[petsc-users] Performance of Fieldsplit PC

[Bernardo Rocha]

Hello everyone,

I have a general question about the performance of the PCFieldSplit
that I'm not sure if I understood properly.

Consider a simple Poisson problem discretized by FEM into a system Ax=b
which is then solved by CG and Jacobi.

Then, I create a "vectorial Poisson" problem by simply adding another block
of this problem to create a block-like version of it.
Something like
[ [A, 0]
  [0, A]]
then I create a PCFieldSplit with CG and Jacobi for each block.

Either with additive or multiplicative fieldsplit, the PC is much better
and solves it
with fewer iterations than the scalar case. However, the execution time
taken by
the PCFieldSplit is much bigger than the simple Jacobi for the scalar case.

(From -log_view I see all the time difference in PCApply)

Why is this happening?

Best regards,
Bernardo
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