Timers: separation of delete and insert operations on rbtree.

Delete/insert operation complexity for a red-black tree is O(log n),
where n is the total number of tree elements.

If all delete operations are performed before all insert operations,
the average number of tree elements during an operation will be
lower than in the mixed-operations case.

src/nxt_timer.c

@@ -155,7 +155,7 @@ nxt_timer_change(nxt_event_engine_t *engine, nxt_timer_t *timer,
 static void
 nxt_timer_changes_commit(nxt_event_engine_t *engine)
 {
-    nxt_timer_t         *timer;
+    nxt_timer_t         *timer, **add, **add_end;
     nxt_timers_t        *timers;
     nxt_timer_change_t  *ch, *end;
 
@@ -166,6 +166,9 @@ nxt_timer_changes_commit(nxt_event_engine_t *engine)
     ch = timers->changes;
     end = ch + timers->nchanges;
 
+    add = (nxt_timer_t **) ch;
+    add_end = add;
+
     while (ch < end) {
         timer = ch->timer;
 
@@ -175,20 +178,16 @@ nxt_timer_changes_commit(nxt_event_engine_t *engine)
             break;
 
         case NXT_TIMER_ADD:
-            if (nxt_timer_is_in_tree(timer)) {
-                nxt_debug(timer->task, "timer rbtree delete: %M", timer->time);
-
-                nxt_rbtree_delete(&timers->tree, &timer->node);
-            }
 
             timer->time = ch->time;
 
-            nxt_debug(timer->task, "timer rbtree insert: %M", timer->time);
+            *add_end++ = timer;
 
-            nxt_rbtree_insert(&timers->tree, &timer->node);
-            nxt_timer_in_tree_set(timer);
+            if (!nxt_timer_is_in_tree(timer)) {
+                break;
+            }
 
-            break;
+            /* Fall through. */
 
         case NXT_TIMER_DELETE:
             nxt_debug(timer->task, "timer rbtree delete: %M", timer->time);
@@ -204,6 +203,17 @@ nxt_timer_changes_commit(nxt_event_engine_t *engine)
         ch++;
     }
 
+    while (add < add_end) {
+        timer = *add;
+
+        nxt_debug(timer->task, "timer rbtree insert: %M", timer->time);
+
+        nxt_rbtree_insert(&timers->tree, &timer->node);
+        nxt_timer_in_tree_set(timer);
+
+        add++;
+    }
+
     timers->nchanges = 0;
 }