[petsc-users] BJACOBI with FIELDSPLIT

[Rossi, Simone]

Dear all,

I'm debugging my application in which I'm trying to use the FIELDSPLIT preconditioner for solving a 2x2 block matrix.


Currently I'm testing the preconditioner on a decoupled system where I solve two identical and independent Poisson problems. Using the default fieldsplit type (multiplicative), I'm expecting the method to be equivalent to a Block Jacobi solver.

Setting

-ksp_rtol 1e-6

while  using gmres/hypre on each subblock with

-fieldsplit_0_ksp_rtol 1e-12

-fieldsplit_1_ksp_rtol 1e-12

I'm expecting to converge in 1 iteration with a single solve for each block.


Asking to output the iteration count for the subblocks with

-ksp_converged_reason

-fieldsplit_0_ksp_converged_reason

-fieldsplit_1_ksp_converged_reason

revealed that the outer solver converges in 1 iteration, but each block is solved for 3 times.

This is the output I get:


  Linear fieldsplit_0_ solve converged due to CONVERGED_RTOL iterations 7
  Linear fieldsplit_1_ solve converged due to CONVERGED_RTOL iterations 7
  0 KSP preconditioned resid norm 9.334948012657e+01 true resid norm 1.280164130222e+02 ||r(i)||/||b|| 1.000000000000e+00

  Linear fieldsplit_0_ solve converged due to CONVERGED_RTOL iterations 7
  Linear fieldsplit_1_ solve converged due to CONVERGED_RTOL iterations 7
  Linear fieldsplit_0_ solve converged due to CONVERGED_RTOL iterations 7
  Linear fieldsplit_1_ solve converged due to CONVERGED_RTOL iterations 7
  1 KSP preconditioned resid norm 1.518151977611e-11 true resid norm 8.123270435936e-12 ||r(i)||/||b|| 6.345491366429e-14

Linear solve converged due to CONVERGED_RTOL iterations 1



Are the subblocks actually solved for multiple times at every outer iteration?


Thanks for the help,


Simone



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