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  1. JDK
  2. JDK-8202578

Revisit location for class unload events


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      Mail conversation during Flight Recorder review JDK-8199712:

      On 5/3/18 10:34 AM, Markus Gronlund wrote:
      > Hi Coleen,
      > thanks for taking a look.
      > We have filed https://bugs.openjdk.java.net/browse/JDK-8202578 to follow up on the placement of where to post the class unload events.
      > About that, it might be possible to at least move the actual post_class_unload_event() function to InstanceKlass::notify_unload_class() as you suggest, with the proviso that we insert the "unloading hook", Jfr::on_unloading_classes(), just after the classes_do(InstanceKlass::notify_unload_class). Another problem, maybe minor, with moving the posting of class unload events to InstanceKlass::notify_unload_class() is how to maintain the same timestamp for each individual unload event.
      > The posting for unloads is "two-stage" in that the first pass fires an unload event (instant, same timestamp) for each klass; the second stage enters the JFR framework via on_unloading_classes() to save all constants for these classes now about to disappear. I think we could do that though, something like this:
      > …
      > // Tell serviceability tools these classes are unloading
      > classes_do(InstanceKlass::notify_unload_class);
      > JFR_ONLY(Jfr::on_unloading_classes();)
      > ...
      > About also getting post_class_load_event() and
      > post_class_define_event() to InstanceKlass, this will be more
      > problematic. The class load event is a duration event, in that it
      > measures the actual time it took to load the class (it is stack
      > allocated inside SystemDictionary::resolve_instance_class_or_null()).
      > Therefore it might not be possible to move it. Maybe for
      > define_event() as it is only an instant event.\

      Yes, thank you for filing the RFE to follow up about this. Can you cut/paste this into the description?
      > We can sort out the intricacies involved in the context of https://bugs.openjdk.java.net/browse/JDK-8202578 .

      > -----Original Message-----
      > From: Coleen Phillimore
      > Sent: den 26 april 2018 19:59
      > To: hotspot-dev@openjdk.java.net
      > Subject: Re: RFR(XL): 8199712: Flight Recorder
      > http://cr.openjdk.java.net/~egahlin/8199712.0/src/hotspot/share/classf
      > ile/classLoaderData.cpp.udiff.html
      > We can file another RFE for this but I think you could call
      > post_class_unload_event() from in InstanceKlass and call from inside of InstanceKlass::notify_unload_class.
      > void ClassLoaderData::unload() {
      > _unloading = true;
      > // Tell serviceability tools these classes are unloading
      > classes_do(InstanceKlass::notify_unload_class);
      > Rather than walking through _klasses again during unloading. I think we should see if this is possible to improve this after this checkin.
      > http://cr.openjdk.java.net/~egahlin/8199712.0/src/hotspot/share/classfile/systemDictionary.cpp.udiff.html
      > Then move post_class_load and post_class_define events to instanceKlass.cpp too.


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              • Assignee:
                hseigel Harold Seigel
                mgronlun Markus Grönlund
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