On Fri, 30 Oct 2020 20:23:04 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. I think I addressed your comments, retesting now. Thank you! ------------- PR: https://git.openjdk.java.net/jdk/pull/967
