PERF-2: red-black symmetric Gauss–Seidel smoother behind a knob (MgSmoother::RedBlack; default lexicographic = the recorded regime, bit-identical) — red/black cell lists per level, the symmetric pair red,black,black,red, the operator cache keyed on the smoother; EmbeddedParameters::poisson_smoother, harness knobs RTX_FSI2O_MG_RB (overset) and RTX_FSI2_MG_RB (embedded, the noise probe); pin: solves the masked problem to the same stop, agrees with lexicographic to 2e-12, cached = uncached bit for bit

Co-Authored-By: Claude Fable 5.1 <[email protected]>
Claude-Session: https://claude.ai/code/session_01YJPeT6WA2e7YvAnS875AHL

crates/specialized/rtx-fsi/tests/fsi2_harness/march.rs

@@ -384,6 +384,14 @@ pub fn run_march(case: BenchmarkCase, config: &MarchConfig) -> MarchResult {
         solver.set_poisson_precision(rtx_cfd::solvers::incompressible::MgPrecision::F32);
         println!("  poisson V-cycle precision: F32 (M1 probe)");
     }
+    // PERF-2: `RTX_FSI2_MG_RB=1` — the red-black smoother on the embedded
+    // solver (the noise probe's regime gate).
+    if std::env::var("RTX_FSI2_MG_RB").is_ok_and(|v| v == "1") {
+        solver.set_poisson_smoother(rtx_cfd::solvers::incompressible::MgSmoother::RedBlack);
+        println!(
+            "  multigrid smoother: RED-BLACK symmetric Gauss–Seidel (PERF-2 regime, RTX_FSI2_MG_RB)"
+        );
+    }
     let dt_fluid = harness.dt_fluid;
     let dt = dt_fluid * subcycle as f64;
     let interface = &harness.interface;

crates/specialized/rtx-fsi/tests/fsi2_harness/overset.rs

@@ -300,6 +300,16 @@ impl OversetFluid {
                 },
                 poisson_solver: PoissonSolverKind::Multigrid,
                 poisson_precision: MgPrecision::F64,
+                // PERF-2: `RTX_FSI2O_MG_RB=1` selects the red-black smoother
+                // (a regime change, band-gated; default = the recorded regime).
+                poisson_smoother: if std::env::var("RTX_FSI2O_MG_RB").is_ok_and(|v| v == "1") {
+                    println!(
+                        "  multigrid smoother: RED-BLACK symmetric Gauss–Seidel (PERF-2 regime, RTX_FSI2O_MG_RB)"
+                    );
+                    rtx_cfd::solvers::incompressible::MgSmoother::RedBlack
+                } else {
+                    rtx_cfd::solvers::incompressible::MgSmoother::Lexicographic
+                },
                 convection_scheme: bg_convection(),
             },
         )?;