Add postponed send loop to QEMU driver

src/msgr_send.cpp

@@ -111,22 +111,28 @@ void osd_messenger_t::outbox_push(osd_op_t *cur_op)
         return;
     }
 #endif
-    if (!ringloop)
+    if (!ringloop && !has_send_loop)
     {
-        // FIXME: It's worse because it doesn't allow batching
+        // "Send loop" should be used, like in QEMU driver, or performance will suffer
+        // due to a large number of sendmsg() syscalls where they can be avoided.
+        // "Send loop" increases 4k T1Q128 randread from ~55k iops to ~90k on a 1 OSD.
+        // The difference is smaller when there are more OSDs though...
         while (cl->outbox.size())
         {
             try_send(cl);
         }
     }
-    else if (cl->write_msg.msg_iovlen > 0 || !try_send(cl))
+    else if (cl->write_msg.msg_iovlen > 0 || has_send_loop || !try_send(cl))
     {
         if (cl->write_state == 0)
         {
             cl->write_state = CL_WRITE_READY;
             write_ready_clients.push_back(cur_op->peer_fd);
         }
-        ringloop->wakeup();
+        if (ringloop)
+        {
+            ringloop->wakeup();
+        }
     }
 }
 
@@ -218,6 +224,12 @@ void osd_messenger_t::send_replies()
     write_ready_clients.clear();
 }
 
+bool osd_messenger_t::can_send()
+{
+    has_send_loop = true;
+    return write_ready_clients.size() > 0;
+}
+
 void osd_messenger_t::handle_send(int result, osd_client_t *cl)
 {
     cl->write_msg.msg_iovlen = 0;