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-fsi/tests/fsi2_harness/overset.rs

@@ -313,6 +313,19 @@ impl OversetFluid {
                 convection_scheme: bg_convection(),
             },
         )?;
+        // PERF-2 P2: `RTX_THREADS=n` — rayon's global pool and the multigrid's
+        // red-black maps on n threads (bit-identical to 1; no effect on the
+        // lexicographic smoother).
+        let threads: usize = std::env::var("RTX_THREADS")
+            .ok()
+            .and_then(|v| v.parse().ok())
+            .unwrap_or(1)
+            .max(1);
+        if threads > 1 {
+            rtx_cfd::solvers::incompressible::configure_threads(threads);
+            background.set_poisson_threads(threads);
+            println!("  multigrid threads: {threads} (RTX_THREADS)");
+        }
         background.set_boundary_velocity(move |x, y, t| {
             if x <= 0.0 {
                 (inflow_for(u_mean, y, t), 0.0)