MueLu: SchurComplement unit tests depend on the nonzero matrix structure
Created by: aprokop
@trilinos/muelu
Schur complement by its nature depends on the result of several matrix-matrix products. Matrix-matrix products for C = A*B in Tpetra at the moment are optimized in the following way: if an entry in A is zero, we skip it and skip fetching a row of B. Thus, the graph structure of the resulting matrix is different from the case when one does not skip zeros, though the numerically the matrix is the same.
One of the SchurComplement tests check the number of entries in the final SchurComplement. Thus, the test depend on whether Tpetra skips zeros or not in the matrix-matrix multiply.
This does not seem like an important issue. I don't think we will be moving from skip-zeros default in Tpetra because it has a lot of benefits. I only got failures in SchurComplement tests because I switched off zero skipping due to running reuse scenarios.