[Cython] CEP: prange for parallel loops

[Robert Bradshaw]

On Tue, Apr 5, 2011 at 8:02 AM, Dag Sverre Seljebotn
<d.s.seljebotn at astro.uio.no> wrote:
> On 04/05/2011 04:58 PM, Dag Sverre Seljebotn wrote:
>>
>> On 04/05/2011 04:53 PM, Robert Bradshaw wrote:
>>>
>>> On Tue, Apr 5, 2011 at 3:51 AM, Stefan Behnel<stefan_ml at behnel.de>
>>>  wrote:
>>>>
>>>> mark florisson, 04.04.2011 21:26:
>>>>>
>>>>> For clarity, I'll add an example:
>>>>>
>>>>> def f(np.ndarray[double] x, double alpha):
>>>>>     cdef double s = 0
>>>>>     cdef double tmp = 2
>>>>>     cdef double other = 6.6
>>>>>
>>>>>     with nogil:
>>>>>         for i in prange(x.shape[0]):
>>>>>             # reading 'tmp' makes it firstprivate in addition to
>>>>> lastprivate
>>>>>             # 'other' is only ever read, so it's shared
>>>>>             printf("%lf %lf %lf\n", tmp, s, other)
>>>>
>>>> So, adding a printf() to your code can change the semantics of your
>>>> variables? That sounds like a really bad design to me.
>>>
>>> That's what I was thinking. Basically, if you do an inlace operation,
>>> then it's a reduction variable, no matter what else you do to it
>>> (including possibly a direct assignment, though we could make that a
>>> compile-time error).
>>
>> -1, I think that's too obscure. Not being able to use inplace operators
>> for certain variables will be at the very least be nagging.

You could still use inplace operators to your hearts content--just
don't bother using the reduced variable outside the loop. (I guess I'm
assuming reducing a variable has negligible performance overhead,
which it should.) For the rare cases that you want the non-aggregated
private, make an assignment to another variable, or use non-inplace
operations.

Not being able to mix inplace operators might be an annoyance. We
could also allow explicit declarations, as per Pauli's suggestion, but
not require them. Essentially, as long as we have

1) Sequential behavior == one thread scheduled (by semantics)
2) one thread scheduled == multiple threads scheduled (user's
responsibility, as it must be)

then I think we should be fine.

>> I think we need to explicitly declare something. Either a simple
>> prange(..., reduce="s:+"), or all-out declaration of thread-local variables.
>
> Sorry: prange(..., reduce="s"), or perhaps &s or cython.address(s). The + is
> of course still specified in code.
>
> Dag Sverre
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