Is there a limit for the message size in mpi using boost::mpi?

Question

I'm currently writing a simulation using boost::mpi on top of openMPI and everything works great. However once I scale up the system and therefore have to send larger std::vectors I get errors.

I've reduced the issue to the following problem:

#include <boost/mpi.hpp>
#include <boost/mpi/environment.hpp>
#include <boost/mpi/communicator.hpp>
#include <boost/serialization/vector.hpp>
#include <iostream>
#include <vector>
namespace mpi = boost::mpi;

int main() {
    mpi::environment env;
    mpi::communicator world;

    std::vector<char> a;
    std::vector<char> b;
    if (world.rank() == 0) {
        for (size_t i = 1; i < 1E10; i *= 2) {
            a.resize(i);
            std::cout << "a " << a.size();
            world.isend(0, 0, a);
            world.recv(0, 0, b);
            std::cout << "\tB " << b.size() << std::endl;
        }
    }
    return 0;
}

prints out:

a 1 B 1
a 2 B 2
a 4 B 4
....
a 16384 B 16384
a 32768 B 32768
a 65536 B 65536
a 131072    B 0
a 262144    B 0
a 524288    B 0
a 1048576   B 0
a 2097152   B 0

I'm aware that there is a limit to a mpi message size, but 65kB seems a little low to me. Is there a way of sending larger messages?

Answer 1

The limit of the message size is the same as for MPI_Send: INT_MAX.

The issue is that you are not waiting for the isend to finish before resizing the vector a in the next iteration. This means that isend will read invalid data due to the reallocations in the vector a. Note that buffer a is passed by reference to boost::mpi and you are thus not allowed to change the buffer a until the isend operation has finished.

If you run your program with valgrind, you will see invalid reads as soon as i = 131072.

The reason your program works till 65536 bytes, is that OpenMPI will send messages directly if they are smaller than the components btl_eager_limit. For the self component (sending to the own process), this happens to be 128*1024 bytes. Since boost::serialization adds the size of the std::vector to the byte stream, you exceed this eager_limit as soon as you use 128*1024 = 131072 as your input size.

To fix your code, save the boost::mpi::request return value from isend() and then add wait() to the end of the loop:

#include <boost/mpi.hpp>
#include <boost/mpi/environment.hpp>
#include <boost/mpi/communicator.hpp>
#include <boost/serialization/vector.hpp>
#include <iostream>
#include <vector>
namespace mpi = boost::mpi;

int main() {
    mpi::environment env;
    mpi::communicator world;

    std::vector<char> a;
    std::vector<char> b;
    if (world.rank() == 0) {
        for (size_t i = 1; i < 1E9; i *= 2) {
            a.resize(i);
            std::cout << "a " << a.size();
            mpi::request req = world.isend(0, 0, a);
            world.recv(0, 0, b);
            std::cout << "\tB " << b.size() << std::endl;
            req.wait();
        }
    }
    return 0;
}