[petsc-users] questions about matrix preallocation

[Dominik Szczerba]

This all was very useful for me to know, thank you. Dominik

On Thu, Nov 24, 2011 at 3:18 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:
> On Thu, Nov 24, 2011 at 07:46, Dominik Szczerba <dominik at itis.ethz.ch>
> wrote:
>>
>> Yes, that is true, but 1) within the overshot preallocation margin 2)
>> I do it at each execution of the loop, so I do not see why only the
>> second time there are mallocs. I display mallocs directly before
>> KSPSolve().
>
> The matrix is compacted. The implementation
> (MatSeqAIJSetPreallocation_SeqAIJ) could be changed to analyze the newly
> prescribed preallocation relative to the old allocation and determine that
> it can fit within the old storage. The problem here is that someone might
> specifically preallocate smaller because they want to free the old storage.
> So maybe we should just recognize an exact match, but what if the user
> called MatCreateMPIAIJWithSplitArrays() (so they still own the arrays) and
> they are calling this preallocation routine because they want to reuse the
> arrays for something else?
> If we can settle on some understandable semantics for when freeing and
> reallocating makes sense, then we can put it in.
> Or you could insert zeros into those locations that might be used later.
>
>>
>> > I suggest assembling all possible entries (even the ones that happen to
>> > be
>> > zero) in the first assembly. Then there will be space and you can reuse
>> > the
>> > data structure.
>>
>> This will require a bigger change in my code. Is always calling
>> MatMPIAIJSetPreallocation a bad (inefficient) alternative?
>
> Depends on the availability of memory, but probably not. Always profile.