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

[Yasumasa Suenaga]

Hi Per,

Will class unloading be occurred after relocation?
If so, I think we can use this approach.

I read the slide [1], it says any garbage does not exist in ZPage after relocation.


Thanks,

Yasumasa


[1] https://www.jfokus.se/jfokus18/preso/ZGC--Low-Latency-GC-for-OpenJDK.pdf


On 2018/08/08 0:28, Per Liden wrote:
> Hi,
> 
> On 08/07/2018 03:02 PM, Yasumasa Suenaga wrote:
>> Hi Per,
>>
>> Currently, class unloading seems not to be implemented.
>> So do not worry about it?
> 
> Support for class unloading is coming, so this assumption would not hold for very long.
> 
> cheers,
> Per
> 
>>
>> http://hg.openjdk.java.net/jdk/jdk/file/10ef3d870cb7/src/hotspot/share/gc/z/zArguments.cpp#l86
>>
>>
>> Thanks,
>>
>> Yasumasa
>>
>>
>> On 2018/08/07 21:32, Per Liden wrote:
>>> Hi Yasumasa,
>>>
>>> I'm afraid this approach of iterating over objects doesn't work. In ZGC, you can't walk over ZPages since some objects in there might be unreachable and hence might have an invalid/dead class pointer, which in turn means you can't tell the size of the object. When iterating over the heap you must start from the roots and only visit live objects. This becomes very complicated to do in the SA, which is why it hasn't been implemented.
>>>
>>> cheers,
>>> Per
>>>
>>> On 07/26/2018 03:52 PM, Yasumasa Suenaga wrote:
>>>> CC'ing to hotspot-gc-dev
>>>>
>>>>
>>>> On 2018/07/26 21:30, Yasumasa Suenaga wrote:
>>>>> PING: Could you review it?
>>>>>
>>>>>>    webrev: http://cr.openjdk.java.net/~ysuenaga/JDK-8207843/webrev.00/
>>>>>
>>>>>
>>>>> Yasumasa
>>>>>
>>>>>
>>>>> On 2018/07/19 23:03, Yasumasa Suenaga wrote:
>>>>>> Hi all,
>>>>>>
>>>>>> Please review this webrev.
>>>>>>
>>>>>>       JBS: https://bugs.openjdk.java.net/browse/JDK-8207843
>>>>>>    webrev: http://cr.openjdk.java.net/~ysuenaga/JDK-8207843/webrev.00/
>>>>>>
>>>>>> I encountered AssertionFailure when I attached HSDB to the process which is working with ZGC as below:
>>>>>>
>>>>>> sun.jvm.hotspot.utilities.AssertionFailure: Unexpected CollectedHeap type: sun.jvm.hotspot.gc.z.ZCollectedHeap
>>>>>>      at jdk.hotspot.agent/sun.jvm.hotspot.utilities.Assert.that(Assert.java:32)
>>>>>>      at jdk.hotspot.agent/sun.jvm.hotspot.oops.ObjectHeap.collectLiveRegions(ObjectHeap.java:448)
>>>>>>      at jdk.hotspot.agent/sun.jvm.hotspot.oops.ObjectHeap.iterate(ObjectHeap.java:173)
>>>>>>      at jdk.hotspot.agent/sun.jvm.hotspot.HSDB$VisitHeap.run(HSDB.java:1741)
>>>>>>      at jdk.hotspot.agent/sun.jvm.hotspot.utilities.WorkerThread$MainLoop.run(WorkerThread.java:70)
>>>>>>      at java.base/java.lang.Thread.run(Thread.java:832)
>>>>>>
>>>>>> ObjectHeap#collectLiveRegions() branches by instance type of CollectedHeap. However it does not support ZCollectedHeap.
>>>>>> So I add ZCollectedHeap to it and add some methods to iterate ZPageTable.
>>>>>>
>>>>>>
>>>>>> Thanks,
>>>>>>
>>>>>> Yasumasa