[petsc-users] reusing matrix created with MatCreateMPIAIJWithSplitArrays

[Barry Smith]

> On Jun 30, 2016, at 2:40 PM, Hassan Raiesi <Hassan.Raiesi at aero.bombardier.com> wrote:
> 
> Hello,
>  
> We are using PETSC in our CFD code, and  noticed that using “MatCreateMPIAIJWithSplitArrays” is almost 60% faster for large problem size (i.e DOF > 725M, using GAMG each time-step only takes 5sec, compared to 8.3 sec when assembling the matrix one row at a time using matsetvaluesblocked()  as recommended). 
>  
> The problem is that the memory usage goes up after each call to MatCreateMPIAIJWithSplitArrays  to update the matrix values. As MatCreateMPIAIJWithSplitArrays is not supposed to copy the values, do we need to call it each time to update the values? We tried to just update the values of the diagonal and off-diagonal part of the arrays passed to “MatCreateMPIAIJWithSplitArrays”, (the sparsity structure is fixed) but it looks like that the values are not updated, what is the proper way to update the values of the matrix created by MatCreateMPIAIJWithSplitArrays?

   Since you have direct access to the two numeric arrays passed to MatCreateMPIAIJWithSplitArrays() you can simply change the values in those locations

   AND THEN immediately CALL MatAssemblyBegin/MatAssemblyEnd() on the matrix; this will increase the the PETSc object state value for the matrix so the matrix routines (and preconditioner) will know you changed the matrix values. If you don't call the MatAssemblyBegin/MatAssemblyEnd() the preconditioner will think the matrix has not been changed so just use its old values as you observed.

   Barry

Of course if you change any nonzero locations in the matrix you need to destroy the matrix and call MatCreateMPIAIJWithSplitArrays() again. 

>  
>  
> Thank you
>  
> Hassan Raiesi,
> Advanced Aerodynamics Department
> Bombardier Aerospace
>  
> hassan.raiesi at aero.bombardier.com
>  
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