/// @file "MPI/examples/exMPI04/.README.txt"
/// @brief Example exMPI04 README

/*! \page exMPI04 Example : exMPI04


Description
===========
An example of dosimetry in a water phantom.
The example is as exMPI03, but adds output in G4analysis ntuple
and demonstrates ntuple merging.

In difference from merging other data (G4Run, scorers, hitograms),
the ntuple data are not sent to the collecting rank(s) at the
end of run but during event processing. That's why (an) extra rank(s)
have to be reserved for this purpose. The number of extra workers requested 
is set in G4MPImanager constructor in main(). While G4MPImanager 
can be created with any number of extra workers (< total number),
the ntuple merging is at present supported only for one.

The extra worker is connected to the run action class via G4MPIextraWorker
object which is set to G4MPImanager in main().
The standard calls to G4AnalysisManager (creating ntuple, open, write and 
close file) trigger creating all necessary analysis tools object
for merging ntuple data on flight.

MPI ntuple merging can be activated in sequential mode only;
this activation is perfomed by creating the G4MPIntupleMerger
object in the RunActionMaster constructor. 
The merger must be created before creating G4AnalysisManager
(= the first call to G4AnalysisManager::Instance())
and deleted only at the end of program.

If multithreading mode is enabled, the ntuples are merged from 
threads to files per ranks.
Combined MT + MPI merging is not yet supported.
Merging ntuples is actually supported only with Root output format.

How to build
============
Use CMake on Geant4 library installed with CMake build.

This example requires G4mpi library to be installed
(see examples/extended/parallel/MPI/source/REDME.md)

Follow these commands,

    > mkdir build
    > cd build
    > cmake -DG4mpi_DIR=<where-G4mpi-wasintalled>/lib[64]/G4mpi -DCMAKE_CXX_COMPILER=mpicxx \
      -DGeant4_DIR=<your Geant4 install path>/lib[64]/Geant4-V.m.n <path-to-source>
      (V.m.n is the version of Geant4, eg. Geant4-9.6.0)
    > make
    > make install

Replace mpicxx with your MPI-compiler wrapper.

*/
