diff --git a/src/drt/src/dr/FlexGridGraph.h b/src/drt/src/dr/FlexGridGraph.h
index 921bbe61a43..dc41ac70c3a 100644
--- a/src/drt/src/dr/FlexGridGraph.h
+++ b/src/drt/src/dr/FlexGridGraph.h
@@ -28,7 +28,7 @@
 
 #pragma once
 
-#include <unordered_map>
+#include <boost/unordered/unordered_flat_map.hpp>
 #include <cstdint>
 #include <cstring>
 #include <iostream>
@@ -1125,7 +1125,7 @@ class FlexGridGraph
   std::vector<bool> srcs_;
   std::vector<bool> dsts_;
   std::vector<bool> guides_;
-  std::unordered_map<frMIdx, frCost> path_costs_;
+  boost::unordered_flat_map<frMIdx, frCost> path_costs_;
   frVector<frCoord> xCoords_;
   frVector<frCoord> yCoords_;
   frVector<frLayerNum> zCoords_;