Question

我正在尝试在Julia中学习并行for循环，而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，我试图确定哪个工作人员经历了哪一步。令人惊讶的是，我从上面的代码得到的是每个工作者都经历了整个迭代，直到最后一个worker（在我的情况下是worker 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人之间分配然后组合在一起，而不是分别为每个工人完成。我的意思是，并行for循环的概念应该是每个工作者只经过2-3步吗？

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：我刚才意识到与@everywhere有关。在我删除@everywhere后，事情按预期工作。

Answer 1

你告诉julia（通过@everywhere）在每个工作者和主持人进程上运行并行循环，因此你以平行的方式从1到10计数5次。（检查在您输出的输出中，从1到10的每个数字恰好发生5次）

稍微详细一点：首先我们注意到进程总数为nprocs() == 5（一个＆＃34;主机＆＃34;和4个工作人员，检查workers()）。 @everywhere告诉Julia在每个进程上运行begin end block的内容，因此在我们的示例中为5次。开始结束块的内容是＆＃34;做一个并行循环，从1到10＆＃34;计数。这正是发生的事情。

删除@everywhere时，只执行一个并行循环。这样你就可以得到你想要的东西：

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

建议阅读：https://docs.julialang.org/en/stable/manual/parallel-computing

Julia：使用@parallel for Loop与SharedArray

1 个答案: