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From Salikh Zakirov <salikh.zaki...@gmail.com>
Subject Re: [jira] Created: (HARMONY-4511) [drlvm][jvmti][gc] GC_gen doesn't collect tagged objects
Date Fri, 27 Jul 2007 00:42:46 GMT
On Tue, Jul 24, 2007 at 01:18:24PM +0400, Gregory Shimansky wrote:
> >On 7/23/07, Alexei Fedotov <alexei.fedotov@gmail.com> wrote:
> >>> These object references are enumerated as weak
> >>There might be a coincidence with the message Harmony VM continues to 
> >>print:
> >>The GC did not provide gc_add_weak_root_set_entry()

I think the analysis by Alexei is correct.
GC_gen does not implement weak roots, so the VM emulates it using
regular gc_add_root_set_entry() function, and the tagged objects
end up not being collected at all.

The last boolean parameter to gc_add_weak_root_set_entry() is not relevant,
as Gregory correctly pointed out, as it controls only if the weak root will
behave as weak reference or as phantom reference, i.e. will be reset as soon
as objects becomes unreachable for the first time, or only after its
finalization has completed.

> Other solution would be to allocate tags inside of objects themself 
> inside of heap, but there is a big problem with it because spec states 
> that tags are local to JVMTI environment. If an agent creates many 
> environments, or if there is more than JVMTI agent, it may be necessary 
> to associate different environments with the same object.

Many environments requirement is not a real problem, at least current
implementation allows using tags only for one environment, and returns
"not available" code for all other environment attempting to request this
capability.

What does prevents putting tags into the objects directly, is the requirement
to enumerate tagged objects quickly, as this is likely to be used by memory
profilers, and the performance of O(tagged) is much more preferrable than
O(all objects). Current implementation puts a pointer to a native tag
structure into objects.

Xiao Feng, what other solutions do you consider?

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