From fd408637a7a172d260f38c73667323148439a09e Mon Sep 17 00:00:00 2001
From: Matthew Parkinson <mattpark@microsoft.com>
Date: Thu, 28 Oct 2021 14:23:15 +0100
Subject: [PATCH] Move some structures to cache lines

Prevent some bad sharing of cache lines by aligning some concurrently
accessed strucutures.
---
 src/ds/mpmcstack.h       |  2 +-
 src/mem/chunkallocator.h | 12 ++++++------
 src/mem/pool.h           |  2 +-
 3 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/src/ds/mpmcstack.h b/src/ds/mpmcstack.h
index d7a4f92..b701c05 100644
--- a/src/ds/mpmcstack.h
+++ b/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;
diff --git a/src/mem/chunkallocator.h b/src/mem/chunkallocator.h
index 0a8e41e..286d7d0 100644
--- a/src/mem/chunkallocator.h
+++ b/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{};
 
diff --git a/src/mem/pool.h b/src/mem/pool.h
index 0c470c7..c773e89 100644
--- a/src/mem/pool.h
+++ b/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: