RFR(S): 8232081: Try to link all classes during dynamic CDS dump

[David Holmes]

Hi Calvin, Ioi,

Looking good - comments below.

A meta-question: normal application classes are rarely loaded but not 
linked so I'm a little surprised this is an issue. What is the main 
source of such classes - generated classes like lambda forms? Also why 
do we need to link them when loading from the archive if they were never 
linked when the application actually executed ??

On 28/02/2020 10:48 am, Calvin Cheung wrote:
> Hi David, Ioi,
> 
> Thanks for your review and suggestions.
> 
> On 2/26/20 9:46 PM, Ioi Lam wrote:
>>
>>
>> On 2/26/20 7:50 PM, David Holmes wrote:
>>> Hi Calvin,
>>>
>>> Adding core-libs-dev as you are messing with their code :)
>>>
>>> On 27/02/2020 1:19 pm, Calvin Cheung wrote:
>>>> JBS: https://bugs.openjdk.java.net/browse/JDK-8232081
>>>> webrev: http://cr.openjdk.java.net/~ccheung/jdk15/8232081/webrev.00/
>>>>
>>>> The proposed changeset for this RFE adds a JVM_LinkClassesForCDS() 
>>>> function to be called from java/lang/Shutdown to notify the JVM to 
>>>> link the classes loaded by the builtin class loaders. The 
>>>
>>> This would be much less disruptive if this was handled purely on the 
>>> VM side once we have started shutdown. No need to make any changes to 
>>> the Java side then, nor jvm.cpp.
>>>
>>
>> Hi David,
>>
>> To link the classes, we need to be able to run Java code -- when 
>> linking a class X loaded by the app loader, X will be verified. During 
>> verification of X, we may need to load additional classes from the app 
>> loader, which executes Java code during its class loading operations.
>>
>> We also need to handle all the exit conditions. As far as I can tell, 
>> an app can exit the JVM in two ways:
>>
>> (1) Explicitly calling System.exit(). This will call 
>> java.lang.Shutdown.exit() which does this:
>>
>> http://hg.openjdk.java.net/jdk/jdk/file/0edc7fd0d7a3/src/java.base/share/classes/java/lang/Shutdown.java#l163 
>>
>>
>>             beforeHalt(); // native
>>             runHooks();
>>             halt(status);
>>
>> (2) When all non-daemon threads have died (e.g., falling out of the 
>> bottom of HelloWorld.main()). There's no explicit call to 
>> System.exit(), but the JVM will proactively call 
>> java.lang.Shutdown.shutdown() inside JavaThread::invoke_shutdown_hooks()
>>
>> http://hg.openjdk.java.net/jdk/jdk/file/0edc7fd0d7a3/src/hotspot/share/runtime/thread.cpp#l4331 
>>
>> http://hg.openjdk.java.net/jdk/jdk/file/0edc7fd0d7a3/src/java.base/share/classes/java/lang/Shutdown.java#l184 
>>
>>
>> If we want to avoid modifying the Java code, I think we can intercept 
>> JVM_BeforeHalt() and JavaThread::invoke_shutdown_hooks(). This way we 
>> should be able to handle all the classes (except those that are loaded 
>> inside Shutdown.runHooks).
> 
> I've implemented the above. No changes to the Java side.
> 
> updated webrev:
> 
>      http://cr.openjdk.java.net/~ccheung/jdk15/8232081/webrev.01/

This looks much better to me. I wasn't sure if you were that concerned 
about catching the classes loaded by the Shutdown hooks, but it is good 
you are not as it seems problematic to me to trigger class loading etc 
after the shutdown hooks have executed - you may hit unexpected 
conditions. So this approach is good.

A few minor comments:

src/hotspot/share/memory/metaspaceShared.cpp
src/hotspot/share/memory/metaspaceShared.hpp

Why the change from TRAPS to "Thread* THREAD"?

---

src/hotspot/share/oops/instanceKlass.cpp

I'm not clear how verify_on is used so am unsure how the additional 
error state check may affect code unrelated to dynamic dumping ??

Also can I suggest (as you are touching this code) to delete the ancient 
comment:

3580     // $$$ This used to be done only for m/s collections.  Doing it
3581     // always seemed a valid generalization.  (DLD -- 6/00)

Thanks,
David
-----

> I also updated the test to test the shutdown hook and System.exit() 
> scenarios.
> 
> All appcds tests passed on my linux host. I'll run more tests using mach5.
> 
> thanks,
> 
> Calvin
> 
> 
>>
>> What do you think?
>>
>> - Ioi
>>
>>> Thanks,
>>> David
>>>
>>>> MetaspaceShared::link_and_cleanup_shared_classes() has been modified 
>>>> to handle both static and dynamic CDS dump. For dynamic CDS dump, 
>>>> only classes loaded by the builtin class loaders will be linked. 
>>>> Local performance testing using javac on HelloWorld.java shows an 
>>>> improvement of >5%.
>>>>
>>>> Passed tier1 - 4 tests.
>>>>
>>>> thanks,
>>>>
>>>> Calvin
>>>>
>>