On Mon, 18 Jul 2022 23:12:00 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. >> >> **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 incrementally with one additional > commit since the last revision: > > Coleen's comments Thanks. I see my comment as pending but have no idea why it was not posted. Let me just repeat it again. src/hotspot/share/prims/jvmtiTagMap.cpp#diff @@ -1330,7 +1352,7 @@ jvmtiError JvmtiTagMap::get_objects_with_tags(const jlong* tags, entry_iterate(&collector); } if (collector.some_dead_found() && env()->is_enabled(JVMTI_EVENT_OBJECT_FREE)) { - post_dead_objects_on_vm_thread(); + remove_and_post_dead_objects(); The function `remove_and_post_dead_objects()` is also called from the `flush_object_free_events()`. This function is using some sync protocol. It is waiting under protection of the monitor `lock()` for `_posting_events` to become false and then sets `_posting_events` to true. It is also notifying the other such waiting threads after clearing the `_posting_events`. Should the function `get_objects_with_tags()` follow the same protocol? Do I understand it right that the goal was to serialize the `remove_and_post_dead_objects()` calls? If not then why do we need this sync protocol in function `flush_object_free_events()`? ------------- PR: https://git.openjdk.org/jdk/pull/9168
