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

[Yasumasa Suenaga]

Hi,

On 2018/08/16 15:55, Per Liden wrote:
> 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.

Thank you for your advice.

I checked VM_HeapDumper::doit() and ZCollectedHeap::collect_as_vm_thread().
ZCollectedHeap::collect_as_vm_thread() will not work, so `-all` option in jmap / jcmd also will not work.

Is it listed somewhere? IMHO it should be documented.


Cheers,

Yasumasa


> 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