[erlang-bugs] erts_debug:flat_size/1 wrong?

[Michael Truog]

On 09/26/2014 06:46 AM, Sverker Eriksson wrote:
> On 09/26/2014 04:19 AM, Michael Truog wrote:
>> Hi,
>>
>> I have been attempting to compare the output of erts_debug:flat_size/1 to the memory info at http://www.erlang.org/doc/efficiency_guide/advanced.html#id68923 and the results show that each term's size is off-by-one (at least for pids local/remote, refs local/remote, floats, integers, bignums, binaries and atoms).  I know the function is experimental, but this is a bug, right?  The problem affects top-level terms and nested terms, so it is likely to understate the memory with large terms.  I wanted to make sure the memory info (in the efficiency guide) was accurate (it seems like it is).  I was testing with R16B03 on 64bits.
>>
>> For example:
>> 1> erts_debug:flat_size(576460752303423488).
>> 2
>> 2> erts_debug:flat_size(576460752303423487).
>> 0
>> 3> erts_debug:flat_size(undefined).
>> 0
>> 4> erts_debug:flat_size([]).
>> 0
>> 5> erts_debug:flat_size([undefined]).
>> 2
>> % 1 word for each element in the list * 2 elements including []
>> 6> erts_debug:flat_size(erlang:make_ref()).
>> 3
>> 7> erts_debug:flat_size(erlang:self()).
>> 0
>> 8> erts_debug:flat_size(1.0).
>> 2
>>
>
>
> erts_debug:flat_size/1 is undocumented and can therefor not be wrong by definition :-)
>
> But no, there is no bug, erts_debug:flat_size/1 works as intended in the current implementation. It returns the number of words on the _heap_ occupied (*) by the term. Excluded are thus off-heap data such as binaries larger than 64 bytes AND the top term word, which is kept in a register or on the stack.
>
> (*) erts_debug:flat_size does not take sharing of sub-terms into account while erts_debug:size/1 do.
>
> 1> A = "Hello".
> "Hello"
> 2> erts_debug:flat_size(A).
> 10
> 3> erts_debug:flat_size({A,A}).
> 23
> 4> erts_debug:size({A,A}).
> 13

Thank you for the information.  However, based on this the Efficiency Guide memory information should be slightly wrong (at http://www.erlang.org/doc/efficiency_guide/advanced.html#id68923), since the information should be
* List: 1 word + |2 word per element| + the size of each element
* Tuple: 2 words + |1 word per element| + the size of each element

and not what it currently is:
* List: 1 word + |1 word per element| + the size of each element
* Tuple: 2 words + the size of each element

That is at least what I have found when using erts_debug:flat_size/1 in the repo https://github.com/okeuday/erlang_term when looking at the total size of an Erlang term.  Do you agree?

Thanks,
Michael
>
>
> /Sverker, Erlang/OTP
>
>
>
>

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