RFR (S): 8213439: Run class initialization for boot loader classes with registered subgraph archiving entry field during CDS dump time

Jiangli Zhou jiangli.zhou at oracle.com
Thu Nov 8 02:29:25 UTC 2018 

Hi David,


On 11/7/18 5:49 PM, David Holmes wrote:
> Hi Jiangli,
>
> On 8/11/2018 11:25 AM, Jiangli Zhou wrote:
>> Hi David,
>>
>> Attached are two logs for class initialization:
>>
>>    init_nodumping: bin/java -Xshare:off
>>
>>    init_cdsumping: bin/java -Xshare:dump
>>
>> The class initialization orderings are identical for 'bin/java 
>> -Xshare:off' and 'bin/java -Xshare:dump' before the initialization of 
>> sun.launcher.LauncherHelper.
>
> Thanks! That's very interesting. How do you actually initiate 
> execution of Java code at dump time - ie how far in the VM init 
> sequence do we go before you do the dump and exit?

Currently the dumping process is done at the very end of 
Threads::create_vm() so we can avoid interfering with the initialization 
order. The module system is initialized in initPhase2, which happens 
before the dumping process. Module system initialization runs some java 
code.

During loading and dumping, we invokes the system loader's loadClass() 
method explicitly for each class specified in the classes list (please 
see ClassListParser::load_current_class). That also triggers java code 
execution.
>
> I compared the dump list with what I see currently, before your patch, 
> and it initializes exactly the same number of classes - 279, but not 
> in exactly the same order, and I can't readily tell that it is the 
> same 279 classes (but suspect it is). So I'm a little unsure as to 
> what exactly your patch does today, and how it might be used in the 
> future for the Long$LongCache issue ?
My current patch does not have any visible effect on the class 
initialization for dump time yet. When the LongCache being adding for 
archiving, we should see Long and Long$LongCache being initialized at 
dump time. Just to verify, I just tried dumping out the initialized 
classes without my patch and I saw the lists is identical (attached 
init_cdsdumping0). The orderings of the initialization are the same with 
and without current patch. My repo was synced earlier this afternoon. 
Maybe there are some differences between our local repos? A library 
change (or other change) could potentially change the ordering.

Really appreciate that you are taking a closer look on this!

Thanks,
Jiangli
>
> Thanks,
> David
>
>> Thanks,
>>
>> Jiangli
>>
>>
>> On 11/7/18 4:56 PM, David Holmes wrote:
>>> Hi Jiangli,
>>>
>>> Messing with the established initialization order still rings alarm 
>>> bells for me. Can you generate the normal class initialization list 
>>> for VM startup, and one for a CDS dump, so that we can see how they 
>>> differ. It would very easy for subtle initialization bugs to creep 
>>> in here. (Even with the existing use of Java code during dump time.)
>>>
>>> Thanks,
>>> David
>>>
>>> On 8/11/2018 10:45 AM, Jiangli Zhou wrote:
>>>> Please review following webrev that explicitly runs class 
>>>> initialization for classes (for boot loader only) with registered 
>>>> subgraph archiving entry field(s) during CDS dump time.
>>>>
>>>> At CDS dump time, some of the loaded classes are initialized due to 
>>>> execution of java code. Currently there are 279 classes are 
>>>> initialized at dump time. The rest of the loaded classes are not 
>>>> initialized. Running class initialization for the boot classes with 
>>>> registered subgraph archiving entry field(s) will allow those 
>>>> static fields (and their directly/indirectly referenced objects) 
>>>> being populated at CDS dump time and archived. The Long$LongCache 
>>>> (planned for JDK-8213033 by Claes, thanks!) is such an example.
>>>>
>>>>    webrev: http://cr.openjdk.java.net/~jiangli/8213439/webrev.00/
>>>>
>>>>    RFE: https://bugs.openjdk.java.net/browse/JDK-8213439
>>>>
>>>> The initialization of a class is triggered explicitly when 
>>>> processing the subgraph entry fields at dump time. For any class 
>>>> that's already initialized at this point, it's essentially a nop. 
>>>> That's the case for current existing subgraph entries.
>>>>
>>>> Tested with tier1-tier3. Thanks David and Ioi's for discussions in 
>>>> the bug report.
>>>>
>>>> Thanks,
>>>>
>>>> Jiangli
>>>>
>>

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