PetscErrorCode GetExactEigenvalues(PetscInt M,PetscInt N,PetscInt P,PetscInt nconv,PetscReal *exact)
{
PetscInt n,i,j,k,l;
PetscReal *evals,ax,ay,az,sx,sy,sz;
PetscErrorCode ierr;
PetscFunctionBeginUser;
ax = PETSC_PI/2/(M+1);
ay = PETSC_PI/2/(N+1);
az = PETSC_PI/2/(P+1);
n = PetscCeilReal(PetscPowReal(nconv,0.33333)+1);
ierr = PetscMalloc1(n*n*n,&evals);CHKERRQ(ierr);
l = 0;
for (i=1;i<=n;i++) {
sx = PetscSinReal(ax*i);
for (j=1;j<=n;j++) {
sy = PetscSinReal(ay*j);
for (k=1;k<=n;k++) {
sz = PetscSinReal(az*k);
evals[l++] = 4.0*(sx*sx+sy*sy+sz*sz);
}
}
}
ierr = PetscSortReal(n*n*n,evals);CHKERRQ(ierr);
for (i=0;i<nconv;i++) exact[i] = evals[i];
ierr = PetscFree(evals);CHKERRQ(ierr);
PetscFunctionReturn(0);
}
// Find the squarest grid, with best[0] <= best[1] <= best[2].
PetscErrorCode ProcessGridFindSquarest(PetscMPIInt nranks,PetscInt best[3]) {
PetscMPIInt target,s,a,b,c;
PetscFunctionBegin;
target = PetscCeilReal(PetscPowReal(nranks,1./3));
if (target*target*target > nranks) target--; // if ceil was overzealous
for (a=target; a>=1; a--) {
if (nranks%a) continue; // Not a candidate factor
s = PetscCeilReal(PetscSqrtReal(nranks/a));
if (s*s > nranks/a) s--; // If our ceil was overzealous
for (b=s; b>=a; b--) {
if (nranks/a % b == 0) { // The first proper divisor is the one I want
c = nranks/a/b;
best[0] = a;
best[1] = b;
best[2] = c;
PetscFunctionReturn(0);
}
}
}
SETERRQ(PETSC_COMM_WORLD,PETSC_ERR_ARG_INCOMP,"Could not find squarest grid");
PetscFunctionReturn(0);
}