Proposal: Add support for TCP_NODELAY option in libmosquitto

[fmoessbauer]

libmosquitto with QoS > 0 shows a very high latency when publishing. This is due to a high latency of messages send from publisher to the broker (~ 40ms).

The reason for the high latency is the TCP stack, because it waits for more incoming packets to group them together. This could be avoided by setting TCP_NODELAY for all sockets. For the mosquitto broker this can be done in the config file. The mosquitto client library has no API functionality to pass flags to the socket. However, the behavior of the Nagel algorithm has to be specified on sender side.

The following diff contains a proof-of-concept showing how to set the TCP_NODELAY flag in libmosquitto (using a compile time option). With this PoC, we were able to reduce the RTT from ~40ms to less than 1ms. For a production ready solution, this option should be selectable at runtime by using e.g. a further API call, or an additional parameter in mosquitto_connect or mosquitto_connect_bind

libmosquitto_nodelay.patch.txt

Xref: #433

[ralight]

Will that do the job?

[fmoessbauer]

@ralight: Thanks for taking care of this. I tested your patch and the results match perfectly with what I measured in the PoC.

Note for future readers: To enable the TCP_NODELAY flag: set

mosquitto_int_option(mosq, MOSQ_OPT_TCP_NODELAY, 1);

In a ping-pong test between two Raspberry Pis, I measure the following latencies:

# unpatched version
lat_circ[ms]:    88.00, lat_step[ms]:    44.00, msg/s:       24
# patched version
lat_circ[ms]:     2.06, lat_step[ms]:     1.03, msg/s:      972

What an improvement. Again: Thanks for your great work.

[ralight]

I'm closing this, it will be part of 1.7.