Re: [Boost-users] [mpi] broadcast performance

Subject: Re: [Boost-users] [mpi] broadcast performance
From: Brian Budge (brian.budge_at_[hidden])
Date: 2012-09-06 10:07:05

Hi Julio -

I may be completely wrong, but I was under the understanding that when
a send call happens, serialization magic occurs that builds an
MPI_Datatype, and that by then handing the data into MPI_Send etc...
we avoid an extra copy?

But perhaps that won't work in my case.  I doubt that MPI_Recv is
capable of building a complex hierarchy back up including pointers,
using operator new etc...  Perhaps you have to have a fully
instantiated object of the same kind in order to use this
functionality with MPI_Recv?

I have a virtual message hierarchy, and the messages (or shared_ptrs
of messages) perform virtual dispatch upon being recv'd.  Is there
anything performance-wise to be gained by using boost::mpi for
send/recv/broadcast?  Or is the MPI_Datatype performance gain only
applicable to classes that have (perhaps complex, but) concrete layout
with object instantiation on the stack?

It seems that if I can't get the MPI_Datatype benefit for my types, I
may be better off maintaining my own buffers for serialization, so I
can potentially lower the number of memory allocations.

Thanks,
  Brian

On Thu, Sep 6, 2012 at 3:29 AM, Júlio Hoffimann
<julio.hoffimann_at_[hidden]> wrote:
> Brian,
>
> You can think of Boost.MPI as a very well-designed wrapper. All it does is
> to call the underlying C (OpenMPI, MPICH, others) implementation when the
> types are covered by the MPI standard.
>
> On other hand, i agree with you, maybe would be possible to specialize a
> template for std::vector<T> that handles it as a raw buffer. Someone has an
> opinion about this?
>
> When i have time, i'll think carefully to see if i can contribute a patch.
>
> Regards,
> Júlio.
>
> 2012/9/6 Brian Budge <brian.budge_at_[hidden]>
>>
>> Okay.  I can do that.  I was just wondering if there was a trick to
>> make it happen under the hood.  I'm curious as to why Bcast doesn't
>> get called by boost::mpi::broadcast for non-trivial types.
>>
>> Thanks,
>>   Brian
>>
>> On Wed, Sep 5, 2012 at 7:00 PM, Júlio Hoffimann
>> <julio.hoffimann_at_[hidden]> wrote:
>> > Hi Brian,
>> >
>> > If i understood correctly, you're actually doing something like:
>> >
>> > std::vector<char> gigaVec;
>> > MPI_Bcast(blah, blah, ..., &gigaVec[0])
>> >
>> >
>> > and want to replace that by boost::mpi::broadcast, is that correct?
>> >
>> > Just do it the same way, if the type of the container is a MPI type,
>> > you're
>> > guaranteed that the underlying MPI implementation will be called.
>> >
>> > Regards,
>> > Júlio.
>> >
>> > _______________________________________________
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