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

[Robert Virding]

Perhaps an easy solution would be to document, unofficially of course so it
is not doucmented, exactly how erts_debug:flat_size/1 works.

Robert


On 26 September 2014 11:27, Michael Truog <mjtruog@REDACTED> wrote:

>  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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>
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