Slow speed after changing from serial to parallel

[Ben Tay]

Hi,

I've a serial 2D CFD code. As my grid size requirement increases, the
simulation takes longer. Also, memory requirement becomes a problem. Grid
size 've reached 1200x1200. Going higher is not possible due to memory
problem.

I tried to convert my code to a parallel one, following the examples given.
I also need to restructure parts of my code to enable parallel looping. I
1st changed the PETSc solver to be parallel enabled and then I restructured
parts of my code. I proceed on as longer as the answer for a simple test
case is correct. I thought it's not really possible to do any speed testing
since the code is not fully parallelized yet. When I finished during most of
the conversion, I found that in the actual run that it is much slower,
although the answer is correct.

So what is the remedy now? I wonder what I should do to check what's wrong.
Must I restart everything again? Btw, my grid size is 1200x1200. I believed
it should be suitable for parallel run of 4 processors? Is that so?

Thank you.
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