Re: [rvm-research] remembered sets processing for a three-generational immix collector

You should also look at the Reference Type processors, as they use a 
simple nursery.  If 'nursery' is true when calling 
ReferenceProcessor#scan, no references created before the last GC will 
be processed - your collector will probably only be correct if you treat 
all reference passes as full-heap.

At the time we wrote it, jess was the only benchmark with a significant 
performance impact, but it's been a while since I checked so it may be 
more important these days.

HTH,

Robin


On 13/03/17 05:32, Shoaib Akram wrote:
> Hi Eliot and others,
>
> Your are right. I would need to remember promotions to the intermediate space in the remset. My current solution is to have two sets of remset buffers. The first set remembers every entry from outside the nursery and is processed and squashed at the end of each nursery collection. The second remembered sets logs all pointers from outside both the nursery plus the intermediate space. Since the intermediate space is placed right next to nursery contiguously, a simple address check is enough for remembering the second set of pointers. Note that the second sets does not need to remember pointers from intermediate to nursery objects since both the nursery and intermediate space is collected together. When the intermediate space is collected, I process the second remembered sets. At the end of intermediate collection, both remsets are cleared. To avoid a couple of complications, I currently evacuate the nursery during intermediate collection directly to mature space. In my debugging exercises, I made sure that every remset entry in the first set (except those from intermediate to nursery) is also found in the second set. Using event counters in the fastPath method, I know that the number of remsets entries being processed during intermediate collection is equal to the number of times new slots were inserted in the remsets in the fastPath. Things don’t seem to work properly after the intermediate collection. Am I missing something? Since the crash happens after GC, I presume there are still stale references somewhere.
>
> Regards,
> Shoaib.
>
>> On Mar 4, 2017, at 10:23 PM, Eliot Moss <[email protected]> wrote:
>>
>> Here is a possibility, if I am understanding your setup correctly:
>>
>> Consider a reference from mature space to a nursery object.  This
>> will correctly be noted in a remset.  However, when the object is
>> promoted to the intermediate space, the fast-path code will now
>> insert the slot into a remset again.  This is in fact correct
>> since you will need to know in the future about this reference
>> from mature to intermediate space, when doing a second-generation
>> collection.
>>
>> I am not sure what you want to do -- this is not necessarily a
>> *problem*.  The entries should land in the remsets for the second
>> generation, not the nursery.  The bad case would be if you end up
>> with new write buffer items directed toward the (supposed now
>> empty) nursery.
>>
>> Regards - Eliot Moss
>>
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