diff --git a/crates/specialized/rtx-fsi/tests/fsi2_interface_noise.rs b/crates/specialized/rtx-fsi/tests/fsi2_interface_noise.rs
index 5e3b5fd..16cd97a 100644
--- a/crates/specialized/rtx-fsi/tests/fsi2_interface_noise.rs
+++ b/crates/specialized/rtx-fsi/tests/fsi2_interface_noise.rs
@@ -106,6 +106,13 @@ fn fsi2_interface_noise_floor() {
         solver.set_poisson_precision(rtx_cfd::solvers::incompressible::MgPrecision::F32);
         println!("  poisson V-cycle precision: F32 (M1 probe)");
     }
+    // PERF-2 regime gate (`RTX_FSI2_MG_RB=1`): the red-black smoother.
+    if env_or("RTX_FSI2_MG_RB", 0.0) != 0.0 {
+        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;
 
     // Rigid march to operating loads (the floor rides with the loads —