On Mon, 16 Nov 2020 23:10:21 GMT, Coleen Phillimore <cole...@openjdk.org> wrote:
>> This change turns the HashTable that JVMTI uses for object tagging into a >> regular Hotspot hashtable - the one in hashtable.hpp with resizing and >> rehashing. Instead of pointing directly to oops so that GC has to walk the >> table to follow oops and then to rehash the table, this table points to >> WeakHandle. GC walks the backing OopStorages concurrently. >> >> The hash function for the table is a hash of the lower 32 bits of the >> address. A flag is set during GC (gc_notification if in a safepoint, and >> through a call to JvmtiTagMap::needs_processing()) so that the table is >> rehashed at the next use. >> >> The gc_notification mechanism of weak oop processing is used to notify Jvmti >> to post ObjectFree events. In concurrent GCs there can be a window of time >> between weak oop marking where the oop is unmarked, so dead (the phantom >> load in peek returns NULL) but the gc_notification hasn't been done yet. In >> this window, a heap walk or GetObjectsWithTags call would not find an object >> before the ObjectFree event is posted. This is dealt with in two ways: >> >> 1. In the Heap walk, there's an unconditional table walk to post events if >> events are needed to post. >> 2. For GetObjectWithTags, if a dead oop is found in the table and posting is >> required, we use the VM thread to post the event. >> >> Event posting cannot be done in a JavaThread because the posting needs to be >> done while holding the table lock, so that the JvmtiEnv state doesn't change >> before posting is done. ObjectFree callbacks are limited in what they can >> do as per the JVMTI Specification. The allowed callbacks to the VM already >> have code to allow NonJava threads. >> >> To avoid rehashing, I also tried to use object->identity_hash() but this >> breaks because entries can be added to the table during heapwalk, where the >> objects use marking. The starting markWord is saved and restored. Adding a >> hashcode during this operation makes restoring the former markWord (locked, >> inflated, etc) too complicated. Plus we don't want all these objects to >> have hashcodes because locking operations after tagging would have to always >> use inflated locks. >> >> Much of this change is to remove serial weak oop processing for the >> weakProcessor, ZGC and Shenandoah. The GCs have been stress tested with >> jvmti code. >> >> It has also been tested with tier1-6. >> >> Thank you to Stefan, Erik and Kim for their help with this change. > > Coleen Phillimore has updated the pull request with a new target base due to > a merge or a rebase. The pull request now contains 15 commits: > > - Reverse remove_dead_entries_locked function names. > - Merge branch 'master' into jvmti-table > - Add shenandoah set_needs_cleaning but this doesn't work. > - fix vmTestbase/nsk/jvmti tests > - improve tagmap cleanup and objectfree event posting > - Add logging to event posting in case of pauses. > - Merge branch 'master' into jvmti-table > - Add back WeakProcessorPhases::Phase enum. > - Serguei 1. > - Code review comments from Kim and Albert. > - ... and 5 more: > https://git.openjdk.java.net/jdk/compare/0357db35...daaa13fe test/hotspot/jtreg/vmTestbase/nsk/jvmti/AttachOnDemand/attach021/attach021Agent00.cpp line 72: > 70: > Java_nsk_jvmti_AttachOnDemand_attach021_attach021Target_shutdownAgent(JNIEnv > * jni, > 71: jclass klass) { > 72: @kimbarrett ? I'm not sure why you made this change. See Serguei's comment. ------------- PR: https://git.openjdk.java.net/jdk/pull/967
