[erlang-questions] message copying overhead atoms vs binaries

[Paolo Negri]

Hi Bob,

Thanks for taking the time to answer and pointing to the resources.
I would also like to to say thanks more in general to the whole list
since as a newcomer being part of this list so far has been very
helpful and interesting.

Thanks,

Paolo

On Mon, Jan 10, 2011 at 12:12 PM, Bob Ippolito <bob@REDACTED> wrote:
> On Mon, Jan 10, 2011 at 6:56 PM, Paolo Negri <paolo.negri@REDACTED> wrote:
>> Dear list,
>>
>> I'm trying to understand what approach of building values contained in
>> messages to pass across processes would be more efficient (in terms of
>> memory and cpu) and if the difference is significant.
>> I could build my message values in two ways
>>
>> a) [{color, red}, {shape, circle}]
>>
>> b) [{color, <<"red">>}, {shape, <<"circle">>}]
>>
>> The set of values is strictly limited (in the order of hundreds and
>> not over 1000) and these values are constantly used by the system so
>> the fact that atoms would be permanently allocated is not a problem in
>> this case.
>>
>> The actual lists are longer and can vary in length between 5 to 10 elements.
>> The processes will exchange thousands of these messages each second
>> and this is the reason why I'm asking this question.
>>
>> I'd like to know which form is cheaper in term of copy operation in
>> both cases internally on a single node or across nodes hosted on
>> different physical machines.
>
> Atoms are cheaper. They are always 1 word on the heap and the rest
> shared in the atom table. Even when binaries are shared, they are
> larger than this [1]. In your case the binaries would not be shared
> anyway, they are smaller than 64 bytes so they are considered heap
> binaries [2]. The external term format also has optimizations that
> allow for an atom table to compactly represent them by reference
> instead of value, where other types to not have this kind of
> optimization [3].
>
> [1] http://www.erlang.org/doc/efficiency_guide/advanced.html
> [2] http://www.erlang.org/doc/efficiency_guide/binaryhandling.html
> [3] http://www.erlang.org/doc/apps/erts/erl_ext_dist.html
>
> -bob
>



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