Measure monitor loop execution time when calculating sleep time

Documentation/Monitors/Monitor-Common.md

@@ -16,12 +16,13 @@ the `monitorpw` parameter, that value will be used instead.
 
 ### `monitor_interval`
 
-This is the time the monitor waits between each cycle of monitoring. The default
+Defines, in milliseconds, how often the monitor updates the status of the
-value of 2000 milliseconds (2 seconds) should be lowered if you want a faster
+servers. The default value is 2000 (2 seconds). Choose a lower value if servers
-response to changes in the server states. The value is defined in milliseconds
+should be queried more often. The smallest possible value is 100. If querying
-and the smallest possible value is 100 milliseconds.
+the servers takes longer than `monitor_interval`, the effective update rate is
+reduced.
 
-The default value of _monitor_interval_ was updated from 10000 milliseconds to
+The default value of `monitor_interval` was updated from 10000 milliseconds to
 2000 milliseconds in MaxScale 2.2.0.
 
 ```

include/maxscale/monitor.hh

@@ -220,6 +220,7 @@ private:
 
     void main();
     static void main(void* pArg);
+    void sleep_until_next_tick(int64_t tick_start_ms);
 };
 
 class MonitorInstanceSimple : public MonitorInstance

server/core/monitor.cc

@@ -2927,6 +2927,9 @@ void MonitorInstance::main()
 
     while (!m_shutdown)
     {
+        /* Measure the time of monitor loop execution. */
+        int64_t loop_start_ms = get_time_ms();
+
         monitor_check_maintenance_requests(m_monitor);
 
         tick();
@@ -2938,24 +2941,34 @@ void MonitorInstance::main()
 
         mon_hangup_failed_servers(m_monitor);
         store_server_journal(m_monitor, m_master);
-
+        sleep_until_next_tick(loop_start_ms);
-        /** Sleep until the next monitoring interval */
-        unsigned int ms = 0;
-        while (ms < m_monitor->interval && !m_shutdown)
-        {
-            if (atomic_load_int(&m_monitor->check_maintenance_flag) != MAINTENANCE_FLAG_NOCHECK)
-            {
-                // Admin has changed something, skip sleep
-                break;
-            }
-            thread_millisleep(MXS_MON_BASE_INTERVAL_MS);
-            ms += MXS_MON_BASE_INTERVAL_MS;
-        }
     }
 
     post_loop();
 }
 
+/**
+ * Sleep until the next monitor tick
+ *
+ * @param tick_start_ms When was the latest tick started
+ */
+void MonitorInstance::sleep_until_next_tick(int64_t tick_start_ms)
+{
+    // Check how much the monitor should sleep to get one full monitor interval.
+    int64_t sleep_time_remaining = m_monitor->interval - (get_time_ms() - tick_start_ms);
+    // Sleep at least one base interval.
+    sleep_time_remaining = MXS_MAX(MXS_MON_BASE_INTERVAL_MS, sleep_time_remaining);
+    /* Sleep in small increments to react fast to changes. */
+    while (sleep_time_remaining > 0 && !should_shutdown() &&
+           atomic_load_int(&m_monitor->check_maintenance_flag) == MAINTENANCE_FLAG_NOCHECK)
+    {
+        int small_sleep_ms = (sleep_time_remaining >= MXS_MON_BASE_INTERVAL_MS) ?
+            MXS_MON_BASE_INTERVAL_MS : sleep_time_remaining;
+        thread_millisleep(small_sleep_ms);
+        sleep_time_remaining -= small_sleep_ms;
+    }
+}
+
 //static
 void MonitorInstance::main(void* pArg)
 {