Hi All.

I have parallel PDE/CFD code in fortran.
Let we consider it consisting of two parts:

1) Startup part; that  includes input reads, splits, distributions, forming neighborhood information arrays, grid arrays, and all related. It includes most of the necessary array declarations.

2) Iterative part; we proceed the solution in time.


Approach One:
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What I do is that during the Startup phase, I declare the most array allocatable and then allocate them sizes depending upon the input reads and domain partitioning. And then In the iterative phase I utilize those arrays. But I "do not" allocate/deallocate new arrays in the iterative part.


Approach Two:
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I think that,  what if I first use to run only the start -up phase of my parallel code having allocatable like things and get the sizes-values required for array allocations for a specific problem size and partitioning. Then I use these values as constant in another version of my code in which I will declare array with the constant values obtained.

So my question is that will there be any significant performance/efficiency difference in the "ITERATIVE part" if the approach two is used (having arrays declared fixed sizes/values)?


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ANOTHER QUESTION ABOUT CALLING SUBROUTINES:
Assume two ways:
1) One way is that we declare arrays in some global module and "USE"
arrays in subroutines which ever is needed there.


2) We pass a large number of arrays (10, 20 or 30 may be) in the header
while making call to a  subroutine.

Which way is quite fast and efficient over the other?

Thank You for your kind attention.

with best regards,
Amjad Ali.