PERF-2 P2: the red-black colour maps, the residual and the V-cycle's per-level maps on rayon threads (MultigridParameters::threads; RTX_THREADS in the harness; rtx_cfd::configure_threads) — each colour's values computed from the unchanged other colour into a scratch and written back, the L1 sum in the serial order: bit-identical to the serial red-black (pin: 4 right-hand sides at 4 threads); the coarsest level stays serial; no effect on the lexicographic regime

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

@@ -178,6 +178,8 @@ pub struct EmbeddedPisoSolver {
     /// PERF-2 P1.1: the prepared multigrid operator, reused across rounds
     /// and steps while the operator's coefficients and mask are unchanged.
     pcg_cache: std::cell::RefCell<super::poisson::PcgCache>,
+    /// PERF-2 P2: threads for the multigrid's red-black maps (default 1).
+    poisson_threads: usize,
     moving: bool,
     /// Mask hysteresis band in multiples of the min cell size (0 = off).
     mask_hysteresis: f64,
@@ -206,6 +208,7 @@ impl EmbeddedPisoSolver {
             inner_stop_factor: 1e-2,
             poisson_profile: std::cell::Cell::new((0, 0, 0, 0)),
             pcg_cache: std::cell::RefCell::new(super::poisson::PcgCache::default()),
+            poisson_threads: 1,
             moving: false,
             mask_hysteresis: 0.0,
             time: 0.0,
@@ -225,6 +228,12 @@ impl EmbeddedPisoSolver {
         self.parameters.poisson_smoother = smoother;
     }
 
+    /// Threads for the multigrid's red-black colour maps (PERF-2 P2; the
+    /// caller configures rayon's global pool). Bit-identical to 1.
+    pub fn set_poisson_threads(&mut self, threads: usize) {
+        self.poisson_threads = threads.max(1);
+    }
+
     /// 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

@@ -300,6 +300,7 @@ impl EmbeddedPisoSolver {
                 &MultigridParameters {
                     precision: self.parameters.poisson_precision,
                     smoother: self.parameters.poisson_smoother,
+                    threads: self.poisson_threads,
                     ..MultigridParameters::default()
                 },
                 inner_stop,