diff --git a/AMSS_NCKU_source/bssn_class.C b/AMSS_NCKU_source/bssn_class.C
index 432747e..1b951d1 100644
--- a/AMSS_NCKU_source/bssn_class.C
+++ b/AMSS_NCKU_source/bssn_class.C
@@ -5745,11 +5745,17 @@ void bssn_class::SHStep()
 
 //================================================================================================
 
-// 0: do not use mixing two levels data for OutBD; 1: do use
-
-#define MIXOUTB 0
-void bssn_class::RestrictProlong(int lev, int YN, bool BB,
-                                 MyList<var> *SL, MyList<var> *OL, MyList<var> *corL)
+// 0: do not use mixing two levels data for OutBD; 1: do use
+
+#define MIXOUTB 0
+// In the cached Restrict->OutBdLow2Hi path, coarse Sync is usually redundant:
+// OutBdLow2Hi_cached reads coarse owned cells (build_owned_gsl type-4), not coarse ghost/buffer cells.
+// Keep a switch to restore the old behavior if needed for debugging.
+#ifndef RP_SYNC_COARSE_AFTER_RESTRICT
+#define RP_SYNC_COARSE_AFTER_RESTRICT 0
+#endif
+void bssn_class::RestrictProlong(int lev, int YN, bool BB,
+                                 MyList<var> *SL, MyList<var> *OL, MyList<var> *corL)
 // we assume
 // StateList      1   -----------
 //
@@ -5811,7 +5817,9 @@ void bssn_class::RestrictProlong(int lev, int YN, bool BB,
 //       misc::tillherecheck(GH->Commlev[GH->mylev],GH->start_rank[GH->mylev],a_stream.str());
 #endif
 
-      Parallel::Sync_cached(GH->PatL[lev - 1], SynchList_pre, Symmetry, sync_cache_rp_coarse[lev]);
+#if (RP_SYNC_COARSE_AFTER_RESTRICT == 1)
+      Parallel::Sync_cached(GH->PatL[lev - 1], SynchList_pre, Symmetry, sync_cache_rp_coarse[lev]);
+#endif
 
 #if (PSTR == 1 || PSTR == 2)
 //       a_stream.clear();
@@ -5862,7 +5870,9 @@ void bssn_class::RestrictProlong(int lev, int YN, bool BB,
 //       misc::tillherecheck(GH->Commlev[GH->mylev],GH->start_rank[GH->mylev],a_stream.str());
 #endif
 
-      Parallel::Sync_cached(GH->PatL[lev - 1], SL, Symmetry, sync_cache_rp_coarse[lev]);
+#if (RP_SYNC_COARSE_AFTER_RESTRICT == 1)
+      Parallel::Sync_cached(GH->PatL[lev - 1], SL, Symmetry, sync_cache_rp_coarse[lev]);
+#endif
 
 #if (PSTR == 1 || PSTR == 2)
 //       a_stream.clear();
@@ -5948,7 +5958,9 @@ void bssn_class::RestrictProlong_aux(int lev, int YN, bool BB,
       Parallel::Restrict_bam(GH->PatL[lev - 1], GH->PatL[lev], SL, SynchList_pre, GH->rsul[lev], Symmetry);
 #endif
 
-      Parallel::Sync_cached(GH->PatL[lev - 1], SynchList_pre, Symmetry, sync_cache_rp_coarse[lev]);
+#if (RP_SYNC_COARSE_AFTER_RESTRICT == 1)
+      Parallel::Sync_cached(GH->PatL[lev - 1], SynchList_pre, Symmetry, sync_cache_rp_coarse[lev]);
+#endif
 
 #if (RPB == 0)
 #if (MIXOUTB == 0)
@@ -5970,7 +5982,9 @@ void bssn_class::RestrictProlong_aux(int lev, int YN, bool BB,
       Parallel::Restrict_bam(GH->PatL[lev - 1], GH->PatL[lev], SL, SL, GH->rsul[lev], Symmetry);
 #endif
 
-      Parallel::Sync_cached(GH->PatL[lev - 1], SL, Symmetry, sync_cache_rp_coarse[lev]);
+#if (RP_SYNC_COARSE_AFTER_RESTRICT == 1)
+      Parallel::Sync_cached(GH->PatL[lev - 1], SL, Symmetry, sync_cache_rp_coarse[lev]);
+#endif
 
 #if (RPB == 0)
 #if (MIXOUTB == 0)
@@ -6035,7 +6049,9 @@ void bssn_class::RestrictProlong(int lev, int YN, bool BB)
       Parallel::Restrict_bam(GH->PatL[lev - 1], GH->PatL[lev], SynchList_cor, SynchList_pre, GH->rsul[lev], Symmetry);
 #endif
 
-      Parallel::Sync_cached(GH->PatL[lev - 1], SynchList_pre, Symmetry, sync_cache_rp_coarse[lev]);
+#if (RP_SYNC_COARSE_AFTER_RESTRICT == 1)
+      Parallel::Sync_cached(GH->PatL[lev - 1], SynchList_pre, Symmetry, sync_cache_rp_coarse[lev]);
+#endif
 
 #if (RPB == 0)
 #if (MIXOUTB == 0)
@@ -6059,7 +6075,9 @@ void bssn_class::RestrictProlong(int lev, int YN, bool BB)
       Parallel::Restrict_bam(GH->PatL[lev - 1], GH->PatL[lev], SynchList_cor, StateList, GH->rsul[lev], Symmetry);
 #endif
 
-      Parallel::Sync_cached(GH->PatL[lev - 1], StateList, Symmetry, sync_cache_rp_coarse[lev]);
+#if (RP_SYNC_COARSE_AFTER_RESTRICT == 1)
+      Parallel::Sync_cached(GH->PatL[lev - 1], StateList, Symmetry, sync_cache_rp_coarse[lev]);
+#endif
 
 #if (RPB == 0)
 #if (MIXOUTB == 0)
@@ -6083,7 +6101,7 @@ void bssn_class::RestrictProlong(int lev, int YN, bool BB)
 
 //================================================================================================
 
-void bssn_class::ProlongRestrict(int lev, int YN, bool BB)
+void bssn_class::ProlongRestrict(int lev, int YN, bool BB)
 {
   if (lev > 0)
   {
@@ -6136,15 +6154,18 @@ void bssn_class::ProlongRestrict(int lev, int YN, bool BB)
 #else
       Parallel::Restrict_after(GH->PatL[lev - 1], GH->PatL[lev], SynchList_cor, StateList, Symmetry);
 #endif
-      Parallel::Sync_cached(GH->PatL[lev - 1], StateList, Symmetry, sync_cache_rp_coarse[lev]);
+#if (RP_SYNC_COARSE_AFTER_RESTRICT == 1)
+      Parallel::Sync_cached(GH->PatL[lev - 1], StateList, Symmetry, sync_cache_rp_coarse[lev]);
+#endif
     }
 
     Parallel::Sync_cached(GH->PatL[lev], SynchList_cor, Symmetry, sync_cache_rp_fine[lev]);
   }
-}
-#undef MIXOUTB
-
-//================================================================================================
+}
+#undef MIXOUTB
+#undef RP_SYNC_COARSE_AFTER_RESTRICT
+
+//================================================================================================