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. 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
