derived: DerivedMeasurments now return DerivedMetrics

DerivedMeasurments processors now return DerviedMetrics rather than
measurments. The notion of an InstrumentChannel doesn't really make
sense in the context of DerivedMeasurments, which are not directly
measured on the target. Since Measurement's require a channel, a simpler
DerviedMetric is added that only requires a name and a type.

devlib/__init__.py

@@ -19,7 +19,7 @@ from devlib.instrument.monsoon import MonsoonInstrument
 from devlib.instrument.netstats import NetstatsInstrument
 from devlib.instrument.gem5power import Gem5PowerInstrument
 
-from devlib.derived import DerivedMeasurements
+from devlib.derived import DerivedMeasurements, DerivedMetric
 from devlib.derived.energy import DerivedEnergyMeasurements
 
 from devlib.trace.ftrace import FtraceCollector

devlib/derived/__init__.py

@@ -12,6 +12,45 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 #
+
+from devlib.instrument import MeasurementType, MEASUREMENT_TYPES
+
+
+class DerivedMetric(object):
+
+    __slots__ = ['name', 'value', 'measurement_type']
+
+    @property
+    def units(self):
+        return self.measurement_type.units
+
+    def __init__(self, name, value, measurement_type):
+        self.name = name
+        self.value = value
+        if isinstance(measurement_type, MeasurementType):
+            self.measurement_type = measurement_type
+        else:
+            try:
+                self.measurement_type = MEASUREMENT_TYPES[measurement_type]
+            except KeyError:
+                msg = 'Unknown measurement type:  {}'
+                raise ValueError(msg.format(measurement_type))
+
+    def __cmp__(self, other):
+        if hasattr(other, 'value'):
+            return cmp(self.value, other.value)
+        else:
+            return cmp(self.value, other)
+
+    def __str__(self):
+        if self.units:
+            return '{}: {} {}'.format(self.name, self.value, self.units)
+        else:
+            return '{}: {}'.format(self.name, self.value)
+
+    __repr__ = __str__
+
+
 class DerivedMeasurements(object):
 
     @staticmethod

devlib/derived/energy.py

@@ -15,8 +15,8 @@
 from __future__ import division
 from collections import defaultdict
 
-from devlib import DerivedMeasurements
+from devlib import DerivedMeasurements, DerivedMetric
-from devlib.instrument import Measurement, MEASUREMENT_TYPES, InstrumentChannel
+from devlib.instrument import  MEASUREMENT_TYPES, InstrumentChannel
 
 
 class DerivedEnergyMeasurements(DerivedMeasurements):
@@ -86,12 +86,12 @@ class DerivedEnergyMeasurements(DerivedMeasurements):
         derived_measurements = []
         for site in energy_results:
             total_energy = energy_results[site]['end'] - energy_results[site]['start']
-            instChannel = InstrumentChannel('cum_energy', site, MEASUREMENT_TYPES['energy'])
+            name = '{}_total_energy'.format(site)
-            derived_measurements.append(Measurement(total_energy, instChannel))
+            derived_measurements.append(DerivedMetric(name, total_energy, MEASUREMENT_TYPES['energy']))
 
         for site in power_results:
             power = power_results[site] / (count + 1)  #pylint: disable=undefined-loop-variable
-            instChannel = InstrumentChannel('avg_power', site, MEASUREMENT_TYPES['power'])
+            name = '{}_average_power'.format(site)
-            derived_measurements.append(Measurement(power, instChannel))
+            derived_measurements.append(DerivedMetric(name, power, MEASUREMENT_TYPES['power']))
 
         return derived_measurements

doc/derived_measurements.rst

@@ -9,7 +9,7 @@ Example
 -------
 
 The following example shows how to use an implementation of a
-:class:`DerivedMeasurement` to obtain a list of calculated ``Measurements``.
+:class:`DerivedMeasurement` to obtain a list of calculated ``DerivedMetric``'s.
 
 .. code-block:: ipython
 
@@ -42,9 +42,39 @@ Derived Measurements
 
 .. method:: DerivedMeasurements.process(measurement_csv)
 
-   Returns a list of :class:`Measurement` objects that have been calculated.
+   Returns a list of :class:`DerivedMetric` objects that have been calculated.
 
 
+Derived Metric
+~~~~~~~~~~~~~~
+
+.. class:: DerivedMetric
+
+  Represents a metric derived from previously collected ``Measurement``s.
+  Unlike, a ``Measurement``, this was not measured directly from the target.
+
+
+.. attribute:: DerivedMetric.name
+
+   The name of the derived metric. This uniquely defines a metric -- two
+   ``DerivedMetric`` objects with the same ``name`` represent to instances of
+   the same metric (e.g. computed from two different inputs).
+
+.. attribute:: DerivedMetric.value
+
+   The ``numeric`` value of the metric that has been computed for a particular
+   input.
+
+.. attribute:: DerivedMetric.measurement_type
+
+   The ``MeasurementType`` of the metric. This indicates which conceptual
+   category the metric falls into, its units, and conversions to other
+   measurement types.
+
+.. attribute:: DerivedMetric.units
+
+   The units in which the metric's value is expressed.
+
 
 Available Derived Measurements
 -------------------------------
@@ -63,7 +93,7 @@ Available Derived Measurements
 
 .. method:: DerivedEnergyMeasurements.process(measurement_csv)
 
-  Returns a list of :class:`Measurement` objects that have been calculated for
+  Returns a list of :class:`DerivedMetric` objects that have been calculated for
   the average power and cumulative energy for each site.