[petsc-users] MatMPIAIJSetPreallocation question

[Barry Smith]

> On Aug 4, 2017, at 2:52 PM, Xiangdong <epscodes at gmail.com> wrote:
> 
> Hello everyone,
> 
> I have a quick questions on the MatMPIAIJSetPreallocation. If we do not pass the exact number of nonzeros for o_nnz and d_nnz, (for example, MAT_INFO_NZ_ALLOCATED is 50% more than MAT_INFO_NZ_USED), will the performance of MatMult be affected? What effect will these allocated but unused memory cause?
   
   It is not a big deal to have extra memory, it won't slow things down. If you under estimate it can slow things down a lot.

> 
> Thank you.
> 
> Best,
> Xiangdong