[Trilinos-Users] vectors on overlapped maps

Heroux, Michael A maherou at sandia.gov
Sat Jun 30 10:43:39 MDT 2012


I forgot to mention that you can make a non-overlapping map using a utility from Epetra.  By default it will assign shared GIDs to the processor with the lowest ID number.  Here is the function:

http://trilinos.sandia.gov/packages/docs/dev/packages/epetra/doc/html/classEpetra__Util.html#a603340e974a58a652eea0c4815e4acf7

Mike

From: erhan turan <turane at gmail.com<mailto:turane at gmail.com>>
Date: Saturday, June 30, 2012 11:23 AM
To: Michael Heroux <maherou at sandia.gov<mailto:maherou at sandia.gov>>
Cc: "trilinos-users at software.sandia.gov<mailto:trilinos-users at software.sandia.gov>" <trilinos-users at software.sandia.gov<mailto:trilinos-users at software.sandia.gov>>
Subject: Re: [Trilinos-Users] [EXTERNAL] vectors on overlapped maps

Hi Mike,

Thanks for your advice. You're right, I forgot to check the return value of Norm2().  Let me think something that can be performed for non-trivial distribution. Maybe I can figure something out.

Erhan

On Fri, Jun 29, 2012 at 10:35 PM, Heroux, Michael A <maherou at sandia.gov<mailto:maherou at sandia.gov>> wrote:
Erhan,

The reason you are getting a result of zero is that the Norm2 method is checking if the global IDs in the vector map are unique.  This is the first line of executable code:


  if (!Map().UniqueGIDs()) {EPETRA_CHK_ERR(-1);}

Because this check fails, the method returns with a error code of -1.  If you were to check the return value of the method call, you should see -1 returned.

The bigger question of trying to compute norms of vectors with overlapped elements is supported in some situations.  In particular, if you create an overlap map such that shared global IDs are at the end of the global ID list on each processor, you can create an additional map with non-overlap IDs and two Epetra vectors where the non-overlap is a sub vector of the overlapped vector.

The following code (adapted from your code) is approximately correct.

I hope this helps.

Mike


  int NumGlobalElements = 4; // global dimension of the problem


  int NumMyElements; // local nodes

  Epetra_IntSerialDenseVector MyGlobalElements;


  if( Comm.MyPID() == 0 ) {

    NumMyElements = 3;

    MyGlobalElements.Size(NumMyElements);

    MyGlobalElements[0] = 0;

    MyGlobalElements[1] = 1;

    MyGlobalElements[2] = 2;

  } else {

    NumMyElements = 3;

    MyGlobalElements.Size(NumMyElements);

    MyGlobalElements[0] = 2;

    MyGlobalElements[1] = 3;

    MyGlobalElements[2] = 1;

  }


  // create a overlap map

  Epetra_Map Map_overlap(-1,MyGlobalElements.Length(),

MyGlobalElements.Values(),0, Comm);


  // create a non-overlap map

  Epetra_Map Map_unique(-1,MyGlobalElements.Length()-1,

 MyGlobalElements.Values(),0, Comm);


  // Create importer to handle data replication

  Epetra_Import x_unique_to_overlap(Map_overlap, Map_unique);


  // create a vector based on overlap map

  Epetra_Vector x_overlap(Map_overlap);


  // create a vector based on non-overlap map

  // Passing in the pointer to the first element of x_overlap

  // means that x_unique and x_overlap share space

  Epetra_Vector x_unique(View, Map_unique,&x_overlap[0]);



  // Now set values in x_unique (trivial in this example, but generally not)

  x_unique.PutScalar(1.0);


  // Now copy replicated values to overlap vector

  x_overlap.Import(x_unique, x_unique_to_overlap, Insert);


  double myNorm;


  assert(x_overlap.Norm2(&myNorm)==-1);


  x_unique.Norm2(&myNorm);



cout<<"norm is ="<<myNorm<< " over " << x_unique.GlobalLength()<< " elements"<<endl;


From: erhan turan <turane at gmail.com<mailto:turane at gmail.com>>
Date: Friday, June 29, 2012 10:38 AM
To: "trilinos-users at software.sandia.gov<mailto:trilinos-users at software.sandia.gov>" <trilinos-users at software.sandia.gov<mailto:trilinos-users at software.sandia.gov>>
Subject: [EXTERNAL] [Trilinos-Users] vectors on overlapped maps

Hello everyone,

I have a question regarding to vector overlapped maps. Attached code is a modification of example 9 in didasko/epetra folder. A vector is created with an overlapping map.

What to expect as a norm value on two cores is 2. what you get is 0 ! Of course, this was an example on Import and Export methods - there you are also playing win Epetra_CombineMode.

My question is, why is "0" the result?  And also, should we always use Export and Import methods to deal overlapped maps? So, it is not safe to use such a naive approach even if the map is created carefully.

Regards,

Erhan Turan


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