Re: [Boost-mpi] mixing boost serialization with C MPI functions

Subject: Re: [Boost-mpi] mixing boost serialization with C MPI functions
From: Jiaxin Han (hanjiaxin_at_[hidden])
Date: 2015-11-19 11:56:34

Hi Lorenz,

Thank you very much! Your code works! But it seems I have to manually
allocate the memory before unpacking the data from the archive? Then if my
data is a vector of vector, does it mean I have to resize every vector as
well?

Back to the MPI error, sorry I was wrong to say that it is related to
boost.MPI. With the new code passing MPI_PACKED data, I am receiving the
same out of memory error. So it seems my MPI installation has a problem in
the message size for packed data. Thanks to both Lorenz and Alain!

Jiaxin

2015-11-19 15:56 GMT+00:00 Lorenz Hübschle-Schneider <huebschle_at_[hidden]>:

> Hi,
>
> On 19/11/15 16:07, Jiaxin Han wrote:
>
>> Hi,
>>
>> Could anyone point me to an example of mixing boost serialization with C
>> MPI functions?
>>
>
> you need to send the archive size first if it's not precisely known. Try
> something like this:
>
> mpi::packed_oarchive oa(comm);
> oa << sendbuf;
> auto sendptr = const_cast<void*>(oa.address());
> // cast to int because MPI uses ints for sizes like it's still 1990
> int sendsize = static_cast<int>(oa.size());
> MPI_Send(&sendsize, 1, MPI_INT, 1, 0, comm);
> MPI_Send(sendptr, sendsize, MPI_PACKED, 1, 0, comm);
>
> The receiving side would look similar:
>
> mpi::packed_iarchive ia(comm);
> int recvsize;
> MPI_Recv(&recvsize, 1, MPI_INT, 0, 0, comm, MPI_STATUS_IGNORE);
> ia.resize(recvsize);
> auto recvptr = ia.address();
> MPI_Recv(recvptr, recvsize, MPI_PACKED, 0, 0, comm, MPI_STATUS_IGNORE);
>
>
> Do I have to make sure buffer is big enough or the oarchive will handle
>> the memory automatically? If the former, what is the correct memory size
>> to hold a vector? I guess it should hold not just vec.data(), but also
>> vec.size().
>>
>
> It does this automatically for the sending size. On the receiver, you have
> to know how much data to expect and reserve enough memory accordingly. This
> is what the first transmission in my code above is for.
>
> And lastly, oa does not appear to be the correct variable to pass to
>> MPI_Send. Then what should I pass to MPI_Send after creating the archive?
>>
>
> You pass in the pointer and its length -- oa.address() and oa.size() in
> the example above.
>
> I am asking because the boost mpi installation on our server appears to
>> have a limitation on the message size. For example, the attached code
>> reports an error as:
>>
>> terminate called after throwing an instance of
>>
>> 'boost::exception_detail::clone_impl<boost::exception_detail::error_info_injector<boost::mpi::exception>
>>  >'
>>    what():  MPI_Alloc_mem: MPI_Alloc_mem: Out of "special" (shared) memory
>> MPI Application rank 0 killed before MPI_Finalize() with signal 6
>>
>> This appears to be a problem only with the boost installation on the
>> server. The code runs correctly on my local machine.
>>
>
> I'm no expert but to me this sounds like an issue with the MPI
> installation on your servers. Note that it's not Boost.MPI that's giving
> you an error - the error is from the C function that it calls.
>
> Cheers,
> Lorenz
>