Fix snmp mac polling performance issue. (sonic-net#95)

When SONiC system get a few hundreds MACs learnt on vlans, snmp-subagent
started to see below errors:
INFO supervisord: snmp-subagent socket.send() raised exception.
ERR snmp-subagent [ax_interface] ERROR: [Errno 32] Broken pipe

This was due to the snmpd getting timeout from subagent and closed the socket.

It was root caused to be the mac polling algorithm inefficiency.
The redis-server was taking average > 60% CPU cycles (peak to 85~90%) of one core.
After the fix in this commit, the redis-server load is average to
~3% (peak to ~6%) of one core.

Author: zhenggen-xu

src/sonic_ax_impl/mibs/ietf/rfc4363.py

@@ -7,13 +7,6 @@
 from ax_interface.util import mac_decimals
 from bisect import bisect_right
 
-def fdb_vlanmac(db_conn, fdb):
-    if 'vlan' in fdb:
-        vlan_id = fdb["vlan"]
-    elif 'bvid' in fdb:
-        vlan_id = port_util.get_vlan_id_from_bvid(db_conn, fdb["bvid"])
-    return (int(vlan_id),) + mac_decimals(fdb["mac"])
-
 class FdbUpdater(MIBUpdater):
     def __init__(self):
         super().__init__()
@@ -27,7 +20,19 @@ def __init__(self):
         self.vlanmac_ifindex_map = {}
         self.vlanmac_ifindex_list = []
         self.if_bpid_map = {}
-
+        self.bvid_vlan_map = {}
+
+    def fdb_vlanmac(self, fdb):
+        if 'vlan' in fdb:
+            vlan_id = fdb["vlan"]
+        elif 'bvid' in fdb:
+            if fdb["bvid"] in self.bvid_vlan_map:
+                vlan_id = self.bvid_vlan_map[fdb["bvid"]]
+            else:
+                vlan_id = port_util.get_vlan_id_from_bvid(self.db_conn, fdb["bvid"])
+                self.bvid_vlan_map[fdb["bvid"]] = vlan_id
+        return (int(vlan_id),) + mac_decimals(fdb["mac"])
+          
     def reinit_data(self):
         """
         Subclass update interface information
@@ -39,7 +44,7 @@ def reinit_data(self):
         self.oid_name_map = mibs.init_sync_d_interface_tables(self.db_conn)
 
         self.if_bpid_map = port_util.get_bridge_port_map(self.db_conn)
-
+        self.bvid_vlan_map.clear()
 
     def update_data(self):
         """
@@ -69,7 +74,7 @@ def update_data(self):
                 continue
             port_id = self.if_bpid_map[bridge_port_id]
 
-            vlanmac = fdb_vlanmac(self.db_conn, fdb)
+            vlanmac = self.fdb_vlanmac(fdb)
             self.vlanmac_ifindex_map[vlanmac] = mibs.get_index(self.if_id_map[port_id])
             self.vlanmac_ifindex_list.append(vlanmac)
         self.vlanmac_ifindex_list.sort()