performance - 为什么我的 Julia 代码运行速度比 javascript 慢？

Question

最近，我对 Julia-lang 很感兴趣，因为它声称是一种具有接近 C 性能的动态语言。但是，到目前为止我对它的体验并不好(至少在性能方面)。
我正在编写的应用程序需要随机访问特定数组索引，然后将它们的值与其他特定数组索引(经过多次迭代)进行比较。以下代码从程序中模拟了我的需求:
我的 Julia 代码在大约 8 秒内完成执行，而 Java 脚本代码在 chrome 环境中需要不到 1 秒的时间!
我的 Julia 代码有问题吗？非常感谢。

Julia 代码在这里:

n=5000;
x=rand(n)
y=rand(n)
mn=ones(n)*1000;
tic();
for i in 1:n;
    for j in 1:n;
        c=abs(x[j]-y[i]);
        if(c<mn[i])
            mn[i]=c;
        end
    end
end
toc();

Javascript 代码:(比上面的 julia 代码快 8 倍以上!)

n=5000; x=[]; y=[]; mn=[];
for(var i=0; i<n; i++){x.push(Math.random(1))}
for(var i=0; i<n; i++){y.push(Math.random(1))}
for(var i=0; i<n; i++){mn.push(1000)}
console.time('test');
for(var i=0; i<n; i++){
    for(var j=0; j<n; j++){
        c=Math.abs(x[j]-y[i]);
        if(c<mn[i]){
            mn[i]=c;
        }       
    }
} 
console.timeEnd('test');

Answer 1

Performance Tips

Avoid global variables

A global variable might have its value, and therefore its type, change at any point. This makes it difficult for the compiler to optimize code using global variables. Variables should be local, or passed as arguments to functions, whenever possible.

Any code that is performance critical or being benchmarked should be inside a function.

We find that global names are frequently constants, and declaring them as such greatly improves performance:

julia> const n = 5000; const x, y = rand(n), rand(n); const mn = fill(1000.0, n);

julia> function foo!(mn, x, y)
           n = length(mn)
           @inbounds for i in 1:n, j in 1:n
               c = abs(x[j] - y[i])
               if(c < mn[i])
                   mn[i] = c
               end
           end
           return mn
       end
foo! (generic function with 1 method)

julia> using BenchmarkTools: @btime

julia> @btime foo!(mn, x, y)
  15.432 ms (0 allocations: 0 bytes)