inspector: memory: use usable of memoryStats if available

Since kernel v4.6, virtio balloon driver commit 5057dcd0f
introduced metric VIRTIO_BALLOON_S_AVAIL,
corresponding to 'Available' in /proc/meminfo.
Libvirt exposed this metric as 'usable'.

As 'Available' of meminfo is an estimate of how much memory
is available for starting new applications, without swapping.

It's a better metric for calculating memory_usage.

Change-Id: I3b935f1fc2ed74ca45b26990c4f2bd5996e1dfea
Signed-off-by: Chen Hanxiao <chenhx@certusnet.com.cn>

ceilometer/compute/virt/libvirt/inspector.py

@@ -151,7 +151,10 @@ class LibvirtInspector(virt_inspector.Inspector):
         memory_swap_in = memory_swap_out = None
         memory_stats = domain.memoryStats()
         # Stat provided from libvirt is in KB, converting it to MB.
-        if 'available' in memory_stats and 'unused' in memory_stats:
+        if 'usable' in memory_stats and 'available' in memory_stats:
+            memory_used = (memory_stats['available'] -
+                           memory_stats['usable']) / units.Ki
+        elif 'available' in memory_stats and 'unused' in memory_stats:
             memory_used = (memory_stats['available'] -
                            memory_stats['unused']) / units.Ki
         if 'rss' in memory_stats:

ceilometer/tests/unit/compute/virt/libvirt/test_inspector.py

@@ -474,6 +474,27 @@ class TestLibvirtInspection(base.BaseTestCase):
             self.assertIsNone(stats.memory_swap_in)
             self.assertIsNone(stats.memory_swap_out)
 
+    def test_inspect_memory_with_usable(self):
+        domain = mock.Mock()
+        domain.info.return_value = (0, 0, 0, 2, 999999)
+        domain.memoryStats.return_value = {'available': 76800,
+                                           'rss': 30000,
+                                           'swap_in': 5120,
+                                           'swap_out': 8192,
+                                           'unused': 25600,
+                                           'usable': 51200}
+        conn = mock.Mock()
+        conn.domainListGetStats.return_value = [({}, {})]
+        conn.lookupByUUIDString.return_value = domain
+
+        with mock.patch('ceilometer.compute.virt.libvirt.utils.'
+                        'refresh_libvirt_connection', return_value=conn):
+            stats = self.inspector.inspect_instance(self.instance, None)
+            self.assertEqual(25600 / units.Ki, stats.memory_usage)
+            self.assertEqual(30000 / units.Ki, stats.memory_resident)
+            self.assertEqual(5120 / units.Ki, stats.memory_swap_in)
+            self.assertEqual(8192 / units.Ki, stats.memory_swap_out)
+
     def test_inspect_perf_events_libvirt_less_than_2_3_0(self):
         domain = mock.Mock()
         domain.info.return_value = (0, 0, 51200, 2, 999999)

releasenotes/notes/use-usable-metric-if-available-970ee58e8fdeece6.yaml

@@ -0,0 +1,3 @@
+---
+features:
+  - use memory usable metric from libvirt memoryStats if available.