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

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 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

...

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 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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