[petsc-users] Problems with FBCGSR

[Barry Smith]

   I don't think FBCGSR  has any guarantees that it will work so you could just be unlucky. 

   If you really want to explore the issue (probably not worth it) you can run with really tight tolerances on the inner solvers (so they become essentially exact solvers and hence linear operators) and see if the FBCGSR converges. If it does convergence then slowly loosen the tolerances until it messes up the convergences of the FBCGSR

   Barry


> On Apr 1, 2016, at 8:19 AM, Elias Karabelas <karabelaselias at gmail.com> wrote:
> 
> 
> Hi,
> 
> I'm using petsc for solving a non symmetric block system with a
> field-split preconditioner. At the moment I use the fgmres as outer
> solver. I recently discovered that there is also a flexible BICG variant
> in petsc. So I wanted to give it a try. However I get strange behaviour
> when using it. With FGMRES i got the following residual history
> 
>  0 KSP unpreconditioned resid norm 8.632122370708e+00 true resid norm
> 8.632122370708e+00 ||r(i)||/||b|| 1.000000000000e+00
>  1 KSP unpreconditioned resid norm 5.881521061968e-04 true resid norm
> 5.881521061969e-04 ||r(i)||/||b|| 6.813528364620e-05
>  2 KSP unpreconditioned resid norm 6.142969823931e-05 true resid norm
> 6.142969823933e-05 ||r(i)||/||b|| 7.116407252031e-06
>  3 KSP unpreconditioned resid norm 4.110742198982e-05 true resid norm
> 4.110742198982e-05 ||r(i)||/||b|| 4.762145417367e-06
>  4 KSP unpreconditioned resid norm 2.628280876832e-05 true resid norm
> 2.628280876817e-05 ||r(i)||/||b|| 3.044767861187e-06
>  5 KSP unpreconditioned resid norm 1.293845872473e-05 true resid norm
> 1.293845872450e-05 ||r(i)||/||b|| 1.498873413612e-06
>  6 KSP unpreconditioned resid norm 6.930434272105e-06 true resid norm
> 6.930434271781e-06 ||r(i)||/||b|| 8.028656191550e-07
>  7 KSP unpreconditioned resid norm 4.654261998874e-06 true resid norm
> 4.654261998975e-06 ||r(i)||/||b|| 5.391793349419e-07
>  8 KSP unpreconditioned resid norm 3.370057395937e-06 true resid norm
> 3.370057396119e-06 ||r(i)||/||b|| 3.904088996184e-07
>  9 KSP unpreconditioned resid norm 2.540901076284e-06 true resid norm
> 2.540901076192e-06 ||r(i)||/||b|| 2.943541538306e-07
> 10 KSP unpreconditioned resid norm 2.039263657470e-06 true resid norm
> 2.039263657746e-06 ||r(i)||/||b|| 2.362412822907e-07
> 11 KSP unpreconditioned resid norm 1.583890355441e-06 true resid norm
> 1.583890355170e-06 ||r(i)||/||b|| 1.834879404102e-07
> 12 KSP unpreconditioned resid norm 1.184174171199e-06 true resid norm
> 1.184174170742e-06 ||r(i)||/||b|| 1.371822733607e-07
> 13 KSP unpreconditioned resid norm 8.493013125005e-07 true resid norm
> 8.493013119686e-07 ||r(i)||/||b|| 9.838846989132e-08
> 14 KSP unpreconditioned resid norm 5.905267399341e-07 true resid norm
> 5.905267385984e-07 ||r(i)||/||b|| 6.841037617843e-08
> 15 KSP unpreconditioned resid norm 4.160983692482e-07 true resid norm
> 4.160983680083e-07 ||r(i)||/||b|| 4.820348347010e-08
> 16 KSP unpreconditioned resid norm 2.929006184135e-07 true resid norm
> 2.929006164636e-07 ||r(i)||/||b|| 3.393147176151e-08
> 17 KSP unpreconditioned resid norm 2.085222908044e-07 true resid norm
> 2.085222901282e-07 ||r(i)||/||b|| 2.415654935984e-08
> 
> 
> Switching to FBCGSR however yielded the following results
>  0 KSP unpreconditioned resid norm 8.632122370749e+00 true resid norm
> 8.632122370749e+00 ||r(i)||/||b|| 1.000000000000e+00
>  1 KSP unpreconditioned resid norm 6.155300069200e-05 true resid norm
> 4.783918927770e+05 ||r(i)||/||b|| 5.541996188539e+04
>  2 KSP unpreconditioned resid norm 5.203978464579e-05 true resid norm
> 4.069231018620e+05 ||r(i)||/||b|| 4.714056223773e+04
>  3 KSP unpreconditioned resid norm 4.124211335014e-05 true resid norm
> 8.257465019947e+05 ||r(i)||/||b|| 9.565973077408e+04
>  4 KSP unpreconditioned resid norm 2.735866558072e-05 true resid norm
> 4.745745445186e+05 ||r(i)||/||b|| 5.497773596523e+04
>  5 KSP unpreconditioned resid norm 1.234049073102e-05 true resid norm
> 3.694211077995e+05 ||r(i)||/||b|| 4.279609254050e+04
>  6 KSP unpreconditioned resid norm 7.813287928316e-06 true resid norm
> 3.595303092238e+05 ||r(i)||/||b|| 4.165027947728e+04
>  7 KSP unpreconditioned resid norm 3.221541950651e-06 true resid norm
> 3.179392486707e+05 ||r(i)||/||b|| 3.683210629034e+04
>  8 KSP unpreconditioned resid norm 1.777674826629e-06 true resid norm
> 3.155842168872e+05 ||r(i)||/||b|| 3.655928441846e+04
>  9 KSP unpreconditioned resid norm 1.698816021362e-06 true resid norm
> 3.145809168718e+05 ||r(i)||/||b|| 3.644305575854e+04
> 10 KSP unpreconditioned resid norm 1.577351929557e-06 true resid norm
> 3.259758397382e+05 ||r(i)||/||b|| 3.776311615354e+04
> 11 KSP unpreconditioned resid norm 1.613962504892e-06 true resid norm
> 3.186021802487e+05 ||r(i)||/||b|| 3.690890450398e+04
> 12 KSP unpreconditioned resid norm 4.564619465810e-07 true resid norm
> 3.142082512956e+05 ||r(i)||/||b|| 3.639988380613e+04
> 13 KSP unpreconditioned resid norm 3.413352474598e-07 true resid norm
> 3.136609638739e+05 ||r(i)||/||b|| 3.633648254765e+04
> 14 KSP unpreconditioned resid norm 2.914787162382e-07 true resid norm
> 3.133747605150e+05 ||r(i)||/||b|| 3.630332692884e+04
> 15 KSP unpreconditioned resid norm 3.607032335094e-07 true resid norm
> 3.108398102641e+05 ||r(i)||/||b|| 3.600966215648e+04
> 16 KSP unpreconditioned resid norm 7.653443566257e-08 true resid norm
> 3.124857549918e+05 ||r(i)||/||b|| 3.620033887039e+04
> 
> 
> So it seems to me something weird is going on here. Could you help me?
> 
> Kind regards
> Elias
> 
> 
>