[Users] CCTK_ReductionArrayHandle

[Luca Baiotti]

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