[Openmcl-devel] memory

[Daniel Weinreb]

Gary Byers wrote:
> When people ask questions like this, they usually means something
> like "what's the total size of this object, plus the size of all things that
> it transitively and uniquely references ?"
>
> Another way of asking that is to ask "if the object in question became 
> garbage, how
> much memory would be freed by the GC ?"
>   
I'm not sure it's exactly that.  Suppose you have a hash
table with two elements, A and B. A is only reachable through
the hash table; B is reachable in other ways.

If there were a size-of-this-object function applied to the hash table,
I don't think you'd want it to include A, even though A would become
garbage if the hash table became garbage.

But I agree with your fundamental point.  If someone asks
"what is the size of this object", "object" can mean more
than one thing depending on the level of abstraction.
In the case of a hash table, probably the caller "means"
to include all the pieces that make up a hash table,
which might be many "objects" in the low-level sense,
since hash tables can be implemented as data structures
with many low-level objects.

Lisp could know about hash tables specially, but
what if the user, or library being used by the user,
has objects in the high-level sense?  They'd have
to provide some kind of method to say "here's how
to measure my size" that would say which references
to follow and where to stop following references.

-- Dan

> (let* ((obj (make-instance ...)))
>   (gc)
>   (let* ((with-obj (%precise-size-of-allocated-memory-at-the-moment)))
>     (setq obj nil)
>     (gc)
>     (let* ((without-obj (%precise-size-of-allocated-memory-at-the-moment)))
>       (- with-obj without-obj))))
>
> There's no such function as the one that I'm calling 
> %PRECISE-SIZE-OF-ALLOCATED-MEMORY-AT-THE-MOMENT; it's very hard to measure 
> this in CCL (because of threads and because of other issues.)  We're also 
> assuming that OBJ is the only thing that becomes garbage between the two 
> GCs, and we're assuming that that object isn't sitting in some machine 
> register even after we set the variable OBJ to NIL.  Neither of those 
> assumptions is valid.
>
> I don't want to say that there's no reliable way to answer this question in 
> general, but it's much, much harder to answer than it might appear to be at 
> first glance.
>
> There are some heap-analysis utilities described at 
> <http://trac.clozure.com/ccl/wiki/MemoryUtils> (some of them are currently 
> x8664-Linux-specific); none of them try to answer this question, but they 
> do try to answer some similar questions.
>
>
>
> --On April 11, 2010 4:50:23 PM +0200 Joakim Sandgren 
> <joakim at joakimsandgren.com> wrote:
>
>   
>> Hi,
>> Are there a way to see how big in Kb  or bytes a clos object is ?
>>
>>
>> sincerely
>> joakim
>>
>>
>>
>>
>>
>>
>>
>>
>> Joakim Sandgren
>> joakim sandgren musik
>> 42, rue de Maubeuge
>> 75009 Paris
>> France
>> +33 (0)1 45 26 43 90
>> info at joakimsandgren.com
>> http://www.joakimsandgren.com
>>     
>
>
>
>
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