[MITgcm-devel] problem in exch2 with small tiles

[Jean-Michel Campin]

Hi,

Looks like I was too optimistic (with only cs-32 in mind):
With a face size of 90x90 (such as in some lat-long cap grid), 
sNx=10 & sNy=9 will always cause problems.
The true condition for the problem to show-up (assuming a full
face-egde connection) is in fact, with GCD = greatest common divisor:
  sNx <> sNy and GCD(sNx,sNy) < 2*Olx 
Probably better to implement a fix (instead of just a stop).

Cheers,
Jean-Michel

On Sun, Mar 25, 2012 at 05:14:30PM -0400, Jean-Michel Campin wrote:
> Hi,
> 
> As I mentioned in the mitgcm-support email, there is a problem
> in pkg/exch2 when sNx <> sNy and sNx < 2*Olx or sNy < 2*Oly
> 
> I did reproduce Yuan's problem with olx=4, sNx=8, sNy=4 and cs-32 grid, 
> and here is an attempt to explain where the problem is:
> 
> tile A & B on face 1, tile-A eastern neighbor = B ; tile-B western neighbor = A
> tile C & D on face 3, tile-A northern neighbors = C & D
>                       tile-C & tile-D western neighbor = A
> 
> tile tile (face-3)
>   C | D |
>   ^   ^
>   |   |
>  tile-A <-> tile-B
>       (face-1)
> 
> tile-B NW-corner (not a face corner) received valid values during 
> the 2nd EXCH call, which are coming from the Northern halo of tile-A 
> (the order in the list of neighbors matters, since it could as well 
>  be updated from tile-B northern neighbors).
> 
> It means that the 1rst EXCH call needs to provide valid values
> in the Northern halo of tile-A (for i=sNx-Olx+1:sNx)
> which should come from tile C & D interior (in order to be valid).
> 
> The problem: with EXCH_UPDATE_CORNERS, not only interior values (1:sNy) will
> be copied to tile-A Northern halo but also Northern & Southern halo values of
> tiles C & D, which are not valid before the 1rst EXCH.
> The second (C or D) in the list of neighbors can overwrite valid values from
> the other (D or C) with invalid ones from (C or D) Northern or Southern halo.
> 
> At the end, the Northern halo of tile-A will be fixed by the 2nd EXCH call,
> but regarding values which are needed to fill tile-B NW corner (during 2nd
> EXCH calls) they should not be overwritten (with invalid values from tile-C halo).
> this translates to the condition: sNx-sNy+Olx < sNx-Olx+1 or sNy >= 2*Olx
> 
> I can see few solutions:
> 1) add a stop if sNx <> sNy and sNx < 2*Olx or sNy < 2*Oly
> 2) add a 3rd EXCH call if sNx <> sNy and sNx < 2*Olx or sNy < 2*Oly
>    I think this will fix the problem but slow down the model 
> 3) change the 1rst EXCH call to use EXCH_IGNORE_CORNERS
>   instead of EXCH_UPDATE_CORNERS, but to leave the 2nd EXCH call
>   with EXCH_UPDATE_CORNERS.
> I think this (3) should work for scalar EXCH (or did I missed something ?)
> since I have the impression that only the 2nd EXCH call needs to do
> tile-corners update. Not sure yet about vector EXCH.
> 
> Cheers,
> Jean-Michel
> 
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