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[netCDF #ULD-680356]: parallel netcdf: writing with increasing number of cores seems to be slower than fewer cores.

Thank you very much for submitting this information and the code! I will take a 
look at it and see if there is in fact a bug on our end! Depending on data 
sizes, disk storage format (SSD, spinning drive, etc), and total number of 
cores available, it's not unknown to see this sort of counterintuitive result.  
During my own graduate work, I observed a loss of performance when I added too 
many processors to a bioinformatics analysis I was doing.   It was eventually 
tracked to a known issue in the hardware scheduler for that intel chipset.

That said, it is far more likely that there is an issue somewhere in the 
software.  Thanks again for reporting this!  It definitely bears examination.

Have a great day,

-Ward

> Dear Sir, Madam,
> 
> 
> I am writing you because I am performing some tests with parallel netcdf. I 
> am writing a 5 dimension array in parallel. The interesting part is that, 
> when I run the code with 2 cores, it performs faster than, say 20 cores.
> 
> 
> Attached is a folder containing the source code for my test. You will notice 
> I am using the Eigen library, as these experiments are related to some code 
> from my PhD project (where we use this specific array format).
> 
> 
> If you'd like to run the code, please just change the paths to the Eigen 
> directory and parallel netcdf directory in the makefile before compiling.
> 
> 
> Maybe I'm just missing a simple bug, but maybe there is actually an issue?
> 
> 
> Many thanks for your help,
> 
> 
> Alexandre Szenicer.
> 
> 
> 
> 
> 
> 


Ticket Details
===================
Ticket ID: ULD-680356
Department: Support netCDF
Priority: Normal
Status: Closed
===================
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