Re: Memory accounting in libgc

[Neil Jerram <neil@ossau.homelinux.net>]

On 2014-03-12 06:57, Mark H Weaver wrote:
> Andy Wingo <wingo@pobox.com> writes:
> 
>> How does this affect libgc?
>> 
>> First of all, it gives an answer to the question of "how much memory
>> does an object use" -- simply stop the world, mark the heap in two 
>> parts
>> (the first time ignoring the object in question, the second time
>> starting from the object), and subtract the live heap size of the 
>> former
>> from the latter.  Libgc could do this without too much problem, it 
>> seems
>> to me, on objects of any kind.  It would be a little extra code but it
>> could be useful.  Or not?  Dunno.
> 
> This could be generalized to the far more useful question: "How much
> memory does this set of objects use?", although that's a slippery
> question that might better be formulated as "How much memory would be
> freed if this set of objects were no longer needed?".
> 
> For example, suppose you have a large data structure that is referenced
> from two small header objects, A and B.  If you ask "How much memory
> does A use?", the answer will be the size of the small header, and 
> ditto
> for B.  Without being able to ask the more general question, there's no
> way to find out how much would be freed by releasing both.
> 
>      Mark

Absolutely agree that this would be useful, but I suspect a problem in 
how far one can push libgc to simulate a set of objects being freed 
without them actually _being_ freed.  For example there could be 
guardians associated with the objects, and I think one can validly 
imagine them doing either of the possible extremes (or anywhere in 
between), namely:

- resurrecting the objects again

- freeing up a whole load more objects/memory that the guardians know to 
be associated with the original objects, but which wasn't (for some 
reason) simply referenced by them.

Hope that's a useful thought - interesting subject!

     Neil