[Users] CCTK_ReductionArrayHandle

[Erik Schnetter]

Luca

I may have been confused. The flesh offers two different reduction
APIs, one (the newer one) implemented by PUGH and LocalReduce, the
other one (the older one) implemented by Carpet. I may have mixed up
the names of the new and the old interfaces...

-erik

On Thu, Aug 2, 2012 at 10:26 PM, Luca Baiotti <baiotti at ile.osaka-u.ac.jp> wrote:
> On 8/2/12 9:19 PM, Erik Schnetter wrote:
>>
>> On 2012-08-02, at 4:26 , Luca Baiotti <baiotti at ile.osaka-u.ac.jp>
>> wrote:
>>
>>> On 8/2/12 4:11 PM, Roland Haas wrote:
>>>>>
>>>>> CCTK_ReductionArrayHandle is used in several thorns but it is
>>>>> not described in the ReferenceManual. Is it (still) supported?
>>>>> How is it different from CCTK_LocalArrayReductionHandle?
>>>>
>>>> It's part of the "old" reduction interface. Which happens to be
>>>> the one Carpet supports, so it is widely used  :-). I doubt
>>>> support will go away anytime soon. The calls are documented in
>>>> the User manual in section C1.7.5 (page 84 in my build).
>>>
>>>
>>> I see, so CCTK_ReductionArrayHandle is described in the User's
>>> Manual but not in the Reference Manual.
>>>
>>>> As for differences I'd consult this section and the reference
>>>> manual. It seems that one is a reduction over local arrays (ie
>>>> double a[100]) while the other reduces grid functions (or grid
>>>> arrays or grid scalars most likely).
>>>
>>>
>>> I found no reference saying that CCTK_ReductionArrayHandle should
>>> be used only for local arrays or only for grid functions.
>>>
>>> Which of the two codes below is correct (in a subroutine called by
>>> the scheduler)? Why?
>>
>>
>> For historic reasons, local reductions (where the caller passes in
>> the grid size) and "regular" reductions (which use grid variables)
>> have different operators. In principle, using the wrong operator
>> should lead to an error message at run time.
>>
>> What happens currently is likely that PUGHReduce and CarpetReduce
>> register the same operators for both local and global reductions in
>> the same order, and (by coincidence!) their handles are thus
>> interchangeable.
>>
>> Below, number (1) is correct, number (2) is wrong.
>>
>> Number (2) obtains a handle for a global reduction operation. This
>> handle will have a certain value, e.g. 15. Later, the code uses the
>> _local_ reduction operation with handle 15, which (apparently) is
>> also the local sum reduction.
>>
>> We should correct this. For example, we could add large, different
>> offsets for local and global handles, so that they would be
>> different. We already do this for other integer IDs in the flesh.
>>
>> -erik
>>
>>> 1)
>>>
>>> CCTK_REAL local_array_in,local_array_out;
>>>
>>> CCTK_INT handle = CCTK_LocalArrayReductionHandle("sum");
>>>
>>> CCTK_ReduceLocArrayToArray1D(cctkGH, -1, handle, &local_array_in,
>>> local_array_out, 4, CCTK_VARIABLE_REAL));
>>>
>>>
>>> 2)
>>>
>>> CCTK_REAL local_array_in,local_array_out;
>>>
>>> CCTK_INT handle = CCTK_ReductionHandle("sum");
>>>
>>>
>>> CCTK_ReduceLocArrayToArray1D(cctkGH, -1, handle, &local_array_in,
>>> local_array_out, 4, CCTK_VARIABLE_REAL));
>
>
> Thanks for the explanation.
>
> Actually what I have in my code is:
>
> 3)
>
> CCTK_REAL local_array_in,local_array_out;
>
> CCTK_INT handle = CCTK_ReductionArrayHandle("sum");
>
>
> CCTK_ReduceLocArrayToArray1D(cctkGH, -1, handle, &local_array_in,
> local_array_out, 4, CCTK_VARIABLE_REAL));
>
>
> Is this correct? If I change the handle call to
> CCTK_LocalArrayReductionHandle, I get this error at run time:
>
>
> WARNING level 1 in thorn Cactus processor 0 host mbaiotti2.local
>   (line 1120 of /Users/baiotti/Cactus/src/comm/Reduction.c):
>   -> CCTK_LocalArrayReductionHandle: No handle: '-2' found for reduction
> operator 'sum'
>
>
> Luca



-- 
Erik Schnetter <schnetter at cct.lsu.edu>
http://www.perimeterinstitute.ca/personal/eschnetter/