - 如何在 julia 中编写并行循环？

Question

我有以下 Julia 代码，我想对其进行并行化。

using DistributedArrays

function f(x)
    return x^2;
end
y = DArray[]
@parallel for i in 1:100
    y[i] = f(i)
end
println(y)

输出为 DistributedArrays.DArray[] .我希望 y 的值如下: y=[1,4,9,16,...,10000]

Answer 1

您可以使用 n 维分布式数组推导式:

首先，您需要添加更多本地或远程进程:

julia> addprocs(CPU_CORES - 1);

那么你必须使用 DistributedArrays在每个衍生的进程中:

julia> @everywhere using DistributedArrays

最后你可以使用 @DArray 宏，像这样:

julia> x = @DArray [@show x^2 for x = 1:10];
        From worker 2:  x ^ 2 = 1
        From worker 2:  x ^ 2 = 4
        From worker 4:  x ^ 2 = 64
        From worker 2:  x ^ 2 = 9
        From worker 4:  x ^ 2 = 81
        From worker 4:  x ^ 2 = 100
        From worker 3:  x ^ 2 = 16
        From worker 3:  x ^ 2 = 25
        From worker 3:  x ^ 2 = 36
        From worker 3:  x ^ 2 = 49

你可以看到它做你所期望的:

julia> x
10-element DistributedArrays.DArray{Int64,1,Array{Int64,1}}:
   1
   4
   9
  16
  25
  36
  49
  64
  81
 100

请记住，它适用于任意数量的维度:

julia> y = @DArray [@show i + j for i = 1:3, j = 4:6];
        From worker 4:  i + j = 7
        From worker 4:  i + j = 8
        From worker 4:  i + j = 9
        From worker 2:  i + j = 5
        From worker 2:  i + j = 6
        From worker 2:  i + j = 7
        From worker 3:  i + j = 6
        From worker 3:  i + j = 7
        From worker 3:  i + j = 8

julia> y
3x3 DistributedArrays.DArray{Int64,2,Array{Int64,2}}:
 5  6  7
 6  7  8
 7  8  9

julia>

恕我直言，这是做你打算做的最朱利安的方式。

我们可以看看 macroexpand输出以查看发生了什么:

注意:为了便于阅读，此输出已稍作编辑， T代表:

DistributedArrays.Tuple{DistributedArrays.Vararg{DistributedArrays.UnitRange{DistributedArrays.Int}}}

julia> macroexpand(:(@DArray [i^2 for i = 1:10]))
  :(
    DistributedArrays.DArray(
      (
        #231#I::T -> begin
          [i ^ 2 for i = (1:10)[#231#I[1]]]
        end
      ),
      DistributedArrays.tuple(DistributedArrays.length(1:10))
    )
  )

这基本上与手动输入相同:

julia> n = 10; dims = (n,);

julia> DArray(x -> [i^2 for i = (1:n)[x[1]]], dims)
10-element DistributedArrays.DArray{Any,1,Array{Any,1}}:
   1
   4
   9
  16
  25
  36
  49
  64
  81
 100

julia>