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Implementation of MPI that supports large counts
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README.md

BigMPI

Interface to MPI for large messages, i.e. those where the count argument exceeds INT_MAX but is still less than SIZE_MAX. BigMPI is designed for the common case where one has a 64b address space and is unable to do MPI communication on more than 2^31 elements despite having sufficient memory to allocate such buffers. BigMPI does not attempt to support large-counts on systems where C int and void* are both 32b.

Motivation

The MPI standard provides a wide range of communication functions that take a C int argument for the element count, thereby limiting this value to INT_MAX or less. This means that one cannot send, e.g. 3 billion bytes using the MPI_BYTE datatype, or a vector of 5 billion integers using the MPI_INT type, as two examples. There is a natural workaround using MPI derived datatypes, but this is a burden on users who today may not be using derived datatypes.

This project aspires to make it as easy as possible to support arbitrarily large counts (2^63 elements exceeds the local storage compacity of computers for the foreseeable future).

This is an example of the code change required to support large counts using BigMPI:

#ifdef BIGMPI
    MPIX_Bcast_x(stuff, large_count /* MPI_Count */, MPI_BYTE, 0, MPI_COMM_WORLD);
#else // cannot use count>INT_MAX
    MPI_Bcast(stuff, not_large_count /* int */, MPI_BYTE, 0, MPI_COMM_WORLD);
#endif

Interface

The API follows the pattern of MPI_Type_size(_x) in that all BigMPI functions are identical to their corresponding MPI ones except that they end with _x to indicate that the count arguments have the type MPI_Count instead of int. BigMPI functions use the MPIX namespace because they are not in the MPI standard.

Limitations

Even though MPI_Count might be 128b, BigMPI only supports 64b counts (because of MPI_Aint limitations and a desire to use size_t in unit tests), so BigMPI is not going to solve your problem if you want to communicate more than 8 EiB of data in a single message. Such computers do not exist nor is it likely that they will exist in the foreseeable future.

BigMPI only supports built-in datatypes. If you are already using derived-datatypes, then you should already be able to handle large counts without BigMPI.

Support for MPI_IN_PLACE is not implemented in some cases and implemented inefficiently in others. Using MPI_IN_PLACE is discouraged at the present time. We hope to support it more effectively in the future.

BigMPI requires C99. If your compiler does not support C99, get a new compiler.

BigMPI only has C bindings right now. Fortran 2003 bindings are planned. If C++ bindings are important to you, please create an issue for this.

Supported Functions

I believe that point-to-point, one-sided, broadcast and reductions are the only functions worth supporting but I added some of the other collectives anyways. The v-collectives require a point-to-point implementation, but we do not believe this causes a significant loss of performance.

Technical details

MPIX_Type_contiguous_x does the heavy lifting. It's pretty obvious how it works. The datatypes engine will turn this into a contiguous datatype internally and thus the underlying communication will be efficient.
MPI implementations need to be count-safe for this to work, but they need to be count-safe period if the Forum is serious about datatypes being the solution rather than MPI_Count everywhere.

All of the communication functions follow the same pattern, which is clearly seen in MPIX_Send_x. I've optimized for the common case when count is smaller than 2^31 with a likely_if macro to minimize the performance hit of BigMPI for this more common use case (hopefully so that users don't insert a branch for this themselves)

The most obvious optimization I can see doing is to implement MPIX_Type_contiguous_x using internals of the MPI implementation instead of calling six MPI datatype functions. I have started implemented this in MPICH already: https://github.com/jeffhammond/mpich/tree/type_contiguous_x.

Authors

  • Jeff Hammond
  • Andreas Schäfer

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