[erlang-questions] Private ets table vs. Process directory

[Charles Hixson]

When should a private ets table be preferred over the process directory?

To give some context, I'm expecting to has nearly as many processes as I 
can run, and that each one will need internal mutable state.  Also, that 
the mutable state will be complex (partially because of the limited 
number of processes), so passing the state as function parameters would 
entail huge amounts of copying.  (Essentially I'd be modifying nodes 
deep within trees.)

Mutable state would allow me to avoid the copying, and the state is not 
exported from the process.  I'm concerned that a huge number of private 
ets tables would use excessive memory, decreasing the number of 
processes I could use...but all the references keep saying not to use 
the process directory.

I'm still designing things now, so this is the ideal time to decide.  An 
alternative is that I could use a single public ets table, with each 
process only accessing its own data, but I suspect that might involve a 
lot of locking overhead, even though in principle nothing should need to 
be locked.