[mpich-discuss] Question on MPI_Pack/Unpack

[Matthieu Dorier]

Ok thanks for all your answers.
I'm going to keep the send/recv code, just to be consistent with the standard.

Matthieu

----- Mail original -----
> De: "Pavan Balaji" <balaji at mcs.anl.gov>
> À: discuss at mpich.org
> Envoyé: Jeudi 18 Juillet 2013 10:52:56
> Objet: Re: [mpich-discuss] Question on MPI_Pack/Unpack
> 
> 
> 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
> _______________________________________________
> discuss mailing list     discuss at mpich.org
> To manage subscription options or unsubscribe:
> https://lists.mpich.org/mailman/listinfo/discuss
>