- update references to use OpenFOAM-v2206 (openfoam2206) as latest

coding/patterns/parallel.md

-<!-- --- title: OpenFOAM C++ Coding Patterns (HashTable) -->
+<!-- --- title: OpenFOAM C++ Coding Patterns (MPI) -->
 
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 [![code](/icons/code.svg "coding patterns")][code-patterns]
@@ -13,9 +13,58 @@ This is a very broad ranging topic, but here are some general ideas...
 
 ### General Considerations
 
-- When using point-to-point communication, optimize for contiguous data.
+- When using point-to-point communication, optimize for contiguous
+  data and use the `UIPstream::read` and `UOPstream::write` methods.
+  ```
+  UIPstream::read
+  (
+      Pstream::commsTypes::scheduled,
+      fromProci,
+      fld1.data_bytes(),
+      fld1.size_bytes(),
+      tag,
+      comm
+  );
+  ```
+  and
+  ```
+  UOPstream::write
+  (
+      Pstream::commsTypes::scheduled,
+      toProci,
+      fld2.cdata_bytes(),
+      fld2.size_bytes(),
+      tag,
+      comm
+  );
+  ```
+  This will use `MPI_Recv` and `MPI_Send` directly, without the
+  expense of a `MPI_Probe` call.
 
 
+### One-sided Gather
+
+- In cases where _flattened_ information is required on the **master**
+  process only, using `globalIndex` will frequently provide the most
+  efficient method. Quite frequently, the overall size/offset
+  information is only needed on the master and not on the
+  sub-processes:
+  ```
+  globalIndex globIdx(send.size(), globalIndex::gatherOnly{});
+  ```
+  Using the `globalIndex::gatherOnly{}` tag will trigger an MPI gather
+  without the expense of needless broadcast to all nodes.
+
+  In many cases, it can be convenient to use the static `gather()`
+  method:
+  ```
+  const scalarField mergedWeights(globalIndex::gather(weights));
+  ```
+  but can also reuse existing size/offset information:
+  ```
+  const scalarField mergedWeights(globalFaces().gather(weights));
+  ```
+
 ----
 
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