eMQTTD 0.5.4-alpha Released

Benchmark this release on a ubuntu/14.04 server with 8 cores, 32G memory from QingCloud:

200K Connections,
30K Messages/Sec,
20Mbps In/Out Traffic,
200K Topics,
200K Subscribers,

Consumed 7G memory, 40% CPU/core

Benchmark code: https://github.com/emqtt/emqttd_benchmark

Change: rewrite emqttd_pubsub to handle more concurrent subscribe requests.

Change: ./bin/emqttd_ctl add 'stats', 'metrics' commands.

Bugfix: issue #71, #72