On Thu, 30 Jun 2022 14:59:55 GMT, Zhengyu Gu <[email protected]> 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. >> >> **Update 2** >> There is a comment for ` JvmtiTagMap::check_hashmap()` that states >> `ObjectFree` events are posted before heap walks. >> >> // This checks for posting and rehashing before operations that >> // this tagmap table. The calls from a JavaThread only rehash, posting is >> // only done before heap walks. >> void JvmtiTagMap::check_hashmap(bool post_events) { >> >> Now, the events are actually posted after heap walks, but I don't think it >> makes any material material difference. >> Even the events are posted earlier in old code, but they are only processed >> after next GC cycle. > > 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 27 additional commits since > the last revision: > > - Merge branch 'master' into JDK-8256811-jdi-missing-class-unloading-event > - Moved TestClassUnloadEvents.java to new location > - Fix test > - Use Shenandoah GC for debuggee for deterministic > - debug test > - Adding log for debugging test failure on Windows > - Removed HiddenClass test from Problem.txt and cleanup test > - Renamed eventHandler_synthesizeUnloadEvent > - v5 > - Improve naming and cleanup > - ... and 17 more: https://git.openjdk.org/jdk/compare/93213306...a07b3737 src/hotspot/share/prims/jvmtiTagMap.cpp line 1144: > 1142: heap_object_callback, > 1143: user_data); > 1144: Arena dead_object_arena(mtInternal); This should be mtServiceability, not Internal. This is an unusual use of Arena in the JVM. Does it get cleaned up completely when it goes out of scope? ------------- PR: https://git.openjdk.org/jdk/pull/9168
