[petsc-users] reusing LU factorization?

[Barry Smith]

On Jan 28, 2014, at 5:39 PM, Matthew Knepley <knepley at gmail.com> wrote:

> On Tue, Jan 28, 2014 at 5:25 PM, Tabrez Ali <stali at geology.wisc.edu> 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<daveliu at mit.edu>  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