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Backend Comparison

Summary Table

Aspect mpi-sys-backend mpi-rt-sys-backend
Feature flag mpi-sys-backend (default) mpi-rt-sys-backend
Crate mpi-sys mpi-rt-sys
Binding generation rust-bindgen at build time gen_rust.py (pre-generated)
Build dependencies C compiler, libclang, system MPI headers None (pure Rust)
Runtime dependencies System MPI library (linked at compile time) MPIwrapper shared library
Symbol resolution Link-time (static or dynamic linking) Runtime (libloading / dlopen)
MPI handle types Implementation-defined (pointers, ints, structs) Always usize (MPIABI standard)
MPI constants extern globals from C header Loaded from shared library symbols
Configuration MPI_PKG_CONFIG, MPICC, or platform-specific env vars MPI_RT_LIB env var at runtime
Cross-compilation Requires target MPI headers Binary is MPI-agnostic
MPI implementation switch Requires rebuild Change MPI_RT_LIB at runtime
Platform support Linux, macOS, Windows Linux, macOS (where dlopen works)
Maturity Production (upstream rsmpi) Experimental

When to Use Each Backend

Use mpi-sys-backend when:

  • You have a stable MPI installation on your build system
  • You need maximum compatibility with existing rsmpi code
  • You're on Windows with MS-MPI or Intel MPI
  • You want the most battle-tested path

Use mpi-rt-sys-backend when:

  • You want to build without MPI installed (e.g., CI/CD, containers)
  • You need to switch MPI implementations without recompiling
  • You're distributing pre-built binaries that should work with any MPI
  • You're cross-compiling and don't have target MPI headers
  • You want to avoid the libclang build dependency

API Compatibility

Both backends expose the same public API to the mpi crate. The ffi module in src/lib.rs conditionally re-exports from the active backend. User code that depends on the mpi crate does not need to change when switching backends.

The key abstraction that enables this is the RSMPI_*_fn() accessor pattern for constants. Both backends provide these functions, but the implementation differs:

  • mpi-sys: wraps extern globals in trivial accessor functions
  • mpi-rt-sys: loads values from the shared library on first call