remove superfluous threads after a grace period

etc/arangodb/arangod.conf.in

@@ -35,14 +35,16 @@ endpoint = tcp://0.0.0.0:8529
 disable-authentication = yes
 
 # number of server threads
-threads = 10
+threads = 4
 
 # the user and group are normally set in the start script
 # uid = arangodb
 # gid = arangodb
 
 [scheduler]
-threads = 3
+
+# number of threads used for I/O
+threads = 1
 
 [javascript]
 startup-directory = @PKGDATADIR@/js

etc/relative/arangod.conf

@@ -5,11 +5,11 @@
 [server]
 disable-authentication = true
 endpoint = tcp://localhost:8529
-threads = 10
+threads = 4
 # reuse-address = false
 
 [scheduler]
-threads = 3
+threads = 1
 
 [javascript]
 startup-directory = ./js

lib/Dispatcher/DispatcherQueue.cpp

@@ -74,6 +74,8 @@ DispatcherQueue::DispatcherQueue (Scheduler* scheduler,
     _nrSpecial(0),
     _nrBlocked(0),
     _nrThreads(nrThreads),
+    _lastChanged(0.0),
+    _gracePeriod(5.0),
     _scheduler(scheduler),
     _dispatcher(dispatcher),
     createDispatcherThread(creator) {
@@ -370,6 +372,7 @@ bool DispatcherQueue::startQueueThread () {
   }
   else {
     _nrStarted++;
+    _lastChanged = TRI_microtime();
   }
 
   return ok;

lib/Dispatcher/DispatcherQueue.h

@@ -316,6 +316,18 @@ namespace triagens {
 
         size_t _nrThreads;
 
+////////////////////////////////////////////////////////////////////////////////
+/// @brief last time we created or deleted a queue thread
+////////////////////////////////////////////////////////////////////////////////
+
+        double _lastChanged;
+
+////////////////////////////////////////////////////////////////////////////////
+/// @brief grace period before shuting down queue threads
+////////////////////////////////////////////////////////////////////////////////
+
+        double _gracePeriod;
+
 ////////////////////////////////////////////////////////////////////////////////
 /// @brief scheduler
 ////////////////////////////////////////////////////////////////////////////////

lib/Dispatcher/DispatcherThread.cpp

@@ -266,6 +266,16 @@ void DispatcherThread::run () {
       tick(true);
       _queue->_accessQueue.lock();
 
+      // there is a chance, that we created more threads than necessary
+      if (_queue->_nrThreads + _queue->_nrBlocked < _queue->_nrRunning + _queue->_nrStarted + _queue->_nrWaiting) {
+        double n = TRI_microtime();
+
+        if (_queue->_lastChanged + _queue->_gracePeriod < n) {
+          _queue->_lastChanged = n;
+          break;
+        }
+      }
+
       // wait, if there are no jobs
       if (_queue->_readyJobs.empty()) {
         _queue->_nrRunning--;