[fpc-devel] C++ gets language-internal concurrency support

[Michael Schnell]

On 08/19/2011 02:15 PM, David W Noon wrote:
> My experience with OpenMP is that it is difficult to write a loop body
> large enough that context switching does not overwhelm the benefits of
> parallelism.
>
Hmmm.

If you do a multiplication of a 100*100 Matrix you could spawn 10000 
threads and this will result in a huge switching overhead.

But if you have 10 cores and you aggregate the 10000 tasks in 10 groups 
of 1000 calculations each, spawn 10 threads and have each go through a 
loop of calculating 1000 cells, I gather that (in a perfect world) no 
task switching overhead at all would be necessary (but at the beginning 
and the end of the complete calculation).

If in Prism you do something like (pseudo-code draft):
------------------------------------------------------------------------
m := 100;
n :=  100

for parallel ij := 0 to m*n-1 do begin
   i := ij mod m;
   j := ij div m;
  calccell (i,j);
end;
------------------------------------------------------------------------

I understand that Prism (or rather .NET) on a 10 core machine 
automatically would create 10 threads each doing 1000 cells.

-Michael
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