[erlang-questions] simple question about list memory consumption
Sun Jul 8 07:22:36 CEST 2012
> Hm, ignore my previous answer, then; I was under the impression that
> small enough integers would be stored as immediates in the
> list cell...
See the Efficiency guide.
"String ... 2 words per character"
Each list cell holds one integer, which *is* an immediate in the
list cell and points nowhere else, and one pointer to the next
element in the list.
>> 2. opening an Erlang shell (few MB consumption - stable);
>> 3. executing:
>> L = lists:map(fun(_) -> 107 end,lists:seq(1,10000000)), ok.
Two things strike me about this.
(A) you construct TWO lists here, so at 8 bytes per list cell
on a 32-bit machine, there's 16 bytes each already.
(B) You are doing this in the shell, where expressions are
interpreted. I have no idea what space overheads that adds.
Is there a way of measuring the amount of heap space currently
_used_ in a process? process_info>heap_size and
process_info>total_heap_size seem to be about the amount of
space allocated for a process, including its stack and any quantity
of free space. So the following only gave me an approximate idea:
element(2, process_info(self(), heap_size)).
Before = heap_size(),
Result = loop(N),
After = heap_size(),
(After-Before)*1.0 / length(Result).
[$k | loop(N-1)].
However, the idea it _did_ give me was "between 2 and 3 words
per character", so it looks like "2 words per character" as
stated in the Efficiency Guide is, after all, true.
It's certainly nowhere near 21 bytes.
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