size also needs to be on a separate line

diff --git a/src/lib/io/atomic_queue.c b/src/lib/io/atomic_queue.c
index 75f31d4eb46306a28e7242999772e9c1c348f201..7a4d7837f69247a69624f8b33786b9b1587175c1 100644 (file)
--- a/src/lib/io/atomic_queue.c
+++ b/src/lib/io/atomic_queue.c
@@ -68,6 +68,10 @@ typedef struct CC_HINT(packed, aligned(CACHE_LINE_SIZE)) {
 
 /** Structure to hold the atomic queue
  *
+ * @note DO NOT redorder these fields without understanding how alignas works
+ * and maintaining separation. The head and tail must be in different cache lines
+ * to reduce contention between producers and consumers. Cold data (size, chunk)
+ * can share a line, but must be separated from head and tail and entry.
  */
 struct fr_atomic_queue_s {
        alignas(CACHE_LINE_SIZE) atomic_int64_t         head;           //!< Position of the producer.
@@ -82,7 +86,10 @@ struct fr_atomic_queue_s {
                                                                        ///< Reads may still need to occur from size
                                                                        ///< whilst the producer is writing to tail.
 
-       size_t                                          size;           //!< The length of the queue.  This is static.
+       alignas(CACHE_LINE_SIZE) size_t                 size;           //!< The length of the queue.  This is static.
+                                                                       ///< Also needs to be cache aligned, otherwise
+                                                                       ///< it can end up directly after tail in memory
+                                                                       ///< and share a cache line.
 
        void                                            *chunk;         //!< The start of the talloc chunk to pass to free.
                                                                        ///< We need to play tricks to get aligned memory