[petsc-users] Varying TAO optimization solve iterations using BLMVM

[Justin Chang]

I solved a transient diffusion across multiple cores using TAO BLMVM. When
I simulate the same problem but on different numbers of processing cores,
the number of solve iterations change quite drastically. The numerical
solution is the same, but these changes are quite vast. I attached a PDF
showing a comparison between KSP and TAO. KSP remains largely invariant
with number of processors but TAO (with bounded constraints) fluctuates.

My question is, why is this happening? I understand that accumulation of
numerical round-offs may attribute to this, but the differences seem quite
vast to me. My initial thought was that

1) the Hessian is only projected and not explicitly computed, which may
have something to do with the rate of convergence

2) local problem size. Certain regions of my domain have different number
of "violations" which need to be corrected by the bounded constraints so
the rate of convergence depends on how these regions are partitioned?

Any thoughts?

Thanks,
Justin
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