UMFPACK WARNING singular matrix

General Discussion
1

Hello,

I have compiled the FreeFEM++ code successfully using the source code on main branch, but cannot use it to solve any problem (even the ones in example) without encountering

" UMFPACK WARNING singular matrix "

This seems to only be a problem on a specific machine running Ubuntu 22.04, as I have been able to use FreeFEM++ without a problem on another machine running Ubuntu 20.04. Is there something wrong with my installation?

Thanks in advance,
Boyuan

2

Sorry about the lack of information in the initial post. Both Machines are actually Ubuntu 22.04. The issue is specific to UMFPACK on one of them, but when I select the solver to be GMRES, it returns the expected result. More specifically, using the default solver for an example case gives

-- FreeFem++ v4.13 (Mon Jan 13 11:07:38 AM PST 2025 - git v4.13)
   file : 2D-test.edp
 Load: lg_fem lg_mesh lg_mesh3 eigenvalue 
    1 : // Parameters
    2 :  real theta = 4.*pi/3.;
    3 :  real a = 2.; //The length of the semimajor axis
    4 :  real b = 1.; //The length of the semiminor axis
    5 :  func z = x;
 *** Warning  The identifier z hide a Global identifier  

    6 : 
    7 :  // Mesh
    8 :  border Gamma1(t=0., theta){x=a*cos(t); y=b*sin(t);}
    9 :  border Gamma2(t=theta, 2.*pi){x=a*cos(t); y=b*sin(t);}
   10 :  mesh Th = buildmesh(Gamma1(100) + Gamma2(50));
   11 : 
   12 :  // Fespace
   13 :  fespace Vh(Th, P2); //P2 conforming triangular FEM
   14 :  Vh phi, w, f=1;
   15 : 
   16 :  // Solve
   17 :  solve Laplace(phi, w)
   18 :  = int2d(Th)(
   19 :  dx(phi)*dx(w)
   20 :  + dy(phi)*dy(w)
   21 :  )
   22 :  - int2d(Th)(
   23 :  f*w
   24 :  )
   25 :  + on(Gamma1, phi=z)
   26 :  ;
   27 : 
   28 :  // Plot
   29 :  plot(phi, wait=true, ps="membrane.eps"); //Plot phi
   30 :  plot(Th, wait=true, ps="membraneTh.eps"); //Plot Th
   31 : 
   32 :  // Save mesh
   33 :  savemesh(Th,"Th.msh");
   34 :  sizestack + 1024 =1896  ( 872 )

  --  mesh:  Nb of Triangles =   2568, Nb of Vertices 1360
1 UMFPACK WARNING singular matrix 
1 UMFPACK WARNING singular matrix 
  -- Solve : 
          min -nan  max -nan
  number of required edges : 0
times: compile 0.09037s, execution 2.72271s,  mpirank:0
 CodeAlloc : nb ptr  3866,  size :515232 mpirank: 0
Ok: Normal End

But GMRES returns the expected outcome

-- FreeFem++ v4.13 (Mon Jan 13 11:07:38 AM PST 2025 - git v4.13)
   file : 2D-test.edp
 Load: lg_fem lg_mesh lg_mesh3 eigenvalue 
    1 : // Parameters
    2 :  real theta = 4.*pi/3.;
    3 :  real a = 2.; //The length of the semimajor axis
    4 :  real b = 1.; //The length of the semiminor axis
    5 :  func z = x;
 *** Warning  The identifier z hide a Global identifier  

    6 : 
    7 :  // Mesh
    8 :  border Gamma1(t=0., theta){x=a*cos(t); y=b*sin(t);}
    9 :  border Gamma2(t=theta, 2.*pi){x=a*cos(t); y=b*sin(t);}
   10 :  mesh Th = buildmesh(Gamma1(100) + Gamma2(50));
   11 : 
   12 :  // Fespace
   13 :  fespace Vh(Th, P2); //P2 conforming triangular FEM
   14 :  Vh phi, w, f=1;
   15 : 
   16 :  // Solve
   17 :  solve Laplace(phi, w, solver=GMRES)
   18 :  = int2d(Th)(
   19 :  dx(phi)*dx(w)
   20 :  + dy(phi)*dy(w)
   21 :  )
   22 :  - int2d(Th)(
   23 :  f*w
   24 :  )
   25 :  + on(Gamma1, phi=z)
   26 :  ;
   27 : 
   28 :  // Plot
   29 :  plot(phi, wait=true, ps="membrane.eps"); //Plot phi
   30 :  plot(Th, wait=true, ps="membraneTh.eps"); //Plot Th
   31 : 
   32 :  // Save mesh
   33 :  savemesh(Th,"Th.msh");
   34 :  sizestack + 1024 =1896  ( 872 )

  --  mesh:  Nb of Triangles =   2568, Nb of Vertices 1360
  **  fgmres has converged in 325 iterations The relative residual is 9.93524e-07 Cl: 0.107376
  -- Solve : 
          min -2  max 2
  number of required edges : 0
times: compile 0.22081s, execution 1.77229s,  mpirank:0

This issue is not present on another machine - does anyone has insights on what the issue might be?

3

There is probably an issue with your installation.
I ran the code, and get on both macOS and Linux:

[...]
  --  mesh:  Nb of Triangles =   2568, Nb of Vertices 1360
  -- Solve : 
          min -2  max 2
[...]
4

Do you know what could be the issue? I tried to reinstalling a few times in different ways and nothing changes.

5

Did you install PETSc via make petsc-slepc?