Arrays vs tuples / lists

[Ulf Wiger]

Ok, that was perhaps a bit too terse, but I was sitting with
an impatient baby in my lap (she hasn't quite grasped the
beauty of Erlang yet, but I'm working on it.)

test:tup(Iterations, DataSize) runs a tight loop accessing
objects via element/2 in a tuple with 10,000 elements.

test:ets/2 does the same thing, but with an ets set containing
10,000 objects.

Due to the very limited precision of erlang:now() on my
windows box, I ran 1,000 iterations where I accessed all
10,000 objects each iterations.

As you can see, tuple accesses are independent of data size,
as the data already resides on the process heap. With ets,
there is always copying, so as the object size increases, so
does the cost of a lookup.

Even with the smallest type of payload (an atom), element/2
is nearly an order of magnitude faster than ets:lookup/2.

Note, this is only read access. As soon as you start using
setelement/3 on a tuple of size 10,000, the picture shifts
rather dramatically.

/Uffe


On Mon, 17 May 2004 21:14:11 +0200, Ulf Wiger <ulf.wiger@REDACTED> wrote:

> On Mon, 17 May 2004 18:13:01 +0200 (CEST), Thomas Johnsson
> <thomas@REDACTED> wrote:
>
>>>
>>> As long as you only look up stuff in large tuples,
>>> access is very fast and O(1).
>>
>> Splendid! Presumably it is faster than an ets table lookup with an
>> integer
>> as key; by how much, roughly?
>>
>> -- Thomas
>>
>
> Well, it's reasonably easy to measure (at least on UNIX...)
> (I've enclosed a small test program, and a test run on a 3GHz XP box.)
>
> /Uffe
>
> Erlang (BEAM) emulator version 5.3 [threads:0]
>
> Eshell V5.3  (abort with ^G)
> 1> cd("c:/cygwin/home/foo/erl").
> c:/cygwin/home/foo/erl
> ok
> 2> c(test).
> {ok,test}
> 3> test:tup(1000,small).
> {625000,ok}
> 4> test:tup(1000,medium).
> {609000,ok}
> 5> test:tup(1000,big).
> {609000,ok}
> 6> test:ets(1000,small).
> {4156000,ok}
> 7> test:ets(1000,medium).
> {8718000,ok}
> 8> test:ets(1000,big).
> {138594000,ok}
> 9> test:empty(1000).
> {531000,ok}
> 10>
>



-- 
Ulf Wiger