4.2.8 Communication primitives

Figure 4.7: Three performance critical parallel primitives are provided by the WRAPPER. These primitives are always used to communicate data between tiles. The figure shows four tiles. The curved arrows indicate exchange primitives which transfer data between the overlap regions at tile edges and interior regions for nearest-neighbor tiles. The straight arrows symbolize global sum operations which connect all tiles. The global sum operation provides both a key arithmetic primitive and can serve as a synchronization primitive. A third barrier primitive is also provided, it behaves much like the global sum primitive.

Optimized communication support is assumed to be possibly available for a small number of communication operations. It is assumed that communication performance optimizations can be achieved by optimizing a small number of communication primitives. Three optimizable primitives are provided by the WRAPPER

• EXCHANGE This operation is used to transfer data between interior and overlap regions of neighboring tiles. A number of different forms of this operation are supported. These different forms handle
  • Data type differences. Sixty-four bit and thirty-two bit fields may be handled separately.
  • Bindings to different communication methods. Exchange primitives select between using shared memory or distributed memory communication.
  • Transformation operations required when transporting data between different grid regions. Transferring data between faces of a cube-sphere grid, for example, involves a rotation of vector components.
  • Forward and reverse mode computations. Derivative calculations require tangent linear and adjoint forms of the exchange primitives.

• GLOBAL SUM The global sum operation is a central arithmetic operation for the pressure inversion phase of the MITgcm algorithm. For certain configurations scaling can be highly sensitive to the performance of the global sum primitive. This operation is a collective operation involving all tiles of the simulated domain. Different forms of the global sum primitive exist for handling
  • Data type differences. Sixty-four bit and thirty-two bit fields may be handled separately.
  • Bindings to different communication methods. Exchange primitives select between using shared memory or distributed memory communication.
  • Forward and reverse mode computations. Derivative calculations require tangent linear and adjoint forms of the exchange primitives.

• BARRIER The WRAPPER provides a global synchronization function called barrier. This is used to synchronize computations over all tiles. The BARRIER and GLOBAL SUM primitives have much in common and in some cases use the same underlying code.

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