Repeatedly creating a PETSc matrix fills up memory and kills process

Hello,

I must solve repeatedly a system arising from two matrices from a fem formulation. The system matrix is just a linear combination of both matrices. However, when I use the 'PETSc' backend I find that my process gets killed after a few iterations in my for loop. I run top and I see python is filling all my memory (up to 4 GB). This does not happen when I use 'uBLAS', for example. It behaves as if the matrix created inside the loop is not destroyed when the new assignment is made in the following iteration of the loop. I have tried to destroy it manually, but I have no clue.

Thank you very much for your help.

MINIMAL CODE SCRIPT:

from dolfin import *

parameters["linear_algebra_backend"] = 'PETSc' #'PETSc', 'uBLAS', 'Epetra', or 'STL'

Element = "Nedelec 2nd kind H(curl)"
Order = 2

mesh = UnitCubeMesh(15,15,15)

V = FunctionSpace(mesh, Element, Order)
v = TestFunction(V)
u = TrialFunction(V)
a = dot(v, u)*dx
b = dot(v, u)*dx
A=assemble(a)
B=assemble(b)

ka=1.0
kb=1.0

print "Number of dofs: ", A.size(0)

for i in range(0,100):
    #C=A #this is assignment!-->not valid
    #C=A.copy() #another option if G is constructed outside loop
    C=Matrix(A)
    C*=ka
    C.axpy(kb,B,True)
    print "Iteration n. ", i

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