L2SS-245: use a pre-created 0-value array to avoid latencies in the read function

devices/test/devices/monitoring_performance_test.py

@@ -28,8 +28,8 @@ ARRAY_SIZE = 2000000
 class Monitoring_Performance_Device(Device):
     global ARRAY_SIZE
     def read_array(self):
-        print("{} {} read_big".format(time.time(), self.get_name()))
-        return numpy.array([random.random(), ] * ARRAY_SIZE)
+        print("{} {}".format(time.time(), self.get_name()))
+        return self._array
 
     array1_r = attribute(
         dtype = (numpy.double,),
@@ -90,6 +90,8 @@ class Monitoring_Performance_Device(Device):
 
         self.set_state(DevState.OFF)
 
+        self._array = numpy.zeros(ARRAY_SIZE)
+
         self.array1_r.set_data_ready_event(True)
         self.set_change_event("array1_r", True, True)
         self.set_archive_event("array1_r", True, True)