Use alignas(folly::cacheline_align_v) to improve NotificationQueue perf

Summary: This also splits queue size counter into two to make sure we do fewer atomic operations on the consumer thread.

Differential Revision: D24490151

fbshipit-source-id: 69297a2fd0c5fddcd7355a28fe600b72320e81cd

folly/io/async/AtomicNotificationQueue.cpp

@@ -98,8 +98,9 @@ AtomicNotificationQueue::AtomicQueue::~AtomicQueue() {
 }
 
 bool AtomicNotificationQueue::AtomicQueue::push(Task&& value) {
-  std::unique_ptr<Queue::Node> node(new Queue::Node(std::move(value)));
+  pushCount_.fetch_add(1, std::memory_order_relaxed);
 
+  std::unique_ptr<Queue::Node> node(new Queue::Node(std::move(value)));
   auto head = head_.load(std::memory_order_relaxed);
   while (true) {
     node->next =
@@ -222,7 +223,9 @@ void AtomicNotificationQueue::setMaxReadAtOnce(uint32_t maxAtOnce) {
 }
 
 int32_t AtomicNotificationQueue::size() const {
-  return queueSize_.load(std::memory_order_relaxed);
+  auto queueSize = atomicQueue_.getPushCount() - taskExecuteCount_;
+  DCHECK(queueSize >= 0);
+  return queueSize;
 }
 
 bool AtomicNotificationQueue::empty() const {
@@ -230,7 +233,6 @@ bool AtomicNotificationQueue::empty() const {
 }
 
 void AtomicNotificationQueue::putMessage(Func&& func) {
-  queueSize_.fetch_add(1, std::memory_order_relaxed);
   if (atomicQueue_.push(Task{std::move(func), RequestContext::saveContext()})) {
     notifyFd();
   }
@@ -283,7 +285,7 @@ bool AtomicNotificationQueue::drive() {
         break;
       }
     }
-    queueSize_.fetch_sub(1, std::memory_order_relaxed);
+    ++taskExecuteCount_;
     std::move(queue_.front()).execute();
     queue_.pop();
   }

folly/io/async/AtomicNotificationQueue.h

@@ -174,9 +174,18 @@ class AtomicNotificationQueue : private EventBase::LoopCallback,
       return successfulArmCount_ - consumerDisarmCount_;
     }
 
+    /*
+     * Returns how many times push was called.
+     * Can be called from any thread.
+     */
+    ssize_t getPushCount() const {
+      return pushCount_.load(std::memory_order_relaxed);
+    }
+
    private:
-    std::atomic<Queue::Node*> head_{};
-    ssize_t successfulArmCount_{0};
+    alignas(folly::cacheline_align_v) std::atomic<Queue::Node*> head_{};
+    std::atomic<ssize_t> pushCount_{0};
+    alignas(folly::cacheline_align_v) ssize_t successfulArmCount_{0};
     ssize_t consumerDisarmCount_{0};
     static constexpr intptr_t kQueueArmedTag = 1;
   };
@@ -296,7 +305,7 @@ class AtomicNotificationQueue : private EventBase::LoopCallback,
 
   AtomicQueue atomicQueue_;
   Queue queue_;
-  std::atomic<int32_t> queueSize_{0};
+  ssize_t taskExecuteCount_{0};
   int32_t maxReadAtOnce_{10};
   int eventfd_{-1};
   int pipeFds_[2]{-1, -1}; // to fallback to on older/non-linux systems