[petsc-users] questions about pc_comosite

[Xiangdong]

Hello everyone,

I have a questions about residuals reported in pc_composite. For the
examples in petsc-3.7.6/src/ksp/ksp/examples/tutorials/ex1.c, I found that
when I use these three options:

1) -pc_type none
2)  -pc_type composite -pc_composite_pcs none,none -pc_composite_type
multiplicative
3) -pc_type composite -pc_composite_pcs none,none -pc_composite_type
multiplicative -ksp_norm_type unpreconditioned

as shown below, it reported different KSP residuals. Given that I use the
identity preconditioner (with multiplicative), why does the residual vary
for different options?

Thanks.

Best,
Xiangdong

mpirun -np 1 ./extest -ksp_monitor -pc_type none

0 KSP Residual norm 1.414213562373e+00
  1 KSP Residual norm 6.324555320337e-01
  2 KSP Residual norm 3.779644730092e-01
  3 KSP Residual norm 2.581988897472e-01
  4 KSP Residual norm 1.906925178491e-01
  5 KSP Residual norm 1.616509124176e-15

mpirun -np 1 ./extest -ksp_monitor -pc_type composite -pc_composite_pcs
none,none -pc_composite_type multiplicative

 0 KSP Residual norm 1.414213562373e+00
  1 KSP Residual norm 1.176696810829e+00
  2 KSP Residual norm 1.096908636191e+00
  3 KSP Residual norm 4.389821446437e-01
  4 KSP Residual norm 2.088364906576e-01
  5 KSP Residual norm 2.725851091482e-13

mpirun -np 1 ./extest -ksp_monitor -pc_type composite -pc_composite_pcs
none,none -pc_composite_type multiplicative -ksp_norm_type unpreconditioned
0 KSP Residual norm 1.414213562373e+00
  1 KSP Residual norm 1.290994448736e+00
  2 KSP Residual norm 1.249516035344e+00
  3 KSP Residual norm 4.836894336642e-01
  4 KSP Residual norm 1.467306491390e-01
  5 KSP Residual norm 9.881776494390e-14
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20170929/57bb72b7/attachment.html>