PETSc's install and use of fftw always requires MPI.
The code in config/BuildSystem/config/packages/fftw.py and src/mat/impls/fft/fftw/fftw.c could relatively easily be "fixed" to also support building and using FFTW without MPI. Just change the fftw.py to conditionally pass in args.append('--enable-mpi') if self.mpi.found and change self.deps = [self.mpi,self.blasLapack] to self.deps = [self.blasLapack] self.odeps = [self.mpi] and in fftw.c add #if defined(PETSC_HAVE_MPI) around the parts of the code that depend on MPI. We'd welcome a merge request with these enhancements. Barry > On Nov 25, 2021, at 5:15 AM, Fabio Rossi via petsc-users > <petsc-users@mcs.anl.gov> wrote: > > I am trying to compile petsc with the following options (Gentoo build system): > > ./configure --prefix=/usr --build=x86_64-pc-linux-gnu > --host=x86_64-pc-linux-gnu --mandir=/usr/share/man --infodir=/usr/share/info > --datadir=/usr/share --sysconfdir=/etc --localstatedir=/var/lib > --libdir=/usr/lib64/petsc/lib64 scrollOutput=1 FFLAGS=-O2 -pipe -fPIC > CFLAGS=-march=core2 -O2 -pipe -fPIC CXXFLAGS=-march=core2 -O2 -pipe -fPIC > LDFLAGS=-Wl,-O1 -Wl,--as-needed --prefix=/usr/lib64/petsc > --with-shared-libraries --with-single-library --with-clanguage=c > --with-petsc-arch=linux-gnu-c-opt --with-precision=double > --with-gnu-compilers --with-blas-lapack-lib=-lblas -llapack > --with-debugging=0 --with-mpi=0 --with-cc=x86_64-pc-linux-gnu-gcc > --with-cxx=x86_64-pc-linux-gnu-g++ --with-fortran=1 > --with-fc=x86_64-pc-linux-gnu-gfortran --with-mpi-compilers=0 > --with-scalar-type=real --with-windows-graphics=0 --with-matlab=0 > --with-cmake:BOOL=1 --with-pthread=1 --with-afterimage=0 --without-hdf5 > --with-hypre=0 --without-suitesparse --with-superlu=0 --with-x --with-x11 > --with-ptscotch=0 > --with- > scalapack=0 --without-mumps --with-imagemagick=0 --with-python=0 --with-boost > --with-fftw > > so MPI support is disabled, and I get the following configure error: > > TESTING: consistencyChecks from > config.packages.fftw(/var/tmp/portage/sci-mathematics/petsc-3.15.0/work/petsc-3.15.0/config/BuildSystem/config/package.py:963) > TESTING: checkDependencies from > config.packages.fftw(/var/tmp/portage/sci-mathematics/petsc-3.15.0/work/petsc-3.15.0/config/BuildSystem/config/package.py:872) > ******************************************************************************* > UNABLE to CONFIGURE with GIVEN OPTIONS (see configure.log for > details): > ------------------------------------------------------------------------------- > Did not find package MPI needed by fftw. > Enable the package using --with-mpi > > Isn't MPI optional? Where is the error coming from? > > If I disable fftw then it compiles successfully. > > Fabio