Broker memory optimization about umqttbroker HOT 3 CLOSED

max_subscriptions and max_retained_topics are just arrays with some meta-infos, small values don't save a lot. Actual subscriptions topic-names and retained topics names and contents are allocated additionally. Each subscribed topic string counts as one subscription per subscribing client, but '#' or other wildcards are just one subscription.

The queue sizes depend on your expected message sizes. If you only have a few bytes from time to time, 512 for both is probably enough.

If you want to block a 3rd connection, just return a "false" from onConnect.

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thanks :) I 'll give it a try.

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just to share my findings :
I noticed that an ESP8266 TCP connection uses about 3KB ram when active. That is just for the core internal stuff.
The MQTT connection, when active, needs by default 3KB more (MQTT_BUF_SIZE + QUEUE_BUFFER_SIZE ).
This is all released when the MQTT client deconnects and TCP is closed.

This is per client, and the sketch surely needs additional RAM on top. It is easy to quickly run out of RAM if the number of simultaneous clients grows. The behavior becomes hazardous up to crash the ESP8266.

I have reduced both buffers to 512B. Also reworked my sketch to save RAM here and there. It works fine now with 1 permanent client and 2 more who connect temporarily. Never more than 2 simultaneous clients and it runs smoothly in my situation.

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