[MITgcm-support] MITgcm on AS4, kernel 2.6

[Ed Hill]

On Thu, 2005-10-06 at 18:54 +0300, Hezi Gildor wrote:
> Hi Ed,
> 
> thanks for the prompt reply.
> 
> The OS was AS4.0 and AS4U1. The compilers tried are gcc and pgi (5.2 and 
> 6.0). Tried using with LAM, MPICH (from pgi as well as built from tar 
> ball), MPICH2. The compiler options used were many. From the default 
> options to the -O3, to -fast, -fastsse, and -kieee (last 3 for pgi).
> 
> Communications is through the internet. (No private network for now).
> 
> Dual-opteron machines with
> 
> AMD Opteron(tm) Processor 250
> 
> The code is based on MITgcm_ss_20050719, checkpoint571_post. We have few 
> hard-wired changes to code but it runs ok on Alpha machine and on 
> cluster with Linux Kernel 2.4 so i don't think that the problem is with 
> the code.
> 
> when we compared fields such as salinity between two runs with same 
> executables, we can get that at most grid points the difference is zero 
> but at many isolated points scattered throughout the domain it is not (O 
> of 10^-6).

Hi Hezi,

If you are using *exactly* the same executable and *exactly* the same
inputs then, assuming you are running it on exactly the same machine, I
have no idea why you're not seeing exactly the same answer.  Of course,
if anything changes from one run to another (eg. different compiler,
different OS, different hardware, etc.) then you won't, in general, see
exactly identical ("zero difference") output due to differences in
numerical roundoff, etc.

And, to be fair, your description is not enough information for us to
have a good idea of what you're doing (or trying to do).  So, can you
narrow this problem down to a (hopefully simple and) repeatable example?
That is, can you create a small but complete setup that includes *all*
the code changes that you've made and *all* your input files?

Once you can create an example that repeatably demonstrates a problem,
then we can work with you to try to determine the reason.  But, without
such an example, I'm afraid that all we can offer you is guesses...

Ed

-- 
Edward H. Hill III, PhD
office:  MIT Dept. of EAPS;  Rm 54-1424;  77 Massachusetts Ave.
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