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

[Barry Smith]

  Note that PetscScalar maxval, minval; should really be PetscReal maxval, minval; for it to work if compiled for complex.

 
> On Mar 18, 2017, at 10:23 AM, Andreas Mang <andreas at ices.utexas.edu> wrote:
> 
> Jed, you are correct. I’m an idiot. I linked against the wrong library. Next time I’ll sleep before I send you an email. Sorry for wasting your time. Have a great weekend. /Andreas 
> 
>> On Mar 18, 2017, at 9:45 AM, Jed Brown <jed at jedbrown.org> wrote:
>> 
>> I can't reproduce this result.  It looks like something that could
>> happen by mixing up the headers used to compile with the library used to
>> link/execute.
>> 
>> Andreas Mang <andreas at ices.utexas.edu> writes:
>> 
>>> 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;
>>> }
>