[mpich-discuss] Question on MPI_Pack/Unpack

[Pavan Balaji]

MPI is allowed to do any form of encoding inside MPI_Pack.  You can't 
rely on what the data format would be.

We don't do that in MPICH (or any other MPI implementation, AFAICT), but 
the standard allows it.

  -- Pavan

On 07/18/2013 10:29 AM, Dries Kimpe wrote:
> There's another function for MPI_Pack.
>
> It deals with encoding for non-homogeneous systems.
>
> For example, it might convert between different representations of double
> floating point numbers.
>
> Normally, this happens automatically (assuming a proper datatype was
> given) in MPI_Send/MPI_Recv.
>
> MPI_Pack does this while it encodes into a contiguous buffer,
> which is why you're supposed to use the MPI_PACKED datatype when sending
> packed data, so the implementation knows not to convert the data.
>
>     Dries
>
> * Antonio J. Peña <apenya at mcs.anl.gov> [2013-07-18 10:16:04]:
>
>
>> 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]
>
>
>> --------
>> [1] http://people.irisa.fr/Matthieu.Dorier
>
>> _______________________________________________
>> discuss mailing list     discuss at mpich.org
>> To manage subscription options or unsubscribe:
>> https://lists.mpich.org/mailman/listinfo/discuss
>
>
>
> _______________________________________________
> discuss mailing list     discuss at mpich.org
> To manage subscription options or unsubscribe:
> https://lists.mpich.org/mailman/listinfo/discuss
>

-- 
Pavan Balaji
http://www.mcs.anl.gov/~balaji