[MITgcm-support] restart from pickup files (cube-sphere)

[Yuan Lian]

Hi Jean-Michel and Dimitris,

Thanks for all your help. I think the reason that causes my problem is 
that I didn't
keep the number of tiles unchanged, which actually led to noticeable 
difference
of results. I will try all the methods you mentioned and see how my 
model works.

Regards,
Yuan

Jean-Michel Campin wrote:
> Hi Yuan,
>
> As Dimitris mentionned, when 
> a) you redistribute tiles between processors 
> and/or 
> b) change the size of the tiles, 
> you shoud expect different truncation errors, specially with global sum.
> If you run just for few iterations, the differences should be small,
> and you can check this.
>
> If you keep the same number of tiles (case a)(just changing nSx,nSy,nPx,nPy)
> then you don't have to change the EXCH2 files, and if you
> 1) change "#undef GLOBAL_SUM_SEND_RECV" to
>    to    "#define GLOBAL_SUM_SEND_RECV" in CPP_EEOPTIONS.h
> 2) compile with zero optimisation
> you should be able to get the same results (but some compiler and/or
>  some very sensitive set-up might still produce little differences ;
>  you can check the results of our daily testing, all very short test 
>  with no optimization, e.g. with g77 & mpi:
>    http://mitgcm.org/testing.html 
>  to get an idea of which type of set-up is in this case).
>
> If you change the size & number of tiles (case b), and you are not using 
> nonHydrostatic, you can try "#define CG2D_SINGLECPU_SUM" in
> CPP_EEOPTIONS.h, with is supposed to be slower but has been 
> added recently to check this kind of thing (again, with no optimization).
>
> Cheers,
> Jean-Michel
>
> On Fri, Mar 28, 2008 at 08:01:19AM -0700, Yuan Lian wrote:
>   
>> Hi,
>>
>> I recently notice that the simulation will produce different results when
>> I switch number of CPUs from pickup files. For example, originally I
>> used 12 CPUs to run the model, then I switched to 6 CPUs due to the
>> lack of CPU resources. The simulation ran but results were different from
>> those of 12-CPU run (I started from same pickup file but used different
>> number of CPUs to compare results) .
>>
>> My question is, when restart a job from pickup file with different number
>> of CPUs, do I need to modify any configuration other than w2_e2setup.F,
>> W2_EXCH2_TOPOLOGY.h and SIZE.h? Thanks!
>>
>>
>> Cheers,
>> Yuan
>> _______________________________________________
>> MITgcm-support mailing list
>> MITgcm-support at mitgcm.org
>> http://mitgcm.org/mailman/listinfo/mitgcm-support
>>     
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