[OVS] Add mac-table-size to be set on each ovs bridge

By default number of MAC addresses which ovs stores in memory
is quite low - 2048.

Any eviction of a MAC learning table entry triggers revalidation.
Such revalidation is very costly so it cause high CPU usage by
ovs-vswitchd process.

To workaround this problem, higher value of mac-table-size
option can be set for bridge. Then this revalidation will happen
less often and CPU usage will be lower.
This patch adds config option for neutron-openvswitch-agent to allow
users tune this setting in bridges managed by agent.
By default this value is set to 50000 which should be enough for most
systems.

Change-Id: If628f52d75c2b5fec87ad61e0219b3286423468c
Closes-Bug: #1775797

neutron/agent/common/ovs_lib.py

@@ -260,6 +260,8 @@ class OVSBridge(BaseOVS):
                                             key=version_from_protocol)
 
     def create(self, secure_mode=False):
+        other_config = {
+            'mac-table-size': str(cfg.CONF.OVS.bridge_mac_table_size)}
         with self.ovsdb.transaction() as txn:
             txn.add(
                 self.ovsdb.add_br(self.br_name,
@@ -271,6 +273,9 @@ class OVSBridge(BaseOVS):
             txn.add(
                 self.ovsdb.db_add('Bridge', self.br_name,
                                   'protocols', self._highest_protocol_needed))
+            txn.add(
+                self.ovsdb.db_set('Bridge', self.br_name,
+                                  ('other_config', other_config)))
             if secure_mode:
                 txn.add(self.ovsdb.set_fail_mode(self.br_name,
                                                  FAILMODE_SECURE))

neutron/conf/agent/ovs_conf.py

@@ -27,6 +27,13 @@ OPTS = [
                help=_('Timeout in seconds for ovsdb commands. '
                       'If the timeout expires, ovsdb commands will fail with '
                       'ALARMCLOCK error.')),
+    cfg.IntOpt('bridge_mac_table_size',
+               default=50000,
+               help=_('The maximum number of MAC addresses to learn on '
+                      'a bridge managed by the Neutron OVS agent. Values '
+                      'outside a reasonable range (10 to 1,000,000) might be '
+                      'overridden by Open vSwitch according to the '
+                      'documentation.')),
 ]
 
 

neutron/tests/functional/agent/test_ovs_lib.py

@@ -18,6 +18,7 @@ import uuid
 
 import mock
 from neutron_lib import constants as const
+from oslo_config import cfg
 from ovsdbapp.backend.ovs_idl import idlutils
 
 from neutron.agent.common import ovs_lib
@@ -603,6 +604,9 @@ class OVSLibTestCase(base.BaseOVSLinuxTestCase):
 
     def test_bridge_lifecycle_ovsbridge(self):
         name = utils.get_rand_name(prefix=net_helpers.BR_PREFIX)
+        mac_table_size = 12345
+        cfg.CONF.set_override(
+            'bridge_mac_table_size', mac_table_size, group='OVS')
         br = ovs_lib.OVSBridge(name)
         self.assertEqual(br.br_name, name)
         # Make sure that instantiating an OVSBridge does not actually create
@@ -610,6 +614,11 @@ class OVSLibTestCase(base.BaseOVSLinuxTestCase):
         self.addCleanup(self.ovs.delete_bridge, name)
         br.create()
         self.assertTrue(self.ovs.bridge_exists(name))
+        br_other_config = self.ovs.ovsdb.db_find(
+            'Bridge', ('name', '=', name), columns=['other_config']
+        ).execute()[0]['other_config']
+        self.assertEqual(str(mac_table_size),
+                         br_other_config['mac-table-size'])
         br.destroy()
         self.assertFalse(self.ovs.bridge_exists(name))
 

releasenotes/notes/ovs-mac-table-size-config-option-d255d5208650f34b.yaml

@@ -0,0 +1,14 @@
+---
+features:
+  - |
+    A new config option ``bridge_mac_table_size`` has been added for
+    Neutron OVS agent.
+    This value will be set on every Open vSwitch bridge managed by the
+    openvswitch-neutron-agent in ``other_config:mac-table-size`` column
+    in ovsdb.
+    Default value for this new option is set to 50000 and it should be enough
+    for most systems.
+    More details about this option can be found in `Open vSwitch documentation
+    <http://www.openvswitch.org/support/dist-docs/ovs-vswitchd.conf.db.5.html>`_
+    For more information see bug
+    `1775797 <https://bugs.launchpad.net/neutron/+bug/1775797>`_.