[Cplex] Integrating OpenMP and Cilk into C++

Tom Scogland tom at scogland.com
Fri Jun 21 23:27:03 CEST 2013

As far as I know, every OpenMP implementation in existence runs the inner
loop serially if the outer loop takes all N available threads (assuming
default settings).

Can you convince it to do otherwise? Yes, you can, but actually only since
nested parallelism was introduced relatively recently.

On Fri, Jun 21, 2013 at 11:21 PM, Pablo Halpern <
lwg at halpernwightsoftware.com> wrote:

> This seems like a good opportunity for me to get educated and, from what
> I've seen, I'm not alone in needing to be educated in this way.  I'm
> wondering how static scheduling can be composable.  Consider the
> following (avoiding any specific syntax for fear that I'll get it wrong):
> void f() {
>      parallel_statically_scheduled_loop (int i = 0; i < 1000; ++i)
>          g(i);
> }
> void g(int i) {
>      parallel_statically_scheduled_loop (int j = 0; j < 1000; ++j)
>          compute(i, j);
> }
> If there are N cores available, how do you avoid having N-squared
> threads in the inner loop?
> Anticipating that you might tell me that the scheduler detects that
> there are no more threads available, and therefore run the inner loop
> serially, I'll ask a follow-up question: do any implementations actually
> do this?  If not, why not? (It's been a well-known problem for a long
> time, so its surprising to me that no implementation would have fixed
> it, if that's the right thing to do.)
> I empathize with the annoyance of people making incorrect assumptions
> about a model they don't understand (they do it with Cilk all the time),
> so I'll try to refrain from doing that with OpenMP in the future.
> Thanks,
> -Pablo
> On 06/21/2013 04:26 PM, Bronis R. de Supinski wrote:
> >
> > Pablo:
> >
> > Re:
> >> There is no need to get defensive.  I am not attacking OpenMP.
> >> However, it is well known that widely-used OpenMP features,
> >> particularly static scheduling, do not compose well with libraries
> >> that also use parallelism. If the author of a piece of code does not
> >> know whether that code might be called in a parallel context, then he
> >> cannot use parallelism without risking exponential oversubscription.
> >> (I have seen this happen, often). If run on a desktop or mobile system
> >> rather than a dedicated HPC system, static scheduling creates load
> >> imbalances that hurt performance.
> >
> > While I will agree that composability with other parallelism
> > models is a weakness of OpenMP, composability with itself is
> > not. The issue that you raise is not one of the model or the
> > specification but rather one of quality of implementation.
> >
> > Omitting static scheduling would be a mistake. Many, many
> > situations are well suited to it and it is a natural, low
> > overhead concept.
> >
> > Bronis
> >
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-Tom Scogland
"A little knowledge is a dangerous thing.
 So is a lot."
-Albert Einstein
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