[erlang-questions] ets vs list

Ulf Wiger <>
Tue Sep 14 00:17:23 CEST 2010


On 09/13/2010 09:05 PM, Morten Krogh wrote:
> 
>> No, there are several other overheads. For instance, you must somehow
>> implement a hash table from immutable dynamically typed data structures,
>> whereas in ETS (in C), you can use mutable arrays. The dict module,
>> for example, uses a tree structure of tuples. Once you've located
>> the leaf tuple where the object is to reside, you have to create
>> a new copy of that tuple, which means you must also update (copy)
>> the parent tuple, etc.
> 
> But it is to some extent self imposed that you want a functional data
> structure.

Sure. I'm not trying to criticize Erlang, but merely to point
out what makes ETS tables implemented in the VM faster than
an Emulation of them in Erlang code.


> You could have used the process dictionary for this or some foreign
> interface.
> Nowadays nif, but that was not available to you then.

But in both cases (for Robert and for me), the whole _point_ was
to implement them in Erlang, since we wanted to get the functionality
in place without having to spend a lot of time with a native
implementation. The goal was not speed at all, but productivity.



BR,
Ulf W


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