[MITgcm-support] Zonal filter with nPx>1

[Roland Young]

Hi Jean,

OK thanks.

I have coded a version that works for nPx>1 (at least for nSx=nSy=1). It gathers the field from all the processors along a longitude row and applies the filter on the west-most processor before scattering the filtered field back to the other processors.

I was wondering whether, in general, the MITgcm is bit-reproducible with respect to domain decomposition? I’d like to know whether my edited zonal filter does the same thing as the standard version but I can’t be sure as the output is not quite bit-reproducible. 

For example, I ran the following configurations:

(a) 16x1 standard zonal filter
(b) 16x1 edited zonal filter

Output from (a) and (b) is bit reproducible.

(c) 4x4 edited zonal filter
(d) 8x2 edited zonal filter
(e) 8x1 standard zonal filter
(f) 32x1 standard zonal filter

Output from (a) is not bit-reproducible wrt (c), (d), (e), or (f) (although identical to 1 part in 10^8 in U and V, and 1 part in 10^16 in theta, after 10 time steps)

(g) 16x1 no zonal filter
(h) 4x4 no zonal filter

Output from (g) and (h) is not bit-reproducible (but are identical to 1 part in 10^8 in U and V, and 1 part in 10^16 in theta after 2 time steps - any more it crashes as the zonal filter isn’t on)

Bit-reproducibility is not critical for my work so I can probably proceed as-is, but are there any parts of the code that are known to be not bit-reproducible across domain decompositions? I’m using the global atmosphere with mom_vecinv.

Thanks,

Roland