This shows up in n-cpu:

The "model server went away" showed up 11 times in the last two weeks with the last one being on Dec. 3. This sample size is too small for me to close at this time.

2013-11-25 15:24:22.099 21076 ERROR nova.servicegroup.drivers.db [-] model server went away
2013-11-25 15:24:32.814 ERROR nova.compute.manager [req-ecacaa21-3f07-4b44-9896-8b5bd2238a19 ServersTestManualDisk-tempest-1962756300-user ServersTestManualDisk-tempest-1962756300-tenant] [instance: 1f872097-8ad8-44f8-ba03-89a14115efe0] Failed to deallocate network for instance.
2013-11-25 15:25:32.855 21076 ERROR root [-] Original exception being dropped: ['Traceback (most recent call last):\n', '  File "/opt/stack/new/nova/nova/compute/manager.py", line 1809, in _try_deallocate_network\n    self._deallocate_network(context, instance, requested_networks)\n', '  File "/opt/stack/new/nova/nova/compute/manager.py", line 1491, in _deallocate_network\n    context, instance, requested_networks=requested_networks)\n', '  File "/opt/stack/new/nova/nova/network/api.py", line 93, in wrapped\n    return func(self, context, *args, **kwargs)\n', '  File "/opt/stack/new/nova/nova/network/api.py", line 318, in deallocate_for_instance\n    self.network_rpcapi.deallocate_for_instance(context, **args)\n', '  File "/opt/stack/new/nova/nova/network/rpcapi.py", line 199, in deallocate_for_instance\n    host=host, requested_networks=requested_networks)\n', '  File "/opt/stack/new/nova/nova/rpcclient.py", line 85, in call\n    return self._invoke(self.proxy.call, ctxt, method, **kwargs)\n', '  File "/opt/stack/new/nova/nova/rpcclient.py", line 63, in _invoke\n    return cast_or_call(ctxt, msg, **self.kwargs)\n', '  File "/opt/stack/new/nova/nova/openstack/common/rpc/proxy.py", line 130, in call\n    exc.info, real_topic, msg.get(\'method\'))\n', 'Timeout: Timeout while waiting on RPC response - topic: "network", RPC method: "deallocate_for_instance" info: "<unknown>"\n']
2013-11-25 15:25:38.371 21076 ERROR nova.openstack.common.periodic_task [-] Error during ComputeManager.update_available_resource: Timeout while waiting on RPC response - topic: "conductor", RPC method: "compute_node_update" info: "<unknown>"
2013-11-25 15:26:32.903 21076 ERROR root [-] Original exception being dropped: ['Traceback (most recent call last):\n', '  File "/opt/stack/new/nova/nova/compute/manager.py", line 1919, in _delete_instance\n    self._shutdown_instance(context, db_inst, bdms)\n', '  File "/opt/stack/new/nova/nova/compute/manager.py", line 1854, in _shutdown_instance\n    self._try_deallocate_network(context, instance, requested_networks)\n', '  File "/opt/stack/new/nova/nova/compute/manager.py", line 1814, in _try_deallocate_network\n    self._set_instance_error_state(context, instance[\'uuid\'])\n', '  File "/opt/stack/new/nova/nova/compute/manager.py", line 484, in _set_instance_error_state\n    vm_state=vm_states.ERROR)\n', '  File "/opt/stack/new/nova/nova/compute/manager.py", line 473, in _instance_update\n    **kwargs)\n', '  File "/opt/stack/new/nova/nova/conductor/api.py", line 389, in instance_update\n    updates, \'conductor\')\n', '  File "/opt/stack/new/nova/nova/conductor/rpcapi.py", line 149, in instance_update\n    service=service)\n', '  File "/opt/stack/new/nova/nova/rpcclient.py", line 85, in call\n    return self._invoke(self.proxy.call, ctxt, method, **kwargs)\n', '  File "/opt/stack/new/nova/nova/rpcclient.py", line 63, in _invoke\n    return cast_or_call(ctxt, msg, **self.kwargs)\n', '  File "/opt/stack/new/nova/nova/openstack/common/rpc/proxy.py", line 130, in call\n    exc.info, real_topic, msg.get(\'method\'))\n', 'Timeout: Timeout while waiting on RPC response - topic: "conductor", RPC method: "instance_update" info: "<unknown>"\n']
2013-11-25 15:26:32.933 21076 ERROR nova.servicegroup.drivers.db [-] Recovered model server connection!