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

[Sverker Eriksson]

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


/Sverker, Erlang/OTP