Double solve on the coarse level?

[ntardieu at giref.ulaval.ca]

Dear PETSc users,

We designed a multigrid solver based on the PCMG framework.
But when one looks at the output, it seems that the system on the coarsest
level is solved twice : indeed the same residuals appear twice. Is it a
consequence of using a W cycle with an iteratif solver on the coarsest
level?
The PC object and a typical output are given below.

Thanks,

Nicolas

=======================================================================
PC Object:
  type: mg
    MG: type is MULTIPLICATIVE, levels=2 cycles=w, pre-smooths=1,
post-smooths=1
  Coarse gride solver -- level 0 -------------------------------
    KSP Object:(mg_coarse_)
      type: cr
      maximum iterations=30, initial guess is zero
      tolerances:  relative=1e-06, absolute=1e-50, divergence=10000
      left preconditioning
    PC Object:(mg_coarse_)
      type: sor
        SOR: type = local_symmetric, iterations = 1, omega = 1
      linear system matrix = precond matrix:
      Matrix Object:
        type=seqaij, rows=24497, cols=24497
        total: nonzeros=936401, allocated nonzeros=0
          not using I-node routines
  Down solver (pre-smoother) on level 1 -------------------------------
    KSP Object:(mg_levels_1_)
      type: chebychev
        Chebychev: eigenvalue estimates:  min = 0.01, max = 100
      maximum iterations=3, initial guess is zero
      tolerances:  relative=1e-05, absolute=1e-50, divergence=10000
      left preconditioning
    PC Object:(mg_levels_1_)
      type: sor
        SOR: type = local_symmetric, iterations = 1, omega = 1
      linear system matrix = precond matrix:
      Matrix Object:
        type=seqaij, rows=59833, cols=59833
        total: nonzeros=670149, allocated nonzeros=670149
          not using I-node routines
  Up solver (post-smoother) on level 1 -------------------------------
    KSP Object:(mg_levels_1_)
      type: chebychev
        Chebychev: eigenvalue estimates:  min = 0.01, max = 100
      maximum iterations=3
      tolerances:  relative=1e-05, absolute=1e-50, divergence=10000
      left preconditioning
    PC Object:(mg_levels_1_)
      type: sor
        SOR: type = local_symmetric, iterations = 1, omega = 1
      linear system matrix = precond matrix:
      Matrix Object:
        type=seqaij, rows=59833, cols=59833
        total: nonzeros=670149, allocated nonzeros=670149
          not using I-node routines
  linear system matrix = precond matrix:
  Matrix Object:
    type=seqaij, rows=59833, cols=59833
    total: nonzeros=670149, allocated nonzeros=670149
      not using I-node routines

=======================================================================
Typical output :

    0 KSP Residual norm 8.529532792227e-01
    1 KSP Residual norm 8.312168822014e-01
    2 KSP Residual norm 7.962289819435e-01
    3 KSP Residual norm 7.498665547818e-01
      0 KSP Residual norm 8.623070846983e-01 <<<<<<<<<<
      1 KSP Residual norm 4.994728639387e-01
      2 KSP Residual norm 4.441045285060e-01
      3 KSP Residual norm 4.036899174922e-01
      4 KSP Residual norm 3.741251351308e-01
      5 KSP Residual norm 3.449669992123e-01
      6 KSP Residual norm 3.220670285389e-01
      7 KSP Residual norm 2.974753633724e-01
      8 KSP Residual norm 2.786493949211e-01
      9 KSP Residual norm 2.666365822398e-01
     10 KSP Residual norm 2.615313007214e-01
     11 KSP Residual norm 2.580757152907e-01
     12 KSP Residual norm 2.549505151556e-01
     13 KSP Residual norm 2.509670840632e-01
     14 KSP Residual norm 2.464315316006e-01
     15 KSP Residual norm 2.416198261586e-01
     16 KSP Residual norm 2.359854861997e-01
     17 KSP Residual norm 2.298769116813e-01
     18 KSP Residual norm 2.232203276735e-01
     19 KSP Residual norm 2.153461917946e-01
     20 KSP Residual norm 2.072492142809e-01
     21 KSP Residual norm 1.983342705665e-01
     22 KSP Residual norm 1.894778760982e-01
     23 KSP Residual norm 1.803772952130e-01
     24 KSP Residual norm 1.718394016178e-01
     25 KSP Residual norm 1.639952865611e-01
     26 KSP Residual norm 1.571843225056e-01
     27 KSP Residual norm 1.512651261054e-01
     28 KSP Residual norm 1.459688114323e-01
     29 KSP Residual norm 1.410245577886e-01
     30 KSP Residual norm 1.364936912778e-01
      0 KSP Residual norm 8.623070846983e-01  <<<<<<<<<<
      1 KSP Residual norm 4.994728639387e-01
      2 KSP Residual norm 4.441045285060e-01
      3 KSP Residual norm 4.036899174922e-01
      4 KSP Residual norm 3.741251351308e-01
      5 KSP Residual norm 3.449669992123e-01
      6 KSP Residual norm 3.220670285389e-01
      7 KSP Residual norm 2.974753633724e-01
      8 KSP Residual norm 2.786493949211e-01
      9 KSP Residual norm 2.666365822398e-01
     10 KSP Residual norm 2.615313007214e-01
     11 KSP Residual norm 2.580757152907e-01
     12 KSP Residual norm 2.549505151556e-01
     13 KSP Residual norm 2.509670840632e-01
     14 KSP Residual norm 2.464315316006e-01
     15 KSP Residual norm 2.416198261586e-01
     16 KSP Residual norm 2.359854861997e-01
     17 KSP Residual norm 2.298769116813e-01
     18 KSP Residual norm 2.232203276735e-01
     19 KSP Residual norm 2.153461917946e-01
     20 KSP Residual norm 2.072492142809e-01
     21 KSP Residual norm 1.983342705665e-01
     22 KSP Residual norm 1.894778760982e-01
     23 KSP Residual norm 1.803772952130e-01
     24 KSP Residual norm 1.718394016178e-01
     25 KSP Residual norm 1.639952865611e-01
     26 KSP Residual norm 1.571843225056e-01
     27 KSP Residual norm 1.512651261054e-01
     28 KSP Residual norm 1.459688114323e-01
     29 KSP Residual norm 1.410245577886e-01
     30 KSP Residual norm 1.364936912778e-01
    0 KSP Residual norm 2.864424649239e+00
    1 KSP Residual norm 2.719941822422e+00
    2 KSP Residual norm 2.486679927440e+00
    3 KSP Residual norm 2.176182306332e+00
  0 KSP preconditioned resid norm 3.133018965496e+02 true resid norm
5.229629619561e+01 ||Ae||/||Ax|| 1.000000000000e+00

=======================================================================