Trilinos is a software package that provides lots of functionality for linear algebra, among other things. For example, it includes implementations of a variety of linear solvers, as well as various different sparse and dense matrix and vector formats. Trilinos also has many subpackages that deal with problems that go far beyond linear algebra, for example nonlinear solvers, automatic differentiation packages, uncertainty propagation engines, etc. Of particular interest to deal.II is their ability to provide this functionality both on sequential and parallel (using MPI) computers. Compared to PETSc, which is written in C, Trilinos is written in C++ and can be considered to be a more modern version of PETSc though both packages are under continuing development at their respective national laboratories.
deal.II has wrapper classes to the linear algebra parts of Trilinos that provide almost the same interfaces as the built-in deal.II linear algebra classes. We use these interfaces for parallel computations based on MPI since the native deal.II linear algebra classes lack this ability. They are used, among other programs, in step-31 and step-32.
While building deal.II with Trilinos is covered in the ReadMe file, we here give an introduction to building Trilinos in such a way that it contains everything that we need from the deal.II side.
Note: The current version of deal.II requires at least Trilinos 11.2. Deal.II is known to work with Trilinos up to 11.14 and 12.4. Others versions of Trilinos should work too but we do not do regression tests with them.
deal.II uses the following libraries from Trilinos and will fail to compile with Trilinos if they are not present:
    cd trilinos-12.4.2
    mkdir build
    cd build
    cmake                                            \
    -DTrilinos_ENABLE_Amesos=ON                      \
    -DTrilinos_ENABLE_Epetra=ON                      \
    -DTrilinos_ENABLE_Ifpack=ON                      \
    -DTrilinos_ENABLE_AztecOO=ON                     \
    -DTrilinos_ENABLE_Sacado=ON                      \
    -DTrilinos_ENABLE_Teuchos=ON                     \
    -DTrilinos_ENABLE_MueLu=ON                       \
    -DTrilinos_ENABLE_ML=ON                          \
    -DTrilinos_VERBOSE_CONFIGURE=OFF                 \
    -DTPL_ENABLE_MPI=ON                              \
    -DBUILD_SHARED_LIBS=ON                           \
    -DCMAKE_VERBOSE_MAKEFILE=OFF                     \
    -DCMAKE_BUILD_TYPE=RELEASE                       \
    -DCMAKE_INSTALL_PREFIX:PATH=$HOME/share/trilinos \
    ../
    make install
      
      You will need to adjust the path into which you want to install Trilinos
      in the CMAKE_INSTALL_PREFIX line. If you do not have MPI installed you
      should use -DTPL_ENABLE_MPI=OFF instead. Additionally, if
      your computer has enough memory available, it may also be useful to pass
      the flag -DTrilinos_ENABLE_EXPLICIT_INSTANTIATION=ON, which
      will improve compilation times of deal.II programs that use Trilinos.
    
    
      If your computer has more than one processor core, use
      make -jN instead of make in the last line
      above, where N is the number of processors you have.
    
      Note: At the time of writing (November 2016) Trilinos (at least up to 
      v12.8.1) cannot link against LAPACK 3.6.0 or later, due to two symbols 
      that were deprecated (and removed by default) in LAPACK 3.6.0 (see the 
      
      release notes). To fix this, edit the Trilinos file 
      packages/epetra/src/Epetra_LAPACK_wrappers.h and change the 
      lines
      
    
    #define DGGSVD_F77  F77_BLAS_MANGLE(dggsvd,DGGSVD)
    #define SGGSVD_F77  F77_BLAS_MANGLE(sggsvd,SGGSVD)
      
      to
      
    
    #define DGGSVD_F77  F77_BLAS_MANGLE(dggsvd3,DGGSVD3)
    #define SGGSVD_F77  F77_BLAS_MANGLE(sggsvd3,SGGSVD3)
      
      before installing Trilinos. (Credit for this fix goes to 
      this page.)
    
    
    
      Trilinos sometimes searches for other libraries but can't find
      them if they are not in the usual directories or have other
      names. A common example are BLAS or LAPACK. In a case like
      this, you may have to specifically pass the directories and/or
      library names under which they can be found
      to cmake. For example, this may mean to add the
      following flags to the call above:
      
	-DBLAS_LIBRARY_NAMES:STRING=goto \
	-DBLAS_LIBRARY_DIRS:STRING=/apps/GotoBLAS/lib64 \
	-DLAPACK_LIBRARY_NAMES:STRING=lapack \
	-DLAPACK_LIBRARY_DIRS:STRING=/apps/lapack-3.2.1/lib64
      
    
    
      Trilinos (via its Amesos package) can interface with a number of direct
      solvers (see, for example,
      this page for Trilinos 11.8). Most of them are external
      packages to Trilinos and you will need to tell Trilinos configuration
      scripts that you want to use them, for example via the 
      TrilinosWrappers::SolverDirect class.  This can be tricky,
      but adding defines similar to the following to the cmake command line
      will achieve the goal to enable the UMFPACK and SuperLU/SuperLUDist
      solvers:
      
        -DTPL_ENABLE_UMFPACK:BOOL=ON \
        -DTPL_ENABLE_SuperLU:BOOL=ON \
        -DTPL_ENABLE_SuperLUDist:BOOL=ON \
        -DTPL_UMFPACK_INCLUDE_DIRS="/usr/include" \
        -DSuperLUDist_INCLUDE_DIRS:FILEPATH="/path/to/SuperLU_DIST_3.2/SRC" \
        -DTPL_SuperLUDist_LIBRARIES:FILEPATH="/path/to/SuperLU_DIST_3.2/lib/libsuperlu_dist.a" \
        -DSuperLU_INCLUDE_DIRS:FILEPATH="/path/to/SuperLU_4.3/SRC" \
        -DTPL_SuperLU_LIBRARIES:FILEPATH="/path/to/SuperLU_4.3/lib/libsuperlu_4.3.a"
      
      Similarly, to enable MUMPS, commands should include
      
        -DTPL_ENABLE_MUMPS:BOOL=ON \
        -DTPL_ENABLE_SCALAPACK:BOOL=ON
      
      and possibly followed by
      
        -DTPL_MUMPS_INCLUDE_DIRS:PATH=/usr/include/openmpi-x86_64 \
        -DSCALAPACK_LIBRARY_DIRS:PATH=/lib64/openmpi/lib \
      
      where you need to adjust the exact paths, of course.