On Thu, 23 Jun 2022 12:34:59 GMT, Zhengyu Gu <z...@openjdk.org> wrote:
>> Currently, jdi only check and process class unloading event when it detects >> a new GC cycle. >> >> After [JDK-8212879](https://bugs.openjdk.org/browse/JDK-8212879), posting >> class events can overlap with GC finish event, that results, sometimes, it >> only captures partial or even empty unloaded class list. The pending list >> usually can be flushed out at next GC cycle. But for the classes unloaded >> during the last GC cycle, the class unloading events may lost forever. >> >> This patch checks and processes class unloading events unconditionally, >> suggested by @kbarrett, the last pending unloaded class list can be flushed >> by other events, such as `VM_DEATH`. >> >> It also performs `commonRef_compact()` only when there are classes unloaded. >> >> New test failed about 20% without patch, none with patch. >> >> **Update** >> There are significant changes from early patch. >> >> The new approach: >> No longer removing dead objects and post events on VM thread. I believe it >> was implemented this way to workaround the following issues: >> - JDI event handler uses JVMTI raw monitor, it requires thread in >> `_in_native` state >> - The thread can not hold lock, which is needed to protect `JvmtiTagMap` >> while walking, when transition to `_in_native` state >> >> The new solution breaks up into two steps: >> - Collect all dead object tags with lock >> - Transition to `_in_native` state and post object free events in one batch >> >> This way, JDI event handler can process object free events upon arrivals >> without delay. > > Zhengyu Gu has updated the pull request with a new target base due to a merge > or a rebase. The incremental webrev excludes the unrelated changes brought in > by the merge/rebase. The pull request contains 18 additional commits since > the last revision: > > - Improve naming and cleanup > - Merge branch 'master' into JDK-8256811-jdi-missing-class-unloading-event > - v4 > - v3 > - v2 > - Merge branch 'master' into JDK-8256811-jdi-missing-class-unloading-event > - Merge branch 'jdi_tmp' into JDK-8256811-jdi-missing-class-unloading-event > - v0 > - v2 > - v1 > - ... and 8 more: https://git.openjdk.org/jdk/compare/c4855a3d...559b4bf1 src/hotspot/share/prims/jvmtiTagMap.cpp line 1200: > 1198: }; > 1199: > 1200: // PostObjectFree can't be called by JavaThread, so call it from the VM > thread. I wish I'd written why here. I think it's because we can't hold a Mutex to do the callback, but now I don't remember. I need to spend some time to understand this. Thanks @plummercj for adding Kim and I. ------------- PR: https://git.openjdk.org/jdk/pull/9168
