[MITgcm-devel] GLOBAL_SUM in cost_final.F

[Jean-Michel Campin]

Hi,

I started to test some changes in pkg/cost/cost_final.F,
to replace the call to           GLOBAL_SUM_RL( fc , myThid ) 
with a call to    GLOBAL_SUM_TILE_RL( tile_fc , fc , myThid )

There is a practical advantage to do this is that I can get
better forward gradient agreement when using MPI compared to non-MPI
(when GLOBAL_SUM_SEND_RECV is defined in CPP_EEOPTIONS.h). 
For instance, with tutorial_tracer_adjsens, forward grad 
testreport with MPI will give 16 digits for forward grad 
whereas now it's only 5 (on baudelaire with gfortran).

There is also a less obvious advantage is that 
 GLOBAL_SUM_RL( fc , myThid )
gives the wrong results when using multi-threads (since
fc is in common block).

I have several versions that I can check-in:
1) only changes in cost_final.F (but this leave on the side
 few other cost contributions.
2) add "tile_fc(bi,bj)" to cost.h and change
 cost_final.F seaice_cost_final.F shelfice_cost_final.F thsice_cost_final.F
  (the only pkg that would need GLOBAL_SUM_RL( fc ) would be pkg/ecco)
3) same as (2) + also change ecco_cost_final.F
  (changes are not major, total of 6 lines, changing
   fc = fc
  to
   tile_fc(bi,bj) = tile_fc(bi,bj)
  )
I think (3) would be cleaner (and easier to follow and check), but I don't
know how many different versions of ecco_cost_final.F users have.

Suggestions ?

Cheers,
Jean-Michel