How to use D parallel functions/library

[thedeemon via Digitalmars-d-learn]

On Tuesday, 24 November 2015 at 18:49:25 UTC, Bishop120 wrote:
> I figured this would be a simple parallel foreach function with 
> an iota range of sizeX and just making int X declared inside 
> the function so that I didnt have to worry about shared 
> variable but I cant get around the alive++ reduction and I dont 
> understand enough about D's reduction/parallel library.
>
> Any ideas?  Thanks in advance for yalls patience and assistance!

Incrementing often the same variable from different parallel 
threads is a very bad idea in terms of performance. I would 
suggest counting number of alive cells for each row independently 
(in a local non-shared variable) and storing it to an array (one 
value per row), then after the loop sum them up.

auto aliveCellsPerRow = new int[N];

foreach(i; iota(N).parallel) {
   int aliveHere;
   //...process a row...
   aliveCellsPerRow[i] = aliveHere;
}

alive = aliveCellsPerRow.sum;

Then everything will be truly parallel, correct and fast.