garbage collection different between petsc4py 3.17.4 and 3.18.3

petsc4py 3.18.3 does not garbage collect as effectively as 3.17.4.

This plot shows memory usage when repeatedly creating and solving a matrix:

This has been observed on macOS, Debian, and Ubuntu.

In both cases, petsc4py was obtained from conda-forge.

This plot was generated by running mprof on the following script in serial (adapted from demo/kspsolve):

from petsc4py import PETSc

def mat_and_vec():
    # grid size and spacing
    m, n  = 32, 32
    hx = 1.0/(m-1)
    hy = 1.0/(n-1)

    # create sparse matrix
    A = PETSc.Mat()
    A.create(PETSc.COMM_WORLD)
    A.setSizes([m*n, m*n])
    A.setType('aij') # sparse
    A.setPreallocationNNZ(5)

    # precompute values for setting
    # diagonal and non-diagonal entries
    diagv = 2.0/hx**2 + 2.0/hy**2
    offdx = -1.0/hx**2
    offdy = -1.0/hy**2

    # loop over owned block of rows on this
    # processor and insert entry values
    Istart, Iend = A.getOwnershipRange()
    for I in range(Istart, Iend) :
        A[I,I] = diagv
        i = I//n    # map row number to
        j = I - i*n # grid coordinates
        if i> 0  : J = I-n; A[I,J] = offdx
        if i< m-1: J = I+n; A[I,J] = offdx
        if j> 0  : J = I-1; A[I,J] = offdy
        if j< n-1: J = I+1; A[I,J] = offdy

    # communicate off-processor values
    # and setup internal data structures
    # for performing parallel operations
    A.assemblyBegin()
    A.assemblyEnd()

    # obtain sol & rhs vectors
    x, b = A.createVecs()
    x.set(0)
    b.set(1)

    return A, x, b

def solve(A, x, b):
    # create linear solver
    ksp = PETSc.KSP()
    ksp.create(PETSc.COMM_WORLD)
    # use conjugate gradients
    ksp.setType('cg')
    # and incomplete Cholesky
    ksp.getPC().setType('icc')
    # and next solve
    ksp.setOperators(A)
    ksp.setFromOptions()
    ksp.solve(b, x)

for sweep in range(50000):
    solve(*mat_and_vec())

Notes

• I am aware that it would be more efficient to retain and repopulate the matrix and vectors, but repeatedly destroying and creating them used to work.
• Manually calling ksp.destroy(), A.destroy(), x.destroy(), and b.destroy() at the end of solve() also resolves the issue, but automatic garbage collection used to work when these variables went out of scope.
• Manually calling PETSc.garbage_cleanup() at each iteration also resolves the issue, but automatic garbage collection used to work.
• -malloc_dump and -malloc_view indicate no leak with either version.
• #1291 may be related.