From 55c81bce1b9c10cbbefac3d94118f303caa21ebe Mon Sep 17 00:00:00 2001
From: ksaurabh-amd <>
Date: Thu, 9 Oct 2025 11:53:34 +0100
Subject: [PATCH] ENH: GAMGAgglomeration: increase repeatability. Fixes #3450

---
 .../pairGAMGAgglomeration/pairGAMGAgglomerate.C       | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/src/OpenFOAM/matrices/lduMatrix/solvers/GAMG/GAMGAgglomerations/pairGAMGAgglomeration/pairGAMGAgglomerate.C b/src/OpenFOAM/matrices/lduMatrix/solvers/GAMG/GAMGAgglomerations/pairGAMGAgglomeration/pairGAMGAgglomerate.C
index 7da7012658..dfc6d6e257 100644
--- a/src/OpenFOAM/matrices/lduMatrix/solvers/GAMG/GAMGAgglomerations/pairGAMGAgglomeration/pairGAMGAgglomerate.C
+++ b/src/OpenFOAM/matrices/lduMatrix/solvers/GAMG/GAMGAgglomerations/pairGAMGAgglomeration/pairGAMGAgglomerate.C
@@ -221,9 +221,14 @@ Foam::tmp<Foam::labelField> Foam::pairGAMGAgglomeration::agglomerate
     auto tcoarseCellMap = tmp<labelField>::New(nFineCells, -1);
     auto& coarseCellMap = tcoarseCellMap.ref();
 
+    // Small tolerance to account for faces potentially having slightly
+    // different truncation error in their weights from run to run
+    // (e.g. due to offloading). If all the largest faces per cell are
+    // within this tolerance use the first one. This guarantees repeatability.
+    const scalar tol = 1E-10;
+
     nCoarseCells = 0;
     label celli;
-
     for (label cellfi=0; cellfi<nFineCells; cellfi++)
     {
         // Change cell ordering depending on direction for this level
@@ -250,7 +255,7 @@ Foam::tmp<Foam::labelField> Foam::pairGAMGAgglomeration::agglomerate
                 (
                     coarseCellMap[upperAddr[facei]] < 0
                  && coarseCellMap[lowerAddr[facei]] < 0
-                 && faceWeights[facei] > maxFaceWeight
+                 && faceWeights[facei] > maxFaceWeight*(1.0 + tol)
                 )
                 {
                     // Match found. Pick up all the necessary data
@@ -282,7 +287,7 @@ Foam::tmp<Foam::labelField> Foam::pairGAMGAgglomeration::agglomerate
                 {
                     label facei = cellFaces[faceOs];
 
-                    if (faceWeights[facei] > clusterMaxFaceCoeff)
+                    if (faceWeights[facei] > clusterMaxFaceCoeff*(1.0 + tol))
                     {
                         clusterMatchFaceNo = facei;
                         clusterMaxFaceCoeff = faceWeights[facei];