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-cfd/src/solvers/incompressible/embedded/mod.rs

@@ -71,6 +71,9 @@ pub struct EmbeddedParameters {
     /// and the stop stay f64; only the preconditioner runs in single
     /// precision. No effect with [`PoissonSolverKind::Sor`].
     pub poisson_precision: MgPrecision,
+    /// The multigrid smoother ordering (PERF-2; default lexicographic, the
+    /// recorded regime).
+    pub poisson_smoother: super::poisson::MgSmoother,
     /// Convective face values in the explicit predictor (default
     /// [`ConvectionScheme::Upwind`], which is bit-identical to the fixed-grid
     /// PISO). The TVD schemes add SIMPLE's limited correction to each
@@ -93,6 +96,7 @@ impl Default for EmbeddedParameters {
             boundaries: AleBoundaries::default(),
             poisson_solver: PoissonSolverKind::Sor,
             poisson_precision: MgPrecision::F64,
+            poisson_smoother: super::poisson::MgSmoother::Lexicographic,
             convection_scheme: ConvectionScheme::Upwind,
         }
     }
@@ -215,6 +219,12 @@ impl EmbeddedPisoSolver {
         self.parameters.poisson_precision = precision;
     }
 
+    /// The multigrid smoother ordering (PERF-2's red-black knob); the
+    /// cached operator is rebuilt on the next solve.
+    pub fn set_poisson_smoother(&mut self, smoother: super::poisson::MgSmoother) {
+        self.parameters.poisson_smoother = smoother;
+    }
+
     /// Mask hysteresis for the moving-body rebuild, as a fraction of the
     /// min cell size (default 0, exactly the plain rebuild). With a band,
     /// a cell within `band * h_min` of the surface keeps the

crates/specialized/rtx-cfd/src/solvers/incompressible/embedded/projection.rs

@@ -299,6 +299,7 @@ impl EmbeddedPisoSolver {
                 &mut p_prime,
                 &MultigridParameters {
                     precision: self.parameters.poisson_precision,
+                    smoother: self.parameters.poisson_smoother,
                     ..MultigridParameters::default()
                 },
                 inner_stop,

crates/specialized/rtx-cfd/src/solvers/incompressible/mod.rs

@@ -65,8 +65,8 @@ pub use piso::{PisoParameters, PisoResult, PisoSolver};
 #[cfg(feature = "cuda")]
 pub use piso_gpu::PisoGpuSolver;
 pub use poisson::{
-    MgPrecision, MultigridParameters, PcgCache, PoissonProblem, PoissonSolution, PoissonSolverKind,
+    MgPrecision, MgSmoother, MultigridParameters, PcgCache, PoissonProblem, PoissonSolution,
-    solve_multigrid_pcg, solve_multigrid_pcg_cached,
+    PoissonSolverKind, solve_multigrid_pcg, solve_multigrid_pcg_cached,
 };
 pub use polygon_sdf::PolygonSdf;
 pub use simple::{ConvectionScheme, SimpleParameters, SimpleResult, SimpleSolver};

crates/specialized/rtx-cfd/src/solvers/incompressible/poisson.rs

@@ -268,11 +268,27 @@ pub enum MgPrecision {
     F32,
 }
 
+/// The V-cycle's smoother ordering (PERF-2, `docs/perf2_campaign.md`).
+/// `Lexicographic` is the recorded regime (row-major symmetric
+/// Gauss–Seidel, a dependency chain through the division per cell);
+/// `RedBlack` updates the two colours of the five-point stencil in turn —
+/// each colour a map of independent cells (threads, vectors, the GPU) —
+/// and is a different preconditioner, gated by the noise probe and the
+/// anchor's band, never bit-identical.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
+pub enum MgSmoother {
+    #[default]
+    Lexicographic,
+    RedBlack,
+}
+
 /// Multigrid preconditioner parameters.
 #[derive(Debug, Clone)]
 pub struct MultigridParameters {
     /// Precision of the V-cycle (see [`MgPrecision`]).
     pub precision: MgPrecision,
+    /// Smoother ordering (see [`MgSmoother`]).
+    pub smoother: MgSmoother,
     /// Symmetric Gauss–Seidel sweeps before AND after the coarse correction
     /// (default 2; a value of 0 is treated as 1). One count for both on
     /// purpose: unequal pre/post counts make the V-cycle non-symmetric and
@@ -290,6 +306,7 @@ impl Default for MultigridParameters {
     fn default() -> Self {
         Self {
             precision: MgPrecision::F64,
+            smoother: MgSmoother::Lexicographic,
             smoother_sweeps: 2,
             coarsest_cells: 32,
             max_iterations: 500,
@@ -431,6 +448,10 @@ struct Level<T: MgScalar> {
     ap: Vec<T>,
     /// Row-major indices of the active cells.
     cells: Vec<usize>,
+    /// The active cells with `i + j` even / odd, each row-major (the
+    /// red-black smoother's two independent maps on the five-point stencil).
+    red: Vec<usize>,
+    black: Vec<usize>,
     /// Fine index → coarse index (empty on the coarsest level).
     coarse_of: Vec<usize>,
 }
@@ -491,6 +512,17 @@ impl<T: MgScalar> Level<T> {
             }
         }
         let cells: Vec<usize> = (0..n).filter(|&idx| active[idx]).collect();
+        let parity = |idx: usize| (idx % nx + idx / nx) % 2;
+        let red: Vec<usize> = cells
+            .iter()
+            .copied()
+            .filter(|&idx| parity(idx) == 0)
+            .collect();
+        let black: Vec<usize> = cells
+            .iter()
+            .copied()
+            .filter(|&idx| parity(idx) == 1)
+            .collect();
         let cast = |v: &[f64]| v.iter().map(|&x| T::from_f64(x)).collect::<Vec<T>>();
         Self {
             ae: cast(&problem.ae),
@@ -501,6 +533,8 @@ impl<T: MgScalar> Level<T> {
             problem,
             active,
             cells,
+            red,
+            black,
             coarse_of: Vec::new(),
         }
     }
@@ -559,6 +593,33 @@ impl<T: MgScalar> Level<T> {
         }
     }
 
+    /// One symmetric red-black Gauss–Seidel sweep: red, black, black, red —
+    /// within a colour every cell reads only the other colour, so each
+    /// half-sweep is a map (symmetric as a preconditioner, like the
+    /// lexicographic pair).
+    fn symmetric_gs_rb(&self, b: &[T], x: &mut [T]) {
+        for &idx in &self.red {
+            x[idx] = (b[idx] + self.neighbour_sum(x, idx)) / self.ap[idx];
+        }
+        for &idx in &self.black {
+            x[idx] = (b[idx] + self.neighbour_sum(x, idx)) / self.ap[idx];
+        }
+        for &idx in &self.black {
+            x[idx] = (b[idx] + self.neighbour_sum(x, idx)) / self.ap[idx];
+        }
+        for &idx in &self.red {
+            x[idx] = (b[idx] + self.neighbour_sum(x, idx)) / self.ap[idx];
+        }
+    }
+
+    /// One symmetric sweep in the chosen ordering.
+    fn smooth(&self, b: &[T], x: &mut [T], smoother: MgSmoother) {
+        match smoother {
+            MgSmoother::Lexicographic => self.symmetric_gs(b, x),
+            MgSmoother::RedBlack => self.symmetric_gs_rb(b, x),
+        }
+    }
+
     /// Galerkin coarsening: coarse face coefficient = sum of the fine
     /// coefficients across that coarse face, coarse `extra_diag` = sum of
     /// the children's. Returns the coarse problem and the parent map.
@@ -601,6 +662,7 @@ pub(crate) struct Hierarchy<T: MgScalar = f64> {
     levels: Vec<Level<T>>,
     work: Vec<Work<T>>,
     sweeps: usize,
+    smoother: MgSmoother,
 }
 
 impl<T: MgScalar> Hierarchy<T> {
@@ -630,6 +692,7 @@ impl<T: MgScalar> Hierarchy<T> {
             levels,
             work,
             sweeps: params.smoother_sweeps.max(1),
+            smoother: params.smoother,
         }
     }
 
@@ -672,7 +735,7 @@ impl<T: MgScalar> Hierarchy<T> {
                 wf.x[idx] = T::ZERO;
             }
             for _ in 0..self.sweeps {
-                fine.symmetric_gs(&wf.b, &mut wf.x);
+                fine.smooth(&wf.b, &mut wf.x, self.smoother);
             }
             fine.residual(&wf.b, &wf.x, &mut wf.r);
             for &idx in &coarse.cells {
@@ -695,7 +758,7 @@ impl<T: MgScalar> Hierarchy<T> {
                 wb.x[idx] = T::ZERO;
             }
             for _ in 0..COARSEST_SWEEPS {
-                bottom.symmetric_gs(&wb.b, &mut wb.x);
+                bottom.smooth(&wb.b, &mut wb.x, self.smoother);
             }
         }
         // Up: prolongate, smooth.
@@ -707,7 +770,7 @@ impl<T: MgScalar> Hierarchy<T> {
                 wf.x[idx] += T::from_f64(COARSE_CORRECTION) * wc.x[fine.coarse_of[idx]];
             }
             for _ in 0..self.sweeps {
-                fine.symmetric_gs(&wf.b, &mut wf.x);
+                fine.smooth(&wf.b, &mut wf.x, self.smoother);
             }
         }
         for &idx in &levels[0].cells {
@@ -762,6 +825,7 @@ struct OperatorKey {
     coefficients: Vec<u64>,
     smoother_sweeps: usize,
     coarsest_cells: usize,
+    smoother: MgSmoother,
 }
 
 impl OperatorKey {
@@ -782,6 +846,7 @@ impl OperatorKey {
             coefficients,
             smoother_sweeps: params.smoother_sweeps,
             coarsest_cells: params.coarsest_cells,
+            smoother: params.smoother,
         }
     }
 
@@ -790,6 +855,7 @@ impl OperatorKey {
             && self.ny == problem.ny
             && self.smoother_sweeps == params.smoother_sweeps
             && self.coarsest_cells == params.coarsest_cells
+            && self.smoother == params.smoother
             && self.active == problem.active
             && self.coefficients.iter().copied().eq(problem
                 .ae

crates/specialized/rtx-cfd/tests/poisson_redblack.rs

@@ -0,0 +1,108 @@
+//! PERF-2 (`docs/perf2_campaign.md`): the red-black symmetric Gauss–Seidel
+//! smoother is a different preconditioner, not a bit-identical one: the pin
+//! is that it solves the same masked problem to the same stop, that its
+//! solution agrees with the lexicographic one to the solver's tolerance,
+//! and that the cached red-black solve is bit-identical to the uncached
+//! red-black solve (the cache keys on the smoother).
+
+use rtx_cfd::solvers::incompressible::{
+    MgSmoother, MultigridParameters, PcgCache, PoissonProblem, solve_multigrid_pcg,
+    solve_multigrid_pcg_cached,
+};
+
+fn problem(nx: usize, ny: usize, seed: u64) -> PoissonProblem {
+    let mut p = PoissonProblem::new(nx, ny);
+    let (dx, dy, dt) = (1.0 / nx as f64, 0.41 / ny as f64, 1e-3);
+    let (ae, an) = (dt * dy / dx, dt * dx / dy);
+    let hole = |i: usize, j: usize| {
+        let (x, y) = ((i as f64 + 0.5) * dx, (j as f64 + 0.5) * dy);
+        (x - 0.2).powi(2) + (y - 0.2).powi(2) < 0.05 * 0.05
+    };
+    for j in 0..ny {
+        for i in 0..nx {
+            let idx = j * nx + i;
+            if hole(i, j) {
+                p.active[idx] = false;
+                continue;
+            }
+            if i + 1 < nx && !hole(i + 1, j) {
+                p.ae[idx] = ae;
+            }
+            if i > 0 && !hole(i - 1, j) {
+                p.aw[idx] = ae;
+            }
+            if j + 1 < ny && !hole(i, j + 1) {
+                p.an[idx] = an;
+            }
+            if j > 0 && !hole(i, j - 1) {
+                p.as_[idx] = an;
+            }
+            if i + 1 == nx {
+                p.extra_diag[idx] = 2.0 * ae;
+            }
+        }
+    }
+    let mut state = seed | 1;
+    for idx in 0..nx * ny {
+        state ^= state << 13;
+        state ^= state >> 7;
+        state ^= state << 17;
+        p.rhs[idx] = if p.active[idx] {
+            1e-6 * ((state >> 11) as f64 / (1u64 << 53) as f64 - 0.5)
+        } else {
+            0.0
+        };
+    }
+    p
+}
+
+#[test]
+fn red_black_solves_the_masked_problem_and_caches_exactly() {
+    let (nx, ny) = (96, 40);
+    let prob = problem(nx, ny, 5);
+    let tol = 1e-12;
+    let lex = MultigridParameters::default();
+    let rb = MultigridParameters {
+        smoother: MgSmoother::RedBlack,
+        ..MultigridParameters::default()
+    };
+    let (mut p_lex, mut p_rb) = (vec![0.0; nx * ny], vec![0.0; nx * ny]);
+    let s_lex = solve_multigrid_pcg(&prob, &mut p_lex, &lex, tol, None);
+    let s_rb = solve_multigrid_pcg(&prob, &mut p_rb, &rb, tol, None);
+    assert!(s_lex.converged && s_rb.converged);
+    assert!(
+        prob.residual_l1(&p_rb) < tol,
+        "red-black residual {:.3e}",
+        prob.residual_l1(&p_rb)
+    );
+    let scale = p_lex.iter().fold(0.0_f64, |m, v| m.max(v.abs()));
+    let diff = p_lex
+        .iter()
+        .zip(&p_rb)
+        .fold(0.0_f64, |m, (a, b)| m.max((a - b).abs()));
+    println!(
+        "  lexicographic {} iterations vs red-black {} iterations; solutions differ by {:.3e} on a scale of {:.3e}",
+        s_lex.iterations, s_rb.iterations, diff, scale
+    );
+    assert!(
+        diff < 1e-6 * scale,
+        "red-black and lexicographic disagree: {diff:.3e} of {scale:.3e}"
+    );
+    // The cached red-black solve is the uncached one, bit for bit.
+    let mut cache = PcgCache::default();
+    for k in 0..3u64 {
+        let mut q = prob.clone();
+        q.rhs = problem(nx, ny, 20 + k).rhs;
+        let (mut a, mut b) = (vec![0.0; nx * ny], vec![0.0; nx * ny]);
+        let sa = solve_multigrid_pcg(&q, &mut a, &rb, tol, None);
+        let sb = solve_multigrid_pcg_cached(&q, &mut b, &rb, tol, None, &mut cache);
+        assert!(a.iter().zip(&b).all(|(x, y)| x.to_bits() == y.to_bits()));
+        assert_eq!(sa.iterations, sb.iterations);
+    }
+    // Switching the smoother is a cache miss (the key carries it), still exact.
+    let (mut a, mut b) = (vec![0.0; nx * ny], vec![0.0; nx * ny]);
+    let sa = solve_multigrid_pcg(&prob, &mut a, &lex, tol, None);
+    let sb = solve_multigrid_pcg_cached(&prob, &mut b, &lex, tol, None, &mut cache);
+    assert!(a.iter().zip(&b).all(|(x, y)| x.to_bits() == y.to_bits()));
+    assert_eq!(sa.iterations, sb.iterations);
+}

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(),
             },
         )?;