[MITgcm-devel] Re: [MITgcm-support] Question: boundary exchange, hrcube condfiguration

[Martin Losch]

Hi Jean-Michel,

I guess I am causing may misunderstandings. I'll try to clarify:
1. I don't understand, what's different between a simple xy or lat- 
lon topogogy and a cubed sphere topography in the middle of a face,  
away from the corners. To my simple mind there isn't a difference. So  
why do we need two exchanges for the cubed sphere and only one for xy/ 
latlon topogogies?
2. the corners of the cube require special attention. I can see that.  
However, why does/(should?) resetting the corners as done within  
#ifdef W2_FILL_NULL_REGIONS [...] #endif not change the results?

what am I missing? Probably a lot. If it's too complicated to explain  
via email (without a blackboard etc), that's fine, but if not I'd  
like to know (but life will go on, if I won't know).

Martin

On 13 Jul 2007, at 17:11, Jean-Michel Campin wrote:

> Hi Martin,
>
> I was not refering to the corner-halo region of 1 face
> but one of a tile "inside" a face:
>>> in the simplest case (a tile in the middle of a face),
> and since this e-mail, I though of how to do like in exch1,
> (meaning with only 1 EXCH2_R81_CUBE call) but it would require
> to split the list of connected neighbours in 3, and then to proceed in
> 3 steps (instead of only 2 trivial set of connections x, then y,
> for what we have in exch1), and it seems painfull to me.
>
> Jean-Michel
>
> On Tue, Jul 10, 2007 at 03:53:10PM +0200, Martin Losch wrote:
>> Hi Jean-Michel (and Chris!),
>>
>> I still don't understand, your argument would apply to non-cube
>> topologies too, but it does not.
>>
>> In exch2_3d_r8 we have:
>>        CALL EXCH2_R81_CUBE( phi, 'T ',
>>      I            OLw, OLe, OLs, OLn, myNz,
>>      I            exchWidthX, exchWidthY,
>>      I            FORWARD_SIMULATION, EXCH_UPDATE_CORNERS, myThid )
>> then there a block within
>> #ifdef W2_FILL_NULL_REGIONS
>> #endif
>> which according to some comment is only used for debugging. defining
>> W2_FILL_NULL_REGIONS fills the face corners with zeros (or whatever
>> the fill value is) and this is not supposed to change the results, so
>> why do we need a second call (after which filling the corners does/
>> should not change the results)?
>>
>> Martin
>>
>> On 27 Jun 2007, at 15:24, Jean-Michel Campin wrote:
>>
>>> Hi,
>>>
>>> I think (but I might be wrong) that,
>>> in the simplest case (a tile in the middle of a face),
>>> we need a 1rst call to fill the halo region next to the tile edges
>>> (i.e., [1-Olx:0,1:sNy]),
>>> and a second call to fill the "corner" parts of the halo regions
>>> (i.e, [1-Olx:0,1-Oly:0]).
>>>
>>> This could be done in just 1 call, but would require to connect to
>>> 8 tiles instead of only 4 in the present case.
>>>
>>> I would be interesting to see how to apply the same solution
>>> as with exch1 (or why it's not possible).
>>>
>>> Now, I am trying to figure out why we need 3 exit2_rl2_cube  
>>> calls ...
>>>
>>> Jean-Michel
>>>
>>> On Wed, Jun 27, 2007 at 01:44:54PM +0200, Martin Losch wrote:
>>>> Hi there,
>>>>
>>>> could someone please explain this to Michael (and me)? Why do we  
>>>> need
>>>> two calls to exch2_rl1_cube?
>>>>
>>>> Martin
>>>>
>>>> On 22 Jun 2007, at 14:14, Michael Schroeter wrote:
>>>>
>>>>> Hi,
>>>>>
>>>>> as Martin already reported in an earlier posting
>>>>> (http://forge.csail.mit.edu/pipermail/mitgcm-support/2007-March/
>>>>> 004719.html)
>>>>> we have some unexpected scaling behaviour on a IBM p690
>>>>> (cubed sphere configuration, 16 passive tracers).
>>>>>
>>>>> We found out that the boundary exchange of the ptracers data seems
>>>>> to be
>>>>> responsible for the slow down of the code. Now we have some ideas
>>>>> for
>>>>> code optimazation.
>>>>>
>>>>> However, there is one thing in the current code I do not  
>>>>> unterstand.
>>>>> In "exch2_3d_rl.F" the subroutine "EXCH2_RL1_CUBE" is called two
>>>>> times
>>>>> with exactly the same values for the argument list parameters
>>>>> (even if the cpp option "W2_FILL_NULL_REGIONS" is undefined).
>>>>> Can anybody explain that to me?
>>>>>
>>>>> Thanks in advance.
>>>>>
>>>>> Regards
>>>>> Michael
>>>>>
>>>>> -- 
>>>>> Dr. Michael Schroeter                          Phone: +49(471)
>>>>> 4831-2084
>>>>> Email: Michael.Schroeter at awi.de                Fax:   +49(471)
>>>>> 4831-1149
>>>>> Alfred-Wegener-Institute for Polar- and Marine Research
>>>>> Am Handelshafen 12, D-27570 Bremerhaven
>>>>> www.awi.de
>>>>> _______________________________________________
>>>>> MITgcm-support mailing list
>>>>> MITgcm-support at mitgcm.org
>>>>> http://mitgcm.org/mailman/listinfo/mitgcm-support
>>>>
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