On Jan 28, 2014, at 5:39 PM, Matthew Knepley <[email protected]> wrote:
> On Tue, Jan 28, 2014 at 5:25 PM, Tabrez Ali <[email protected]> wrote: > Hello > > This is my observation as well (with MUMPS). The first solve (after assembly > which is super fast) takes a few mins (for ~1 million unknowns on 12/24 > cores) but from then on only a few seconds for each subsequent solve for each > time step. > > Perhaps symbolic factorization in MUMPS is all serial? > > Yes, it is. I missed this. I was just assuming a PETSc LU. Yes, I have no idea of relative time of symbolic and numeric for those other packages. Barry > > Matt > > Like the OP I often do multiple runs on the same problem but I dont know if > MUMPS or any other direct solver can save the symbolic factorization info to > a file that perhaps can be utilized in subsequent reruns to avoid the costly > "first solves". > > Tabrez > > > On 01/28/2014 04:04 PM, Barry Smith wrote: > On Jan 28, 2014, at 1:36 PM, David Liu<[email protected]> wrote: > > Hi, I'm writing an application that solves a sparse matrix many times using > Pastix. I notice that the first solves takes a very long time, > Is it the first “solve” or the first time you put values into that matrix > that “takes a long time”? If you are not properly preallocating the matrix > then the initial setting of values will be slow and waste memory. See > http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Mat/MatXAIJSetPreallocation.html > > The symbolic factorization is usually much faster than a numeric > factorization so that is not the cause of the slow “first solve”. > > Barry > > > > while the subsequent solves are very fast. I don't fully understand what's > going on behind the curtains, but I'm guessing it's because the very first > solve has to read in the non-zero structure for the LU factorization, while > the subsequent solves are faster because the nonzero structure doesn't change. > > My question is, is there any way to save the information obtained from the > very first solve, so that the next time I run the application, the very first > solve can be fast too (provided that I still have the same nonzero structure)? > > > -- > No one trusts a model except the one who wrote it; Everyone trusts an > observation except the one who made it- Harlow Shapley > > > > > -- > What most experimenters take for granted before they begin their experiments > is infinitely more interesting than any results to which their experiments > lead. > -- Norbert Wiener
