Ben,

I'm trying to add a single DoF to my Trilinos system (not associated with the FE system, similar to the question asked here <https://groups.google.com/d/msg/dealii/FpDMRyhsmqI/XqiHH2qZpWEJ>). I followed the advice there, and set it up as a block system, where block (0,0) is the usual FE matrix, and block (1,1) is a 1x1 "matrix". Setting this up to work in parallel with Trilinos was tricky, but I got it working by using the usual partitioning IndexSet and an IndexSet with 1 entry owned by processor 0, which I use to reinit the new matrix blocks (the 1x1 block, and the 1xn and nx1 off-diagonal blocks).

The code I've written works, however, as I've scaled up the system it's become a huge bottleneck.

Can you elaborate? Is it setting up the matrices that is the bottleneck, or solving the linear system?

If it is solving the linear system, have you thought about forming the Schur complement with regard to the 1x1 block? This will just yield a rank-1 update to the other diagonal block.

Best
 W.

--
------------------------------------------------------------------------
Wolfgang Bangerth          email:                 bange...@colostate.edu
                           www: http://www.math.colostate.edu/~bangerth/

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