[Mini] Docs info for blocking_factor

Docs/source/running_cpp/parallelization.rst

@@ -1,3 +1,5 @@
+.. _parallelization_warpx:
+
 Parallelization in  WarpX
 =========================
 
@@ -17,13 +19,23 @@ the parallelization. The main user-defined parameters are
 AMReX ``max_grid_size`` and ``blocking_factor``
 -----------------------------------------------
 
-* ``amr.max_grid_size`` is the maximum number of points per **grid** along each
+* ``amr.max_grid_size`` is the maximum number of cells per **grid** along each
   direction (default ``amr.max_grid_size=32`` in 3D).
 
-* ``amr.blocking_factor``: The size of each **grid** must be divisible by the
-  `blocking_factor` along all dimensions (default ``amr.blocking_factor=8``).
+* ``amr.blocking_factor``: is the minimum number of cells per **grid** along each
+  direction (default ``amr.blocking_factor=8``).
   Note that the ``max_grid_size`` also has to be divisible by ``blocking_factor``.
 
+  .. note::
+
+     You can use the parameters above if you want the same number of cells in all directions.
+     Or you can set ``amr.max_grid_size_x``, ``amr.max_grid_size_y`` and ``amr.max_grid_size_z``;
+     ``amr.blocking_factor_x``, ``amr.blocking_factor_x`` and ``amr.blocking_factor_x`` to different numbers of cells.
+
+The total number of **grids** is determined using those two restrictions and the number of
+ranks used to run the simulation. You can visit `AMReX <https://amrex-codes.github.io/amrex/docs_html/GridCreation.html?highlight=blocking_factor>`_
+documentation for more information on the two parameters.
+
 These parameters can have a dramatic impact on the code performance. Each
 **grid** in the decomposition is surrounded by guard cells, thus increasing the
 amount of data, computation and communication. Hence having a too small

Docs/source/running_cpp/parameters.rst

@@ -1252,7 +1252,7 @@ In-situ capabilities can be used by turning on Sensei or Ascent (provided they a
 * ``<diag_name>.coarsening_ratio`` (list of `int`) optional (default `1 1 1`)
     Reduce size of the field output by this ratio in each dimension.
     (This is done by averaging the field over 1 or 2 points along each direction, depending on the staggering).
-    ``plot_coarsening_ratio`` should be an integer divisor of ``blocking_factor``.
+    ``plot_coarsening_ratio`` should be an integer divisor of ``blocking_factor``, deined in the :ref:`parallelization <parallelization_warpx>` section.
 
 * ``<diag_name>.file_prefix`` (`string`) optional (default `diags/plotfiles/plt`)
     Root for output file names. Supports sub-directories.