Recommendation for parallelism with nested for loops?

[bachmeier]

On Friday, 19 August 2022 at 02:02:57 UTC, Adam D Ruppe wrote:

> Even if they aren't equal, you'll get decent benefit from 
> parallel on the outer one alone, but not as good since the work 
> won't be balanced.

Unless there's some kind of blocking going on in D's 
implementation, if the number of passes on the outer loop is 
large enough relative to the number of cores, applying parallel 
to the outer loop is the best you can do - uneven amounts of work 
on the inner loop will get spread out across cores. There are 
always counterexamples, but the ratio of passes to cores needs to 
be pretty low for further optimizations to have any chance to 
help.