rewrite northpole folding in the nogather case

[smasson]

Context

There is a list of points which worries me about the nogather folding:

• The code is quite complicated to understand...
• We are sending halos values (which is usually not what should be done in MPI exchanges!) -> We assume that the east-west communications were done before applying the folding
• T, U, V and F grids have the same neighbours, which is strange as the communication patterns are not the same for these grids (there are potential shifts of +/-1). I think we have no error as we send the halos.
• I am not sure if it is really working when nn_hls > 1
• There is a duplication of code between lbc_nfd_nogather_generic.h90 and lbc_nfd_generic.h90

Analysis

• lbc_nfd_generic.h90 was already cleaned to make it compatible with nn_hls > 1 and to make sure we don't use halos values. It can be called independently of the east-west periodicity. We should take advantage of this!
• The idea is to use the "gather" folding during mppini to know which neighbour is sending which data and therefore build a mapping of the communications associated with the folding. This will allow us to skip lbc_nfd_nogather_generic. This strategy is possible as the folding is symmetrical: sending data to neighbour N <-> receiving data from neighbour N.

Beware as the folding is also filling the e-w halos, the number of point exchanged between 2 neighbourgs can be different.

Recommendation

• Limite the size and the number of allocated buffers
• Use mpiwaitall and mpi_Irecv.
• We could also code a "collective neighbourhood" version for the folding.
• change the name of ln_nnogather

[smasson]

In 15267:

Error: Failed to load processor CommitTicketReference

No macro or processor named 'CommitTicketReference' found

[smasson]

[15267] passes all sette tests with nn_hls = 1 or 2 (./sette.sh -A -C -t -F -c -v ACFt and ./sette.sh -A -C -t -e -F -c -v ACeFt ) and gives the same results as [15266]

[acc]

Nice one Seb. I can confirm it passes and matches on my system too. Just to be clear: you didn't change the name of ln_nnogather after all?

[smasson]

no I did not change it (yet...).
I was a little bit afraid of the consequences (-> all the changes to do in sette)

[smasson]

In 15270:

Error: Failed to load processor CommitTicketReference

No macro or processor named 'CommitTicketReference' found