[petsc-users] MatMPIAIJSetPreallocation question

Barry Smith bsmith at mcs.anl.gov
Fri Aug 4 15:52:12 CDT 2017

> 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 

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