[petsc-dev] DMCreateMatrix_Moab vs. DMCreateMatrix

[Jed Brown]

On Tue, Mar 5, 2013 at 10:03 AM, Tim Tautges <tautges at mcs.anl.gov> wrote:

>   In a previous pull request, Jed suggested "Unless you have grand plans
> for matrices "in MOAB", you should avoid masquerading as a matrix format
> and just implement DMCreateMatrix_MOAB (preferring a BAIJ matrix, it
> seems)."  But, in the documentation for many specific matrix types (I
> checked BAIJ and AIJ), it says "For good matrix assembly performance the
> user should preallocate the matrix storage by setting the parameters d_nz
> (or d_nnz) and o_nz (or o_nnz). By setting these parameters accurately,
> performance can be increased by more than a factor of 50."  So if a matrix
> can be created DMCreateMatrix (which passes just the dm and a Mat*), how
> does one efficiently create this matrix generically without calling the
> type-specific function?  Do you set options for nz and nnz, or are there
> MatSet-type functions for setting these?


See the other DMCreateMatrix_ImplName(). The DM can use
MatMPIBAIJSetPreallocation() (and others for different formats, only the
matching routine is used) or MatXAIJSetPreallocation() to preallocate for
any format that can understand this specification.
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