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Subject: Re: [OMPI users] MPI_Init() and MPI_Init_thread()
From: Richard Treumann (treumann_at_[hidden])
Date: 2010-03-04 08:36:55


A call to MPI_Init allows the MPI library to return any level of thread
support it chooses. This MPI 1.1 call does not let the application say what
it wants and does not let the implementation reply with what it can
guarantee.

If you are using only one MPI implementation and your code will never be
run on another implementation you can check the user doc for that MPI
implementation and see if says that you can do what you want to do. If
your application is single threaded you can use MPI_Init and be portable.

If your application has threads, you should use MPI_Init_thread and check
the response to be portable or to make sure you get thread safety from an
MPI implementation that has both a thread safe and a thread unsafe mode. A
thread unsafe mode can skip some locking and maybe give better performance.

The MPI standard wanted to allow for the possibility that some
implementation of MPI would not be able to tolerate threads in the
application at all so SINGLE was one answer that could be returned. For
what you want to do, you need a library that can return FUNNELED or better.
If you ask for FUNNELED and the MPI implementation you are using returns
SINGLE it is telling you that using OpenMP threads is not allowed. It does
not say why. It may work fine but the implementation is telling you not to
do it and you cannot know if there are good reasons.

Consider a made up example:

Imagine some system supports Mutex lock/unlock but with terrible
performance. As a work around, it offers a non-standard substitute for
malloc called st_malloc (single thread malloc) that does not do locking.
Normal malloc is also available and it does use locking. The documentation
for this system says that it is safe to use both st_malloc and malloc in a
single threaded application because the only difference is that st_malloc
skips the locking. It also warns that if one thread calls st_malloc while
another calls malloc, heap corruption is likely.

Next imagine that the MPI implementation uses st_malloc to provide best
performance and the OpenMP threads use malloc. When this particular MPI
implementation returns SINGLE, it really means SINGLE. If there is only one
application thread using regular malloc it is safe but if there is a malloc
call on one thread while the main thread is in an MPI call, heap corruption
is likely.

Dick Treumann - MPI Team
IBM Systems & Technology Group
Dept X2ZA / MS P963 -- 2455 South Road -- Poughkeepsie, NY 12601
Tele (845) 433-7846 Fax (845) 433-8363

                                                                                                                        
  From: Yuanyuan ZHANG <zhang.yuanyuan_at_[hidden]>
                                                                                                                        
  To: Open MPI Users <users_at_[hidden]>
                                                                                                                        
  Date: 03/03/2010 07:34 PM
                                                                                                                        
  Subject: Re: [OMPI users] MPI_Init() and MPI_Init_thread()
                                                                                                                        
  Sent by: users-bounces_at_[hidden]
                                                                                                                        

Hi guys,

Thanks for your help, but unfortunately I am still not clear.

> You are right Dave, FUNNELED allows the application to have multiple
> threads but only the man thread calls MPI.
My understanding is that even if I use SINGLE or MPI_Init, I can still
have multiple threads if I use OpenMP PARALLEL directive, and only
the main thread makes MPI calls. Am I correct?

> An OpenMP/MPI hybrid program that makes MPI calls only in between
parallel
> sections is usually a FUNNELED user of MPI
For an OpenMP/MPI hybrid program, if I only want to make MPI calls using
the main thread, ie., only in between parallel sections, can I just use
SINGLE or MPI_Init? What's the benefit of FUNNELED?

Thanks.

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