From 15348ff994e678d2873473eb27fa8be9d6f7ceea Mon Sep 17 00:00:00 2001
From: Yannay <yannayjaffe@mail.tau.ac.il>
Date: Fri, 5 Apr 2024 20:05:49 +0300
Subject: [PATCH] fix: put key_gen_ in a separate cache line

Prevents false sharing when one thread writes to key_gen_ and others read other class members (that reside on the same cache line)
---
 benchmark/DB_oindex.hh | 11 ++++++++---
 benchmark/DB_uindex.hh | 11 ++++++++---
 2 files changed, 16 insertions(+), 6 deletions(-)

diff --git a/benchmark/DB_oindex.hh b/benchmark/DB_oindex.hh
index b59f0910..9ec25ea1 100644
--- a/benchmark/DB_oindex.hh
+++ b/benchmark/DB_oindex.hh
@@ -773,7 +773,10 @@ protected:
 
 private:
     table_type table_;
-    uint64_t key_gen_;
+    union alignas(CACHE_LINE_SIZE) {
+        uint64_t key_gen_;
+        uint8_t _[CACHE_LINE_SIZE];
+    };
 
     static bool
     access_all(std::array<access_t, value_container_type::num_versions>& cell_accesses, std::array<TransItem*,
@@ -1450,8 +1453,10 @@ public:
 
 //private:
     table_type table_;
-    uint64_t key_gen_;
-
+    union alignas(CACHE_LINE_SIZE) {
+        uint64_t key_gen_;
+        uint8_t _[CACHE_LINE_SIZE];
+    };
     //static bool
     //access_all(std::array<access_t, internal_elem::num_versions>&, std::array<TransItem*, internal_elem::num_versions>&, internal_elem*) {
     //    always_assert(false, "Not implemented.");
diff --git a/benchmark/DB_uindex.hh b/benchmark/DB_uindex.hh
index 8c4c75c9..8a3700b6 100644
--- a/benchmark/DB_uindex.hh
+++ b/benchmark/DB_uindex.hh
@@ -84,8 +84,10 @@ private:
     Hash hasher_;
     Pred pred_;
 
-    uint64_t key_gen_;
-
+    union alignas(CACHE_LINE_SIZE) {
+        uint64_t key_gen_;
+        uint8_t _[CACHE_LINE_SIZE];
+    };
     // used to mark whether a key is a bucket (for bucket version checks)
     // or a pointer (which will always have the lower 3 bits as 0)
     static constexpr uintptr_t bucket_bit = C::item_key_tag;
@@ -681,7 +683,10 @@ private:
     Hash hasher_;
     Pred pred_;
 
-    uint64_t key_gen_;
+    union alignas(CACHE_LINE_SIZE) {
+        uint64_t key_gen_;
+        uint8_t _[CACHE_LINE_SIZE];
+    };
 
     // used to mark whether a key is a bucket (for bucket version checks)
     // or a pointer (which will always have the lower 3 bits as 0)