MQTT Agent - Configuration values to modify timing performance

Agent queue block time:

Some TCP stacks provide a select functionality which allows any given observer to be notified of any incoming packet on the socket. This allows us to make the MQTT-Agent event driven by calling MQTTAgent_ProcessLoop when the observer is notified of any incoming packets on the socket. FreeRTOS+TCP can also use socket callbacks to notify the socket owner of incoming data on the socket. If your TCP stack does not provide this functionality, the agent will block on its event queue for a maximum of MQTT_AGENT_MAX_EVENT_QUEUE_WAIT_TIME milliseconds after which it calls MQTTAgent_ProcessLoop. This leads to delay between when the packet actually arrives and when it actually gets processed. Thus, if you want to make the MQTT Agent more responsive, you can make the MQTT_AGENT_MAX_EVENT_QUEUE_WAIT_TIME value smaller. This would cause the agent to check the socket more often thereby reducing the time between when an incoming packet is received and when it is processed.

NOTE: While making the above change it is very important to understand that reducing the MQTT_AGENT_MAX_EVENT_QUEUE_WAIT_TIME value would cause the agent process loop to run more often, which might have an impact on the runtime of other tasks.

Socket read/recv timeout:

Most TCP stacks allow applications to block on a socket read (also called recv()) call when there is no data to be read. If you are using the MQTT Agent, then the socket read timeout must be set to 0 – that is, the socket must be non-blocking. When the socket is set to blocking, every read operation of the socket blocks the Agent for the duration of the timeout. This stops the agent from processing other incoming requests such as processing a message which the application wants to publish. This is emphasized in cases where QoS0 publishes are involved because, in this case, there is no incoming packet (e.g. PUB-ACK) to unblock the agent from the socket read operation. Thus, when the application decides to publish a message, that message has to wait in the agent queue since the agent is blocked. This leads to a very slow turn-around time.

Agent task priority:

The MQTT Agent is designed to work best when its priority is set to be higher than all the tasks using it. This allows the agent to quickly process all the requests from the application as well as incoming data from the MQTT message broker. If the agent is of lower priority than the tasks using it, then the user-task can continuously put requests in the agent queue without giving the agent a chance to process those requests. This process would go on till the agent queue is full and the producer task blocks. Only then the Agent task might get a chance to process a message from the queue.