Issue in tests/C/pres_temp_wr.c

[Sjaardema, Gregory D]

While looking through the parallel-netcdf-1.7.0 tests, I stumbled across the press_temp_wr.c file.  I think the logic in this file is failed.


  *   Each processor has the exact same view of the 3D pressure and temperature data.  This is OK for a test.
  *   The code below is used for each processor to write out a portion of the pressure and temperature data to the file:

   /* These settings tell netcdf to write one timestep of data. (The
     setting of start[0] inside the loop below tells netCDF which
                    &data[0][0][0]);
     timestep to write.) */
   count[0] = 1;
   count[1] = NLVL/nprocs;
   count[2] = NLAT;
   count[3] = NLON;
   start[1] = 0;
   start[2] = 0;
   start[3] = 0;

   /* Write the pretend data. This will write our surface pressure and
      surface temperature data. The arrays only hold one timestep worth
      of data. We will just rewrite the same data for each timestep. In
      a real application, the data would change between timesteps. */

   for (rec = 0; rec < NREC; rec++)
   {
      start[0] = rec;
      err = ncmpi_put_vara_float_all(ncid, pres_varid, start, count, &pres_out[0]][0][0]);
      CHECK_ERR
      err = ncmpi_put_vara_float_all(ncid, temp_varid, start, count, &temp_out[0][0][0]);
      CHECK_ERR
   }

  *   I think that the start[1] value and the first dimension of the pres_out and temp_out array in the calls to ncmpi_put_vara_float are incorrect.  With these values, only 1/nprocs portion of the array is written and the remainder is zero-filled.
     *   This is verified by running press_temp_wr on 1, 2, and 4 processors and doing an ncdump of the file.
     *   All runs should give the same file, but they don’t.
  *   I think that the correct code should be something similar to:

   /* These settings tell netcdf to write one timestep of data. (The
     setting of start[0] inside the loop below tells netCDF which
                    &data[0][0][0]);
     timestep to write.) */
   count[0] = 1;
   count[1] = NLVL/nprocs;
   count[2] = NLAT;
   count[3] = NLON;
   start[1] = rank*(NLVL/nprocs);
   start[2] = 0;
   start[3] = 0;

   /* Write the pretend data. This will write our surface pressure and
      surface temperature data. The arrays only hold one timestep worth
      of data. We will just rewrite the same data for each timestep. In
      a real application, the data would change between timesteps. */

   for (rec = 0; rec < NREC; rec++)
   {
      start[0] = rec;
      err = ncmpi_put_vara_float_all(ncid, pres_varid, start, count, &pres_out[start[1]][0][0]);
      CHECK_ERR
      err = ncmpi_put_vara_float_all(ncid, temp_varid, start, count, &temp_out[start[1]][0][0]);
      CHECK_ERR
   }


  *   With these changes, the exact same file is written for 1, 2, and 4 processors.
  *   Similar changes are needed for the press_temp_rd.c
  *   Note that a “real” application would probably not replicate the pres_out and temp_out arrays on all processes and instead each process would have its own portion of the array; however, for this test in which the arrays are replicated on all processors, I think the changes shown above are needed to give correct results.

..Greg

--
"A supercomputer is a device for turning compute-bound problems into I/O-bound problems”
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