[mpich-discuss] Question on MPI_Pack/Unpack

[Antonio J. Peña]

Hi Matthieu,

These functions only copy all the data of the datatype into a contiguous 
memory region. They don't do any further action. I'd say these are a more 
natural and efficient solution for your problem than the self send-receive 
approach. Note that the send is internally performing the pack before 
sending, so you'd effectively do the same but avoiding the send/receive 
step.

  Antonio


On Thursday, July 18, 2013 04:35:28 PM Matthieu Dorier wrote:


Hi,


I'd like some clarification on MPI_Pack/Unpack: the standard only says that 
"some communication libraries provide pack/unpack functions for sending 
non-contiguous data", but it's not clear to me what MPI_Pack/Unpack are 
supposed to do. For example, if I have a MPI_Datatype representing a 
structure with holes between fields, is MPI_Pack just supposed to put the 
elements of the structure close to each other so that there is no more 
holes, or can it do something different (compressing, adding metadata, 
etc.)?


The reason I'm asking this is because I'm designing a code to extract 
chunks of multi-dimentional arrays (basically a big array has ghost zones 
and I want to extract the non-ghost data from this big array into a smaller 
array, contiguous in memory). This extraction is local and does not involve 
communication so right now after using MPI_Type_create_subarray to 
create a datatype adapted to my need, I post an MPI_Irecv with a 
contiguous array of MPI_CHAR as type, and then an MPI_Send with one 
element of the type I just created. I was wondering if MPI_Pack would be 
equivalent (and maybe more efficient)?


Thanks



Matthieu Dorier

PhD student at ENS Cachan Brittany and IRISA

http://people.irisa.fr/Matthieu.Dorier[1]


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[1] http://people.irisa.fr/Matthieu.Dorier
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