[petsc-users] single precision vs double: strange behavior

[Andreas Mang]

Hey guys:

I was trying to run my code with single precision which resulted in strange behavior. The code essentially already breaks within the test case creation (computing some sinusoidal functions). I discovered that. e.g., norms and max and min values do not make sense. I created a simple test example that demonstrates strange behavior on my machine. I am trying to find the min and max values of a PETSc vector with numbers ranging from 0 to 15. I removed all non-essential compiler flags and dependencies.

If I compile PETSc with double precision I get

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
min 0 at 0
max 15 at 15

If I compile with single precision I get

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
min 15 at 8
max 14 at 7

The difference is that I add "--with-precision=single” to the configure command of PETSc. I use intel/17.0 with mpich2/3.1.4. The code can be found below. I also provide the configure options and the command line for compiling the code. 

Thanks /Andreas


mpicxx -O3 -I./include -isystem/h1/andreas/code/develop/cold/external/libs/petsc_single/build/include -isystem/h1/andreas/code/develop/cold/external/libs/petsc_single/build/cxx_opt/include -c apps/checkcoldreg.cpp -o obj/checkcoldreg.o  

Configure Options: --with-cc=mpicc --CFLAGS= --with-cxx=mpicxx --CXXFLAGS= --download-f2cblaslapack --with-debugging=1 --with-64-bit-indices --with-shared-libraries=0 --with-x=0 --with-fc=0 --with-precision=single

#include <iostream>
#include "petsc.h"

int main(int argc, char **argv) {
    PetscErrorCode ierr;
    PetscScalar maxval, minval;
    PetscInt posmin, posmax;
    Vec x; PetscScalar* p_x = NULL;

    ierr = PetscInitialize(0, reinterpret_cast<char***>(NULL),
                              reinterpret_cast<char*>(NULL),
                              reinterpret_cast<char*>(NULL)); CHKERRQ(ierr);
    PetscInt n = 16;

    ierr = VecCreate(PETSC_COMM_WORLD, &x); CHKERRQ(ierr);
    ierr = VecSetSizes(x, n, n); CHKERRQ(ierr);
    ierr = VecSetFromOptions(x); CHKERRQ(ierr);

    ierr = VecGetArray(x, &p_x); CHKERRQ(ierr);
    for (PetscInt i = 0; i < n; ++i) {
        p_x[i] = static_cast<PetscScalar>(i);
        std::cout << p_x[i] << " ";
    }
    ierr = VecRestoreArray(x, &p_x); CHKERRQ(ierr);

    std::cout << std::endl;

    ierr = VecMin(x, &posmin, &minval); CHKERRQ(ierr);
    ierr = VecMax(x, &posmax, &maxval); CHKERRQ(ierr);

    std::cout << "min " << minval << " at " << posmin << std::endl;
    std::cout << "max " << maxval << " at " << posmax << std::endl;

    ierr = VecDestroy(&x); CHKERRQ(ierr);
    ierr = PetscFinalize(); CHKERRQ(ierr);
    return 0;
}