[MITgcm-devel] impldiff/solve_tridiagnonal

[Jean-Michel Campin]

Hi Martin,

I am going to try few things there, and will let you know 
(probably earlier next week) how things go.

Cheers,
Jean-Michel

On Thu, Sep 29, 2016 at 07:35:15AM +0200, Martin Losch wrote:
> Hi Jean-Michel,
> 
> I am all for it. Do you know what to do exactly or should I try to figure it out?
> 
> Martin
> > On 29 Sep 2016, at 01:58, Jean-Michel Campin <jmc at mit.edu> wrote:
> > 
> > Hi Martin,
> > 
> >> I could use mom/salt/tempImplVertAdv = .TRUE., right?
> > It's not really available, would need to change the code (change 2 or 3 "IF ()")
> > so that the routines that deal with implicit vertical advection
> > (i.e., mom_u_implicit_r.F, mom_v_implicit_r.F & gad_implicit_r.F ) 
> > are also called when just implicit diffusion/viscosity is turned on (even without 
> > implicit advection) since these routines are also doing the vertical diffusion.
> > 
> > Cheers,
> > Jean-Michel
> > 
> > On Wed, Sep 28, 2016 at 09:04:00PM +0200, Martin Losch wrote:
> >> Hi Jean-Michel,
> >> 
> >> interesting that you remember that. Yes, we have a new computer (o: So optimization starts again from scratch ???
> >> 
> >> no, I am not using CD_CODE. I could use mom/salt/tempImplVertAdv = .TRUE., right?
> >> 
> >> Martin
> >> 
> >>> On 28 Sep 2016, at 19:16, Jean-Michel Campin <jmc at mit.edu> wrote:
> >>> 
> >>> Hi Martin,
> >>> 
> >>> I found an email on the list iwith an interesting Post-Scriptum that was arguing 
> >>> in the other direction, i.e., to allow NOT to use SOLVE_DIAGONAL_KINNER code:
> >>> http://mitgcm.org/pipermail/mitgcm-devel/2011-October/004995.html
> >>> 
> >>> But appart from that (and I agree that this performance issue is platform/compiler 
> >>> dependent), I thought we could try not to use impldiff.F anymore (most of the code 
> >>> is there except for CD_CODE) and leave impldiff.F to the retirement track.
> >>> Are you using the CD_CODE ?
> >>> 
> >>> Cheers,
> >>> Jean-Michel
> >>> 
> >>> On Wed, Sep 28, 2016 at 06:41:07PM +0200, Martin Losch wrote:
> >>>> Hi there,
> >>>> 
> >>>> impldiff.F turns out to be one of the expensive routines on some platforms. One of the reasons is apparently poor cache efficiency. In solve_tridiagonal this is ???solved??? by moving the k-loop inside the i,j loops. There???s a CPP flag SOLVE_DIAGONAL_KINNER to turn that on in solve_tridiagonal.
> >>>> 
> >>>> Would anyone object to me implementing something like this in impldiff.F? I would use SOLVE_DIAGONAL_KINNER (off by default) to turn this feature on.
> >>>> 
> >>>> Martin
> >>>> 
> >>>> PS. would be much better if impldiff would actually call solve_tridiagonal, right?
> >>>> 
> >>>> 
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