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While browsing All/New I stumbled upon some threads about instances receiving DDOS and that Lemmy has some really database intensive operations which can cause the whole instance to run out of memory with a relatively small amount of traffic if they all hit those expensive queries. I hope these will get fixed in the future releases, but they might be at least partial cause for the problem.

Some database tuning might help, even considerably if current settings are somehow incompatible with the load, but there's still a limit on what that can achieve. I'm not an expert, but Lemmys own documentation refers to this configuration tuner and there's plenty of others around, so that might be worth looking for. Also enabling slow query log might give some info on what's the root cause for these.

If the issue is with the database I don't think that swapping will help since if the memory is full "expanding" it with a slow storage only makes the issue worse. In general at least some swap space is recommended tho, but for this kind of load I doubt it'll help.