[Trilinos-Users] Epetra_BlockMap

John Mitchell jamitch at sandia.gov
Wed Feb 10 10:36:08 MST 2010

Thanks Mike and Chris.  Yes -- that does answer my question.

Heroux, Michael A wrote:
> John,
> Since Epetra_Map and Epetra_BlockMap objects are "read-only" once 
> constructed, the copy constructors use a reference counting mechanism 
> to reduce costs.  In other words, each Epetra_Map object points to an 
> Epetra_MapData object using a reference counted pointer.  When a copy 
> of a map is created the new map points to the same MapData object and 
> the reference counter is increased by one.  As a result, the copy of 
> the original map results in a minimal increase in memory.
> The memory cost of the original map varies greatly with the type of 
> constructor used.  The simplest constructor produces just a few 
> integer data values per processor (e.g., min/max GID on the given 
> processor and global min/max).  The more complicated constructors, 
> where lists of global IDs are stored on each processor, result in a 
> storage cost proportional to the number of elements in the map, which 
> is roughly n/p if the map is evenly distributed across the machine 
> (where n is the global length of the map and p  is the number of 
> processors used).
> Does this answer your question?
> Mike
> On 2/10/10 10:43 AM, "John Mitchell" <jamitch at sandia.gov> wrote:
>     What is the cost in memory/cpu time of using a copy
>     constructor/assignment operator with Epetra_Maps?
>     For example, if I have a function "createMap" that computes an
>     Epetra_BlockMap -- is that an ok use case?  My anecdotal experience is
>     that it seems to work fine but I wanted to checkin on that.
>     Epetra_BlockMap overlapMap = createMap(myArgs);
>     Thanks,
>     John
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