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Presently VASP breaks up immediate MPI send (MPI_isend) and MPI receive (MPI_irecv) calls using large data blocks into smaller ones. We found that large blocks cause a dramatic bandwidth reduction on LINUX clusters linked by a 100 Mbit and/or Gbit Ethernet (all Kernels, all mpi versions including 2.6.X Linux kernels, lam.7.1.1). MPI_BLOCK determines the block size. If use_collective is used, MPI_BLOCK is used only for the fast global sum routine (search for M_sumf_d in mpi.F).

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