Hello Kev, just to make sure: did you compile the deal.II library and your code in Optimized mode/Release mode <https://www.dealii.org/current/readme.html#configuration> ?
Marc On Thursday, December 8, 2022 at 2:35:22 PM UTC-7 Wolfgang Bangerth wrote: > > Kev, > > > I've done some layman's benchmarking of the individual "steps" (setup, > > assembly, solve, ...) in my current version of the code. It looks as if > the > > assembly takes several orders of magnitude (~100 at least) longer than > the > > solving part. > > > > My question is now: What is the best strategy to speed up assembly, is > there > > any experience with this? I've read different approaches and am confused > > what's promising for small-scale problems. So far I'm considering: > > > > 1) Using a matrix-free approach rather than PETSc - this seems to be a > win in > > most cases, would however consider rewriting large parts of the code > and I am > > not sure if I will gain a lot given my small system size. > > > > 2) Only assemble the Jacobian every few steps, but the residual in every > step. > > This is probably easier to implement. I know from experience with my > problem > > that I pretty quickly land in a situation where I need only one or two > Newton > > steps to find the solution to my nonlinear equation, so there saving > will be > > small at best. > > Like Bruno, it seems rather unlikely to me that assembly would take 100x > times > longer than other things, and I would suggest you use something like > TimerOutput to time individual sections to narrow down where the issue > lies. > > Beyond this, only updating the Jacobian is indeed a fairly usual strategy. > One > can of course implement this by hand, but it's quite cumbersome to > implement > optimal algorithms to determine when updating is necessary, and I would > encourage you to take a look at step-77 to see how one can solve nonlinear > problems efficiently through interfaces to advanced libraries such as > SUNDIALS: > https://dealii.org/developer/doxygen/deal.II/step_77.html > > I don't remember if KINSOL has ways to solve a sequence of nonlinear > system, > re-using the Jacobian between systems. But if it does not, you can always > just > store a pointer to the last Jacobian matrix and whenever you start solving > a > linear system for the first time, copy the stored matrix over. > > In any case, I think the first step should be to look at (i) whether > assembly > really takes that long for you, (ii) understand why it takes that long. If > step-77 is any indication, then assembling the Jacobian should really not > take > that much longer than actually solving linear systems. > > Best > W. > > > -- > ------------------------------------------------------------------------ > Wolfgang Bangerth email: [email protected] > www: http://www.math.colostate.edu/~bangerth/ > > > -- The deal.II project is located at http://www.dealii.org/ For mailing list/forum options, see https://groups.google.com/d/forum/dealii?hl=en --- You received this message because you are subscribed to the Google Groups "deal.II User Group" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/dealii/5bb618cf-52ce-4871-944b-aeb56d1b4c50n%40googlegroups.com.
