Handling of timers with bias.

Timers that don't require maximum precision (most of them, actually) can be
triggered earlier or later within the bias interval.

To reduce wakeups by timers, the expire function now triggers not only all
timers that fall within the elapsed time, but also those whose bias falls
within this interval.

src/nxt_timer.c

@@ -75,7 +75,8 @@ nxt_timer_add(nxt_event_engine_t *engine, nxt_timer_t *timer,
 
     time = engine->timers.now + timeout;
 
-    nxt_debug(timer->task, "timer add: %M %M:%M", timer->time, timeout, time);
+    nxt_debug(timer->task, "timer add: %M±%d %M:%M",
+              timer->time, timer->bias, timeout, time);
 
     timer->enabled = 1;
 
@@ -84,11 +85,12 @@ nxt_timer_add(nxt_event_engine_t *engine, nxt_timer_t *timer,
         diff = nxt_msec_diff(time, timer->time);
         /*
          * Use the previous timer if difference between it and the
-         * new timer is less than required precision milliseconds: this
-         * decreases number of rbtree operations for fast connections.
+         * new timer is within bias: this decreases number of rbtree
+         * operations for fast connections.
          */
-        if (nxt_abs(diff) < timer->precision) {
-            nxt_debug(timer->task, "timer previous: %M", time);
+        if (nxt_abs(diff) <= timer->bias) {
+            nxt_debug(timer->task, "timer previous: %M±%d",
+                      time, timer->bias);
 
             nxt_timer_change(engine, timer, NXT_TIMER_NOPE, 0);
             return;
@@ -102,7 +104,8 @@ nxt_timer_add(nxt_event_engine_t *engine, nxt_timer_t *timer,
 nxt_bool_t
 nxt_timer_delete(nxt_event_engine_t *engine, nxt_timer_t *timer)
 {
-    nxt_debug(timer->task, "timer delete: %M", timer->time);
+    nxt_debug(timer->task, "timer delete: %M±%d",
+              timer->time, timer->bias);
 
     timer->enabled = 0;
 
@@ -142,7 +145,8 @@ nxt_timer_change(nxt_event_engine_t *engine, nxt_timer_t *timer,
         timer->change = timers->nchanges;
     }
 
-    nxt_debug(timer->task, "timer change: %M:%d", time, change);
+    nxt_debug(timer->task, "timer change: %M±%d:%d",
+              time, timer->bias, change);
 
     ch = &timers->changes[timer->change - 1];
 
@@ -190,7 +194,8 @@ nxt_timer_changes_commit(nxt_event_engine_t *engine)
             /* Fall through. */
 
         case NXT_TIMER_DELETE:
-            nxt_debug(timer->task, "timer rbtree delete: %M", timer->time);
+            nxt_debug(timer->task, "timer rbtree delete: %M±%d",
+                      timer->time, timer->bias);
 
             nxt_rbtree_delete(&timers->tree, &timer->node);
             nxt_timer_in_tree_clear(timer);
@@ -206,7 +211,8 @@ nxt_timer_changes_commit(nxt_event_engine_t *engine)
     while (add < add_end) {
         timer = *add;
 
-        nxt_debug(timer->task, "timer rbtree insert: %M", timer->time);
+        nxt_debug(timer->task, "timer rbtree insert: %M±%d",
+                  timer->time, timer->bias);
 
         nxt_rbtree_insert(&timers->tree, &timer->node);
         nxt_timer_in_tree_set(timer);
@@ -252,10 +258,10 @@ nxt_timer_find(nxt_event_engine_t *engine)
 
         if (timer->enabled) {
             time = timer->time;
-            timers->minimum = time;
+            timers->minimum = time - timer->bias;
 
-            nxt_debug(timer->task, "timer found minimum: %M:%M",
-                      time, timers->now);
+            nxt_debug(timer->task, "timer found minimum: %M±%d:%M",
+                      time, timer->bias, timers->now);
 
             delta = nxt_msec_diff(time, timers->now);
 
@@ -297,14 +303,15 @@ nxt_timer_expire(nxt_event_engine_t *engine, nxt_msec_t now)
     {
         timer = (nxt_timer_t *) node;
 
-                       /* timer->time > now */
-        if (nxt_msec_diff(timer->time , now) > 0) {
+                       /* timer->time > now + timer->bias */
+        if (nxt_msec_diff(timer->time , now) > (int32_t) timer->bias) {
             return;
         }
 
         next = nxt_rbtree_node_successor(tree, node);
 
-        nxt_debug(timer->task, "timer expire delete: %M", timer->time);
+        nxt_debug(timer->task, "timer expire delete: %M±%d",
+                  timer->time, timer->bias);
 
         nxt_rbtree_delete(tree, &timer->node);
         nxt_timer_in_tree_clear(timer);