LINELET segfaults, no LINELET regression tests

[bigfooted]

Describe the bug
There are no regression tests for LINELET and there are reports about LINELET crashing with a segmentation fault for certain testcases. The crashes need to be confirmed preferably with an existing testcase using LINELET.

[Syphonicc]

I'd like to look at this. Starting by enabling LINELET on some existing wall-bounded cases (s809, SST flat plate, laminar NACA0012) to try to reproduce a crash.

[Syphonicc]

I had a look at this. No segfault so far, but LINELET does cause a clean divergence on one case and degrades convergence on another.

Three wall-bounded cases, 500 iterations, only LINEAR_SOLVER_PREC changed (all other settings identical):

Case	Default prec	LINELET
rans/s809/turb_S809 (unstructured)	−5.22	−1.14, with 226 non-physical points
rans/flatplate/turb_SST_flatplate (structured)	−7.15	−6.10
navierstokes/naca0012/lam_NACA0012 (structured)	−8.19	−8.66

So it helps slightly on laminar NACA0012, hurts on the SST flat plate, and is bad on s809.

Running s809 further, it diverges outright at iteration 1140:

Error in "void CSolver::SetResidual_RMS(const CGeometry*, const CConfig*)":
SU2 has diverged (Residual > 10^20 detected).

The same config without LINELET converges to −9.02 at 10k iterations.

One thing that might be relevant: with multigrid, the linelets become very short on coarse levels. SU2 reports:

lam_NACA0012:        23, 3, 4 points per line (avg) across MG levels
turb_SST_flatplate:  32, 13, 3 points per line (avg)
turb_S809:           32 points per line (avg)

Three or four points per line seems close to degenerate - I'd guess the linelet structure isn't being rebuilt meaningfully on the coarse grids.

I also confirmed there are currently no configs in TestCases that set LINEAR_SOLVER_PREC= LINELET - the only occurrence is a comment listing the available options in turbomachinery/Aachen_turbine/aachen_3D_MP_restart.cfg. So there's no existing coverage at all.

Happy to add regression tests for LINELET once there's a case that behaves sensibly with it - lam_NACA0012 is the only one of the three I'd currently trust. Also happy to test other cases if you know which ones the segfault reports came from.

Built from develop, serial (-Dwith-mpi=disabled), GCC 15.2.1, Fedora 42.

[SiddDart]

The coarse-grid behavior is particularly interesting. If the linelets are collapsing to only a few points on the coarser MG levels, that could definitely explain why the preconditioner behaves so differently across cases.

lam_NACA0012 seems like a good starting point for regression coverage given that it currently behaves sensibly with LINELET. I’ll report back if I find any other cases where LINELET gives stable convergence

[Syphonicc]

The coarse-grid behavior is particularly interesting. If the linelets are collapsing to only a few points on the coarser MG levels, that could definitely explain why the preconditioner behaves so differently across cases.

lam_NACA0012 seems like a good starting point for regression coverage given that it currently behaves sensibly with LINELET. I’ll report back if I find any other cases where LINELET gives stable convergence

Thanks that'd be useful. I'll start putting together a LINELET regression case based on lam_NACA0012 in the meantime. If you turn up other cases where it's stable, especially anything without multigrid, that'd help narrow down whether the coarse-level linelets are actually the cause