Move some structures to cache lines

Prevent some bad sharing of cache lines by aligning some concurrently
accessed strucutures.

Author: mjp41

src/ds/mpmcstack.h

@@ -11,7 +11,7 @@ namespace snmalloc
     using ABAT = ABA<T, c>;
 
   private:
-    ABAT stack;
+    alignas(CACHELINE_SIZE) ABAT stack;
 
   public:
     constexpr MPMCStack() = default;

src/mem/chunkallocator.h

@@ -79,6 +79,12 @@ namespace snmalloc
     ModArray<NUM_SLAB_SIZES, MPMCStack<ChunkRecord, RequiresInit>>
       decommitted_chunk_stack;
 
+    /**
+     * Which is the current epoch to place dealloced chunks, and the
+     * first place we look for allocating chunks.
+     */
+    alignas(CACHELINE_SIZE) std::atomic<size_t> epoch{0};
+
     /**
      * All memory issued by this address space manager
      */
@@ -88,12 +94,6 @@ namespace snmalloc
 
     std::atomic<ChunkAllocatorLocalState*> all_local{nullptr};
 
-    /**
-     * Which is the current epoch to place dealloced chunks, and the
-     * first place we look for allocating chunks.
-     */
-    std::atomic<size_t> epoch{0};
-
     // Flag to ensure one-shot registration with the PAL for notifications.
     std::atomic_flag register_decay{};
 

src/mem/pool.h

@@ -28,8 +28,8 @@ namespace snmalloc
     friend class Pool;
 
   private:
-    std::atomic_flag lock = ATOMIC_FLAG_INIT;
     MPMCStack<T, PreZeroed> stack;
+    std::atomic_flag lock = ATOMIC_FLAG_INIT;
     T* list{nullptr};
 
   public: