[petsc-users] prefix (i.e. cumulative) sum

[Jed Brown]

Barry Smith <bsmith at mcs.anl.gov> writes:

>   Here you go. In 20+ years of PETSc no one has needed this before.

This is disgusting O(P) critical path.  Do it in O(log P):

int mysum=0,mystart;
for (i=0; i<nlocal; i++) {
  mysum += xx[i];
}
MPI_Scan(&mysum,&mystart,1,MPI_INT,MPI_SUM,comm);
mystart -= mysum;
xx[0] += mystart;
for (i=1; i<nlocal; i++) {
  xx[i] += xx[i-1];
}
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