Questions on concurrent class unloading

[Erik Österlund]

Hi Zhengyu,

On 2019-04-08 21:02, Zhengyu Gu wrote:
> 
> 
> On 4/8/19 2:49 PM, Erik Osterlund wrote:
>> Hi Zhengyu,
>>
>> The code is called by concurrent GC threads, so 
>> is_ConcurrentGC_thread() should return true.
>>
> E.g ZNMethodUnlinkClosure is only used by ZNMethodUnlinkTask, and the 
> task is executed by worker threads, which are not concurrent gc thread. No?

The workgang we use for concurrent execution, uses concurrent GC threads.

> I also added assert(false, "debug") inside under nm->is_unloading() 
> branch, it never fired.

So... are you running a workload that needs to unload nmethods? Because 
we are definitely unloading nmethods. I did the same experiment, and hit 
the assert as expected.

/Erik

> Thanks,
> 
> -Zhengyu
> 
> 
> 
>> is_unloading() returns true if the nmethod has a dead oop due to GC.
>>
>> /Erik
>>
>>> On 8 Apr 2019, at 17:27, Zhengyu Gu <zgu at redhat.com> wrote:
>>>
>>> Hi,
>>>
>>> I am studying concurrent class unloading in ZGC, it looks to me that 
>>> nothing gets unlinked (ZNMethod::unlink())and purged 
>>> (ZNMethod::purge()).
>>>
>>> Inside ZNMethodUnlinkClosure::do_nmethod() [1]
>>>
>>> It appears that nm->is_unloading() can never be true. Otherwise, 
>>> assertion should fail inside nm->flush_dependencies(false), cause 
>>> neither Universe::heap()->is_gc_active() (true for STW GC) nor 
>>> is_ConcurrentGC_thread() is true.
>>>
>>> Similar with ZNMethodPurgeClosure [2] , nm->make_unloaded() has 
>>> similar assertion.
>>>
>>> What did I miss?
>>>
>>> Thanks,
>>>
>>> -Zhengyu
>>>
>>>
>>> [1] 
>>> http://hg.openjdk.java.net/jdk/jdk/file/542735f2a53e/src/hotspot/share/gc/z/zNMethod.cpp#l279 
>>>
>>>
>>> [2] 
>>> http://hg.openjdk.java.net/jdk/jdk/file/542735f2a53e/src/hotspot/share/gc/z/zNMethod.cpp#l357 
>>>
>>>
>>>
>>>
>>