Datatypes episode

[robertodr]

To consider

• Give less space to the communicators and groups episodes in favor of more in-depth treatment of datatypes.

To change

• Clarify the meaning of displacements in the typemap
• In the my_pair type-along, the addresses are relative to whatever MPI thinks is the first byte in memory. Make it so that they are relative to the first field in the data structure
• Clarify why/when to use pack/unpack and why/when to define datatypes.
  • The former can incur overhead, e.g. calls to functions in a dynamically linked library (-lmpi) are slower than calls to functions in your executable.
  • The latter is essential if we want to perform reductions with a custom operation.

To add

• Defining custom operations on custom datatypes. This could fit in the collective episode.
• Short discussion of whether it is possible to automatically generate the MPI representation of custom datatypes.
  • Automatic serialization/deserialization libraries (like Cereal for C++) but using these is not exactly the same as defining datatypes, e.g. you can't do custom reductions.
  • Introspection. Possible in C++ with metaprogramming, but do you really want to go there? 😄
  • Code generation. Build is in two passes then: first generate the code (the generation of the code needs some tool to analyze the AST) then compile it.