scikit-fem: InteriorBasis slow for Hex1

While working with MeshHex on #279 and #281, I noticed that InteriorBasis seemed slow for MeshHex and ElementHex1.

I solved a Laplace equation (like ex13) and this was the bottleneck; e.g. for mesh.p.shape[1] == basis.N == 34001:

• load: 0.6 s
• InteriorBasis: 82.5 s
• asm: 8.2s
• preconditioner: 0.05 s (pyamgcl.amgcl)
• solve: 0.36 s (solver_iter_pcg)

So the algebraic multigrid #264 is great and assembly is O. K. but building the basis is slow.

I’ve started to quantify this by comparing the time for InteriorBasis with MeshTet and ElementTetP1 using .init_tensor with the same points (and so same basis.N but with five times as many tetrahedral elements).

 from pathlib import Path
from time import time

from matplotlib.pyplot import subplots
import numpy as np

import skfem

number = []
tetrahedral = []
hexahedral = []
for n in range(2, 15):
    times = []
    points = [np.arange(n)]*3
    for cls, elt in [(skfem.MeshTet, skfem.ElementTetP1()),
                     (skfem.MeshHex, skfem.ElementHex1())]:
        mesh = cls.init_tensor(*points)
        tic = time()
        basis = skfem.InteriorBasis(mesh, elt)
        times.append(time() - tic)
    number.append(basis.N)
    tetrahedral.append(times[-2])
    hexahedral.append(times[-1])

fig, ax = subplots()
fig.suptitle('InteriorBasis')
ax.loglog(number, hexahedral, linestyle='none', marker='s',
        label=skfem.MeshHex.name)
ax.loglog(number, tetrahedral, linestyle='none', marker='^',
        label=skfem.MeshTet.name)
ax.set_xlabel('basis.N')
ax.set_ylabel('time / s')
ax.legend()
fig.savefig(Path(__file__).with_suffix('.png'))

The hexahedral case is a hundred times slower.