xref: /petsc/src/mat/utils/compressedrow.c (revision 11e456e13129ef92940db30c75583eb122d1f227)

#include <petsc-private/matimpl.h>  /*I   "petscmat.h"  I*/

#undef __FUNCT__
#define __FUNCT__ "MatCheckCompressedRow"
/*@C
   MatCheckCompressedRow - Determines whether the compressed row matrix format should be used.
      If the format is to be used, this routine creates Mat_CompressedRow struct.
      Compressed row format provides high performance routines by taking advantage of zero rows.
      Supported types are MATAIJ, MATBAIJ and MATSBAIJ.

   Collective

   Input Parameters:
+  A             - the matrix
.  nrows         - number of rows with nonzero entries
.  compressedrow - pointer to the struct Mat_CompressedRow
.  ai            - row pointer used by seqaij and seqbaij
.  mbs           - number of (block) rows represented by ai
-  ratio         - ratio of (num of zero rows)/m, used to determine if the compressed row format should be used

   Notes: By default PETSc will not check for compressed rows on sequential matrices. Call MatSetOption(Mat,MAT_CHECK_COMPRESSED_ROW,PETSC_TRUE); before
          MatAssemblyBegin() to have it check.

   Developer Note: The reason this takes the compressedrow, ai and mbs arguments is because it is called by both the SeqAIJ and SEQBAIJ matrices and
                   the values are not therefore obtained by directly taking the values from the matrix object.
                   This is not a general public routine and hence is not listed in petscmat.h (it exposes a private data structure) but it is used
                   by some preconditioners and hence is labeled as PETSC_EXTERN

   Level: developer
@*/
PETSC_EXTERN PetscErrorCode MatCheckCompressedRow(Mat A,PetscInt nrows,Mat_CompressedRow *compressedrow,PetscInt *ai,PetscInt mbs,PetscReal ratio)
{
  PetscErrorCode ierr;
  PetscInt       *cpi=NULL,*ridx=NULL,nz,i,row;

  PetscFunctionBegin;
  /* in case this is being reused, delete old space */
  ierr = PetscFree2(compressedrow->i,compressedrow->rindex);CHKERRQ(ierr);

  compressedrow->i      = NULL;
  compressedrow->rindex = NULL;


  /* compute number of zero rows */
  nrows = mbs - nrows;

  /* if a large number of zero rows is found, use compressedrow data structure */
  if (nrows < ratio*mbs) {
    compressedrow->use = PETSC_FALSE;

    ierr = PetscInfo3(A,"Found the ratio (num_zerorows %d)/(num_localrows %d) < %G. Do not use CompressedRow routines.\n",nrows,mbs,ratio);CHKERRQ(ierr);
  } else {
    compressedrow->use = PETSC_TRUE;

    ierr = PetscInfo3(A,"Found the ratio (num_zerorows %d)/(num_localrows %d) > %G. Use CompressedRow routines.\n",nrows,mbs,ratio);CHKERRQ(ierr);

    /* set compressed row format */
    nrows  = mbs - nrows; /* num of non-zero rows */
    ierr   = PetscMalloc2(nrows+1,PetscInt,&cpi,nrows,PetscInt,&ridx);CHKERRQ(ierr);
    row    = 0;
    cpi[0] = 0;
    for (i=0; i<mbs; i++) {
      nz = ai[i+1] - ai[i];
      if (nz == 0) continue;
      cpi[row+1]  = ai[i+1];    /* compressed row pointer */
      ridx[row++] = i;          /* compressed row local index */
    }
    compressedrow->nrows  = nrows;
    compressedrow->i      = cpi;
    compressedrow->rindex = ridx;
  }
  PetscFunctionReturn(0);
}