Julia：将 @parallel for Loop 与 SharedArray 一起使用时出现意外结果

Question

我正在尝试在 Julia 中学习并行 for 循环，但我对以下示例代码的结果感到困惑：

addprocs(4)
@everywhere begin
N = 10
V = SharedArray{Int64,1}(N)
@sync @parallel for i = 1:N
    V[i] = i
    println(V[i])
end 
end

通过使用 println，我试图确定哪个工人经历了哪个步骤。令人惊讶的是，我从上面的代码中得到的是，每个工人都在不断地进行整个迭代，直到最后一个工人（在我的例子中是工人 3）完成 for 循环：

From worker 4:  1
From worker 4:  2
From worker 4:  3
From worker 5:  1
From worker 5:  2
From worker 5:  3
From worker 5:  4
From worker 5:  5
From worker 5:  6
From worker 5:  4
From worker 5:  5
From worker 5:  6
From worker 2:  7
From worker 2:  8
From worker 2:  4
From worker 2:  5
From worker 2:  6
From worker 2:  9
From worker 2:  10
From worker 4:  1
From worker 4:  2
From worker 2:  7
From worker 2:  8
From worker 3:  9
From worker 5:  4
From worker 5:  5
From worker 5:  6
From worker 3:  10
From worker 3:  7
From worker 3:  8
From worker 3:  7
From worker 3:  8
From worker 3:  9
From worker 3:  10
From worker 2:  4
From worker 2:  5
From worker 2:  6
From worker 3:  7
From worker 3:  8
From worker 4:  3
From worker 4:  9
From worker 4:  10
From worker 4:  1
From worker 4:  2
From worker 4:  3
From worker 4:  1
From worker 4:  2
From worker 4:  3
From worker 5:  9
From worker 5:  10

这不是我对并行 for 循环的期望，因为我认为工作应该在工作人员之间分配然后组合在一起，而不是为每个工作人员单独完成。我的意思是，并行for循环的概念应该是每个worker只经过2-3个步骤吗？

我的代码有问题还是我误解了并行 for 循环的概念？

谢谢！

---

编辑：我刚刚意识到@everywhere 与此有关。在我消除了@everywhere 之后，一切正常。

Answer 1

您告诉 julia（通过@everywhere）在每个工作人员和主机进程上运行并行循环，因此您以并行方式从 1 计数到 10 5 次。 （检查您发布的输出中从 1 到 10 的每个数字是否恰好出现 5 次）

稍微详细一点：首先我们注意到进程总数为nprocs() == 5（一个“主机”和4个工作人员，请查看workers()）。 @everywhere 告诉 Julia 在每个进程上运行 begin end 块的内容，因此在我们的示例中运行 5 次。 begin end 块的内容是“做一个并行循环，从 1 数到 10”。这正是发生的事情。

当您删除 @everywhere 时，您只会执行一个并行循环。因此，您将得到您想要的：

julia> N = 10
       V = SharedArray{Int64,1}(N)
       @sync @parallel for i = 1:N
           V[i] = i
           println(V[i])
       end
        From worker 5:  9
        From worker 5:  10
        From worker 3:  4
        From worker 3:  5
        From worker 3:  6
        From worker 2:  1
        From worker 2:  2
        From worker 2:  3

推荐阅读：https://docs.julialang.org/en/stable/manual/parallel-computing