Looks like in 2.2+ changing the memory_allocator field in cassandra.yaml has no effect:
The instructions in comments on how to use jemalloc haven't been updated to reflect this change. Robert Stupp, is that an accurate assessment?
[~iamaleksey] How do we want to handle the deprecation more generally? Warn on 2.2, remove in 3.0?
EDIT: I described this in a way that isn't very clear to those unfamiliar with Robert's change. To be clear: You can still use JEMAlloc with Cassandra. The memory_allocator option was deprecated in favor of automatically detecting if JEMAlloc is installed, and, if so, using it.