[Vm-dev] Detecting objects that point to an object

Mariano Martinez Peck marianopeck at gmail.com
Tue Jul 20 08:47:06 UTC 2010


On Mon, Jul 19, 2010 at 7:59 PM, Eliot Miranda <eliot.miranda at gmail.com>wrote:

>
>
>
> On Mon, Jul 19, 2010 at 7:35 AM, Igor Stasenko <siguctua at gmail.com> wrote:
>
>>
>> Mariano, can you tell us, why you need to know, how many objects pointing
>> to
>> a specific one?
>> The reference counting is well known solution to this, but it is less
>> effective than garbage collection
>> (in terms of memory management), since you have to touch the counters
>> at each memory write.
>>
>
> and recursively when objects are freed.
>
> Further, for two reasons one will also need a scan-mark garbage collector
> to collect all garbage.  It is inevitable that one won't waste space on a
> reference count that can hold the max number of references and so the system
> will have to deal with a max count and have some objects with an overflowed
> count.  Reference counting cannot easily deal with circularities and so a
> cycle of references will result in non-zero ref counts for nodes in the
> cycle and prevent garbage collection.
>
>
>
Thanks for the answers. Maybe I should have explained why I needed that.
What I had in mind, is to be able to detect subgraphs. But not any kind of
subgraphs, but only those on which ALL objects are ONLY reachable from the
root of the subgraph. This means, that there are no objects outside the
graph, pointing to objects inside the graph. In ImageSegment word, would
mean to detect subgraphs that don't have outPointers.

So I was thinking a way to detect them. It is impossible to scan all memory
for every "possible" subgraph. I thought that only maintaining a counter
could be enough: if the counter is 1, then it is perfect. If it is more than
1, it can be perfect but only if all the pointers are from inside the graph.
To do this, I need to scan the subgraphs again and count objects...

So my "solution" sucks. But that's why I got the asking myself if I could
maintain a counter. Not to replace the actual GC.

Thanks

Mariano



>
>
>>
>> On 19 July 2010 12:55, Mariano Martinez Peck <marianopeck at gmail.com>
>> wrote:
>> >
>> > Ok...thanks for the answers...I think I will have to go in another way.
>> But anyway, thanks
>> >
>> > mariano
>> >
>> > On Thu, Jul 15, 2010 at 6:43 PM, Andreas Raab <andreas.raab at gmx.de>
>> wrote:
>> >>
>> >> On 7/15/2010 5:40 AM, Mariano Martinez Peck wrote:
>> >>>
>> >>> Hi folks. I need to know the objects (actually, only how many) that
>> are
>> >>> pointing to any object. So...I take an object X, and I want to know
>> how
>> >>> many objects are pointing to X.
>> >>>
>> >>> The only possible I see in Squeak VM is to do a full mark over all
>> >>> objects in memory (or starting by the roots). This is time consuming
>> and
>> >>> I even need (maybe) to do this check (to know how many objects are
>> >>> pointing to an object) for every single object.
>> >>>
>> >>> So...is there a faster solution for this? The only thing I can imagine
>> >>> is to modify the object header, maybe create a new type, and add an
>> >>> array with the addresses of the objects that are pointing to that
>> >>> object. Or maybe just a counter that I can increase or decrease ?
>> >>
>> >> That's called "reference counting". It's not supported in Squeak.
>> >>
>> >> Cheers,
>> >>  - Andreas
>> >
>> >
>> >
>>
>>
>>
>> --
>> Best regards,
>> Igor Stasenko AKA sig.
>>
>
>
>
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