Question

After installing gcc and mpich library in my linux I can compile my codes with mpicxx compiler. Is it possible to use c++11 with mpi library with just upgrading gcc compiler?

Answer 1

Changing the compiler with a newer version should work in general unless some strong code generation changes are observed (e.g. different data alignment or different ABIs). MPI is a library and as such it doesn’t care what language constructs you are using as long as those constructs don’t mess up with its internals. Since you are going to use C++11 for the threading it provides, there are some things that you should be aware of.

First, multithreading doesn’t always play nice with MPI. Most MPI implementations are internally threaded themselves but are not thread safe by default.

Second, MPI defines four levels of threading support:

• MPI_THREAD_SINGLE: no threading support – MPI would function safely only when used by a single-threaded application;
• MPI_THREAD_FUNNELED: partial threading support – MPI can be used in a multithreaded application but only the main thread may call to MPI;
• MPI_THREAD_SERIALIZED: partial threading support – MPI can be used in a multithreaded application but no concurrent calls in different threads are allowed. That is, each thread can call into MPI but a serialisation mechanism has to be in place;
• MPI_THREAD_MULTIPLE: full threading support – MPI can be called freely from many threads.

Truth is most MPI implementations support out of the box MPI_THREAD_FUNNELED at max with most of them supporting only MPI_THERAD_SINGLE. Open MPI for example has to be compiled with a non-default option in order to get the full threading support.

Multithreaded applications should initialise the MPI library using MPI_Init_thread() instead of MPI_Init() and the thread that makes the initialisation call becomes the main thread – the very same main thread that is only allowed to call into MPI when the supported level is MPI_THREAD_FUNNELED. One gives MPI_Thread_init() the desired level of threading support and the function returns the supported level which might be lower than desired. In the latter case correct and portable programs are supposed to act accordingly and either switch to non-threaded operation or abort with the respective error message to the user.

More information about how MPI works together with threads can be found in the MPI Standard v2.2.