From: Michael Starkie <Starfire@LL.MIT.EDU>
Newsgroups: comp.parallel.mpi
Subject: Re: Confused about Request::Wait(status) and tag = -1 !!
Date: Mon, 28 Dec 1998 12:05:50 -0500
Organization: MIT Lincoln Laboratory
Message-Id: <3687BA6D.27E1BE89@LL.MIT.EDU>
References: <3682A45B.442E2F76@LL.MIT.EDU>
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Michael Starkie wrote:

    The problem is with my usage of MPI::Request and the receive buffer.
Polling on RTImpiManager::receive( char* message ) is confusing MPI
because for every entry into receive a new req handle is being allocated
inside MPI.  Furthermore this handle is being associated with a dynamic
buffer whose life only exists inside the receive method.  The solution
is to call req = comm.Irecv( ... ) only once with a persistant req
variable and buffer.  Then poll on the req variable with req.Test(
status ).  This solved the problem.  Here is how I broke it up.

main ( ... ) {
...
  char* msg = new char[256];
  mpiMsgEnvelope env;
  env.type = MPI::CHAR;
  env.source = MPI::ANY_SOURCE;
  env.tag = MPI::ANY_TAG;
  int tag;
  MPI::Request req = RTIstreamManagerMpi::mpiMgr.reqIReceive( msg, 256,
env );
  while ( _done == RTI::RTI_FALSE )
  {
    if( RTIstreamManagerMpi::mpiMgr.iReceive( req, tag ) )
      if( tag == RTI_MPI_EXIT )
        _done == RTI::RTI_TRUE
  }

  MPI::Finalize()
} /* main */

MPI::Request
RTImpiManager::reqIReceive( char* buf, int buf_size, mpiMsgEnvelope env
)
{

  MPI::Request req;

  req = comm.Irecv( buf, buf_size, env.type, env.source, env.tag );

  return req;
}

int
RTImpiManager::iReceive( MPI::Request _req, int& _tag )
{
  MPI::Status status;
  if( _req.Test( status ) )
  {
    _tag = status.Get_tag();
    cout << "rank " << getRank() << " received tag " << _tag << endl;
    return 1;
  }
  else
    return 0;
}


>  If I comment out req.Wait() then req.Test(status) always fails!!!
> Why?
> Do I really have to wait here? What is the purpose of a non blocking
> receive then?
> I would like to poll on RTImpiManager::receive without having to wait.
>
> req is a public member of RTImpiManager and is defined in the
> constructor.
>
> Also when I do get a message, status.Get_tag() is always = -1. Why?
> The send method is defined at the bottom of this text in
> terminateProcesses.
>
> int
> RTImpiManager::receive( char* message )
> {
>   char* buf = new char( STD_BUFFER_SIZE );
>
>   req = comm.Irecv( buf, STD_BUFFER_SIZE, MPI::CHAR,
>     MPI::ANY_SOURCE, MPI::ANY_TAG );
>
>   req.Wait();
>   if( req.Test( status ) )
>   {
>     cout << "rank " << getRank() << " received tag " <<
> status.Get_tag() << endl;
>     if( status.Get_tag() == RTI_MPI_EXIT )
>     {
>       cout << "rank " << getRank() << " received terminate." << endl;
>       terminate();
>       return 1;
>     }
>     else
>     {
>       int len = strlen( buf ) + 1;
>       message = new char( len );
>       strcpy( message, buf );
>       delete buf;
>       return 1;
>     }
>   }
>   else
>     return 0;
> }
>
> ----------
> void
> RTImpiManager::terminateProcesses()
> {
>   char* none = 0;
>   for( int i = 1; i < getSize(); i++ )
>   {
>     cout << "rank " << getRank() << " is sending to " << i << endl;
>     comm.Send( none, 0, MPI::CHAR, i, RTI_MPI_EXIT );
>   }
>   MPI::Finalize();
>   exit(0);
> }
>
> --
> Cordially,
> Michael Starkie
>
>



--
Cordially,
Michael Starkie