Example codes of use for LAPACK:

Example codes of use for ScaLAPACK:

Distribution of matrices, collecting back the result

• distribute a dense general matrix with an generate_element_ij_of_A(int i,int j,double *a) function
  • prerequise: you provide a function generate_element_ij_of_A(int ia,int ja,double *a) which for any processors is able to construct the entry A(ia,ja) of the matrix A
  • example1 is very straight-forward but not too efficient.
  • example2 takes advantage of 2d block cyclic format but the code is slightly more complicated.
• distribute a dense general matrix with an generate_k_rows_of_A(int k,int i,double *a) function
  • prerequise: you provide a function generate_k_rows_of_A(int k,int ia,double *a) which for any processors is able to construct the submatrix A(ia:ia+k-1,1:n), i.e. the k rows of the matrix A starting from line ia ending at line ia+k-1
  • example3 has each processor generates several complete rows and use PBLAS for communication to redistribute the matrix.
• useful routines from SCALAPACK/TOOLS to keep in mind:
  • infog2l -- map global (ia,ja) index to (prow,pcol) processor and (lrindx,lcindx) local array indices (picture)
  • indxg2p -- map global index to processor
  • numroc -- compute local extent, useful in allocating storage for local array
  • infog1l -- map global to local index
  • indxl2g -- map local index to global index
  • p_elset/p_elget -- assign or get global matrix entry

Use of ScaLAPACK routines

• Examples of use for ScaLAPACK can be found at: http://www.netlib.org/scalapack/examples/