Reference documentation for deal.II version Git 6454e3f 2017-06-26 22:46:01 +0200
Public Member Functions | List of all members
Utilities::MPI::MPI_InitFinalize Class Reference

#include <deal.II/base/mpi.h>

Public Member Functions

 MPI_InitFinalize (int &argc, char **&argv, const unsigned int max_num_threads=numbers::invalid_unsigned_int)
 
 ~MPI_InitFinalize ()
 

Detailed Description

A class that is used to initialize the MPI system at the beginning of a program and to shut it down again at the end. It also allows you to control the number threads used in each MPI task.

If deal.II is configured with PETSc, the library will be initialized in the beginning and destroyed at the end automatically (internally by calling PetscInitialize() and PetscFinalize()).

If deal.II is configured with p4est, that library will also be initialized in the beginning, and destroyed at the end automatically (internally by calling sc_init(), p4est_init(), and sc_finalize()).

If a program uses MPI one would typically just create an object of this type at the beginning of main(). The constructor of this class then runs MPI_Init() with the given arguments. At the end of the program, the compiler will invoke the destructor of this object which in turns calls MPI_Finalize to shut down the MPI system.

This class is used in step-32, for example.

Definition at line 375 of file mpi.h.

Constructor & Destructor Documentation

Utilities::MPI::MPI_InitFinalize::MPI_InitFinalize ( int &  argc,
char **&  argv,
const unsigned int  max_num_threads = numbers::invalid_unsigned_int 
)

Initialize MPI (and, if deal.II was configured to use it, PETSc) and set the number of threads used by deal.II (via the underlying Threading Building Blocks library) to the given parameter.

Parameters
[in,out]argcA reference to the 'argc' argument passed to main. This argument is used to initialize MPI (and, possibly, PETSc) as they read arguments from the command line.
[in,out]argvA reference to the 'argv' argument passed to main.
[in]max_num_threadsThe maximal number of threads this MPI process should utilize. If this argument is set to numbers::invalid_unsigned_int (the default value), then the number of threads is determined automatically in the following way: the number of threads to run on this MPI process is set in such a way that all of the cores in your node are spoken for. In other words, if you have started one MPI process per node, setting this argument is equivalent to setting it to the number of cores present in the node this MPI process runs on. If you have started as many MPI processes per node as there are cores on each node, then this is equivalent to passing 1 as the argument. On the other hand, if, for example, you start 4 MPI processes on each 16-core node, then this option will start 4 worker threads for each node. If you start 3 processes on an 8 core node, then they will start 3, 3 and 2 threads, respectively.
Note
This function calls MultithreadInfo::set_thread_limit() with either max_num_threads or, following the discussion above, a number of threads equal to the number of cores allocated to this MPI process. However, MultithreadInfo::set_thread_limit() in turn also evaluates the environment variable DEAL_II_NUM_THREADS. Finally, the worker threads can only be created on cores to which the current MPI process has access to; some MPI implementations limit the number of cores each process may access to one or a subset of cores in order to ensure better cache behavior. Consequently, the number of threads that will really be created will be the minimum of the argument passed here, the environment variable (if set), and the number of cores accessible to the thread.
MultithreadInfo::set_thread_limit() can only work if it is called before any threads are created. The safest place for a call to it is therefore at the beginning of main(). Consequently, this extends to the current class: the best place to create an object of this type is also at or close to the top of main().

Definition at line 295 of file mpi.cc.

Utilities::MPI::MPI_InitFinalize::~MPI_InitFinalize ( )

Destructor. Calls MPI_Finalize() in case this class owns the MPI process.

Definition at line 431 of file mpi.cc.


The documentation for this class was generated from the following files: