Measure node latency

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Hi,

Do you know how to easily measure the latency induced by a ros node (one I haven't written my self, for exemple right now I'm interested in the pointcloud_to_laserscan node).

I'm interested in nodes that subscribe to topic A, and for each topic received from topic A will publish exactly one message on topic B after some delay (scheduler delays + ros delays + processing time ...). How to get the delay (either the delay for each message, or some min/average/max as for the rostopic hz command.

Do you have any idea?

Thanks a lot in advance