xref: /petsc/src/mat/utils/compressedrow.c (revision c6db04a5321582041def2b1e244c75985478b3ef)

#include <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
.  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.

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

  PetscFunctionBegin;
  if (!compressedrow->check) PetscFunctionReturn(0);

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


  /* compute number of zero rows */
  nrows = 0;
  for (i=0; i<mbs; i++){        /* for each row */
    nz = ai[i+1] - ai[i];       /* number of nonzeros */
    if (nz == 0) 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);
}