Re: [petsc-users] strange convergence

Hi Lukasz, thanks for sharing very interesting slide. Both of you are right, the mortar method starts from continuum argument then reduce to discrete space by discretizing the Lagrange multiplier. However, the way to choose the interpolation space has some implication on the properties of the

Re: [petsc-users] strange convergence

On Wed, May 3, 2017 at 2:29 AM, Hoang Giang Bui wrote: > Dear Jed > > If I understood you correctly you suggest to avoid penalty by using the > Lagrange multiplier for the mortar constraint? In this case it leads to the > use of discrete Lagrange multiplier space. > Sorry

Re: [petsc-users] strange convergence

On 3 May 2017, at 08:29, Hoang Giang Bui > wrote: Dear Jed If I understood you correctly you suggest to avoid penalty by using the Lagrange multiplier for the mortar constraint? In this case it leads to the use of discrete Lagrange multiplier

Re: [petsc-users] strange convergence

On Wed, 3 May 2017 at 09:29, Hoang Giang Bui wrote: > Dear Jed > > If I understood you correctly you suggest to avoid penalty by using the > Lagrange multiplier for the mortar constraint? In this case it leads to the > use of discrete Lagrange multiplier space. Do you or

Re: [petsc-users] strange convergence

Thanks Barry Running with that option gives the output for the first solve: BoomerAMG SETUP PARAMETERS: Max levels = 25 Num levels = 7 Strength Threshold = 0.10 Interpolation Truncation Factor = 0.00 Maximum Row Sum Threshold for Dependency Weakening = 0.90 Coarsening Type =

Re: [petsc-users] strange convergence

> On Apr 29, 2017, at 8:34 AM, Jed Brown wrote: > > Hoang Giang Bui writes: > >> Hi Barry >> >> The first block is from a standard solid mechanics discretization based on >> balance of momentum equation. There is some material involved but in >>

Re: [petsc-users] strange convergence

Hoang Giang Bui writes: > Hi Barry > > The first block is from a standard solid mechanics discretization based on > balance of momentum equation. There is some material involved but in > principal it's well-posed elasticity equation with positive definite > tangent operator.

Re: [petsc-users] strange convergence

Hi Barry The first block is from a standard solid mechanics discretization based on balance of momentum equation. There is some material involved but in principal it's well-posed elasticity equation with positive definite tangent operator. The "gluing business" uses the mortar method to keep the

Re: [petsc-users] strange convergence

Ok, so boomerAMG algebraic multigrid is not good for the first block. You mentioned the first block has two things glued together? AMG is fantastic for certain problems but doesn't work for everything. Tell us more about the first block, what PDE it comes from, what discretization, and

Re: [petsc-users] strange convergence

It's in fact quite good Residual norms for fieldsplit_u_ solve. 0 KSP Residual norm 4.014715925568e+00 1 KSP Residual norm 2.160497019264e-10 Residual norms for fieldsplit_wp_ solve. 0 KSP Residual norm 0.e+00 0 KSP preconditioned resid norm 4.014715925568e+00

Re: [petsc-users] strange convergence

Run again using LU on both blocks to see what happens. > On Apr 27, 2017, at 2:14 AM, Hoang Giang Bui wrote: > > I have changed the way to tie the nonconforming mesh. It seems the matrix now > is better > > with -pc_type lu the output is > 0 KSP preconditioned resid

Re: [petsc-users] strange convergence

I have changed the way to tie the nonconforming mesh. It seems the matrix now is better with -pc_type lu the output is 0 KSP preconditioned resid norm 3.308678584240e-01 true resid norm 9.006493082896e+06 ||r(i)||/||b|| 1.e+00 1 KSP preconditioned resid norm 2.004313395301e-12

Re: [petsc-users] strange convergence

This can happen in the matrix is singular or nearly singular or if the factorization generates small pivots, which can occur for even nonsingular problems if the matrix is poorly scaled or just plain nasty. > On Apr 24, 2017, at 5:10 PM, Hoang Giang Bui wrote: > > It

Re: [petsc-users] strange convergence

It took a while, here I send you the output 0 KSP preconditioned resid norm 3.129073545457e+05 true resid norm 9.015150492169e+06 ||r(i)||/||b|| 1.e+00 1 KSP preconditioned resid norm 7.442444222843e-01 true resid norm 1.003356247696e+02 ||r(i)||/||b|| 1.112966720375e-05 2 KSP

Re: [petsc-users] strange convergence

> On Apr 24, 2017, at 12:47 PM, Hoang Giang Bui wrote: > > Good catch. I get this for the very first step, maybe at that time the rhs_w > is zero. With the multiplicative composition the right hand side of the second solve is the initial right hand side of the second

Re: [petsc-users] strange convergence

Good catch. I get this for the very first step, maybe at that time the rhs_w is zero. In the later step, it shows 2 step convergence Residual norms for fieldsplit_u_ solve. 0 KSP Residual norm 3.165886479830e+04 1 KSP Residual norm 2.905922877684e-01 Residual norms for fieldsplit_wp_

Re: [petsc-users] strange convergence

> On Apr 24, 2017, at 3:16 AM, Hoang Giang Bui wrote: > > Thanks Barry, trying with -fieldsplit_u_type lu gives better convergence. I > still used 4 procs though, probably with 1 proc it should also be the same. > > The u block used a Nitsche-type operator to connect two

Re: [petsc-users] strange convergence

Thanks Barry, trying with -fieldsplit_u_type lu gives better convergence. I still used 4 procs though, probably with 1 proc it should also be the same. The u block used a Nitsche-type operator to connect two non-matching domains. I don't think it will leave some rigid body motion leads to not

Re: [petsc-users] strange convergence

> On Apr 23, 2017, at 2:42 PM, Hoang Giang Bui wrote: > > Dear Matt/Barry > > With your options, it results in > > 0 KSP preconditioned resid norm 1.106709687386e+31 true resid norm > 9.015150491938e+06 ||r(i)||/||b|| 1.e+00 > Residual norms for

Re: [petsc-users] strange convergence

Dear Matt/Barry With your options, it results in 0 KSP preconditioned resid norm 1.106709687386e+31 true resid norm 9.015150491938e+06 ||r(i)||/||b|| 1.e+00 Residual norms for fieldsplit_u_ solve. 0 KSP Residual norm 2.407308987203e+36 1 KSP Residual norm

Re: [petsc-users] strange convergence

Huge preconditioned norms but normal unpreconditioned norms almost always come from a very small pivot in an LU or ILU factorization. The first thing to do is monitor the two sub solves. Run with the additional options -fieldsplit_u_ksp_type richardson -fieldsplit_u_ksp_monitor

Re: [petsc-users] strange convergence

On Sun, Apr 23, 2017 at 12:22 PM, Hoang Giang Bui wrote: > Hello > > I encountered a strange convergence behavior that I have trouble to > understand > > KSPSetFromOptions completed > 0 KSP preconditioned resid norm 1.106709687386e+31 true resid norm > 9.015150491938e+06