tree ef2e427daf88c697171e4e580b31b8b77b02a36b
parent d28b3c86d36088aabc2331b8bab8dc15fb4f0eaa
author Sameer Agarwal <sameeragarwal@google.com> 1338963059 -0700
committer Sameer Agarwal <sameeragarwal@google.com> 1339045337 -0700

Block oriented fill reducing orderings.

By virtue of the modeling layer in Ceres being block oriented,
all the matrices used by Ceres are also block oriented.
When doing sparse direct factorization of these matrices, the
fill-reducing ordering algorithms can either be run on the
block or the scalar form of these matrices. Running it on the
block form exposes more of the super-nodal structure of the
matrix to the Cholesky factorization routines. This leads to
substantial gains in factorization performance.

This changelist adds support for approximate minimium degree
orderings to be computed on the block structure of the
Schur complement matrix. This affects, SchurComplementSolver
and VisibilityBasedPreconditioner and SparseNormalCholesky
 when using SuiteSparse.

A bool, use_block_amd has been added to Solver::Options and
bundle_adjuster.cc has been updated to allow testing with it.

When combined with a multithreaded Schur elimination, speed ups
can be seen quite uniformly across the board. For some problems
this can be dramatic, reducing the factorization time from 70
seconds down to 17 seconds.

Change-Id: I15ebb0afcbc85ada032ec8d179ee3a2f7c8d3e46