ZGC: RFR: 8207843: HSDB cannot show Object Histogram when ZGC is working

[Per Liden]

Hi,

On 08/16/2018 03:52 AM, Yasumasa Suenaga wrote:
> Hi,
> 
> On 2018/08/15 2:36, Per Liden wrote:
>> Hi,
>>
>> On 2018-08-11 14:06, Yasumasa Suenaga wrote:
>>> Hi Per,
>>>
>>> SA needs memory regions to generate class histogram. So it is 
>>> difficult to use visitor model for it.
>>> I want to back to this issue when class unloading is implemented.
>>
>> If you want to pursue the path of walking the live set, then you can 
>> do that today, you don't need to wait for class unloading support to 
>> appear.
> 
> I might be able to implement it. However ObjectHeap in SA needs memory 
> region, so it might be so complex code, and have large different code 
> from other GC algorithms.

Yes, this would be a fairly big undertaking.

> (Other GC algorithms can provide live spaces to ObjectHeap.)
> 
> BTW I concern class unloading issue also occurs in heap dump.
> For example, if the user requests heap dump with -all option in ZGC 
> concurrent cycle, it might have invalid objects in it.

This isn't an issue, because heap dumps are always generated by walking 
the live-set (see ZHeapIterator), so we never walk over unreachable objects.

cheers,
Per

> 
> 
> cheers,
> 
> Yasumasa
> 
> 
>> cheers,
>> Per
>>
>>>
>>>
>>> Thanks,
>>>
>>> Yasumasa
>>>
>>>
>>> On 2018/08/08 20:45, Per Liden wrote:
>>>> Hi,
>>>>
>>>> On 08/08/2018 10:34 AM, Yasumasa Suenaga wrote:
>>>>> On 2018/08/08 17:07, Per Liden wrote:
>>>>>> On 08/08/2018 10:02 AM, Per Liden wrote:
>>>>>>> Hi,
>>>>>>>
>>>>>>> On 08/08/2018 10:00 AM, Yasumasa Suenaga wrote:
>>>>>>>> Hi Per,
>>>>>>>>
>>>>>>>> Will class unloading be occurred after relocation?
>>>>>>>
>>>>>>> No, before relocation.
>>>>>>
>>>>>> But it doesn't really matter when it happens, since there can 
>>>>>> still be unreachable objects with dead class pointers in ZPages 
>>>>>> that were not relocated.
>>>>>
>>>>> I think we can include all unreachable objects in the histogram.
>>>>> For example, ParallelScavengeHeap in SA collects all objects 
>>>>> between bottom() and top() in all spaces like `jcmd 
>>>>> GC.class_histogram` with -all option.
>>>>>
>>>>> So I think we can fix Object Histogram for ZGC if we resolve dead 
>>>>> class pointer.
>>>>> If SA can know ZGC phase, we might resolve it as following:
>>>>>
>>>>>
>>>>>    * GC is working:
>>>>>        Check colored pointer to collect oops.
>>>>>
>>>>>    * GC is NOT working:
>>>>>        Collect all oops in ZPage.
>>>>
>>>> Unfortunately that's not good enough. It doesn't matter if the GC is 
>>>> "working" or not, there might be unreachable objects from several GC 
>>>> cycles back which you can't touch. The only safe way to do this is 
>>>> to walk the live graph, which means starting from the roots. As you 
>>>> might have noticed, ZCollectedHeap::object_iterate() is implemented 
>>>> like this, for this exact reason.
>>>>
>>>> cheers,
>>>> Per
>>>>
>>>>>
>>>>>
>>>>> Thanks,
>>>>>
>>>>> Yasumasa
>>>>>
>>>>>
>>>>>> /Per