Strange behavior in ncmpio_file_set_view

[Wei-Keng Liao]

Both constants NC_REQ_INDEP and NC_REQ_COLL are defined internally in PnetCDF.
They are not visible to user applications. I wonder how you switch them.
In PnetCDF, NC_REQ_INDEP is used when the program is in independent mode
and NC_REQ_COLL in collective mode. Users can switch mode by calling
ncmpi_begin_indep_data() and ncmpi_end_indep_data().

Googling keyword "cost_calc" leads me to this page.
https://github.com/open-mpi/ompi/issues/6758
Where it has a test program that can produce the error: crash_mpiio.txt
Maybe you can give it a try to see if the openmpi-4.0.5 you are using has
incorporated the fix?

If that is not the case, could you provide me a short program or a code
fragment?

Wei-keng

> On Oct 28, 2021, at 4:24 PM, Sjaardema, Gregory D <gdsjaar at sandia.gov> wrote:
> 
> I am getting a floating point exception core dump down below `ncmpio_file_set_view` with certain compilers…
>  
> I’ve been trying to trace it down, but am confused by the code in `get_varm` for the case when one rank has no items to get and the other rank has items to read with a non-unity stride (7 in this case).
> This is from a code using netCDF to call down into PnetCDF.
>  
> Originally, the variable being read was NC_REQ_INDEP, so the rank with zero items to read would return from `get_varm` at line 464 and the other rank would continue.  It would eventually end up in `ncmpio_file_set_view` and call down and finally throw the floating point exception.
>  
> Since there is a comment that `MPI_File_set_view` is collective, I figured that might be the issue that only one rank was calling down that path, so changed the variable to be NC_REC_COLL.  Both ranks now call down into `ncmpio_file_set_view`, but then inside that routine, the rank with zero bytes to read falls into the first if block `if (filetype == MPI_BYTE)` and the second rank goes down further and hits the next if block `if (rank == 0) `.
>  
> Both ranks end up calling a MPI_File_set_view, but with different types.  The end result is that I still get a floating point exception on the rank that does have bytes to read.  The execption seems to be in `cost_calc`.
>  
> This is with pnetcdf-1.12.1, clang-12.0.0 (also with clang-10.0.0) and openmpi-4.0.5.
>  
> I’m basically looking for guidance at this point that the calling paths look correct or where to look in more depth…   Any help appreciated.
>  
> ..GReg