Simplest multithreading example

[ag0aep6g via Digitalmars-d-learn]

On 09/01/2017 07:27 AM, Brian wrote:
> double [] hugeCalc(int i){
>      // Code that takes a long time
> }
> 
> so if I do
> 
> 
> double[][int] _hugeCalcCache;
> foreach(i ; I)
>     _hugeCalcCache[i] = hugeCalc(i);
> 
> of course the required time is I.length * (a long time), so I wanted to 
> shorten this by splitting to different threads :
> 
> foreach(i ; parallel(I) )
>     _hugeCalcCache[i] = hugeCalc(i);
> 
> but as you can guess, it doesn't work that easily.

Works pretty well for me:

----
double [] hugeCalc(int i)
{
     // Code that takes a long time
     import core.thread: Thread;
     import std.datetime: seconds;
     Thread.sleep(1.seconds);
     return [i];
}

void main()
{
     static import std.range;
     import std.parallelism: parallel;
     auto I = std.range.iota(0, 10);
     double[][int] _hugeCalcCache;
     foreach(i ; parallel(I))
         _hugeCalcCache[i] = hugeCalc(i);
}
----

That runs in about 3 seconds here. The serial version would of course 
take about 10 seconds. So, parallelism achieved!

Though I don't know if it's safe to access an associative array 
concurrently like that. I'd use a normal dynamic array instead and 
initialize it before going parallel:

----
auto _hugeCalcCache = new double[][](10);
----