yup, synapse is heavy (as the rest of the thread says). this is mainly due to the DB schema being a bit naive, and it caching everything in RAM to speed things up. There are also some operations it does which spike RAM usage (which is then never reclaimed, thanks to Python2's malloc being a bit dumb). As others have said, Synapse is doing a lot more than something like Prosody. Dendrite on the other hand should be good for running on an RPi - we should have an idea this week, where the first monolithic Dendrite binary is due to land.
