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Subject: Re: [OMPI users] MPI and C++ - now Send and Receive of Classes and STL containers
From: Luis Vitorio Cargnini (lvcargnini_at_[hidden])
Date: 2009-07-06 15:24:07


Thanks, but I really do not want to use Boost.
Is easier ? certainly is, but I want to make it using only MPI itself
and not been dependent of a Library, or templates like the majority of
boost a huge set of templates and wrappers for different libraries,
implemented in C, supplying a wrapper for C++.
I admit Boost is a valuable tool, but in my case, as much independent
I could be from additional libs, better.

Le 09-07-06 à 04:49, Number Cruncher a écrit :

> I strongly suggest you take a look at boost::mpi, http://www.boost.org/doc/libs/1_39_0/doc/html/mpi.html
>
> It handles serialization transparently and has some great natural
> extensions to the MPI C interface for C++, e.g.
>
> bool global = all_reduce(comm, local, logical_and<bool>());
>
> This sets "global" to "local_0 && local_1 && ... && local_N-1"
>
>
> Luis Vitorio Cargnini wrote:
>> Thank you very much John, the explanation of &v[0], was the kind of
>> think that I was looking for, thank you very much.
>> This kind of approach solves my problems.
>> Le 09-07-05 à 22:20, John Phillips a écrit :
>>> Luis Vitorio Cargnini wrote:
>>>> Hi,
>>>> So, after some explanation I start to use the bindings of C
>>>> inside my C++ code, then comme my new doubt:
>>>> How to send a object through Send and Recv of MPI ? Because the
>>>> types are CHAR, int, double, long double, you got.
>>>> Someone have any suggestion ?
>>>> Thanks.
>>>> Vitorio.
>>>
>>> Vitorio,
>>>
>>> If you are sending collections of built in data types (ints,
>>> doubles, that sort of thing), then it may be easy, and it isn't
>>> awful. You want the data in a single stretch of continuous memory.
>>> If you are using an STL vector, this is already true. If you are
>>> using some other container, then no guarantees are provided for
>>> whether the memory is continuous.
>>>
>>> Imagine you are using a vector, and you know the number of entries
>>> in that vector. You want to send that vector to processor 2 on the
>>> world communicator with tag 0. Then, the code snippet would be;
>>>
>>> std::vector<double> v;
>>>
>>> ... code that fills v with something ...
>>>
>>> int send_error;
>>>
>>> send_error = MPI_Send(&v[0], v.size(), MPI_DOUBLE, 2,
>>> 0, MPI_COMM_WORLD);
>>>
>>> The &v[0] part provides a pointer to the first member of the array
>>> that holds the data for the vector. If you know how long it will
>>> be, you could use that constant instead of using the v.size()
>>> function. Knowing the length also simplifies the send, since the
>>> remote process also knows the length and doesn't need a separate
>>> send to provide that information.
>>>
>>> It is also possible to provide a pointer to the start of storage
>>> for the character array that makes up a string. Both of these
>>> legacy friendly interfaces are part of the standard, and should be
>>> available on any reasonable implementation of the STL.
>>>
>>> If you are using a container that is not held in continuous
>>> memory, and the data is all of a single built in data type, then
>>> you need to first serialize the data into a block of continuous
>>> memory before sending it. (If the data block is large, then you
>>> may actually have to divide it into pieces and send them
>>> separately.)
>>>
>>> If the data is not a block of all a single built in type, (It may
>>> include several built in types, or it may be a custom data class
>>> with complex internal structure, for example.) then the
>>> serialization problem gets harder. In this case, look at the MPI
>>> provided facilities for dealing with complex data types and
>>> compare to the boost provided facilities. There is an initial
>>> learning curve for the boost facilities, but in the long run it
>>> may provide a substantial development time savings if you need to
>>> transmit and receive several complex types. In most cases, the run
>>> time cost is small for using the boost facilities. (according to
>>> the tests run during library development and documented with the
>>> library)
>>>
>>> John Phillips
>>>
>>> _______________________________________________
>>> users mailing list
>>> users_at_[hidden]
>>> http://www.open-mpi.org/mailman/listinfo.cgi/users
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