Hi team, I have the attached python code which is a mixed formulation on extruded mesh. It is possible that there are other bugs in my formulation, but I'm running into problems using the non-linear solver. My intent is to use direct solver in the non-linear iteration, and my parameters are: solver_parameters={'ksp_type': 'preonly', 'pc_type': 'lu', 'snes_atol':1e-10, 'ksp_atol':1e-10}) Then PETSc complains that I couldn't use direct solver for nested matrix. Google guides me to a post by Christian and Lawrence https://github.com/firedrakeproject/firedrake/issues/431 So I added nest=False in solve(), but then the system run out of memory: [0] Out of memory. Allocated: 0, Used by process: 5061259264 [0] Memory requested 8589934188 I guess it's not quite likely to use 8 GB of RAM for this problem size (388 x 584 x 2 layers). Lawrence did say that "it doesn't work with vector function spaces" earlier in the post, I'm not sure if that is related? My mixed space is [W, W, V] where W = DG(0) x CG(1), V is vector space of dim=2. Many thanks, -TJ