The RandomAccess algorithm is similar in principle to the algorithm
submitted for HPCC in 2005, i.e., it employs software routing and
aggregation on a 3D-torus topology, routing every update along the three dimensions in dimension ordered routing manner and ensuring that the application does not have more than 1024 updates with it at any point in time. The significant changes for Blue Gene/P are as follows: Since an MPI node has 4 cores at its service, the functionality is distributed on the 4 cores with core 0 generating the updates and routing them along the X dimension, core 1 receiving from X dimension and routing along Y, core 2 receiving from Y and routing along Z and core 3 receiving from Z and performing the updates to the local table. The code bypasses MPI and directly uses the lower layer DMA based SPI communication layer (available as standard software library of Blue Gene/P).

The IBM Blue Gene/P system supports direct use of messaging DMA hardware in parallel with use of MPI for applications messaging. To enable this direct use mode for DMA an initialization call to setup the DMA fifos must be executed before invoking the MPI_Init call. The optimized HPCC code has introduced a function call dma_init  which is invoked just before MPI_init for this purpose. This is a method that has been put in to support special messaging situations and that is used in a number of production codes  including QCD. It is also well documented in the Blue Gene redbook.

For MPIFFT we changed the algorithm of parallel 1D FFT from 9-step FFT to basic 6-step algorithm. And then we modified the parallelized FFT codes under HPCC_pzfft1d function to fit to Blue Gene system. We modified all the functions in "fft235.c" to SIMDize radix 2,3,4,5 and 8 FFT routines by using intrinsic functions of IBM XLC compiler to generate appropriate double FPU instructions.