> On Sep 16, 2017, at 10:39 PM, Hao Zhang <[email protected]> wrote: > > hi, > > I am using KSPBCGSL method and HYPRE boomeramg precondtioner passed into > KSPsolve for poisson equation as part of incompressible NS. > > I have two questions > Q1 is number of iterations for the same solver. max_iter = 20 for the same > poisson function > > KSPGetIterationNumber() is used to set up max_iter = 20
What do you mean, KSPGetIterationNumber() tells you the current number of iterations. You cannot use it to set the maximum number of iterations. Use KSPSetTolerances() or -ksp_max_it <its> to set the number > > > 0 KSP Residual norm 4.512245447770e-04 > 2 KSP Residual norm 6.396709069731e-07 > 4 KSP Residual norm 1.757784220489e-10 > 6 KSP Residual norm 1.501464709144e-14 > 8 KSP Residual norm 1.376381429122e-18 > 10 KSP Residual norm 1.072650548945e-19 > In poisson_func(): num_iter = 10, rel_residual = 1.072651e-19 > > and the very same function at the next time step gives: remind you guys > max_iter = 20. I assume the number of iterations should stop at num_iter = 2. Why would it stop at num_iter 2. The number of iterations allowed is the same for each solve, it does not accumulate over all solves. > but the solver seems to keep iterating util hitting the max_iter. > 0 KSP Residual norm 5.107005838093e-16 > 2 KSP Residual norm 3.353634821198e-20 > 4 KSP Residual norm 1.835096039266e-23 > 6 KSP Residual norm 1.645496102409e-23 > 8 KSP Residual norm 1.645496099837e-23 > 10 KSP Residual norm 1.645496099836e-23 > 12 KSP Residual norm 1.645496099530e-23 > 14 KSP Residual norm 1.645496034473e-23 > 16 KSP Residual norm 1.645461130961e-23 > 18 KSP Residual norm 1.645460451075e-23 > In poisson_func(): num_iter = 18, rel_residual = 1.645460e-23 But the way it is almost never realistic to try to get residual norms to be this small. You might consider using something like -ksp_atol 1.e-16 or something > > Q2 is: it seems HYPRE with KSPsolve works with coarse mesh but when I double > the mesh size. the solve halted indefinitely. ksp_monitor didn't produce any > results. > is there a way to understand what's happening? You could run in the debugger and use control c to interrupt the code when it is "hanging" and type bt to see the stack trace for the hanging. > > -- > Hao Zhang > Dept. of Applid Mathematics and Statistics, > Stony Brook University, > Stony Brook, New York, 11790
