# AGRIF - Subnest processor map

**URL:** <https://nemo-ocean.discourse.group/t/agrif-subnest-processor-map/974>\
**Category:** Uncategorized\
**Created:** [17 December 2025 13:21 UTC](https://nemo-ocean.discourse.group/t/agrif-subnest-processor-map/974 "2025-12-17T13:21:23Z")\
**Posts on this page:** 9\
**Page:** 1

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**Author:** ![robinson](https://avatars.discourse-cdn.com/v4/letter/r/848f3c/32.png) [@robinson](https://nemo-ocean.discourse.group/u/robinson)\
**Post date:** [17 December 2025 13:21 UTC](https://nemo-ocean.discourse.group/t/agrif-subnest-processor-map/974/1 "2025-12-17T13:21:23Z")

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Dear All,

I’m trying to run AGRIF. With one child level (1\_ ), it runs fine. But i have problems with the processor map when trying to run a subnest (so a child in the child).

I do not specify jpni/jpnj, at none of the levels (they are set to zero), and i get the following error at level 2\_ (please check below). I’ve tried to change jpni/jpnj at level 2\_, and a lot of other tricks, but nothing works. Can anyone help ?

Thanks in advance,

Robinson

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mpp\_init:

```auto
  The number of mpi processes: 966
  exceeds the maximum number of subdomains (ocean+land) = 961
  defined by the following domain decomposition: jpni = 31 jpnj = 31
   You should: 
     - either prescribe your domain decomposition with the namelist variables
       jpni and jpnj to match the number of mpi process you want to use, 
       even IF it not the best choice...
     - or keep the automatic and optimal domain decomposition by picking up one
       of the number of mpi process proposed in the list bellow

                  For your information:
  list of the best partitions including land supression
  -----------------------------------------------------

nb_cores oce: 1849, land domains excluded: 0 ( 0.0%), largest oce domain: 49 ( 7 x 7 )
nb_cores oce: 1643, land domains excluded: 0 ( 0.0%), largest oce domain: 54 ( 6 x 9 )
nb_cores oce: 1548, land domains excluded: 0 ( 0.0%), largest oce domain: 56 ( 7 x 8 )
nb_cores oce: 1431, land domains excluded: 0 ( 0.0%), largest oce domain: 60 ( 6 x 10 )
nb_cores oce: 1333, land domains excluded: 0 ( 0.0%), largest oce domain: 63 ( 7 x 9 )
nb_cores oce: 1296, land domains excluded: 0 ( 0.0%), largest oce domain: 64 ( 8 x 8 )
nb_cores oce: 1272, land domains excluded: 0 ( 0.0%), largest oce domain: 66 ( 6 x 11 )
nb_cores oce: 1161, land domains excluded: 0 ( 0.0%), largest oce domain: 70 ( 7 x 10 )
nb_cores oce: 1116, land domains excluded: 0 ( 0.0%), largest oce domain: 72 ( 8 x 9 )
nb_cores oce: 1032, land domains excluded: 0 ( 0.0%), largest oce domain: 77 ( 7 x 11 )
nb_cores oce: 972, land domains excluded: 0 ( 0.0%), largest oce domain: 80 ( 8 x 10 )

```

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<div class="post-metadata">

**Author:** ![smasson](https://avatars.discourse-cdn.com/v4/letter/s/eb9ed0/32.png) [@smasson](https://nemo-ocean.discourse.group/u/smasson)\
**Post date:** [17 December 2025 13:49 UTC](https://nemo-ocean.discourse.group/t/agrif-subnest-processor-map/974/2 "2025-12-17T13:49:21Z")

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**If you do not specify jpni and jpnj,** NEMO will pick-up the best partition and try to match it with the number of proc you are providing by keeping some land subdomain if needed. For example, if you provide N cores, NEMO will compute jpni, jpnj, noce, land\_exclude and land\_keep with

```auto
noce is an optimal domain decomposition
noce = jpni * jpnj - land_exclude 
noce <= N
N = noce + land_keep 
0 <= land_keep <= land_exclude

```

See section 2.1 of Irrmann et al. 2022 for all details.

If NEMO is unable to find a suitable solution, because `N = noce + land_keep` would require `land_keep > land_exclude`, the model stops with the error message you got.

The solution is then to specify jpni and jpnj in the namelist. The simplest way is to define them such as `N = jpni * jpnj`. You will not use an optimal domain decomposition but the model will run. 🫤

We plan to fix this in the future release by allowing some idle cores when running NEMO, we will the have:

```auto
if land_keep <= land_exclude then
  noce = jpni * jpnj - land_keep 
else 
  noce = jpni * jpnj - land_exclude - n_idle_procs

```

Sébastien

Irrmann, G., Masson, S., Maisonnave, É., Guibert, D., & Raffin, E. (2022). Improving ocean modeling software NEMO 4.0 benchmarking and communication efficiency. _Geoscientific Model Development_, _15_(4), 1567–1582. [GMD - Improving ocean modeling software NEMO 4.0 benchmarking and communication efficiency](https://doi.org/10.5194/gmd-15-1567-2022)

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<div class="post-metadata">

**Author:** ![robinson](https://avatars.discourse-cdn.com/v4/letter/r/848f3c/32.png) [@robinson](https://nemo-ocean.discourse.group/u/robinson)\
**Post date:** [17 December 2025 14:01 UTC](https://nemo-ocean.discourse.group/t/agrif-subnest-processor-map/974/3 "2025-12-17T14:01:48Z")

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Merci Seb, I will try.

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<div class="post-metadata">

**Author:** ![robinson](https://avatars.discourse-cdn.com/v4/letter/r/848f3c/32.png) [@robinson](https://nemo-ocean.discourse.group/u/robinson)\
**Post date:** [17 December 2025 14:50 UTC](https://nemo-ocean.discourse.group/t/agrif-subnest-processor-map/974/4 "2025-12-17T14:50:28Z")

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ok, now it works. So i write below how i did through an example, to help others to whom it may happen.

jpni\*jpnj=966 is the total number of processor used to run Nemo, and none of the numbers suggested in the list when it crashes. I just searched for a division of 966 as squared as possible.

ln\_listonly = .false. ! do nothing else than listing the best domain decompositions (with land domains suppression)  
! ! if T: the largest number of cores tested is defined by max(mppsize, jpni\*jpnj)  
ln\_nnogather = .false. ! activate code to avoid mpi\_allgather use at the northfold  
jpni = 42 ! number of processors following i (set automatically if \< 1), see also ln\_listonly = T  
jpnj = 23 ! number of processors following j (set automatically if \< 1), see also ln\_listonly = T  
nn\_hls = 1 ! halo width (applies to both rows and columns)  
nn\_comm = 1 ! comm choice

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<div class="post-metadata">

**Author:** ![smasson](https://avatars.discourse-cdn.com/v4/letter/s/eb9ed0/32.png) [@smasson](https://nemo-ocean.discourse.group/u/smasson)\
**Post date:** [17 December 2025 15:23 UTC](https://nemo-ocean.discourse.group/t/agrif-subnest-processor-map/974/5 "2025-12-17T15:23:14Z")

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Robinson, which version are you using? With NEMO 5 you must have `nn_hls = 2`.

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<div class="post-metadata">

**Author:** ![robinson](https://avatars.discourse-cdn.com/v4/letter/r/848f3c/32.png) [@robinson](https://nemo-ocean.discourse.group/u/robinson)\
**Post date:** [17 December 2025 15:30 UTC](https://nemo-ocean.discourse.group/t/agrif-subnest-processor-map/974/6 "2025-12-17T15:30:46Z")

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I’m using Nemo 4.2.2

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<div class="post-metadata">

**Author:** ![robinson](https://avatars.discourse-cdn.com/v4/letter/r/848f3c/32.png) [@robinson](https://nemo-ocean.discourse.group/u/robinson)\
**Post date:** [17 December 2025 16:44 UTC](https://nemo-ocean.discourse.group/t/agrif-subnest-processor-map/974/7 "2025-12-17T16:44:47Z")

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Ah now it crashes again, I saw on a webpage you answered to a similar question Sébastien, i get the error below, should i modify the code as in here ([Ice suppression on child grid fails when writing restarts through XIOS (#604) · Issues · NEMO Workspace / Nemo · GitLab](https://forge.nemo-ocean.eu/nemo/nemo/-/issues/604)), although all levels should have sea ice indeed.

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iccontext.cpp terminate called after throwing an instance of ‘xios::CException’  
void cxios\_context\_handle\_create(xios::CContext \*\*, const char \*, int) 34  
terminate called after throwing an instance of ‘xios::CException’  
In file “iccontext.cpp”, function “void cxios\_context\_handle\_create(xios::CContext \*\*, const char \*, int)”, line 54 → Context 2\_nemo unknown

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<div class="post-metadata">

**Author:** ![Franziska](https://avatars.discourse-cdn.com/v4/letter/f/8797f3/32.png) [@Franziska](https://nemo-ocean.discourse.group/u/Franziska)\
**Post date:** [18 December 2025 05:09 UTC](https://nemo-ocean.discourse.group/t/agrif-subnest-processor-map/974/8 "2025-12-18T05:09:19Z")

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The error message (Context 2\_nemo unknown) you get indicates that you probably missed to specify a context for your second level child (“subnest”) in iodef.xml.

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<div class="post-metadata">

**Author:** ![robinson](https://avatars.discourse-cdn.com/v4/letter/r/848f3c/32.png) [@robinson](https://nemo-ocean.discourse.group/u/robinson)\
**Post date:** [18 December 2025 06:02 UTC](https://nemo-ocean.discourse.group/t/agrif-subnest-processor-map/974/9 "2025-12-18T06:02:54Z")

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Ah yes, indeed. I went a bit too quick. Thanks a lot Franziska.