[petsc-users] SNESVI convergence spped

[Ataollah Mesgarnejad]

Dear all,

I'm trying to use SNESVI to solve a quadratic problem with box constraints. My problem in FE context reads:

(\int_{Omega} E phi_i phi_j + \alpha \epsilon dphi_i dphi_j dx) V_i - (\int_{Omega} \alpha \frac{phi_j}{\epsilon} dx) = 0 , 0<= V <= 1

or:

[A]{V}-{b}={0}

here phi is the basis function, E and \alpha are positive constants, and \epsilon is a positive regularization parameter  in order of mesh resolution. In this problem we expect V  =1 a.e. and go to zero very fast at some places. 
I'm running this on a rather small problem (<500000 DOFS) on small number of processors (<72). I expected SNESVI to converge in couple of iterations (<10) since my A matrix doesn't change, however I'm experiencing a slow convergence (~50-70 iterations). I checked KSP solver for SNES and it converges with a few iterations. 

I would appreciate  any suggestions or observations to increase the convergence speed?  

Best,
Ata