Finite difference

Example stencil for derivative:

• Where are the data?
  • Data parallel: specified in layout directives (or default)
    • HPF code:

               real f(1:nx),fdprime(1:nx)
               !HPF DISTRUBITE f(BLOCK) 
               !HPF DISTRUBITE fdprime(BLOCK)
               fdprime(3:nx-2) = c0*(-f(5:nx) + 
                                      16*f(4:nx-1) -
                                      30*f(3:nx-2) +
                                      16*f(2:nx-3) -
                                         f(1:nx-4))
      

    • f(i-2), f(i-1), f(i+1), and/or f(i+2) may be on different PE(s) than f(i) (and fdprime(i)).

    • Worst choice: all 5 pieces of data on different PEs
      • One comm. for each term (for each grid point)

    • Comm. may be specially optimized (CSHIFT)

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