[Vm-dev] Root flag & root objects

Eliot Miranda eliot.miranda at gmail.com
Sat Feb 21 19:58:37 UTC 2009

On Sat, Feb 21, 2009 at 11:01 AM, Andreas Raab <andreas.raab at gmx.de> wrote:

> Eliot Miranda wrote:
>> So yes, when creating a new image, all objects are in oldSpace and so no
>> objects need to be in the rot table or have their root bit set.
> Correct.
>  This raises another point of ignorance for me.  I would have thought that
>> it is OK for the root bit to be set or the root table to contain objects
>> that are not in old space or do not/no longer refer to objects in youngSpace
>> because the GC will remove these from the root table on GC.  But I don't see
>> where objects get cleared from the rootTable.  I only see the RootTable
>> being voided when it gets full (rather drastic).  Can anyone say if this
>> happens or not, and if so, where it happens?
> I think you're right. Short of a full GC the root table may never get
> cleared.

I think next week I'll instrument the VM and we should put it underneath a
server image which we should run for some time ad then interrupt and print
the occupancy of the root table.  It could be that it is filling up with
objects that are no loner roots and incremental collection is slowing down.

>  - Andreas
>  Looks to me like either markAndTrace: or a specialized version, say
>> markAndTraceRootObject:, should maintain a flag, refersToYoungObject, and
>> clear the object from the rootTable (and clear the root bit) if
>> refersToYoungObject is false at the end of the scan.   But with the pointer
>> reversal algorithm it is hard to see where to maintain this flag.
>> Can those who know enlighten this ignorant soul?
>> TIA
>> On Sat, Feb 21, 2009 at 4:42 AM, Igor Stasenko <siguctua at gmail.com<mailto:
>> siguctua at gmail.com>> wrote:
>>    Hi,
>>    i'd like to ask, is it safe to clear the root flag for every object
>>    in system.
>>    I'm artificially creating a new image, and i'd like to know, what
>>    objects should be marked as roots initially, or VM can care about it
>>    itself?
>>    --
>>    Best regards,
>>    Igor Stasenko AKA sig.
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