/* * Copyright (c) 2001, 2016, Oracle and/or its affiliates. All rights reserved. * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. * */ #include "precompiled.hpp" #include "classfile/javaClasses.inline.hpp" #include "classfile/systemDictionary.hpp" #include "gc/shared/collectedHeap.hpp" #include "gc/shared/collectedHeap.inline.hpp" #include "gc/shared/gcTimer.hpp" #include "gc/shared/gcTraceTime.inline.hpp" #include "gc/shared/referencePolicy.hpp" #include "gc/shared/referenceProcessor.inline.hpp" #include "logging/log.hpp" #include "memory/allocation.hpp" #include "memory/resourceArea.hpp" #include "oops/oop.inline.hpp" #include "runtime/heapMonitoring.hpp" #include "runtime/java.hpp" #include "runtime/jniHandles.hpp" ReferencePolicy* ReferenceProcessor::_always_clear_soft_ref_policy = NULL; ReferencePolicy* ReferenceProcessor::_default_soft_ref_policy = NULL; jlong ReferenceProcessor::_soft_ref_timestamp_clock = 0; void referenceProcessor_init() { ReferenceProcessor::init_statics(); } void ReferenceProcessor::init_statics() { // We need a monotonically non-decreasing time in ms but // os::javaTimeMillis() does not guarantee monotonicity. jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC; // Initialize the soft ref timestamp clock. _soft_ref_timestamp_clock = now; // Also update the soft ref clock in j.l.r.SoftReference java_lang_ref_SoftReference::set_clock(_soft_ref_timestamp_clock); _always_clear_soft_ref_policy = new AlwaysClearPolicy(); if (is_server_compilation_mode_vm()) { _default_soft_ref_policy = new LRUMaxHeapPolicy(); } else { _default_soft_ref_policy = new LRUCurrentHeapPolicy(); } if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) { vm_exit_during_initialization("Could not allocate reference policy object"); } guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery || RefDiscoveryPolicy == ReferentBasedDiscovery, "Unrecognized RefDiscoveryPolicy"); } void ReferenceProcessor::enable_discovery(bool check_no_refs) { #ifdef ASSERT // Verify that we're not currently discovering refs assert(!_discovering_refs, "nested call?"); if (check_no_refs) { // Verify that the discovered lists are empty verify_no_references_recorded(); } #endif // ASSERT // Someone could have modified the value of the static // field in the j.l.r.SoftReference class that holds the // soft reference timestamp clock using reflection or // Unsafe between GCs. Unconditionally update the static // field in ReferenceProcessor here so that we use the new // value during reference discovery. _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock(); _discovering_refs = true; } ReferenceProcessor::ReferenceProcessor(MemRegion span, bool mt_processing, uint mt_processing_degree, bool mt_discovery, uint mt_discovery_degree, bool atomic_discovery, BoolObjectClosure* is_alive_non_header) : _discovering_refs(false), _enqueuing_is_done(false), _is_alive_non_header(is_alive_non_header), _processing_is_mt(mt_processing), _next_id(0) { _span = span; _discovery_is_atomic = atomic_discovery; _discovery_is_mt = mt_discovery; _num_q = MAX2(1U, mt_processing_degree); _max_num_q = MAX2(_num_q, mt_discovery_degree); _discovered_refs = NEW_C_HEAP_ARRAY(DiscoveredList, _max_num_q * number_of_subclasses_of_ref(), mtGC); if (_discovered_refs == NULL) { vm_exit_during_initialization("Could not allocated RefProc Array"); } _discoveredSoftRefs = &_discovered_refs[0]; _discoveredWeakRefs = &_discoveredSoftRefs[_max_num_q]; _discoveredFinalRefs = &_discoveredWeakRefs[_max_num_q]; _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_q]; // Initialize all entries to NULL for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) { _discovered_refs[i].set_head(NULL); _discovered_refs[i].set_length(0); } setup_policy(false /* default soft ref policy */); } #ifndef PRODUCT void ReferenceProcessor::verify_no_references_recorded() { guarantee(!_discovering_refs, "Discovering refs?"); for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) { guarantee(_discovered_refs[i].is_empty(), "Found non-empty discovered list at %u", i); } } #endif void ReferenceProcessor::weak_oops_do(OopClosure* f) { for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) { if (UseCompressedOops) { f->do_oop((narrowOop*)_discovered_refs[i].adr_head()); } else { f->do_oop((oop*)_discovered_refs[i].adr_head()); } } } void ReferenceProcessor::update_soft_ref_master_clock() { // Update (advance) the soft ref master clock field. This must be done // after processing the soft ref list. // We need a monotonically non-decreasing time in ms but // os::javaTimeMillis() does not guarantee monotonicity. jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC; jlong soft_ref_clock = java_lang_ref_SoftReference::clock(); assert(soft_ref_clock == _soft_ref_timestamp_clock, "soft ref clocks out of sync"); NOT_PRODUCT( if (now < _soft_ref_timestamp_clock) { log_warning(gc)("time warp: " JLONG_FORMAT " to " JLONG_FORMAT, _soft_ref_timestamp_clock, now); } ) // The values of now and _soft_ref_timestamp_clock are set using // javaTimeNanos(), which is guaranteed to be monotonically // non-decreasing provided the underlying platform provides such // a time source (and it is bug free). // In product mode, however, protect ourselves from non-monotonicity. if (now > _soft_ref_timestamp_clock) { _soft_ref_timestamp_clock = now; java_lang_ref_SoftReference::set_clock(now); } // Else leave clock stalled at its old value until time progresses // past clock value. } size_t ReferenceProcessor::total_count(DiscoveredList lists[]) { size_t total = 0; for (uint i = 0; i < _max_num_q; ++i) { total += lists[i].length(); } return total; } ReferenceProcessorStats ReferenceProcessor::process_discovered_references( BoolObjectClosure* is_alive, OopClosure* keep_alive, VoidClosure* complete_gc, AbstractRefProcTaskExecutor* task_executor, GCTimer* gc_timer) { assert(!enqueuing_is_done(), "If here enqueuing should not be complete"); // Stop treating discovered references specially. disable_discovery(); // If discovery was concurrent, someone could have modified // the value of the static field in the j.l.r.SoftReference // class that holds the soft reference timestamp clock using // reflection or Unsafe between when discovery was enabled and // now. Unconditionally update the static field in ReferenceProcessor // here so that we use the new value during processing of the // discovered soft refs. _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock(); ReferenceProcessorStats stats( total_count(_discoveredSoftRefs), total_count(_discoveredWeakRefs), total_count(_discoveredFinalRefs), total_count(_discoveredPhantomRefs)); // Soft references { GCTraceTime(Debug, gc, ref) tt("SoftReference", gc_timer); process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true, is_alive, keep_alive, complete_gc, task_executor); } update_soft_ref_master_clock(); // Weak references { GCTraceTime(Debug, gc, ref) tt("WeakReference", gc_timer); process_discovered_reflist(_discoveredWeakRefs, NULL, true, is_alive, keep_alive, complete_gc, task_executor); } // Final references { GCTraceTime(Debug, gc, ref) tt("FinalReference", gc_timer); process_discovered_reflist(_discoveredFinalRefs, NULL, false, is_alive, keep_alive, complete_gc, task_executor); } // Phantom references { GCTraceTime(Debug, gc, ref) tt("PhantomReference", gc_timer); process_discovered_reflist(_discoveredPhantomRefs, NULL, true, is_alive, keep_alive, complete_gc, task_executor); } // Weak global JNI references. It would make more sense (semantically) to // traverse these simultaneously with the regular weak references above, but // that is not how the JDK1.2 specification is. See #4126360. Native code can // thus use JNI weak references to circumvent the phantom references and // resurrect a "post-mortem" object. { GCTraceTime(Debug, gc, ref) tt("JNI Weak Reference", gc_timer); if (task_executor != NULL) { task_executor->set_single_threaded_mode(); } process_phaseJNI(is_alive, keep_alive, complete_gc); } HeapMonitoring::weak_oops_do(task_executor, is_alive, keep_alive, complete_gc); log_debug(gc, ref)("Ref Counts: Soft: " SIZE_FORMAT " Weak: " SIZE_FORMAT " Final: " SIZE_FORMAT " Phantom: " SIZE_FORMAT, stats.soft_count(), stats.weak_count(), stats.final_count(), stats.phantom_count()); log_develop_trace(gc, ref)("JNI Weak Reference count: " SIZE_FORMAT, count_jni_refs()); return stats; } #ifndef PRODUCT // Calculate the number of jni handles. size_t ReferenceProcessor::count_jni_refs() { class CountHandleClosure: public OopClosure { private: size_t _count; public: CountHandleClosure(): _count(0) {} void do_oop(oop* unused) { _count++; } void do_oop(narrowOop* unused) { ShouldNotReachHere(); } size_t count() { return _count; } }; CountHandleClosure global_handle_count; JNIHandles::weak_oops_do(&global_handle_count); return global_handle_count.count(); } #endif void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive, OopClosure* keep_alive, VoidClosure* complete_gc) { JNIHandles::weak_oops_do(is_alive, keep_alive); complete_gc->do_void(); } void ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) { // Enqueue references that are not made active again, and // clear the decks for the next collection (cycle). enqueue_discovered_reflists(task_executor); // Stop treating discovered references specially. disable_discovery(); } void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list) { // Given a list of refs linked through the "discovered" field // (java.lang.ref.Reference.discovered), self-loop their "next" field // thus distinguishing them from active References, then // prepend them to the pending list. // // The Java threads will see the Reference objects linked together through // the discovered field. Instead of trying to do the write barrier updates // in all places in the reference processor where we manipulate the discovered // field we make sure to do the barrier here where we anyway iterate through // all linked Reference objects. Note that it is important to not dirty any // cards during reference processing since this will cause card table // verification to fail for G1. log_develop_trace(gc, ref)("ReferenceProcessor::enqueue_discovered_reflist list " INTPTR_FORMAT, p2i(&refs_list)); oop obj = NULL; oop next_d = refs_list.head(); // Walk down the list, self-looping the next field // so that the References are not considered active. while (obj != next_d) { obj = next_d; assert(obj->is_instance(), "should be an instance object"); assert(InstanceKlass::cast(obj->klass())->is_reference_instance_klass(), "should be reference object"); next_d = java_lang_ref_Reference::discovered(obj); log_develop_trace(gc, ref)(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT, p2i(obj), p2i(next_d)); assert(java_lang_ref_Reference::next(obj) == NULL, "Reference not active; should not be discovered"); // Self-loop next, so as to make Ref not active. java_lang_ref_Reference::set_next_raw(obj, obj); if (next_d != obj) { oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), next_d); } else { // This is the last object. // Swap refs_list into pending list and set obj's // discovered to what we read from the pending list. oop old = Universe::swap_reference_pending_list(refs_list.head()); java_lang_ref_Reference::set_discovered_raw(obj, old); // old may be NULL oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), old); } } } // Parallel enqueue task class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask { public: RefProcEnqueueTask(ReferenceProcessor& ref_processor, DiscoveredList discovered_refs[], int n_queues) : EnqueueTask(ref_processor, discovered_refs, n_queues) { } virtual void work(unsigned int work_id) { assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds"); // Simplest first cut: static partitioning. int index = work_id; // The increment on "index" must correspond to the maximum number of queues // (n_queues) with which that ReferenceProcessor was created. That // is because of the "clever" way the discovered references lists were // allocated and are indexed into. assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected"); for (int j = 0; j < ReferenceProcessor::number_of_subclasses_of_ref(); j++, index += _n_queues) { _ref_processor.enqueue_discovered_reflist(_refs_lists[index]); _refs_lists[index].set_head(NULL); _refs_lists[index].set_length(0); } } }; // Enqueue references that are not made active again void ReferenceProcessor::enqueue_discovered_reflists(AbstractRefProcTaskExecutor* task_executor) { if (_processing_is_mt && task_executor != NULL) { // Parallel code RefProcEnqueueTask tsk(*this, _discovered_refs, _max_num_q); task_executor->execute(tsk); } else { // Serial code: call the parent class's implementation for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) { enqueue_discovered_reflist(_discovered_refs[i]); _discovered_refs[i].set_head(NULL); _discovered_refs[i].set_length(0); } } } void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) { _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref); oop discovered = java_lang_ref_Reference::discovered(_ref); assert(_discovered_addr && discovered->is_oop_or_null(), "Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered)); _next = discovered; _referent_addr = java_lang_ref_Reference::referent_addr(_ref); _referent = java_lang_ref_Reference::referent(_ref); assert(Universe::heap()->is_in_reserved_or_null(_referent), "Wrong oop found in java.lang.Reference object"); assert(allow_null_referent ? _referent->is_oop_or_null() : _referent->is_oop(), "Expected an oop%s for referent field at " PTR_FORMAT, (allow_null_referent ? " or NULL" : ""), p2i(_referent)); } void DiscoveredListIterator::remove() { assert(_ref->is_oop(), "Dropping a bad reference"); oop_store_raw(_discovered_addr, NULL); // First _prev_next ref actually points into DiscoveredList (gross). oop new_next; if (_next == _ref) { // At the end of the list, we should make _prev point to itself. // If _ref is the first ref, then _prev_next will be in the DiscoveredList, // and _prev will be NULL. new_next = _prev; } else { new_next = _next; } // Remove Reference object from discovered list. Note that G1 does not need a // pre-barrier here because we know the Reference has already been found/marked, // that's how it ended up in the discovered list in the first place. oop_store_raw(_prev_next, new_next); NOT_PRODUCT(_removed++); _refs_list.dec_length(1); } void DiscoveredListIterator::clear_referent() { oop_store_raw(_referent_addr, NULL); } // NOTE: process_phase*() are largely similar, and at a high level // merely iterate over the extant list applying a predicate to // each of its elements and possibly removing that element from the // list and applying some further closures to that element. // We should consider the possibility of replacing these // process_phase*() methods by abstracting them into // a single general iterator invocation that receives appropriate // closures that accomplish this work. // (SoftReferences only) Traverse the list and remove any SoftReferences whose // referents are not alive, but that should be kept alive for policy reasons. // Keep alive the transitive closure of all such referents. void ReferenceProcessor::process_phase1(DiscoveredList& refs_list, ReferencePolicy* policy, BoolObjectClosure* is_alive, OopClosure* keep_alive, VoidClosure* complete_gc) { assert(policy != NULL, "Must have a non-NULL policy"); DiscoveredListIterator iter(refs_list, keep_alive, is_alive); // Decide which softly reachable refs should be kept alive. while (iter.has_next()) { iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */)); bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive(); if (referent_is_dead && !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) { log_develop_trace(gc, ref)("Dropping reference (" INTPTR_FORMAT ": %s" ") by policy", p2i(iter.obj()), iter.obj()->klass()->internal_name()); // Remove Reference object from list iter.remove(); // keep the referent around iter.make_referent_alive(); iter.move_to_next(); } else { iter.next(); } } // Close the reachable set complete_gc->do_void(); log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " dead Refs out of " SIZE_FORMAT " discovered Refs by policy, from list " INTPTR_FORMAT, iter.removed(), iter.processed(), p2i(&refs_list)); } // Traverse the list and remove any Refs that are not active, or // whose referents are either alive or NULL. void ReferenceProcessor::pp2_work(DiscoveredList& refs_list, BoolObjectClosure* is_alive, OopClosure* keep_alive) { assert(discovery_is_atomic(), "Error"); DiscoveredListIterator iter(refs_list, keep_alive, is_alive); while (iter.has_next()) { iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */)); DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());) assert(next == NULL, "Should not discover inactive Reference"); if (iter.is_referent_alive()) { log_develop_trace(gc, ref)("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)", p2i(iter.obj()), iter.obj()->klass()->internal_name()); // The referent is reachable after all. // Remove Reference object from list. iter.remove(); // Update the referent pointer as necessary: Note that this // should not entail any recursive marking because the // referent must already have been traversed. iter.make_referent_alive(); iter.move_to_next(); } else { iter.next(); } } NOT_PRODUCT( if (iter.processed() > 0) { log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT " Refs in discovered list " INTPTR_FORMAT, iter.removed(), iter.processed(), p2i(&refs_list)); } ) } void ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList& refs_list, BoolObjectClosure* is_alive, OopClosure* keep_alive, VoidClosure* complete_gc) { assert(!discovery_is_atomic(), "Error"); DiscoveredListIterator iter(refs_list, keep_alive, is_alive); while (iter.has_next()) { iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */)); HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj()); oop next = java_lang_ref_Reference::next(iter.obj()); if ((iter.referent() == NULL || iter.is_referent_alive() || next != NULL)) { assert(next->is_oop_or_null(), "Expected an oop or NULL for next field at " PTR_FORMAT, p2i(next)); // Remove Reference object from list iter.remove(); // Trace the cohorts iter.make_referent_alive(); if (UseCompressedOops) { keep_alive->do_oop((narrowOop*)next_addr); } else { keep_alive->do_oop((oop*)next_addr); } iter.move_to_next(); } else { iter.next(); } } // Now close the newly reachable set complete_gc->do_void(); NOT_PRODUCT( if (iter.processed() > 0) { log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT " Refs in discovered list " INTPTR_FORMAT, iter.removed(), iter.processed(), p2i(&refs_list)); } ) } // Traverse the list and process the referents, by either // clearing them or keeping them (and their reachable // closure) alive. void ReferenceProcessor::process_phase3(DiscoveredList& refs_list, bool clear_referent, BoolObjectClosure* is_alive, OopClosure* keep_alive, VoidClosure* complete_gc) { ResourceMark rm; DiscoveredListIterator iter(refs_list, keep_alive, is_alive); while (iter.has_next()) { iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */)); if (clear_referent) { // NULL out referent pointer iter.clear_referent(); } else { // keep the referent around iter.make_referent_alive(); } log_develop_trace(gc, ref)("Adding %sreference (" INTPTR_FORMAT ": %s) as pending", clear_referent ? "cleared " : "", p2i(iter.obj()), iter.obj()->klass()->internal_name()); assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference"); iter.next(); } // Close the reachable set complete_gc->do_void(); } void ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) { oop obj = NULL; oop next = refs_list.head(); while (next != obj) { obj = next; next = java_lang_ref_Reference::discovered(obj); java_lang_ref_Reference::set_discovered_raw(obj, NULL); } refs_list.set_head(NULL); refs_list.set_length(0); } void ReferenceProcessor::abandon_partial_discovery() { // loop over the lists for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) { if ((i % _max_num_q) == 0) { log_develop_trace(gc, ref)("Abandoning %s discovered list", list_name(i)); } clear_discovered_references(_discovered_refs[i]); } } class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask { public: RefProcPhase1Task(ReferenceProcessor& ref_processor, DiscoveredList refs_lists[], ReferencePolicy* policy, bool marks_oops_alive) : ProcessTask(ref_processor, refs_lists, marks_oops_alive), _policy(policy) { } virtual void work(unsigned int i, BoolObjectClosure& is_alive, OopClosure& keep_alive, VoidClosure& complete_gc) { _ref_processor.process_phase1(_refs_lists[i], _policy, &is_alive, &keep_alive, &complete_gc); } private: ReferencePolicy* _policy; }; class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask { public: RefProcPhase2Task(ReferenceProcessor& ref_processor, DiscoveredList refs_lists[], bool marks_oops_alive) : ProcessTask(ref_processor, refs_lists, marks_oops_alive) { } virtual void work(unsigned int i, BoolObjectClosure& is_alive, OopClosure& keep_alive, VoidClosure& complete_gc) { _ref_processor.process_phase2(_refs_lists[i], &is_alive, &keep_alive, &complete_gc); } }; class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask { public: RefProcPhase3Task(ReferenceProcessor& ref_processor, DiscoveredList refs_lists[], bool clear_referent, bool marks_oops_alive) : ProcessTask(ref_processor, refs_lists, marks_oops_alive), _clear_referent(clear_referent) { } virtual void work(unsigned int i, BoolObjectClosure& is_alive, OopClosure& keep_alive, VoidClosure& complete_gc) { _ref_processor.process_phase3(_refs_lists[i], _clear_referent, &is_alive, &keep_alive, &complete_gc); } private: bool _clear_referent; }; #ifndef PRODUCT void ReferenceProcessor::log_reflist_counts(DiscoveredList ref_lists[], uint active_length, size_t total_refs) { if (!log_is_enabled(Trace, gc, ref)) { return; } stringStream st; for (uint i = 0; i < active_length; ++i) { st.print(SIZE_FORMAT " ", ref_lists[i].length()); } log_develop_trace(gc, ref)("%s= " SIZE_FORMAT, st.as_string(), total_refs); #ifdef ASSERT for (uint i = active_length; i < _max_num_q; i++) { assert(ref_lists[i].length() == 0, SIZE_FORMAT " unexpected References in %u", ref_lists[i].length(), i); } #endif } #endif void ReferenceProcessor::set_active_mt_degree(uint v) { _num_q = v; _next_id = 0; } // Balances reference queues. // Move entries from all queues[0, 1, ..., _max_num_q-1] to // queues[0, 1, ..., _num_q-1] because only the first _num_q // corresponding to the active workers will be processed. void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[]) { // calculate total length size_t total_refs = 0; log_develop_trace(gc, ref)("Balance ref_lists "); for (uint i = 0; i < _max_num_q; ++i) { total_refs += ref_lists[i].length(); } log_reflist_counts(ref_lists, _max_num_q, total_refs); size_t avg_refs = total_refs / _num_q + 1; uint to_idx = 0; for (uint from_idx = 0; from_idx < _max_num_q; from_idx++) { bool move_all = false; if (from_idx >= _num_q) { move_all = ref_lists[from_idx].length() > 0; } while ((ref_lists[from_idx].length() > avg_refs) || move_all) { assert(to_idx < _num_q, "Sanity Check!"); if (ref_lists[to_idx].length() < avg_refs) { // move superfluous refs size_t refs_to_move; // Move all the Ref's if the from queue will not be processed. if (move_all) { refs_to_move = MIN2(ref_lists[from_idx].length(), avg_refs - ref_lists[to_idx].length()); } else { refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs, avg_refs - ref_lists[to_idx].length()); } assert(refs_to_move > 0, "otherwise the code below will fail"); oop move_head = ref_lists[from_idx].head(); oop move_tail = move_head; oop new_head = move_head; // find an element to split the list on for (size_t j = 0; j < refs_to_move; ++j) { move_tail = new_head; new_head = java_lang_ref_Reference::discovered(new_head); } // Add the chain to the to list. if (ref_lists[to_idx].head() == NULL) { // to list is empty. Make a loop at the end. java_lang_ref_Reference::set_discovered_raw(move_tail, move_tail); } else { java_lang_ref_Reference::set_discovered_raw(move_tail, ref_lists[to_idx].head()); } ref_lists[to_idx].set_head(move_head); ref_lists[to_idx].inc_length(refs_to_move); // Remove the chain from the from list. if (move_tail == new_head) { // We found the end of the from list. ref_lists[from_idx].set_head(NULL); } else { ref_lists[from_idx].set_head(new_head); } ref_lists[from_idx].dec_length(refs_to_move); if (ref_lists[from_idx].length() == 0) { break; } } else { to_idx = (to_idx + 1) % _num_q; } } } #ifdef ASSERT size_t balanced_total_refs = 0; for (uint i = 0; i < _num_q; ++i) { balanced_total_refs += ref_lists[i].length(); } log_reflist_counts(ref_lists, _num_q, balanced_total_refs); assert(total_refs == balanced_total_refs, "Balancing was incomplete"); #endif } void ReferenceProcessor::balance_all_queues() { balance_queues(_discoveredSoftRefs); balance_queues(_discoveredWeakRefs); balance_queues(_discoveredFinalRefs); balance_queues(_discoveredPhantomRefs); } void ReferenceProcessor::process_discovered_reflist( DiscoveredList refs_lists[], ReferencePolicy* policy, bool clear_referent, BoolObjectClosure* is_alive, OopClosure* keep_alive, VoidClosure* complete_gc, AbstractRefProcTaskExecutor* task_executor) { bool mt_processing = task_executor != NULL && _processing_is_mt; // If discovery used MT and a dynamic number of GC threads, then // the queues must be balanced for correctness if fewer than the // maximum number of queues were used. The number of queue used // during discovery may be different than the number to be used // for processing so don't depend of _num_q < _max_num_q as part // of the test. bool must_balance = _discovery_is_mt; if ((mt_processing && ParallelRefProcBalancingEnabled) || must_balance) { balance_queues(refs_lists); } // Phase 1 (soft refs only): // . Traverse the list and remove any SoftReferences whose // referents are not alive, but that should be kept alive for // policy reasons. Keep alive the transitive closure of all // such referents. if (policy != NULL) { if (mt_processing) { RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/); task_executor->execute(phase1); } else { for (uint i = 0; i < _max_num_q; i++) { process_phase1(refs_lists[i], policy, is_alive, keep_alive, complete_gc); } } } else { // policy == NULL assert(refs_lists != _discoveredSoftRefs, "Policy must be specified for soft references."); } // Phase 2: // . Traverse the list and remove any refs whose referents are alive. if (mt_processing) { RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/); task_executor->execute(phase2); } else { for (uint i = 0; i < _max_num_q; i++) { process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc); } } // Phase 3: // . Traverse the list and process referents as appropriate. if (mt_processing) { RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/); task_executor->execute(phase3); } else { for (uint i = 0; i < _max_num_q; i++) { process_phase3(refs_lists[i], clear_referent, is_alive, keep_alive, complete_gc); } } } inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) { uint id = 0; // Determine the queue index to use for this object. if (_discovery_is_mt) { // During a multi-threaded discovery phase, // each thread saves to its "own" list. Thread* thr = Thread::current(); id = thr->as_Worker_thread()->id(); } else { // single-threaded discovery, we save in round-robin // fashion to each of the lists. if (_processing_is_mt) { id = next_id(); } } assert(id < _max_num_q, "Id is out-of-bounds id %u and max id %u)", id, _max_num_q); // Get the discovered queue to which we will add DiscoveredList* list = NULL; switch (rt) { case REF_OTHER: // Unknown reference type, no special treatment break; case REF_SOFT: list = &_discoveredSoftRefs[id]; break; case REF_WEAK: list = &_discoveredWeakRefs[id]; break; case REF_FINAL: list = &_discoveredFinalRefs[id]; break; case REF_PHANTOM: list = &_discoveredPhantomRefs[id]; break; case REF_NONE: // we should not reach here if we are an InstanceRefKlass default: ShouldNotReachHere(); } log_develop_trace(gc, ref)("Thread %d gets list " INTPTR_FORMAT, id, p2i(list)); return list; } inline void ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list, oop obj, HeapWord* discovered_addr) { assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller"); // First we must make sure this object is only enqueued once. CAS in a non null // discovered_addr. oop current_head = refs_list.head(); // The last ref must have its discovered field pointing to itself. oop next_discovered = (current_head != NULL) ? current_head : obj; oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr, NULL); if (retest == NULL) { // This thread just won the right to enqueue the object. // We have separate lists for enqueueing, so no synchronization // is necessary. refs_list.set_head(obj); refs_list.inc_length(1); log_develop_trace(gc, ref)("Discovered reference (mt) (" INTPTR_FORMAT ": %s)", p2i(obj), obj->klass()->internal_name()); } else { // If retest was non NULL, another thread beat us to it: // The reference has already been discovered... log_develop_trace(gc, ref)("Already discovered reference (" INTPTR_FORMAT ": %s)", p2i(obj), obj->klass()->internal_name()); } } #ifndef PRODUCT // Non-atomic (i.e. concurrent) discovery might allow us // to observe j.l.References with NULL referents, being those // cleared concurrently by mutators during (or after) discovery. void ReferenceProcessor::verify_referent(oop obj) { bool da = discovery_is_atomic(); oop referent = java_lang_ref_Reference::referent(obj); assert(da ? referent->is_oop() : referent->is_oop_or_null(), "Bad referent " INTPTR_FORMAT " found in Reference " INTPTR_FORMAT " during %satomic discovery ", p2i(referent), p2i(obj), da ? "" : "non-"); } #endif // We mention two of several possible choices here: // #0: if the reference object is not in the "originating generation" // (or part of the heap being collected, indicated by our "span" // we don't treat it specially (i.e. we scan it as we would // a normal oop, treating its references as strong references). // This means that references can't be discovered unless their // referent is also in the same span. This is the simplest, // most "local" and most conservative approach, albeit one // that may cause weak references to be enqueued least promptly. // We call this choice the "ReferenceBasedDiscovery" policy. // #1: the reference object may be in any generation (span), but if // the referent is in the generation (span) being currently collected // then we can discover the reference object, provided // the object has not already been discovered by // a different concurrently running collector (as may be the // case, for instance, if the reference object is in CMS and // the referent in DefNewGeneration), and provided the processing // of this reference object by the current collector will // appear atomic to every other collector in the system. // (Thus, for instance, a concurrent collector may not // discover references in other generations even if the // referent is in its own generation). This policy may, // in certain cases, enqueue references somewhat sooner than // might Policy #0 above, but at marginally increased cost // and complexity in processing these references. // We call this choice the "RefeferentBasedDiscovery" policy. bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) { // Make sure we are discovering refs (rather than processing discovered refs). if (!_discovering_refs || !RegisterReferences) { return false; } // We only discover active references. oop next = java_lang_ref_Reference::next(obj); if (next != NULL) { // Ref is no longer active return false; } HeapWord* obj_addr = (HeapWord*)obj; if (RefDiscoveryPolicy == ReferenceBasedDiscovery && !_span.contains(obj_addr)) { // Reference is not in the originating generation; // don't treat it specially (i.e. we want to scan it as a normal // object with strong references). return false; } // We only discover references whose referents are not (yet) // known to be strongly reachable. if (is_alive_non_header() != NULL) { verify_referent(obj); if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) { return false; // referent is reachable } } if (rt == REF_SOFT) { // For soft refs we can decide now if these are not // current candidates for clearing, in which case we // can mark through them now, rather than delaying that // to the reference-processing phase. Since all current // time-stamp policies advance the soft-ref clock only // at a full collection cycle, this is always currently // accurate. if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) { return false; } } ResourceMark rm; // Needed for tracing. HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj); const oop discovered = java_lang_ref_Reference::discovered(obj); assert(discovered->is_oop_or_null(), "Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered)); if (discovered != NULL) { // The reference has already been discovered... log_develop_trace(gc, ref)("Already discovered reference (" INTPTR_FORMAT ": %s)", p2i(obj), obj->klass()->internal_name()); if (RefDiscoveryPolicy == ReferentBasedDiscovery) { // assumes that an object is not processed twice; // if it's been already discovered it must be on another // generation's discovered list; so we won't discover it. return false; } else { assert(RefDiscoveryPolicy == ReferenceBasedDiscovery, "Unrecognized policy"); // Check assumption that an object is not potentially // discovered twice except by concurrent collectors that potentially // trace the same Reference object twice. assert(UseConcMarkSweepGC || UseG1GC, "Only possible with a concurrent marking collector"); return true; } } if (RefDiscoveryPolicy == ReferentBasedDiscovery) { verify_referent(obj); // Discover if and only if EITHER: // .. reference is in our span, OR // .. we are an atomic collector and referent is in our span if (_span.contains(obj_addr) || (discovery_is_atomic() && _span.contains(java_lang_ref_Reference::referent(obj)))) { // should_enqueue = true; } else { return false; } } else { assert(RefDiscoveryPolicy == ReferenceBasedDiscovery && _span.contains(obj_addr), "code inconsistency"); } // Get the right type of discovered queue head. DiscoveredList* list = get_discovered_list(rt); if (list == NULL) { return false; // nothing special needs to be done } if (_discovery_is_mt) { add_to_discovered_list_mt(*list, obj, discovered_addr); } else { // We do a raw store here: the field will be visited later when processing // the discovered references. oop current_head = list->head(); // The last ref must have its discovered field pointing to itself. oop next_discovered = (current_head != NULL) ? current_head : obj; assert(discovered == NULL, "control point invariant"); oop_store_raw(discovered_addr, next_discovered); list->set_head(obj); list->inc_length(1); log_develop_trace(gc, ref)("Discovered reference (" INTPTR_FORMAT ": %s)", p2i(obj), obj->klass()->internal_name()); } assert(obj->is_oop(), "Discovered a bad reference"); verify_referent(obj); return true; } bool ReferenceProcessor::has_discovered_references() { for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) { if (!_discovered_refs[i].is_empty()) { return true; } } return false; } // Preclean the discovered references by removing those // whose referents are alive, and by marking from those that // are not active. These lists can be handled here // in any order and, indeed, concurrently. void ReferenceProcessor::preclean_discovered_references( BoolObjectClosure* is_alive, OopClosure* keep_alive, VoidClosure* complete_gc, YieldClosure* yield, GCTimer* gc_timer) { // Soft references { GCTraceTime(Debug, gc, ref) tm("Preclean SoftReferences", gc_timer); for (uint i = 0; i < _max_num_q; i++) { if (yield->should_return()) { return; } preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive, keep_alive, complete_gc, yield); } } // Weak references { GCTraceTime(Debug, gc, ref) tm("Preclean WeakReferences", gc_timer); for (uint i = 0; i < _max_num_q; i++) { if (yield->should_return()) { return; } preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive, keep_alive, complete_gc, yield); } } // Final references { GCTraceTime(Debug, gc, ref) tm("Preclean FinalReferences", gc_timer); for (uint i = 0; i < _max_num_q; i++) { if (yield->should_return()) { return; } preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive, keep_alive, complete_gc, yield); } } // Phantom references { GCTraceTime(Debug, gc, ref) tm("Preclean PhantomReferences", gc_timer); for (uint i = 0; i < _max_num_q; i++) { if (yield->should_return()) { return; } preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive, keep_alive, complete_gc, yield); } } } // Walk the given discovered ref list, and remove all reference objects // whose referents are still alive, whose referents are NULL or which // are not active (have a non-NULL next field). NOTE: When we are // thus precleaning the ref lists (which happens single-threaded today), // we do not disable refs discovery to honor the correct semantics of // java.lang.Reference. As a result, we need to be careful below // that ref removal steps interleave safely with ref discovery steps // (in this thread). void ReferenceProcessor::preclean_discovered_reflist(DiscoveredList& refs_list, BoolObjectClosure* is_alive, OopClosure* keep_alive, VoidClosure* complete_gc, YieldClosure* yield) { DiscoveredListIterator iter(refs_list, keep_alive, is_alive); while (iter.has_next()) { iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */)); oop obj = iter.obj(); oop next = java_lang_ref_Reference::next(obj); if (iter.referent() == NULL || iter.is_referent_alive() || next != NULL) { // The referent has been cleared, or is alive, or the Reference is not // active; we need to trace and mark its cohort. log_develop_trace(gc, ref)("Precleaning Reference (" INTPTR_FORMAT ": %s)", p2i(iter.obj()), iter.obj()->klass()->internal_name()); // Remove Reference object from list iter.remove(); // Keep alive its cohort. iter.make_referent_alive(); if (UseCompressedOops) { narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj); keep_alive->do_oop(next_addr); } else { oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj); keep_alive->do_oop(next_addr); } iter.move_to_next(); } else { iter.next(); } } // Close the reachable set complete_gc->do_void(); NOT_PRODUCT( if (iter.processed() > 0) { log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " Refs out of " SIZE_FORMAT " Refs in discovered list " INTPTR_FORMAT, iter.removed(), iter.processed(), p2i(&refs_list)); } ) } const char* ReferenceProcessor::list_name(uint i) { assert(i <= _max_num_q * number_of_subclasses_of_ref(), "Out of bounds index"); int j = i / _max_num_q; switch (j) { case 0: return "SoftRef"; case 1: return "WeakRef"; case 2: return "FinalRef"; case 3: return "PhantomRef"; } ShouldNotReachHere(); return NULL; }