# PETSc basics

$\newcommand\inv{^{-1}}\newcommand\invt{^{-t}} \newcommand\bbP{\mathbb{P}} \newcommand\bbR{\mathbb{R}} \newcommand\defined{ \mathrel{\lower 5pt \hbox{{\equiv\atop\mathrm{\scriptstyle D}}}}}$ 31.1 : What is PETSc and why?
31.1.1 : What is in PETSc?
31.1.2 : Programming model
31.1.3 : Design philosophy
31.1.4 : Language support
31.1.4.1 : C/C++
31.1.4.2 : Fortran
31.1.4.3 : Python
31.1.5 : Documentation
31.2 : Basics of running a PETSc program
31.2.1 : Compilation
31.2.2 : Running
31.2.3 : Initialization and finalization
31.3 : PETSc installation
31.3.1 : Debug
31.3.2 : Environment options
31.3.3 : Variants
31.3.4 : External packages

# 31 PETSc basics

## 31.1 What is PETSc and why?

crumb trail: > petsc-design > What is PETSc and why?

PETSc is a library with a great many uses, but for now let's say that it's primarily a library for dealing with the sort of linear algebra that comes from discretized PDEs . On a single processor, the basics of such computations can be coded out by a grad student during a semester course in numerical analysis, but on large scale issues get much more complicated and a library becomes indispensible.

PETSc's prime justification is then that it helps you realize scientific computations at large scales, meaning large problem sizes on large numbers of processors.

There are two points to emphasize here:

• Linear algebra with dense matrices is relatively simple to formulate. For sparse matrices the amount of logistics in dealing with nonzero patterns increases greatly. PETSc does most of that for you.
• Linear algebra on a single processor, even a multicore one, is managable; distributed memory parallelism is much harder, and distributed memory sparse linear algebra operations are doubly so. Using PETSc will save you many, many, Many! hours of coding over developing everything yourself from scratch.

Remark

The PETSc library has hundreds of routines. In this chapter and the next few we will only touch on a basic subset of these. The full list of man pages can be found at https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/singleindex.html . Each man page comes with links to related routines, as well as (usually) example codes for that routine.

### 31.1.1 What is in PETSc?

crumb trail: > petsc-design > What is PETSc and why? > What is in PETSc?

The routines in PETSc (of which there are hundreds) can roughly be divided in these classes:

• Basic linear algebra tools: dense and sparse matrices, both sequential and parallel, their construction and simple operations.
• Solvers for linear systems, and to a lesser extent nonlinear systems; also time-stepping methods.
• Profiling and tracing: after a successful run, timing for various routines can be given. In case of failure, there are traceback and memory tracing facilities.

### 31.1.2 Programming model

crumb trail: > petsc-design > What is PETSc and why? > Programming model

PETSc, being based on MPI, uses the SPMD programming model (section~ 2.1 ), where all processes execute the same executable. Even more than in regular MPI codes, this makes sense here, since most PETSc objects are collectively created on some communicator, often MPI_COMM_WORLD . With the object-oriented design (section~ 31.1.3 ) this means that a PETSc program almost looks like a sequential program.

MatMult(A,x,y);      // y <- Ax
VecCopy(y,res);      // r <- y
VecAXPY(res,-1.,b);  // r <- r - b


This is sometimes called sequential semantics .

### 31.1.3 Design philosophy

crumb trail: > petsc-design > What is PETSc and why? > Design philosophy

PETSc has an object-oriented design, even though it is written in C. There are classes of objects, such \clstinline{Mat} for matrices and \clstinline{Vec} for Vectors, but there is also the \clstinline{KSP} (for "Krylov SPace solver") class of linear system solvers, and \clstinline{PetscViewer} for outputting matrices and vectors to screen or file.

Part of the object-oriented design is the polymorphism of objects: after you have created a \clstinline{Mat} matrix as sparse or dense, all methods such as MatMult (for the matrix-vector product) take the same arguments: the matrix, and an input and output vector.

This design where the programmer manipulates a handle' also means that the internal of the object, the actual storage of the elements, is hidden from the programmer. This hiding goes so far that even filling in elements is not done directly but through function calls:

VecSetValue(i,j,v,mode)
MatSetValue(i,j,v,mode)
MatSetValues(ni,is,nj,js,v,mode)


### 31.1.4 Language support

crumb trail: > petsc-design > What is PETSc and why? > Language support

#### 31.1.4.1 C/C++

crumb trail: > petsc-design > What is PETSc and why? > Language support > C/C++

PETSc is implemented in C, so there is a natural interface to C. There is no separate C++ interface.

#### 31.1.4.2 Fortran

crumb trail: > petsc-design > What is PETSc and why? > Language support > Fortran

Fortran90 interface exists. The Fortran77 interface is only of interest for historical reasons.

To use Fortran, include both a module and a cpp header file:

#include "petsc/finclude/petscXXX.h"
use petscXXX


(here XXX stands for one of the PETSc types, but including \flstinline{petsc.h} and using \flstinline{use petsc} gives inclusion of the whole library.)

Variables can be declared with their type (\clstinline{Vec}, \clstinline{Mat}, \clstinline{KSP} et cetera), but internally they are Fortran \clstinline{Type} objects so they can be declared as such.

Example:

#include "petsc/finclude/petscvec.h"
use petscvec
Vec b
type(tVec) x


The output arguments of many query routines are optional in PETSc. While in C a generic NULL can be passed, Fortran has type-specific nulls, such as

#### 31.1.4.3 Python

crumb trail: > petsc-design > What is PETSc and why? > Language support > Python

A python interface was written by Lisandro Dalcin, and requires separate installation, based on already defined PETSC_DIR and PETSC_ARCH variables. This can be downloaded at https://bitbucket.org/petsc/petsc4py/src/master/ , with documentation at https://www.mcs.anl.gov/petsc/petsc4py-current/docs/ .

### 31.1.5 Documentation

crumb trail: > petsc-design > What is PETSc and why? > Documentation

PETSc comes with a manual in pdf form and web pages with the documentation for every routine. The starting point is the web page https://www.mcs.anl.gov/petsc/documentation/index.html .

There is also a mailing list with excellent support for questions and bug reports.

TACC note

For questions specific to using PETSc on TACC resources, submit tickets to the TACC or XSEDE portal .

## 31.2 Basics of running a PETSc program

crumb trail: > petsc-design > Basics of running a PETSc program

### 31.2.1 Compilation

crumb trail: > petsc-design > Basics of running a PETSc program > Compilation

A PETSc compilation needs a number of include and library paths, probably too many to specify interactively. The easiest solution is to create a makefile:

include ${PETSC_DIR}/lib/petsc/conf/variables include${PETSC_DIR}/lib/petsc/conf/rules
program : program.o
${CLINKER} -o$@ $^${PETSC_LIB}


The two include lines provide the compilation rule and the library variable.

If you want to write your own compile rule, there is an example makefile $PETSC_DIR/share/petsc/Makefile.user you can take for inspiration. Invoked without arguments it prints out the relevant variables: [c:246] make -f !$PETSC_DIR/share/petsc/Makefile.user
CC=/Users/eijkhout/Installation/petsc/petsc-3.13/macx-clang-debug/bin/mpicc
CXX=/Users/eijkhout/Installation/petsc/petsc-3.13/macx-clang-debug/bin/mpicxx
FC=/Users/eijkhout/Installation/petsc/petsc-3.13/macx-clang-debug/bin/mpif90
CFLAGS=-Wall -Wwrite-strings -Wno-strict-aliasing -Wno-unknown-pragmas -fstack-protector -Qunused-arguments -fvisibility=hidden -g3
CXXFLAGS=-Wall -Wwrite-strings -Wno-strict-aliasing -Wno-unknown-pragmas -fstack-protector -fvisibility=hidden -g
FFLAGS=-m64 -g
CPPFLAGS=-I/Users/eijkhout/Installation/petsc/petsc-3.13/macx-clang-debug/include -I/Users/eijkhout/Installation/petsc/petsc-3.13/include
LDFLAGS=-L/Users/eijkhout/Installation/petsc/petsc-3.13/macx-clang-debug/lib -Wl,-rpath,/Users/eijkhout/Installation/petsc/petsc-3.13/macx-clang-debug/lib
LDLIBS=-lpetsc -lm


You can use these rules:

% : %.F90
$(LINK.F) -o$@ $^$(LDLIBS)
%.o: %.F90
$(COMPILE.F)$(OUTPUT_OPTION) $< % : %.cxx$(LINK.cc) -o $@$^ $(LDLIBS) %.o: %.cxx$(COMPILE.cc) $(OUTPUT_OPTION)$<

# app : a.o b.o c.o
#       $(LINK.F) -o$@ $^$(LDLIBS)


(The PETSC_CC_INCLUDES variable contains all paths for compilation of C programs; correspondingly there is PETSC_FC_INCLUDES for Fortran source.)

If don't want to include those configuration files, you can find out the include options by:

cd $PETSC_DIR make getincludedirs  and copying the results into your compilation script. The build process assumes that variables PETSC_DIR and PETSC_ARCH have been set. These depend on your local installation. Usually there will be one installation with debug settings and one with production settings. Develop your code with the former: it will do memory and bound checking. Then recompile and run your code with the optimized production installation. TACC note On TACC clusters, a petsc installation is loaded by commands such as module load petsc/3.11  Use module avail petsc to see what configurations exist. The basic versions are # development module load petsc/3.11-debug # production module load petsc/3.11  Other installations are real versus complex, or 64bit integers instead of the default 32. The command module spider petsc  tells you all the available petsc versions. The listed modules have a naming convention such as petsc/3.11-i64debug where the 3.11 is the PETSc release (minor patches are not included in this version; TACC aims to install only the latest patch, but generally several versions are available), and i64debug describes the debug version of the installation with 64bit integers. ### 31.2.2 Running crumb trail: > petsc-design > Basics of running a PETSc program > Running PETSc programs use MPI for parallelism, so they are started like any other MPI program: mpiexec -n 5 -machinefile mf \ your_petsc_program option1 option2 option3  TACC note On TACC clusters, use ibrun . ### 31.2.3 Initialization and finalization PETSc has an call that initializes both PETSc and MPI, so normally you would replace MPI_Init by C: PetscErrorCode PetscInitialize (int *argc,char ***args,const char file[],const char help[]) Input Parameters: argc - count of number of command line arguments args - the command line arguments file - [optional] PETSc database file. help - [optional] Help message to print, use NULL for no message Fortran: call PetscInitialize(file,ierr) Input parameters: ierr - error return code file - [optional] PETSc database file, use PETSC_NULL_CHARACTER to not check for code specific file.  PetscInitialize . Unlike with MPI, you do not want to use a NULL value for the argc,argv arguments, since PETSc makes extensive use of commandline options; see section 38.3 . // init.c ierr = PetscInitialize(&argc,&argv,(char*)0,help); CHKERRQ(ierr); int flag; MPI_Initialized(&flag); if (flag) printf("MPI was initialized by PETSc\n"); else printf("MPI not yet initialized\n");  There are two further arguments to PetscInitialize : 1. the name of an options database file; and 2. a help string, that is displayed if you run your program with the -h option. Fortran note • The Fortran version has no arguments for commandline options; it also doesn't take a help string. • If no help string is passed, give • If your main program is in C, but some of your PETSc calls are in Fortran files, it is necessary to call PetscInitializeFortran after PetscInitialize . // init.F90 call PetscInitialize(PETSC_NULL_CHARACTER,ierr) CHKERRA(ierr) call MPI_Initialized(flag,ierr) CHKERRA(ierr) if (flag) then print *,"MPI was initialized by PETSc"  Python note The following works if you don't need commandline options. from petsc4py import PETSc  To pass commandline arguments to PETSc, do: import sys from petsc4py import init init(sys.argv) from petsc4py import PETSc  After initialization, you can use MPI_COMM_WORLD or PETSC_COMM_WORLD (which is created by MPI_Comm_dup and used internally by PETSc): MPI_Comm comm = PETSC_COMM_WORLD; MPI_Comm_rank(comm,&mytid); MPI_Comm_size(comm,&ntids);  Python note comm = PETSc.COMM_WORLD nprocs = comm.getSize(self) procno = comm.getRank(self)  The corresponding call to replace MPI_Finalize is PetscFinalize . You can elegantly capture and return the error code by the idiom return PetscFinalize();  at the end of your main program. ## 31.3 PETSc installation crumb trail: > petsc-design > PETSc installation PETSc has a large number of installation options. These can roughly be divided into: 1. Options to describe the environment in which PETSc is being installed, such as the names of the compilers or the location of the MPI library; 2. Options to specify the type of PETSc installation: real versus complex, 32 versus 64-bit integers, et cetera; 3. Options to specify additional packages to download. For an existing installation, you can find the options used, and other aspects of the build history, in the configure.log / make.log files : $PETSC_DIR/$PETSC_ARCH/lib/petsc/conf/configure.log$PETSC_DIR/$PETSC_ARCH/lib/petsc/conf/make.log  ### 31.3.1 Debug crumb trail: > petsc-design > PETSc installation > Debug For any set of options, you will typically make two installations: one with -with-debugging=yes and once no . See section 38.1.1 for more detail. ### 31.3.2 Environment options crumb trail: > petsc-design > PETSc installation > Environment options Compilers, compiler options, MPI. While it is possible to specify download_mpich , this should only be done on machines that you are certain do not already have an MPI library, such as your personal laptop. Supercomputer clusters are likely to have an optimized MPI library, and letting PETSc download its own will lead to degraded performance. ### 31.3.3 Variants crumb trail: > petsc-design > PETSc installation > Variants • Scalars: the option with-scalar-type has values real , complex ; with-precision has values single , double , __float128 , __fp16 . ### 31.3.4 External packages crumb trail: > petsc-design > PETSc installation > External packages PETSc can extend its functionality through external packages such as mumps , Hypre , fftw . These can be specified in two ways: 1. Referring to an installation already on your system: --with-hdf5-include=${TACC_HDF5_INC}
--with-hf5_lib=\${TACC_HDF5_LIB}

--with-parmetis=1 --download-parmetis=1