Disable garbage collector when reading sensors to improve timing

CHANGES.rst

@@ -57,8 +57,9 @@ Development
 - Prettify logging
 - Optionally start modeserver, defaulting to true
 - Add basic HTTP API subsystem. Thanks `@vkuhlen`_ and `@DieDiren`_.
-- Add ``make wifi-ap`` command for starting the access point interactively
+- Add ``make provide-wifi`` command for starting the access point interactively
 - Start WiFi in STA_AP mode by default
+- Disable garbage collector when reading sensors to improve timing
 
 
 2019-06-22 0.5.1

terkin/datalogger.py

@@ -13,7 +13,7 @@
 from terkin.network import SystemWiFiMetrics
 from terkin.sensor import SensorManager, AbstractSensor
 from terkin.sensor.system import SystemMemoryFree, SystemTemperature, SystemBatteryLevel, SystemUptime
-from terkin.util import dformat
+from terkin.util import dformat, gc_disabled
 
 log = logging.getLogger(__name__)
 
@@ -274,14 +274,21 @@ def read_sensors(self):
             sensorname = sensor.__class__.__name__
             log.info('Reading sensor port "%s"', sensorname)
 
+            # Read sensor port.
             try:
-                reading = sensor.read()
+
+                # Disable garbage collector to guarantee reasonable
+                # realtime behavior before invoking sensor reading.
+                with gc_disabled():
+                    reading = sensor.read()
                 if reading is None or reading is AbstractSensor.SENSOR_NOT_INITIALIZED:
                     continue
                 data.update(reading)
 
-            except:
-                log.exception('Reading sensor "%s" failed', sensorname)
+            except Exception as ex:
+                # Because of the ``gc_disabled`` context manager used above,
+                # the propagation of exceptions has to be tweaked like that.
+                log.exc(ex, 'Reading sensor "%s" failed', sensorname)
 
             self.device.watchdog.feed()
 

terkin/util.py

@@ -144,3 +144,35 @@ def flatten(input_object, separator='_'):
     https://github.com/evamayerova/python-flatten/blob/master/flatten_to_dict/flatten_to_dict.py
     """
     return _flatten(input_object, key_prefix="", separator=separator)
+
+
+class gc_disabled:
+    """
+    Context manager to temporarily disable the garbage collector.
+
+    Please be aware this piece does not account for thread safety in any way.
+
+    https://community.hiveeyes.org/t/timing-things-on-micropython-for-esp32/2329
+    https://bugs.python.org/issue31356
+
+    Synopsis::
+
+        with gc_disabled():
+            # Do something that needs realtime guarantees
+            # such as a pair trade, robotic braking, etc.
+            run_some_timing_critical_stuff()
+
+    """
+
+    def __enter__(self):
+        import gc
+        gc.disable()
+        return self
+
+    def __exit__(self, *exc_details):
+        # exc_info: (<class 'NameError'>, NameError("name 'asdf' is not defined",), None)
+        received_exc = exc_details[0] is not None
+        import gc
+        gc.enable()
+        if received_exc:
+            raise exc_details[1]