yt runs SlicePlot in-suite analysis failed with Enzo embedded Python2.7 with MPI rank > 1

[cindytsai]

Bug report

Bug summary yt failed while running SlicePlot in Enzo inline-analysis with embedded python. yt/frontends/enzo/io.py (class IOHandlerInMemory) cannot handle cases when grids needed doesn't exist in self.grids_in_memory in that specific MPI rank inside the function def _read_fluid_selection. As in contrast, both find_max and ProjectionPlot work well with MPI rank > 1.

Code for reproduction

1. Get Enzo
2. Build the Makefile Fill in LOCAL_PYTHON_INSTALL, LOCAL_HDF5_INSTALL, LOCAL_INCLUDES_PYTHON, LOCAL_LIBS_PYTHON, and make sure the last two variables should contain numpy package as well.
   • A working makefile on the machine I'm using: Make.mach.eureka-python2
   • Run Enzo with embedded python (this should be switched on):

     make python-yes

   • Show the configuration of current build system:

     make show-config

3. Name the in-suite analysis script as user_script.py: Here, we do SlicePlot operation.

   
   import yt
   yt.enable_parallelism()

def main(): ds = yt.frontends.enzo.EnzoDatasetInMemory()

# Do slice plot, but won't work when Enzo Embedded Python MPI rank > 1
sz = yt.SlicePlot(ds, 'z', 'density')

# Output the result on root only
if yt.is_root():
    sz.save()

Some useful information, see [Embedded Python in Enzo](https://enzo.readthedocs.io/en/enzo-2.5/user_guide/EmbeddedPython.html). _(Beware that this site is out-dated, and it hasn't update for Enzo2.6.)_

**Actual outcome**

File "", line 1, in File "./user_script.py", line 8, in main sz = yt.SlicePlot(ds, 'z', 'density') File "/work1/cindytsai/Software/SysPython27/lib/python2.7/site-packages/yt/visualization/plot_window.py", line 2039, in SlicePlot return AxisAlignedSlicePlot(ds, normal, fields, *args, **kwargs) File "/work1/cindytsai/Software/SysPython27/lib/python2.7/site-packages/yt/visualization/plot_window.py", line 1314, in init slc.get_data(fields) File "/work1/cindytsai/Software/SysPython27/lib/python2.7/site-packages/yt/data_objects/data_containers.py", line 1621, in get_data fluids, self, self._current_chunk) File "/work1/cindytsai/Software/SysPython27/lib/python2.7/site-packages/yt/geometry/geometry_handler.py", line 304, in _read_fluid_fields chunk_size) File "/work1/cindytsai/Software/SysPython27/lib/python2.7/site-packages/yt/frontends/enzo/io.py", line 279, in _read_fluid_selection data_view = self.grids_in_memory[g.id][fname][self.my_slice].swapaxes(0, 2) P000 yt : [ERROR ] 2020-08-27 20:32:24,411 KeyError: 4 P000 yt : [ERROR ] 2020-08-27 20:32:24,411 Error occured on rank 0.

**Expected outcome**
Successfully produce slice plot image, while Enzo is running.

**Version Information**
  * Operating System: CentOS 7
  * Python Version: Python 2.7
  * yt version: yt-3.6.0
  * Enzo 2.6

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[matthewturk]

Hi @cindytsai , this is indeed a change in behavior.

What used to happen was that we would chunk up the slice object and then replicate that slice object across processors. With the transition to yt-3.0 this changed, and we no longer parallelized the slicing operation.

What might work best for this moving forward may be to have the parallel operation happen inside the generation of the fixed resolution buffer. This would be somewhat straightforward to implement, in that what you could do is have a parallel reduction operation in the coordinate handler following the call to the pixelization operation.