Yes,I receive the trace after changing RequestSenderDirect to use zmq.NOBLOCK in sending.

Here are the reasons for using zmq.NOBLOCK:
1、nova-compute send CALL message to nova-conductor , this is normal procedure:
[root@host224 home(keystone_admin)]# tail -f /var/log/nova/nova-compute.log
2017-02-08 10:59:51.588 28874 DEBUG oslo_messaging._drivers.zmq_driver.client.zmq_routing_table [-] Processing target ROUTER/conductor for round-robin. _fetch_round_robin_hosts_from_matchmaker /usr/lib/python2.7/site-packages/oslo_messaging/_drivers/zmq_driver/client/zmq_routing_table.py:59
2017-02-08 10:59:51.588 28874 DEBUG oslo_messaging._drivers.zmq_driver.client.zmq_routing_table [-] Host resolved for the current connection is host224:62374 get_round_robin_host /usr/lib/python2.7/site-packages/oslo_messaging/_drivers/zmq_driver/client/zmq_routing_table.py:48
2017-02-08 10:59:51.588 28874 DEBUG oslo_messaging._drivers.zmq_driver.zmq_socket [-] Connecting DEALER socket 3937d22b-4335-4859-8a32-43a834d13675 to tcp://host224:62374 connect_to_address /usr/lib/python2.7/site-packages/oslo_messaging/_drivers/zmq_driver/zmq_socket.py:188
2017-02-08 10:59:51.588 28874 DEBUG oslo_messaging._drivers.zmq_driver.client.zmq_routing_table [-] Processing target ROUTER/conductor for round-robin. _fetch_round_robin_hosts_from_matchmaker /usr/lib/python2.7/site-packages/oslo_messaging/_drivers/zmq_driver/client/zmq_routing_table.py:59
2017-02-08 10:59:51.589 28874 DEBUG oslo_messaging._drivers.zmq_driver.zmq_socket [-] Connecting DEALER socket 3937d22b-4335-4859-8a32-43a834d13675 to tcp://host224:53730 connect_to_address /usr/lib/python2.7/site-packages/oslo_messaging/_drivers/zmq_driver/zmq_socket.py:188
2017-02-08 10:59:51.589 28874 DEBUG oslo_messaging._drivers.zmq_driver.zmq_socket [-] Connecting DEALER socket 3937d22b-4335-4859-8a32-43a834d13675 to tcp://host224:53038 connect_to_address /usr/lib/python2.7/site-packages/oslo_messaging/_drivers/zmq_driver/zmq_socket.py:188
2017-02-08 10:59:51.589 28874 DEBUG oslo_messaging._drivers.zmq_driver.client.zmq_senders [-] Sending CALL message a8ef9d4e-edaa-11e6-9e4b-fa163e6b3caa to target <Target topic=conductor, version=3.0> (v1.0) send /usr/lib/python2.7/site-packages/oslo_messaging/_drivers/zmq_driver/client/zmq_senders.py:155
2017-02-08 10:59:51.589 28874 DEBUG oslo_messaging._drivers.zmq_driver.poller.green_poller [-] Registering socket 3937d22b-4335-4859-8a32-43a834d13675 register /usr/lib/python2.7/site-packages/oslo_messaging/_drivers/zmq_driver/poller/green_poller.py:32
2017-02-08 10:59:51.600 28874 DEBUG oslo_messaging._drivers.zmq_driver.client.zmq_receivers [-] Received REPLY for a8ef9d4e-edaa-11e6-9e4b-fa163e6b3caa _run_loop /usr/lib/python2.7/site-packages/oslo_messaging/_drivers/zmq_driver/client/zmq_receivers.py:125
2017-02-08 10:59:51.605 28874 DEBUG oslo_messaging._drivers.zmq_driver.poller.green_poller [-] Unregistering socket 3937d22b-4335-4859-8a32-43a834d13675 unregister /usr/lib/python2.7/site-packages/oslo_messaging/_drivers/zmq_driver/poller/green_poller.py:40
2017-02-08 10:59:51.605 28874 DEBUG oslo_messaging._drivers.zmq_driver.zmq_socket [-] Socket 3937d22b-4335-4859-8a32-43a834d13675 closed close /usr/lib/python2.7/site-packages/oslo_messaging/_drivers/zmq_driver/zmq_socket.py:179
[root@host224 home(keystone_admin)]# redis-cli smembers ROUTER/conductor
1) "host224:53038"
2) "host224:53730"
3) "host224:58548"
4) "host224:62374"

2、When restart nova-conductor, nova-compute send CALL message to conductor, this is abnormal procedure, socket.send() function has been blocked.
[root@host224 home(keystone_admin)]# systemctl restart openstack-nova-conductor.service 
[root@host224 home(keystone_admin)]# redis-cli smembers ROUTER/conductor/host224
1) "host224:50534"
2) "host224:61061"
3) "host224:65061"
4) "host224:49290"
[root@host224 home(keystone_admin)]# tail -f /var/log/nova/nova-compute.log
2017-02-08 11:00:01.588 28874 DEBUG oslo_messaging._drivers.zmq_driver.client.zmq_routing_table [-] Processing target ROUTER/conductor for round-robin. _fetch_round_robin_hosts_from_matchmaker /usr/lib/python2.7/site-packages/oslo_messaging/_drivers/zmq_driver/client/zmq_routing_table.py:59
2017-02-08 11:00:01.588 28874 DEBUG oslo_messaging._drivers.zmq_driver.client.zmq_routing_table [-] Host resolved for the current connection is host224:53730 get_round_robin_host /usr/lib/python2.7/site-packages/oslo_messaging/_drivers/zmq_driver/client/zmq_routing_table.py:48
2017-02-08 11:00:01.588 28874 DEBUG oslo_messaging._drivers.zmq_driver.zmq_socket [-] Connecting DEALER socket f7381ccd-831f-4a20-b677-86f93f3cef91 to tcp://host224:53730 connect_to_address /usr/lib/python2.7/site-packages/oslo_messaging/_drivers/zmq_driver/zmq_socket.py:188
2017-02-08 11:00:01.588 28874 DEBUG oslo_messaging._drivers.zmq_driver.client.zmq_routing_table [-] Processing target ROUTER/conductor for round-robin. _fetch_round_robin_hosts_from_matchmaker /usr/lib/python2.7/site-packages/oslo_messaging/_drivers/zmq_driver/client/zmq_routing_table.py:59
2017-02-08 11:00:01.588 28874 DEBUG oslo_messaging._drivers.zmq_driver.zmq_socket [-] Connecting DEALER socket f7381ccd-831f-4a20-b677-86f93f3cef91 to tcp://host224:53038 connect_to_address /usr/lib/python2.7/site-packages/oslo_messaging/_drivers/zmq_driver/zmq_socket.py:188
2017-02-08 11:00:45.347 28874 DEBUG oslo_service.periodic_task [req-522af5ab-10aa-47d9-8ba5-7f5a4edc35e4 - - - - -] Running periodic task ComputeManager._poll_volume_usage run_periodic_tasks /usr/lib/python2.7/site-packages/oslo_service/periodic_task.py:215
2017-02-08 11:00:45.348 28874 DEBUG oslo_service.periodic_task [req-522af5ab-10aa-47d9-8ba5-7f5a4edc35e4 - - - - -] Running periodic task ComputeManager.update_available_resource run_periodic_tasks /usr/lib/python2.7/site-packages/oslo_service/periodic_task.py:215
2017-02-08 11:00:45.350 28874 DEBUG oslo_messaging._drivers.zmq_driver.client.zmq_routing_table [req-522af5ab-10aa-47d9-8ba5-7f5a4edc35e4 - - - - -] Processing target ROUTER/conductor for round-robin. _fetch_round_robin_hosts_from_matchmaker /usr/lib/python2.7/site-packages/oslo_messaging/_drivers/zmq_driver/client/zmq_routing_table.py:59
2017-02-08 11:00:45.350 28874 DEBUG oslo_messaging._drivers.zmq_driver.client.zmq_routing_table [req-522af5ab-10aa-47d9-8ba5-7f5a4edc35e4 - - - - -] Host resolved for the current connection is host224:53038 get_round_robin_host /usr/lib/python2.7/site-packages/oslo_messaging/_drivers/zmq_driver/client/zmq_routing_table.py:48
2017-02-08 11:00:45.351 28874 DEBUG oslo_messaging._drivers.zmq_driver.zmq_socket [req-522af5ab-10aa-47d9-8ba5-7f5a4edc35e4 - - - - -] Connecting DEALER socket d05b6c5d-69cf-4e4f-887f-ca80db8a2153 to tcp://host224:53038 connect_to_address /usr/lib/python2.7/site-packages/oslo_messaging/_drivers/zmq_driver/zmq_socket.py:188
2017-02-08 11:00:45.351 28874 DEBUG oslo_messaging._drivers.zmq_driver.client.zmq_routing_table [req-522af5ab-10aa-47d9-8ba5-7f5a4edc35e4 - - - - -] Processing target ROUTER/conductor for round-robin. _fetch_round_robin_hosts_from_matchmaker /usr/lib/python2.7/site-packages/oslo_messaging/_drivers/zmq_driver/client/zmq_routing_table.py:59
2017-02-08 11:00:45.351 28874 DEBUG oslo_messaging._drivers.zmq_driver.zmq_socket [req-522af5ab-10aa-47d9-8ba5-7f5a4edc35e4 - - - - -] Connecting DEALER socket d05b6c5d-69cf-4e4f-887f-ca80db8a2153 to tcp://host224:53730 connect_to_address /usr/lib/python2.7/site-packages/oslo_messaging/_drivers/zmq_driver/zmq_socket.py:188
2017-02-08 11:01:45.884 28874 DEBUG oslo_messaging._drivers.zmq_driver.client.zmq_routing_table [-] Updating routing table from the matchmaker. 2 target(s) updated ['ROUTER/scheduler', 'ROUTER/conductor']. _update_routing_table /usr/lib/python2.7/site-packages/oslo_messaging/_drivers/zmq_driver/client/zmq_routing_table.py:194
2017-02-08 11:04:45.887 28874 DEBUG oslo_messaging._drivers.zmq_driver.client.zmq_routing_table [-] Updating routing table from the matchmaker. 2 target(s) updated ['ROUTER/scheduler', 'ROUTER/conductor']. _update_routing_table /usr/lib/python2.7/site-packages/oslo_messaging/_drivers/zmq_driver/client/zmq_routing_table.py:194
2017-02-08 11:07:45.890 28874 DEBUG oslo_messaging._drivers.zmq_driver.client.zmq_routing_table [-] Updating routing table from the matchmaker. 2 target(s) updated ['ROUTER/scheduler', 'ROUTER/conductor']. _update_routing_table /usr/lib/python2.7/site-packages/oslo_messaging/_drivers/zmq_driver/client/zmq_routing_table.py:194
2017-02-08 11:10:45.893 28874 DEBUG oslo_messaging._drivers.zmq_driver.client.zmq_routing_table [-] Updating routing table from the matchmaker. 2 target(s) updated ['ROUTER/scheduler', 'ROUTER/conductor']. _update_routing_table /usr/lib/python2.7/site-packages/oslo_messaging/_drivers/zmq_driver/client/zmq_routing_table.py:194.
....................