Good question!
The outcome is the same (I think). The difference is in performance.
* Parallel-safe accumulation happens once per thread with reduce intents
vs. once per loop iteration in your case.
In the reduce-intent case, "pi += inter;" in the body of the loop
accumulates onto a task-private variable. This does not require parallel
safety and so can be done fast, get vectorized, etc.
* With reduce intents, the parallel-safe accumulation is done using an
atomic add. When using a sync variable, even a single += operation is
quite expensive, e.g. it involves grabbing and releasing a lock.
Vassily
On 07/22/15 11:56, Russel Winder wrote:
> On Wed, 2015-07-22 at 10:16 -0700, Vassily Litvinov wrote:
>> […]
>>
>> var pi: real;
>> forall j in 0..intervals with (+ reduce pi) {
>> const x = (j-0.5)*delta;
>> const inter = 4.0/(1.0+x**2);
>> pi += inter;
>> }
>> writeln("Pi: ", pi*delta);
>>
> […]
>
> Will this be different from:
>
> var sum: sync real = 0.0;
> forall i in 1..n do { sum += 1.0 / (1.0 + ((i - 0.5) * delta) ** 2);
> }
> const pi = 4.0 * delta * sum;
>
> ?
>
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