xref: /petsc/src/ksp/pc/impls/gamg/util.c (revision b41ce5d507ea9a58bfa83cf403107a702e77a67d)
1 /*
2  GAMG geometric-algebric multigrid PC - Mark Adams 2011
3  */
4 #include <petsc/private/matimpl.h>
5 #include <../src/ksp/pc/impls/gamg/gamg.h>           /*I "petscpc.h" I*/
6 
7 /*
8    Produces a set of block column indices of the matrix row, one for each block represented in the original row
9 
10    n - the number of block indices in cc[]
11    cc - the block indices (must be large enough to contain the indices)
12 */
13 PETSC_STATIC_INLINE PetscErrorCode MatCollapseRow(Mat Amat,PetscInt row,PetscInt bs,PetscInt *n,PetscInt *cc)
14 {
15   PetscInt       cnt = -1,nidx,j;
16   const PetscInt *idx;
17   PetscErrorCode ierr;
18 
19   PetscFunctionBegin;
20   ierr = MatGetRow(Amat,row,&nidx,&idx,NULL);CHKERRQ(ierr);
21   if (nidx) {
22     cnt = 0;
23     cc[cnt] = idx[0]/bs;
24     for (j=1; j<nidx; j++) {
25       if (cc[cnt] < idx[j]/bs) cc[++cnt] = idx[j]/bs;
26     }
27   }
28   ierr = MatRestoreRow(Amat,row,&nidx,&idx,NULL);CHKERRQ(ierr);
29   *n = cnt+1;
30   PetscFunctionReturn(0);
31 }
32 
33 /*
34     Produces a set of block column indices of the matrix block row, one for each block represented in the original set of rows
35 
36     ncollapsed - the number of block indices
37     collapsed - the block indices (must be large enough to contain the indices)
38 */
39 PETSC_STATIC_INLINE PetscErrorCode MatCollapseRows(Mat Amat,PetscInt start,PetscInt bs,PetscInt *w0,PetscInt *w1,PetscInt *w2,PetscInt *ncollapsed,PetscInt **collapsed)
40 {
41   PetscInt       i,nprev,*cprev = w0,ncur = 0,*ccur = w1,*merged = w2,*cprevtmp;
42   PetscErrorCode ierr;
43 
44   PetscFunctionBegin;
45   ierr = MatCollapseRow(Amat,start,bs,&nprev,cprev);CHKERRQ(ierr);
46   for (i=start+1; i<start+bs; i++) {
47     ierr  = MatCollapseRow(Amat,i,bs,&ncur,ccur);CHKERRQ(ierr);
48     ierr  = PetscMergeIntArray(nprev,cprev,ncur,ccur,&nprev,&merged);CHKERRQ(ierr);
49     cprevtmp = cprev; cprev = merged; merged = cprevtmp;
50   }
51   *ncollapsed = nprev;
52   if (collapsed) *collapsed  = cprev;
53   PetscFunctionReturn(0);
54 }
55 
56 
57 /* -------------------------------------------------------------------------- */
58 /*
59    PCGAMGCreateGraph - create simple scaled scalar graph from matrix
60 
61  Input Parameter:
62  . Amat - matrix
63  Output Parameter:
64  . a_Gmaat - eoutput scalar graph (symmetric?)
65  */
66 PetscErrorCode PCGAMGCreateGraph(Mat Amat, Mat *a_Gmat)
67 {
68   PetscErrorCode ierr;
69   PetscInt       Istart,Iend,Ii,jj,kk,ncols,nloc,NN,MM,bs;
70   MPI_Comm       comm;
71   Mat            Gmat;
72   MatType        mtype;
73 
74   PetscFunctionBegin;
75   ierr = PetscObjectGetComm((PetscObject)Amat,&comm);CHKERRQ(ierr);
76   ierr = MatGetOwnershipRange(Amat, &Istart, &Iend);CHKERRQ(ierr);
77   ierr = MatGetSize(Amat, &MM, &NN);CHKERRQ(ierr);
78   ierr = MatGetBlockSize(Amat, &bs);CHKERRQ(ierr);
79   nloc = (Iend-Istart)/bs;
80 
81 #if defined PETSC_GAMG_USE_LOG
82   ierr = PetscLogEventBegin(petsc_gamg_setup_events[GRAPH],0,0,0,0);CHKERRQ(ierr);
83 #endif
84 
85   if (bs > 1) {
86     const PetscScalar *vals;
87     const PetscInt    *idx;
88     PetscInt          *d_nnz, *o_nnz,*w0,*w1,*w2;
89     PetscBool         ismpiaij,isseqaij;
90 
91     /*
92        Determine the preallocation needed for the scalar matrix derived from the vector matrix.
93     */
94 
95     ierr = PetscObjectBaseTypeCompare((PetscObject)Amat,MATSEQAIJ,&isseqaij);CHKERRQ(ierr);
96     ierr = PetscObjectBaseTypeCompare((PetscObject)Amat,MATMPIAIJ,&ismpiaij);CHKERRQ(ierr);
97 
98     ierr = PetscMalloc2(nloc, &d_nnz,isseqaij ? 0 : nloc, &o_nnz);CHKERRQ(ierr);
99 
100     if (isseqaij) {
101       PetscInt       max_d_nnz;
102 
103       /*
104           Determine exact preallocation count for (sequential) scalar matrix
105       */
106       ierr = MatSeqAIJGetMaxRowNonzeros(Amat,&max_d_nnz);CHKERRQ(ierr);
107       max_d_nnz = PetscMin(nloc,bs*max_d_nnz);CHKERRQ(ierr);
108       ierr = PetscMalloc3(max_d_nnz, &w0,max_d_nnz, &w1,max_d_nnz, &w2);CHKERRQ(ierr);
109       for (Ii = 0, jj = 0; Ii < Iend; Ii += bs, jj++) {
110         ierr = MatCollapseRows(Amat,Ii,bs,w0,w1,w2,&d_nnz[jj],NULL);CHKERRQ(ierr);
111       }
112       ierr = PetscFree3(w0,w1,w2);CHKERRQ(ierr);
113 
114     } else if (ismpiaij) {
115       Mat            Daij,Oaij;
116       const PetscInt *garray;
117       PetscInt       max_d_nnz;
118 
119       ierr = MatMPIAIJGetSeqAIJ(Amat,&Daij,&Oaij,&garray);CHKERRQ(ierr);
120 
121       /*
122           Determine exact preallocation count for diagonal block portion of scalar matrix
123       */
124       ierr = MatSeqAIJGetMaxRowNonzeros(Daij,&max_d_nnz);CHKERRQ(ierr);
125       max_d_nnz = PetscMin(nloc,bs*max_d_nnz);CHKERRQ(ierr);
126       ierr = PetscMalloc3(max_d_nnz, &w0,max_d_nnz, &w1,max_d_nnz, &w2);CHKERRQ(ierr);
127       for (Ii = 0, jj = 0; Ii < Iend - Istart; Ii += bs, jj++) {
128         ierr = MatCollapseRows(Daij,Ii,bs,w0,w1,w2,&d_nnz[jj],NULL);CHKERRQ(ierr);
129       }
130       ierr = PetscFree3(w0,w1,w2);CHKERRQ(ierr);
131 
132       /*
133          Over estimate (usually grossly over), preallocation count for off-diagonal portion of scalar matrix
134       */
135       for (Ii = 0, jj = 0; Ii < Iend - Istart; Ii += bs, jj++) {
136         o_nnz[jj] = 0;
137         for (kk=0; kk<bs; kk++) { /* rows that get collapsed to a single row */
138           ierr = MatGetRow(Oaij,Ii+kk,&ncols,0,0);CHKERRQ(ierr);
139           o_nnz[jj] += ncols;
140           ierr = MatRestoreRow(Oaij,Ii+kk,&ncols,0,0);CHKERRQ(ierr);
141         }
142         if (o_nnz[jj] > (NN/bs-nloc)) o_nnz[jj] = NN/bs-nloc;
143       }
144 
145     } else {
146       SETERRQ(PETSC_COMM_WORLD,PETSC_ERR_USER,"Require AIJ matrix type");
147     }
148 
149     /* get scalar copy (norms) of matrix */
150     ierr = MatGetType(Amat,&mtype);CHKERRQ(ierr);
151     ierr = MatCreate(comm, &Gmat);CHKERRQ(ierr);
152     ierr = MatSetSizes(Gmat,nloc,nloc,PETSC_DETERMINE,PETSC_DETERMINE);CHKERRQ(ierr);
153     ierr = MatSetBlockSizes(Gmat, 1, 1);CHKERRQ(ierr);
154     ierr = MatSetType(Gmat, mtype);CHKERRQ(ierr);
155     ierr = MatSeqAIJSetPreallocation(Gmat,0,d_nnz);CHKERRQ(ierr);
156     ierr = MatMPIAIJSetPreallocation(Gmat,0,d_nnz,0,o_nnz);CHKERRQ(ierr);
157     ierr = PetscFree2(d_nnz,o_nnz);CHKERRQ(ierr);
158 
159     for (Ii = Istart; Ii < Iend; Ii++) {
160       PetscInt dest_row = Ii/bs;
161       ierr = MatGetRow(Amat,Ii,&ncols,&idx,&vals);CHKERRQ(ierr);
162       for (jj=0; jj<ncols; jj++) {
163         PetscInt    dest_col = idx[jj]/bs;
164         PetscScalar sv       = PetscAbs(PetscRealPart(vals[jj]));
165         ierr = MatSetValues(Gmat,1,&dest_row,1,&dest_col,&sv,ADD_VALUES);CHKERRQ(ierr);
166       }
167       ierr = MatRestoreRow(Amat,Ii,&ncols,&idx,&vals);CHKERRQ(ierr);
168     }
169     ierr = MatAssemblyBegin(Gmat,MAT_FINAL_ASSEMBLY);CHKERRQ(ierr);
170     ierr = MatAssemblyEnd(Gmat,MAT_FINAL_ASSEMBLY);CHKERRQ(ierr);
171   } else {
172     /* just copy scalar matrix - abs() not taken here but scaled later */
173     ierr = MatDuplicate(Amat, MAT_COPY_VALUES, &Gmat);CHKERRQ(ierr);
174   }
175 
176 #if defined PETSC_GAMG_USE_LOG
177   ierr = PetscLogEventEnd(petsc_gamg_setup_events[GRAPH],0,0,0,0);CHKERRQ(ierr);
178 #endif
179 
180   *a_Gmat = Gmat;
181   PetscFunctionReturn(0);
182 }
183 
184 /* -------------------------------------------------------------------------- */
185 /*@C
186    PCGAMGFilterGraph - filter (remove zero and possibly small values from the) graph and make it symmetric if requested
187 
188    Collective on Mat
189 
190    Input Parameter:
191 +   a_Gmat - the graph
192 .   vfilter - threshold paramter [0,1)
193 -   symm - make the result symmetric
194 
195    Level: developer
196 
197    Notes: This is called before graph coarsers are called.
198 
199 .seealso: PCGAMGSetThreshold()
200 @*/
201 PetscErrorCode PCGAMGFilterGraph(Mat *a_Gmat,PetscReal vfilter,PetscBool symm)
202 {
203   PetscErrorCode    ierr;
204   PetscInt          Istart,Iend,Ii,jj,ncols,nnz0,nnz1, NN, MM, nloc;
205   PetscMPIInt       rank;
206   Mat               Gmat  = *a_Gmat, tGmat, matTrans;
207   MPI_Comm          comm;
208   const PetscScalar *vals;
209   const PetscInt    *idx;
210   PetscInt          *d_nnz, *o_nnz;
211   Vec               diag;
212   MatType           mtype;
213 
214   PetscFunctionBegin;
215 #if defined PETSC_GAMG_USE_LOG
216   ierr = PetscLogEventBegin(petsc_gamg_setup_events[GRAPH],0,0,0,0);CHKERRQ(ierr);
217 #endif
218   /* scale Gmat for all values between -1 and 1 */
219   ierr = MatCreateVecs(Gmat, &diag, 0);CHKERRQ(ierr);
220   ierr = MatGetDiagonal(Gmat, diag);CHKERRQ(ierr);
221   ierr = VecReciprocal(diag);CHKERRQ(ierr);
222   ierr = VecSqrtAbs(diag);CHKERRQ(ierr);
223   ierr = MatDiagonalScale(Gmat, diag, diag);CHKERRQ(ierr);
224   ierr = VecDestroy(&diag);CHKERRQ(ierr);
225 
226   if (vfilter < 0.0 && !symm) {
227     /* Just use the provided matrix as the graph but make all values positive */
228     MatInfo     info;
229     PetscScalar *avals;
230     PetscBool isaij,ismpiaij;
231     ierr = PetscObjectBaseTypeCompare((PetscObject)Gmat,MATSEQAIJ,&isaij);CHKERRQ(ierr);
232     ierr = PetscObjectBaseTypeCompare((PetscObject)Gmat,MATMPIAIJ,&ismpiaij);CHKERRQ(ierr);
233     if (!isaij && !ismpiaij) SETERRQ(PETSC_COMM_WORLD,PETSC_ERR_USER,"Require (MPI)AIJ matrix type");
234     if (isaij) {
235       ierr = MatGetInfo(Gmat,MAT_LOCAL,&info);CHKERRQ(ierr);
236       ierr = MatSeqAIJGetArray(Gmat,&avals);CHKERRQ(ierr);
237       for (jj = 0; jj<info.nz_used; jj++) avals[jj] = PetscAbsScalar(avals[jj]);
238       ierr = MatSeqAIJRestoreArray(Gmat,&avals);CHKERRQ(ierr);
239     } else {
240       Mat_MPIAIJ  *aij = (Mat_MPIAIJ*)Gmat->data;
241       ierr = MatGetInfo(aij->A,MAT_LOCAL,&info);CHKERRQ(ierr);
242       ierr = MatSeqAIJGetArray(aij->A,&avals);CHKERRQ(ierr);
243       for (jj = 0; jj<info.nz_used; jj++) avals[jj] = PetscAbsScalar(avals[jj]);
244       ierr = MatSeqAIJRestoreArray(aij->A,&avals);CHKERRQ(ierr);
245       ierr = MatGetInfo(aij->B,MAT_LOCAL,&info);CHKERRQ(ierr);
246       ierr = MatSeqAIJGetArray(aij->B,&avals);CHKERRQ(ierr);
247       for (jj = 0; jj<info.nz_used; jj++) avals[jj] = PetscAbsScalar(avals[jj]);
248       ierr = MatSeqAIJRestoreArray(aij->B,&avals);CHKERRQ(ierr);
249     }
250 #if defined PETSC_GAMG_USE_LOG
251     ierr = PetscLogEventEnd(petsc_gamg_setup_events[GRAPH],0,0,0,0);CHKERRQ(ierr);
252 #endif
253     PetscFunctionReturn(0);
254   }
255 
256   ierr = PetscObjectGetComm((PetscObject)Gmat,&comm);CHKERRQ(ierr);
257   ierr = MPI_Comm_rank(comm,&rank);CHKERRQ(ierr);
258   ierr = MatGetOwnershipRange(Gmat, &Istart, &Iend);CHKERRQ(ierr);
259   nloc = Iend - Istart;
260   ierr = MatGetSize(Gmat, &MM, &NN);CHKERRQ(ierr);
261 
262   if (symm) {
263     ierr = MatTranspose(Gmat, MAT_INITIAL_MATRIX, &matTrans);CHKERRQ(ierr);
264   }
265 
266   /* Determine upper bound on nonzeros needed in new filtered matrix */
267   ierr = PetscMalloc2(nloc, &d_nnz,nloc, &o_nnz);CHKERRQ(ierr);
268   for (Ii = Istart, jj = 0; Ii < Iend; Ii++, jj++) {
269     ierr      = MatGetRow(Gmat,Ii,&ncols,NULL,NULL);CHKERRQ(ierr);
270     d_nnz[jj] = ncols;
271     o_nnz[jj] = ncols;
272     ierr      = MatRestoreRow(Gmat,Ii,&ncols,NULL,NULL);CHKERRQ(ierr);
273     if (symm) {
274       ierr       = MatGetRow(matTrans,Ii,&ncols,NULL,NULL);CHKERRQ(ierr);
275       d_nnz[jj] += ncols;
276       o_nnz[jj] += ncols;
277       ierr       = MatRestoreRow(matTrans,Ii,&ncols,NULL,NULL);CHKERRQ(ierr);
278     }
279     if (d_nnz[jj] > nloc) d_nnz[jj] = nloc;
280     if (o_nnz[jj] > (MM-nloc)) o_nnz[jj] = MM - nloc;
281   }
282   ierr = MatGetType(Gmat,&mtype);CHKERRQ(ierr);
283   ierr = MatCreate(comm, &tGmat);CHKERRQ(ierr);
284   ierr = MatSetSizes(tGmat,nloc,nloc,MM,MM);CHKERRQ(ierr);
285   ierr = MatSetBlockSizes(tGmat, 1, 1);CHKERRQ(ierr);
286   ierr = MatSetType(tGmat, mtype);CHKERRQ(ierr);
287   ierr = MatSeqAIJSetPreallocation(tGmat,0,d_nnz);CHKERRQ(ierr);
288   ierr = MatMPIAIJSetPreallocation(tGmat,0,d_nnz,0,o_nnz);CHKERRQ(ierr);
289   ierr = PetscFree2(d_nnz,o_nnz);CHKERRQ(ierr);
290   if (symm) {
291     ierr = MatDestroy(&matTrans);CHKERRQ(ierr);
292   } else {
293     /* all entries are generated locally so MatAssembly will be slightly faster for large process counts */
294     ierr = MatSetOption(tGmat,MAT_NO_OFF_PROC_ENTRIES,PETSC_TRUE);CHKERRQ(ierr);
295   }
296 
297   for (Ii = Istart, nnz0 = nnz1 = 0; Ii < Iend; Ii++) {
298     ierr = MatGetRow(Gmat,Ii,&ncols,&idx,&vals);CHKERRQ(ierr);
299     for (jj=0; jj<ncols; jj++,nnz0++) {
300       PetscScalar sv = PetscAbs(PetscRealPart(vals[jj]));
301       if (PetscRealPart(sv) > vfilter) {
302         nnz1++;
303         if (symm) {
304           sv  *= 0.5;
305           ierr = MatSetValues(tGmat,1,&Ii,1,&idx[jj],&sv,ADD_VALUES);CHKERRQ(ierr);
306           ierr = MatSetValues(tGmat,1,&idx[jj],1,&Ii,&sv,ADD_VALUES);CHKERRQ(ierr);
307         } else {
308           ierr = MatSetValues(tGmat,1,&Ii,1,&idx[jj],&sv,ADD_VALUES);CHKERRQ(ierr);
309         }
310       }
311     }
312     ierr = MatRestoreRow(Gmat,Ii,&ncols,&idx,&vals);CHKERRQ(ierr);
313   }
314   ierr = MatAssemblyBegin(tGmat,MAT_FINAL_ASSEMBLY);CHKERRQ(ierr);
315   ierr = MatAssemblyEnd(tGmat,MAT_FINAL_ASSEMBLY);CHKERRQ(ierr);
316 
317 #if defined PETSC_GAMG_USE_LOG
318   ierr = PetscLogEventEnd(petsc_gamg_setup_events[GRAPH],0,0,0,0);CHKERRQ(ierr);
319 #endif
320 
321 #if defined(PETSC_USE_INFO)
322   {
323     double t1 = (!nnz0) ? 1. : 100.*(double)nnz1/(double)nnz0, t2 = (!nloc) ? 1. : (double)nnz0/(double)nloc;
324     ierr = PetscInfo4(*a_Gmat,"\t %g%% nnz after filtering, with threshold %g, %g nnz ave. (N=%D)\n",t1,vfilter,t2,MM);CHKERRQ(ierr);
325   }
326 #endif
327   ierr    = MatDestroy(&Gmat);CHKERRQ(ierr);
328   *a_Gmat = tGmat;
329   PetscFunctionReturn(0);
330 }
331 
332 /* -------------------------------------------------------------------------- */
333 /*
334    PCGAMGGetDataWithGhosts - hacks into Mat MPIAIJ so this must have npe > 1
335 
336    Input Parameter:
337    . Gmat - MPIAIJ matrix for scattters
338    . data_sz - number of data terms per node (# cols in output)
339    . data_in[nloc*data_sz] - column oriented data
340    Output Parameter:
341    . a_stride - numbrt of rows of output
342    . a_data_out[stride*data_sz] - output data with ghosts
343 */
344 PetscErrorCode PCGAMGGetDataWithGhosts(Mat Gmat,PetscInt data_sz,PetscReal data_in[],PetscInt *a_stride,PetscReal **a_data_out)
345 {
346   PetscErrorCode ierr;
347   MPI_Comm       comm;
348   Vec            tmp_crds;
349   Mat_MPIAIJ     *mpimat = (Mat_MPIAIJ*)Gmat->data;
350   PetscInt       nnodes,num_ghosts,dir,kk,jj,my0,Iend,nloc;
351   PetscScalar    *data_arr;
352   PetscReal      *datas;
353   PetscBool      isMPIAIJ;
354 
355   PetscFunctionBegin;
356   ierr = PetscObjectGetComm((PetscObject)Gmat,&comm);CHKERRQ(ierr);
357   ierr      = PetscObjectBaseTypeCompare((PetscObject)Gmat, MATMPIAIJ, &isMPIAIJ);CHKERRQ(ierr);
358   ierr      = MatGetOwnershipRange(Gmat, &my0, &Iend);CHKERRQ(ierr);
359   nloc      = Iend - my0;
360   ierr      = VecGetLocalSize(mpimat->lvec, &num_ghosts);CHKERRQ(ierr);
361   nnodes    = num_ghosts + nloc;
362   *a_stride = nnodes;
363   ierr      = MatCreateVecs(Gmat, &tmp_crds, 0);CHKERRQ(ierr);
364 
365   ierr = PetscMalloc1(data_sz*nnodes, &datas);CHKERRQ(ierr);
366   for (dir=0; dir<data_sz; dir++) {
367     /* set local, and global */
368     for (kk=0; kk<nloc; kk++) {
369       PetscInt    gid = my0 + kk;
370       PetscScalar crd = (PetscScalar)data_in[dir*nloc + kk]; /* col oriented */
371       datas[dir*nnodes + kk] = PetscRealPart(crd);
372 
373       ierr = VecSetValues(tmp_crds, 1, &gid, &crd, INSERT_VALUES);CHKERRQ(ierr);
374     }
375     ierr = VecAssemblyBegin(tmp_crds);CHKERRQ(ierr);
376     ierr = VecAssemblyEnd(tmp_crds);CHKERRQ(ierr);
377     /* get ghost datas */
378     ierr = VecScatterBegin(mpimat->Mvctx,tmp_crds,mpimat->lvec,INSERT_VALUES,SCATTER_FORWARD);CHKERRQ(ierr);
379     ierr = VecScatterEnd(mpimat->Mvctx,tmp_crds,mpimat->lvec,INSERT_VALUES,SCATTER_FORWARD);CHKERRQ(ierr);
380     ierr = VecGetArray(mpimat->lvec, &data_arr);CHKERRQ(ierr);
381     for (kk=nloc,jj=0;jj<num_ghosts;kk++,jj++) datas[dir*nnodes + kk] = PetscRealPart(data_arr[jj]);
382     ierr = VecRestoreArray(mpimat->lvec, &data_arr);CHKERRQ(ierr);
383   }
384   ierr        = VecDestroy(&tmp_crds);CHKERRQ(ierr);
385   *a_data_out = datas;
386   PetscFunctionReturn(0);
387 }
388 
389 
390 /*
391  *
392  *  PCGAMGHashTableCreate
393  */
394 
395 PetscErrorCode PCGAMGHashTableCreate(PetscInt a_size, PCGAMGHashTable *a_tab)
396 {
397   PetscErrorCode ierr;
398   PetscInt       kk;
399 
400   PetscFunctionBegin;
401   a_tab->size = a_size;
402   ierr = PetscMalloc1(a_size, &a_tab->table);CHKERRQ(ierr);
403   ierr = PetscMalloc1(a_size, &a_tab->data);CHKERRQ(ierr);
404   for (kk=0; kk<a_size; kk++) a_tab->table[kk] = -1;
405   PetscFunctionReturn(0);
406 }
407 
408 PetscErrorCode PCGAMGHashTableDestroy(PCGAMGHashTable *a_tab)
409 {
410   PetscErrorCode ierr;
411 
412   PetscFunctionBegin;
413   ierr = PetscFree(a_tab->table);CHKERRQ(ierr);
414   ierr = PetscFree(a_tab->data);CHKERRQ(ierr);
415   PetscFunctionReturn(0);
416 }
417 
418 PetscErrorCode PCGAMGHashTableAdd(PCGAMGHashTable *a_tab, PetscInt a_key, PetscInt a_data)
419 {
420   PetscInt kk,idx;
421 
422   PetscFunctionBegin;
423   if (a_key<0) SETERRQ1(PETSC_COMM_SELF,PETSC_ERR_USER,"Negative key %d.",a_key);
424   for (kk = 0, idx = GAMG_HASH(a_key);
425        kk < a_tab->size;
426        kk++, idx = (idx==(a_tab->size-1)) ? 0 : idx + 1) {
427 
428     if (a_tab->table[idx] == a_key) {
429       /* exists */
430       a_tab->data[idx] = a_data;
431       break;
432     } else if (a_tab->table[idx] == -1) {
433       /* add */
434       a_tab->table[idx] = a_key;
435       a_tab->data[idx]  = a_data;
436       break;
437     }
438   }
439   if (kk==a_tab->size) {
440     /* this is not to efficient, waiting until completely full */
441     PetscInt       oldsize = a_tab->size, new_size = 2*a_tab->size + 5, *oldtable = a_tab->table, *olddata = a_tab->data;
442     PetscErrorCode ierr;
443 
444     a_tab->size = new_size;
445 
446     ierr = PetscMalloc1(a_tab->size, &a_tab->table);CHKERRQ(ierr);
447     ierr = PetscMalloc1(a_tab->size, &a_tab->data);CHKERRQ(ierr);
448 
449     for (kk=0;kk<a_tab->size;kk++) a_tab->table[kk] = -1;
450     for (kk=0;kk<oldsize;kk++) {
451       if (oldtable[kk] != -1) {
452         ierr = PCGAMGHashTableAdd(a_tab, oldtable[kk], olddata[kk]);CHKERRQ(ierr);
453        }
454     }
455     ierr = PetscFree(oldtable);CHKERRQ(ierr);
456     ierr = PetscFree(olddata);CHKERRQ(ierr);
457     ierr = PCGAMGHashTableAdd(a_tab, a_key, a_data);CHKERRQ(ierr);
458   }
459   PetscFunctionReturn(0);
460 }
461 
462