Wed, 26 Jun 2013 16:58:37 +0200
8013590: NPG: Add a memory pool MXBean for Metaspace
Reviewed-by: jmasa, mgerdin
1 /*
2 * Copyright (c) 2001, 2013, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
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23 */
25 #include "precompiled.hpp"
26 #include "classfile/javaClasses.hpp"
27 #include "classfile/systemDictionary.hpp"
28 #include "gc_implementation/shared/gcTimer.hpp"
29 #include "gc_implementation/shared/gcTraceTime.hpp"
30 #include "gc_interface/collectedHeap.hpp"
31 #include "gc_interface/collectedHeap.inline.hpp"
32 #include "memory/referencePolicy.hpp"
33 #include "memory/referenceProcessor.hpp"
34 #include "oops/oop.inline.hpp"
35 #include "runtime/java.hpp"
36 #include "runtime/jniHandles.hpp"
38 ReferencePolicy* ReferenceProcessor::_always_clear_soft_ref_policy = NULL;
39 ReferencePolicy* ReferenceProcessor::_default_soft_ref_policy = NULL;
40 bool ReferenceProcessor::_pending_list_uses_discovered_field = false;
41 jlong ReferenceProcessor::_soft_ref_timestamp_clock = 0;
43 void referenceProcessor_init() {
44 ReferenceProcessor::init_statics();
45 }
47 void ReferenceProcessor::init_statics() {
48 // We need a monotonically non-deccreasing time in ms but
49 // os::javaTimeMillis() does not guarantee monotonicity.
50 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
52 // Initialize the soft ref timestamp clock.
53 _soft_ref_timestamp_clock = now;
54 // Also update the soft ref clock in j.l.r.SoftReference
55 java_lang_ref_SoftReference::set_clock(_soft_ref_timestamp_clock);
57 _always_clear_soft_ref_policy = new AlwaysClearPolicy();
58 _default_soft_ref_policy = new COMPILER2_PRESENT(LRUMaxHeapPolicy())
59 NOT_COMPILER2(LRUCurrentHeapPolicy());
60 if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) {
61 vm_exit_during_initialization("Could not allocate reference policy object");
62 }
63 guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||
64 RefDiscoveryPolicy == ReferentBasedDiscovery,
65 "Unrecongnized RefDiscoveryPolicy");
66 _pending_list_uses_discovered_field = JDK_Version::current().pending_list_uses_discovered_field();
67 }
69 void ReferenceProcessor::enable_discovery(bool verify_disabled, bool check_no_refs) {
70 #ifdef ASSERT
71 // Verify that we're not currently discovering refs
72 assert(!verify_disabled || !_discovering_refs, "nested call?");
74 if (check_no_refs) {
75 // Verify that the discovered lists are empty
76 verify_no_references_recorded();
77 }
78 #endif // ASSERT
80 // Someone could have modified the value of the static
81 // field in the j.l.r.SoftReference class that holds the
82 // soft reference timestamp clock using reflection or
83 // Unsafe between GCs. Unconditionally update the static
84 // field in ReferenceProcessor here so that we use the new
85 // value during reference discovery.
87 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
88 _discovering_refs = true;
89 }
91 ReferenceProcessor::ReferenceProcessor(MemRegion span,
92 bool mt_processing,
93 uint mt_processing_degree,
94 bool mt_discovery,
95 uint mt_discovery_degree,
96 bool atomic_discovery,
97 BoolObjectClosure* is_alive_non_header,
98 bool discovered_list_needs_barrier) :
99 _discovering_refs(false),
100 _enqueuing_is_done(false),
101 _is_alive_non_header(is_alive_non_header),
102 _discovered_list_needs_barrier(discovered_list_needs_barrier),
103 _bs(NULL),
104 _processing_is_mt(mt_processing),
105 _next_id(0)
106 {
107 _span = span;
108 _discovery_is_atomic = atomic_discovery;
109 _discovery_is_mt = mt_discovery;
110 _num_q = MAX2(1U, mt_processing_degree);
111 _max_num_q = MAX2(_num_q, mt_discovery_degree);
112 _discovered_refs = NEW_C_HEAP_ARRAY(DiscoveredList,
113 _max_num_q * number_of_subclasses_of_ref(), mtGC);
115 if (_discovered_refs == NULL) {
116 vm_exit_during_initialization("Could not allocated RefProc Array");
117 }
118 _discoveredSoftRefs = &_discovered_refs[0];
119 _discoveredWeakRefs = &_discoveredSoftRefs[_max_num_q];
120 _discoveredFinalRefs = &_discoveredWeakRefs[_max_num_q];
121 _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_q];
123 // Initialize all entries to NULL
124 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
125 _discovered_refs[i].set_head(NULL);
126 _discovered_refs[i].set_length(0);
127 }
129 // If we do barriers, cache a copy of the barrier set.
130 if (discovered_list_needs_barrier) {
131 _bs = Universe::heap()->barrier_set();
132 }
133 setup_policy(false /* default soft ref policy */);
134 }
136 #ifndef PRODUCT
137 void ReferenceProcessor::verify_no_references_recorded() {
138 guarantee(!_discovering_refs, "Discovering refs?");
139 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
140 guarantee(_discovered_refs[i].is_empty(),
141 "Found non-empty discovered list");
142 }
143 }
144 #endif
146 void ReferenceProcessor::weak_oops_do(OopClosure* f) {
147 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
148 if (UseCompressedOops) {
149 f->do_oop((narrowOop*)_discovered_refs[i].adr_head());
150 } else {
151 f->do_oop((oop*)_discovered_refs[i].adr_head());
152 }
153 }
154 }
156 void ReferenceProcessor::update_soft_ref_master_clock() {
157 // Update (advance) the soft ref master clock field. This must be done
158 // after processing the soft ref list.
160 // We need a monotonically non-deccreasing time in ms but
161 // os::javaTimeMillis() does not guarantee monotonicity.
162 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
163 jlong soft_ref_clock = java_lang_ref_SoftReference::clock();
164 assert(soft_ref_clock == _soft_ref_timestamp_clock, "soft ref clocks out of sync");
166 NOT_PRODUCT(
167 if (now < _soft_ref_timestamp_clock) {
168 warning("time warp: "INT64_FORMAT" to "INT64_FORMAT,
169 _soft_ref_timestamp_clock, now);
170 }
171 )
172 // The values of now and _soft_ref_timestamp_clock are set using
173 // javaTimeNanos(), which is guaranteed to be monotonically
174 // non-decreasing provided the underlying platform provides such
175 // a time source (and it is bug free).
176 // In product mode, however, protect ourselves from non-monotonicty.
177 if (now > _soft_ref_timestamp_clock) {
178 _soft_ref_timestamp_clock = now;
179 java_lang_ref_SoftReference::set_clock(now);
180 }
181 // Else leave clock stalled at its old value until time progresses
182 // past clock value.
183 }
185 size_t ReferenceProcessor::total_count(DiscoveredList lists[]) {
186 size_t total = 0;
187 for (uint i = 0; i < _max_num_q; ++i) {
188 total += lists[i].length();
189 }
190 return total;
191 }
193 ReferenceProcessorStats ReferenceProcessor::process_discovered_references(
194 BoolObjectClosure* is_alive,
195 OopClosure* keep_alive,
196 VoidClosure* complete_gc,
197 AbstractRefProcTaskExecutor* task_executor,
198 GCTimer* gc_timer) {
199 NOT_PRODUCT(verify_ok_to_handle_reflists());
201 assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
202 // Stop treating discovered references specially.
203 disable_discovery();
205 // If discovery was concurrent, someone could have modified
206 // the value of the static field in the j.l.r.SoftReference
207 // class that holds the soft reference timestamp clock using
208 // reflection or Unsafe between when discovery was enabled and
209 // now. Unconditionally update the static field in ReferenceProcessor
210 // here so that we use the new value during processing of the
211 // discovered soft refs.
213 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
215 bool trace_time = PrintGCDetails && PrintReferenceGC;
217 // Soft references
218 size_t soft_count = 0;
219 {
220 GCTraceTime tt("SoftReference", trace_time, false, gc_timer);
221 soft_count =
222 process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
223 is_alive, keep_alive, complete_gc, task_executor);
224 }
226 update_soft_ref_master_clock();
228 // Weak references
229 size_t weak_count = 0;
230 {
231 GCTraceTime tt("WeakReference", trace_time, false, gc_timer);
232 weak_count =
233 process_discovered_reflist(_discoveredWeakRefs, NULL, true,
234 is_alive, keep_alive, complete_gc, task_executor);
235 }
237 // Final references
238 size_t final_count = 0;
239 {
240 GCTraceTime tt("FinalReference", trace_time, false, gc_timer);
241 final_count =
242 process_discovered_reflist(_discoveredFinalRefs, NULL, false,
243 is_alive, keep_alive, complete_gc, task_executor);
244 }
246 // Phantom references
247 size_t phantom_count = 0;
248 {
249 GCTraceTime tt("PhantomReference", trace_time, false, gc_timer);
250 phantom_count =
251 process_discovered_reflist(_discoveredPhantomRefs, NULL, false,
252 is_alive, keep_alive, complete_gc, task_executor);
253 }
255 // Weak global JNI references. It would make more sense (semantically) to
256 // traverse these simultaneously with the regular weak references above, but
257 // that is not how the JDK1.2 specification is. See #4126360. Native code can
258 // thus use JNI weak references to circumvent the phantom references and
259 // resurrect a "post-mortem" object.
260 {
261 GCTraceTime tt("JNI Weak Reference", trace_time, false, gc_timer);
262 if (task_executor != NULL) {
263 task_executor->set_single_threaded_mode();
264 }
265 process_phaseJNI(is_alive, keep_alive, complete_gc);
266 }
268 return ReferenceProcessorStats(soft_count, weak_count, final_count, phantom_count);
269 }
271 #ifndef PRODUCT
272 // Calculate the number of jni handles.
273 uint ReferenceProcessor::count_jni_refs() {
274 class AlwaysAliveClosure: public BoolObjectClosure {
275 public:
276 virtual bool do_object_b(oop obj) { return true; }
277 };
279 class CountHandleClosure: public OopClosure {
280 private:
281 int _count;
282 public:
283 CountHandleClosure(): _count(0) {}
284 void do_oop(oop* unused) { _count++; }
285 void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
286 int count() { return _count; }
287 };
288 CountHandleClosure global_handle_count;
289 AlwaysAliveClosure always_alive;
290 JNIHandles::weak_oops_do(&always_alive, &global_handle_count);
291 return global_handle_count.count();
292 }
293 #endif
295 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,
296 OopClosure* keep_alive,
297 VoidClosure* complete_gc) {
298 #ifndef PRODUCT
299 if (PrintGCDetails && PrintReferenceGC) {
300 unsigned int count = count_jni_refs();
301 gclog_or_tty->print(", %u refs", count);
302 }
303 #endif
304 JNIHandles::weak_oops_do(is_alive, keep_alive);
305 complete_gc->do_void();
306 }
309 template <class T>
310 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref,
311 AbstractRefProcTaskExecutor* task_executor) {
313 // Remember old value of pending references list
314 T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr();
315 T old_pending_list_value = *pending_list_addr;
317 // Enqueue references that are not made active again, and
318 // clear the decks for the next collection (cycle).
319 ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor);
320 // Do the oop-check on pending_list_addr missed in
321 // enqueue_discovered_reflist. We should probably
322 // do a raw oop_check so that future such idempotent
323 // oop_stores relying on the oop-check side-effect
324 // may be elided automatically and safely without
325 // affecting correctness.
326 oop_store(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr));
328 // Stop treating discovered references specially.
329 ref->disable_discovery();
331 // Return true if new pending references were added
332 return old_pending_list_value != *pending_list_addr;
333 }
335 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) {
336 NOT_PRODUCT(verify_ok_to_handle_reflists());
337 if (UseCompressedOops) {
338 return enqueue_discovered_ref_helper<narrowOop>(this, task_executor);
339 } else {
340 return enqueue_discovered_ref_helper<oop>(this, task_executor);
341 }
342 }
344 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,
345 HeapWord* pending_list_addr) {
346 // Given a list of refs linked through the "discovered" field
347 // (java.lang.ref.Reference.discovered), self-loop their "next" field
348 // thus distinguishing them from active References, then
349 // prepend them to the pending list.
350 // BKWRD COMPATIBILITY NOTE: For older JDKs (prior to the fix for 4956777),
351 // the "next" field is used to chain the pending list, not the discovered
352 // field.
354 if (TraceReferenceGC && PrintGCDetails) {
355 gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list "
356 INTPTR_FORMAT, (address)refs_list.head());
357 }
359 oop obj = NULL;
360 oop next_d = refs_list.head();
361 if (pending_list_uses_discovered_field()) { // New behaviour
362 // Walk down the list, self-looping the next field
363 // so that the References are not considered active.
364 while (obj != next_d) {
365 obj = next_d;
366 assert(obj->is_instanceRef(), "should be reference object");
367 next_d = java_lang_ref_Reference::discovered(obj);
368 if (TraceReferenceGC && PrintGCDetails) {
369 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
370 obj, next_d);
371 }
372 assert(java_lang_ref_Reference::next(obj) == NULL,
373 "Reference not active; should not be discovered");
374 // Self-loop next, so as to make Ref not active.
375 java_lang_ref_Reference::set_next(obj, obj);
376 if (next_d == obj) { // obj is last
377 // Swap refs_list into pendling_list_addr and
378 // set obj's discovered to what we read from pending_list_addr.
379 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
380 // Need oop_check on pending_list_addr above;
381 // see special oop-check code at the end of
382 // enqueue_discovered_reflists() further below.
383 java_lang_ref_Reference::set_discovered(obj, old); // old may be NULL
384 }
385 }
386 } else { // Old behaviour
387 // Walk down the list, copying the discovered field into
388 // the next field and clearing the discovered field.
389 while (obj != next_d) {
390 obj = next_d;
391 assert(obj->is_instanceRef(), "should be reference object");
392 next_d = java_lang_ref_Reference::discovered(obj);
393 if (TraceReferenceGC && PrintGCDetails) {
394 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
395 obj, next_d);
396 }
397 assert(java_lang_ref_Reference::next(obj) == NULL,
398 "The reference should not be enqueued");
399 if (next_d == obj) { // obj is last
400 // Swap refs_list into pendling_list_addr and
401 // set obj's next to what we read from pending_list_addr.
402 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
403 // Need oop_check on pending_list_addr above;
404 // see special oop-check code at the end of
405 // enqueue_discovered_reflists() further below.
406 if (old == NULL) {
407 // obj should be made to point to itself, since
408 // pending list was empty.
409 java_lang_ref_Reference::set_next(obj, obj);
410 } else {
411 java_lang_ref_Reference::set_next(obj, old);
412 }
413 } else {
414 java_lang_ref_Reference::set_next(obj, next_d);
415 }
416 java_lang_ref_Reference::set_discovered(obj, (oop) NULL);
417 }
418 }
419 }
421 // Parallel enqueue task
422 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
423 public:
424 RefProcEnqueueTask(ReferenceProcessor& ref_processor,
425 DiscoveredList discovered_refs[],
426 HeapWord* pending_list_addr,
427 int n_queues)
428 : EnqueueTask(ref_processor, discovered_refs,
429 pending_list_addr, n_queues)
430 { }
432 virtual void work(unsigned int work_id) {
433 assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds");
434 // Simplest first cut: static partitioning.
435 int index = work_id;
436 // The increment on "index" must correspond to the maximum number of queues
437 // (n_queues) with which that ReferenceProcessor was created. That
438 // is because of the "clever" way the discovered references lists were
439 // allocated and are indexed into.
440 assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected");
441 for (int j = 0;
442 j < ReferenceProcessor::number_of_subclasses_of_ref();
443 j++, index += _n_queues) {
444 _ref_processor.enqueue_discovered_reflist(
445 _refs_lists[index], _pending_list_addr);
446 _refs_lists[index].set_head(NULL);
447 _refs_lists[index].set_length(0);
448 }
449 }
450 };
452 // Enqueue references that are not made active again
453 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr,
454 AbstractRefProcTaskExecutor* task_executor) {
455 if (_processing_is_mt && task_executor != NULL) {
456 // Parallel code
457 RefProcEnqueueTask tsk(*this, _discovered_refs,
458 pending_list_addr, _max_num_q);
459 task_executor->execute(tsk);
460 } else {
461 // Serial code: call the parent class's implementation
462 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
463 enqueue_discovered_reflist(_discovered_refs[i], pending_list_addr);
464 _discovered_refs[i].set_head(NULL);
465 _discovered_refs[i].set_length(0);
466 }
467 }
468 }
470 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
471 _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);
472 oop discovered = java_lang_ref_Reference::discovered(_ref);
473 assert(_discovered_addr && discovered->is_oop_or_null(),
474 "discovered field is bad");
475 _next = discovered;
476 _referent_addr = java_lang_ref_Reference::referent_addr(_ref);
477 _referent = java_lang_ref_Reference::referent(_ref);
478 assert(Universe::heap()->is_in_reserved_or_null(_referent),
479 "Wrong oop found in java.lang.Reference object");
480 assert(allow_null_referent ?
481 _referent->is_oop_or_null()
482 : _referent->is_oop(),
483 "bad referent");
484 }
486 void DiscoveredListIterator::remove() {
487 assert(_ref->is_oop(), "Dropping a bad reference");
488 oop_store_raw(_discovered_addr, NULL);
490 // First _prev_next ref actually points into DiscoveredList (gross).
491 oop new_next;
492 if (_next == _ref) {
493 // At the end of the list, we should make _prev point to itself.
494 // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
495 // and _prev will be NULL.
496 new_next = _prev;
497 } else {
498 new_next = _next;
499 }
501 if (UseCompressedOops) {
502 // Remove Reference object from list.
503 oopDesc::encode_store_heap_oop((narrowOop*)_prev_next, new_next);
504 } else {
505 // Remove Reference object from list.
506 oopDesc::store_heap_oop((oop*)_prev_next, new_next);
507 }
508 NOT_PRODUCT(_removed++);
509 _refs_list.dec_length(1);
510 }
512 // Make the Reference object active again.
513 void DiscoveredListIterator::make_active() {
514 // For G1 we don't want to use set_next - it
515 // will dirty the card for the next field of
516 // the reference object and will fail
517 // CT verification.
518 if (UseG1GC) {
519 BarrierSet* bs = oopDesc::bs();
520 HeapWord* next_addr = java_lang_ref_Reference::next_addr(_ref);
522 if (UseCompressedOops) {
523 bs->write_ref_field_pre((narrowOop*)next_addr, NULL);
524 } else {
525 bs->write_ref_field_pre((oop*)next_addr, NULL);
526 }
527 java_lang_ref_Reference::set_next_raw(_ref, NULL);
528 } else {
529 java_lang_ref_Reference::set_next(_ref, NULL);
530 }
531 }
533 void DiscoveredListIterator::clear_referent() {
534 oop_store_raw(_referent_addr, NULL);
535 }
537 // NOTE: process_phase*() are largely similar, and at a high level
538 // merely iterate over the extant list applying a predicate to
539 // each of its elements and possibly removing that element from the
540 // list and applying some further closures to that element.
541 // We should consider the possibility of replacing these
542 // process_phase*() methods by abstracting them into
543 // a single general iterator invocation that receives appropriate
544 // closures that accomplish this work.
546 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
547 // referents are not alive, but that should be kept alive for policy reasons.
548 // Keep alive the transitive closure of all such referents.
549 void
550 ReferenceProcessor::process_phase1(DiscoveredList& refs_list,
551 ReferencePolicy* policy,
552 BoolObjectClosure* is_alive,
553 OopClosure* keep_alive,
554 VoidClosure* complete_gc) {
555 assert(policy != NULL, "Must have a non-NULL policy");
556 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
557 // Decide which softly reachable refs should be kept alive.
558 while (iter.has_next()) {
559 iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
560 bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
561 if (referent_is_dead &&
562 !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
563 if (TraceReferenceGC) {
564 gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s" ") by policy",
565 iter.obj(), iter.obj()->klass()->internal_name());
566 }
567 // Remove Reference object from list
568 iter.remove();
569 // Make the Reference object active again
570 iter.make_active();
571 // keep the referent around
572 iter.make_referent_alive();
573 iter.move_to_next();
574 } else {
575 iter.next();
576 }
577 }
578 // Close the reachable set
579 complete_gc->do_void();
580 NOT_PRODUCT(
581 if (PrintGCDetails && TraceReferenceGC) {
582 gclog_or_tty->print_cr(" Dropped %d dead Refs out of %d "
583 "discovered Refs by policy, from list " INTPTR_FORMAT,
584 iter.removed(), iter.processed(), (address)refs_list.head());
585 }
586 )
587 }
589 // Traverse the list and remove any Refs that are not active, or
590 // whose referents are either alive or NULL.
591 void
592 ReferenceProcessor::pp2_work(DiscoveredList& refs_list,
593 BoolObjectClosure* is_alive,
594 OopClosure* keep_alive) {
595 assert(discovery_is_atomic(), "Error");
596 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
597 while (iter.has_next()) {
598 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
599 DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
600 assert(next == NULL, "Should not discover inactive Reference");
601 if (iter.is_referent_alive()) {
602 if (TraceReferenceGC) {
603 gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
604 iter.obj(), iter.obj()->klass()->internal_name());
605 }
606 // The referent is reachable after all.
607 // Remove Reference object from list.
608 iter.remove();
609 // Update the referent pointer as necessary: Note that this
610 // should not entail any recursive marking because the
611 // referent must already have been traversed.
612 iter.make_referent_alive();
613 iter.move_to_next();
614 } else {
615 iter.next();
616 }
617 }
618 NOT_PRODUCT(
619 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
620 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
621 "Refs in discovered list " INTPTR_FORMAT,
622 iter.removed(), iter.processed(), (address)refs_list.head());
623 }
624 )
625 }
627 void
628 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList& refs_list,
629 BoolObjectClosure* is_alive,
630 OopClosure* keep_alive,
631 VoidClosure* complete_gc) {
632 assert(!discovery_is_atomic(), "Error");
633 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
634 while (iter.has_next()) {
635 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
636 HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
637 oop next = java_lang_ref_Reference::next(iter.obj());
638 if ((iter.referent() == NULL || iter.is_referent_alive() ||
639 next != NULL)) {
640 assert(next->is_oop_or_null(), "bad next field");
641 // Remove Reference object from list
642 iter.remove();
643 // Trace the cohorts
644 iter.make_referent_alive();
645 if (UseCompressedOops) {
646 keep_alive->do_oop((narrowOop*)next_addr);
647 } else {
648 keep_alive->do_oop((oop*)next_addr);
649 }
650 iter.move_to_next();
651 } else {
652 iter.next();
653 }
654 }
655 // Now close the newly reachable set
656 complete_gc->do_void();
657 NOT_PRODUCT(
658 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
659 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
660 "Refs in discovered list " INTPTR_FORMAT,
661 iter.removed(), iter.processed(), (address)refs_list.head());
662 }
663 )
664 }
666 // Traverse the list and process the referents, by either
667 // clearing them or keeping them (and their reachable
668 // closure) alive.
669 void
670 ReferenceProcessor::process_phase3(DiscoveredList& refs_list,
671 bool clear_referent,
672 BoolObjectClosure* is_alive,
673 OopClosure* keep_alive,
674 VoidClosure* complete_gc) {
675 ResourceMark rm;
676 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
677 while (iter.has_next()) {
678 iter.update_discovered();
679 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
680 if (clear_referent) {
681 // NULL out referent pointer
682 iter.clear_referent();
683 } else {
684 // keep the referent around
685 iter.make_referent_alive();
686 }
687 if (TraceReferenceGC) {
688 gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
689 clear_referent ? "cleared " : "",
690 iter.obj(), iter.obj()->klass()->internal_name());
691 }
692 assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");
693 iter.next();
694 }
695 // Remember to update the next pointer of the last ref.
696 iter.update_discovered();
697 // Close the reachable set
698 complete_gc->do_void();
699 }
701 void
702 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
703 oop obj = NULL;
704 oop next = refs_list.head();
705 while (next != obj) {
706 obj = next;
707 next = java_lang_ref_Reference::discovered(obj);
708 java_lang_ref_Reference::set_discovered_raw(obj, NULL);
709 }
710 refs_list.set_head(NULL);
711 refs_list.set_length(0);
712 }
714 void
715 ReferenceProcessor::abandon_partial_discovered_list(DiscoveredList& refs_list) {
716 clear_discovered_references(refs_list);
717 }
719 void ReferenceProcessor::abandon_partial_discovery() {
720 // loop over the lists
721 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
722 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
723 gclog_or_tty->print_cr("\nAbandoning %s discovered list", list_name(i));
724 }
725 abandon_partial_discovered_list(_discovered_refs[i]);
726 }
727 }
729 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
730 public:
731 RefProcPhase1Task(ReferenceProcessor& ref_processor,
732 DiscoveredList refs_lists[],
733 ReferencePolicy* policy,
734 bool marks_oops_alive)
735 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
736 _policy(policy)
737 { }
738 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
739 OopClosure& keep_alive,
740 VoidClosure& complete_gc)
741 {
742 Thread* thr = Thread::current();
743 int refs_list_index = ((WorkerThread*)thr)->id();
744 _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy,
745 &is_alive, &keep_alive, &complete_gc);
746 }
747 private:
748 ReferencePolicy* _policy;
749 };
751 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
752 public:
753 RefProcPhase2Task(ReferenceProcessor& ref_processor,
754 DiscoveredList refs_lists[],
755 bool marks_oops_alive)
756 : ProcessTask(ref_processor, refs_lists, marks_oops_alive)
757 { }
758 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
759 OopClosure& keep_alive,
760 VoidClosure& complete_gc)
761 {
762 _ref_processor.process_phase2(_refs_lists[i],
763 &is_alive, &keep_alive, &complete_gc);
764 }
765 };
767 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
768 public:
769 RefProcPhase3Task(ReferenceProcessor& ref_processor,
770 DiscoveredList refs_lists[],
771 bool clear_referent,
772 bool marks_oops_alive)
773 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
774 _clear_referent(clear_referent)
775 { }
776 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
777 OopClosure& keep_alive,
778 VoidClosure& complete_gc)
779 {
780 // Don't use "refs_list_index" calculated in this way because
781 // balance_queues() has moved the Ref's into the first n queues.
782 // Thread* thr = Thread::current();
783 // int refs_list_index = ((WorkerThread*)thr)->id();
784 // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent,
785 _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
786 &is_alive, &keep_alive, &complete_gc);
787 }
788 private:
789 bool _clear_referent;
790 };
792 void ReferenceProcessor::set_discovered(oop ref, oop value) {
793 if (_discovered_list_needs_barrier) {
794 java_lang_ref_Reference::set_discovered(ref, value);
795 } else {
796 java_lang_ref_Reference::set_discovered_raw(ref, value);
797 }
798 }
800 // Balances reference queues.
801 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
802 // queues[0, 1, ..., _num_q-1] because only the first _num_q
803 // corresponding to the active workers will be processed.
804 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
805 {
806 // calculate total length
807 size_t total_refs = 0;
808 if (TraceReferenceGC && PrintGCDetails) {
809 gclog_or_tty->print_cr("\nBalance ref_lists ");
810 }
812 for (uint i = 0; i < _max_num_q; ++i) {
813 total_refs += ref_lists[i].length();
814 if (TraceReferenceGC && PrintGCDetails) {
815 gclog_or_tty->print("%d ", ref_lists[i].length());
816 }
817 }
818 if (TraceReferenceGC && PrintGCDetails) {
819 gclog_or_tty->print_cr(" = %d", total_refs);
820 }
821 size_t avg_refs = total_refs / _num_q + 1;
822 uint to_idx = 0;
823 for (uint from_idx = 0; from_idx < _max_num_q; from_idx++) {
824 bool move_all = false;
825 if (from_idx >= _num_q) {
826 move_all = ref_lists[from_idx].length() > 0;
827 }
828 while ((ref_lists[from_idx].length() > avg_refs) ||
829 move_all) {
830 assert(to_idx < _num_q, "Sanity Check!");
831 if (ref_lists[to_idx].length() < avg_refs) {
832 // move superfluous refs
833 size_t refs_to_move;
834 // Move all the Ref's if the from queue will not be processed.
835 if (move_all) {
836 refs_to_move = MIN2(ref_lists[from_idx].length(),
837 avg_refs - ref_lists[to_idx].length());
838 } else {
839 refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
840 avg_refs - ref_lists[to_idx].length());
841 }
843 assert(refs_to_move > 0, "otherwise the code below will fail");
845 oop move_head = ref_lists[from_idx].head();
846 oop move_tail = move_head;
847 oop new_head = move_head;
848 // find an element to split the list on
849 for (size_t j = 0; j < refs_to_move; ++j) {
850 move_tail = new_head;
851 new_head = java_lang_ref_Reference::discovered(new_head);
852 }
854 // Add the chain to the to list.
855 if (ref_lists[to_idx].head() == NULL) {
856 // to list is empty. Make a loop at the end.
857 set_discovered(move_tail, move_tail);
858 } else {
859 set_discovered(move_tail, ref_lists[to_idx].head());
860 }
861 ref_lists[to_idx].set_head(move_head);
862 ref_lists[to_idx].inc_length(refs_to_move);
864 // Remove the chain from the from list.
865 if (move_tail == new_head) {
866 // We found the end of the from list.
867 ref_lists[from_idx].set_head(NULL);
868 } else {
869 ref_lists[from_idx].set_head(new_head);
870 }
871 ref_lists[from_idx].dec_length(refs_to_move);
872 if (ref_lists[from_idx].length() == 0) {
873 break;
874 }
875 } else {
876 to_idx = (to_idx + 1) % _num_q;
877 }
878 }
879 }
880 #ifdef ASSERT
881 size_t balanced_total_refs = 0;
882 for (uint i = 0; i < _max_num_q; ++i) {
883 balanced_total_refs += ref_lists[i].length();
884 if (TraceReferenceGC && PrintGCDetails) {
885 gclog_or_tty->print("%d ", ref_lists[i].length());
886 }
887 }
888 if (TraceReferenceGC && PrintGCDetails) {
889 gclog_or_tty->print_cr(" = %d", balanced_total_refs);
890 gclog_or_tty->flush();
891 }
892 assert(total_refs == balanced_total_refs, "Balancing was incomplete");
893 #endif
894 }
896 void ReferenceProcessor::balance_all_queues() {
897 balance_queues(_discoveredSoftRefs);
898 balance_queues(_discoveredWeakRefs);
899 balance_queues(_discoveredFinalRefs);
900 balance_queues(_discoveredPhantomRefs);
901 }
903 size_t
904 ReferenceProcessor::process_discovered_reflist(
905 DiscoveredList refs_lists[],
906 ReferencePolicy* policy,
907 bool clear_referent,
908 BoolObjectClosure* is_alive,
909 OopClosure* keep_alive,
910 VoidClosure* complete_gc,
911 AbstractRefProcTaskExecutor* task_executor)
912 {
913 bool mt_processing = task_executor != NULL && _processing_is_mt;
914 // If discovery used MT and a dynamic number of GC threads, then
915 // the queues must be balanced for correctness if fewer than the
916 // maximum number of queues were used. The number of queue used
917 // during discovery may be different than the number to be used
918 // for processing so don't depend of _num_q < _max_num_q as part
919 // of the test.
920 bool must_balance = _discovery_is_mt;
922 if ((mt_processing && ParallelRefProcBalancingEnabled) ||
923 must_balance) {
924 balance_queues(refs_lists);
925 }
927 size_t total_list_count = total_count(refs_lists);
929 if (PrintReferenceGC && PrintGCDetails) {
930 gclog_or_tty->print(", %u refs", total_list_count);
931 }
933 // Phase 1 (soft refs only):
934 // . Traverse the list and remove any SoftReferences whose
935 // referents are not alive, but that should be kept alive for
936 // policy reasons. Keep alive the transitive closure of all
937 // such referents.
938 if (policy != NULL) {
939 if (mt_processing) {
940 RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/);
941 task_executor->execute(phase1);
942 } else {
943 for (uint i = 0; i < _max_num_q; i++) {
944 process_phase1(refs_lists[i], policy,
945 is_alive, keep_alive, complete_gc);
946 }
947 }
948 } else { // policy == NULL
949 assert(refs_lists != _discoveredSoftRefs,
950 "Policy must be specified for soft references.");
951 }
953 // Phase 2:
954 // . Traverse the list and remove any refs whose referents are alive.
955 if (mt_processing) {
956 RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/);
957 task_executor->execute(phase2);
958 } else {
959 for (uint i = 0; i < _max_num_q; i++) {
960 process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
961 }
962 }
964 // Phase 3:
965 // . Traverse the list and process referents as appropriate.
966 if (mt_processing) {
967 RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/);
968 task_executor->execute(phase3);
969 } else {
970 for (uint i = 0; i < _max_num_q; i++) {
971 process_phase3(refs_lists[i], clear_referent,
972 is_alive, keep_alive, complete_gc);
973 }
974 }
976 return total_list_count;
977 }
979 void ReferenceProcessor::clean_up_discovered_references() {
980 // loop over the lists
981 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
982 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
983 gclog_or_tty->print_cr(
984 "\nScrubbing %s discovered list of Null referents",
985 list_name(i));
986 }
987 clean_up_discovered_reflist(_discovered_refs[i]);
988 }
989 }
991 void ReferenceProcessor::clean_up_discovered_reflist(DiscoveredList& refs_list) {
992 assert(!discovery_is_atomic(), "Else why call this method?");
993 DiscoveredListIterator iter(refs_list, NULL, NULL);
994 while (iter.has_next()) {
995 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
996 oop next = java_lang_ref_Reference::next(iter.obj());
997 assert(next->is_oop_or_null(), "bad next field");
998 // If referent has been cleared or Reference is not active,
999 // drop it.
1000 if (iter.referent() == NULL || next != NULL) {
1001 debug_only(
1002 if (PrintGCDetails && TraceReferenceGC) {
1003 gclog_or_tty->print_cr("clean_up_discovered_list: Dropping Reference: "
1004 INTPTR_FORMAT " with next field: " INTPTR_FORMAT
1005 " and referent: " INTPTR_FORMAT,
1006 iter.obj(), next, iter.referent());
1007 }
1008 )
1009 // Remove Reference object from list
1010 iter.remove();
1011 iter.move_to_next();
1012 } else {
1013 iter.next();
1014 }
1015 }
1016 NOT_PRODUCT(
1017 if (PrintGCDetails && TraceReferenceGC) {
1018 gclog_or_tty->print(
1019 " Removed %d Refs with NULL referents out of %d discovered Refs",
1020 iter.removed(), iter.processed());
1021 }
1022 )
1023 }
1025 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
1026 uint id = 0;
1027 // Determine the queue index to use for this object.
1028 if (_discovery_is_mt) {
1029 // During a multi-threaded discovery phase,
1030 // each thread saves to its "own" list.
1031 Thread* thr = Thread::current();
1032 id = thr->as_Worker_thread()->id();
1033 } else {
1034 // single-threaded discovery, we save in round-robin
1035 // fashion to each of the lists.
1036 if (_processing_is_mt) {
1037 id = next_id();
1038 }
1039 }
1040 assert(0 <= id && id < _max_num_q, "Id is out-of-bounds (call Freud?)");
1042 // Get the discovered queue to which we will add
1043 DiscoveredList* list = NULL;
1044 switch (rt) {
1045 case REF_OTHER:
1046 // Unknown reference type, no special treatment
1047 break;
1048 case REF_SOFT:
1049 list = &_discoveredSoftRefs[id];
1050 break;
1051 case REF_WEAK:
1052 list = &_discoveredWeakRefs[id];
1053 break;
1054 case REF_FINAL:
1055 list = &_discoveredFinalRefs[id];
1056 break;
1057 case REF_PHANTOM:
1058 list = &_discoveredPhantomRefs[id];
1059 break;
1060 case REF_NONE:
1061 // we should not reach here if we are an InstanceRefKlass
1062 default:
1063 ShouldNotReachHere();
1064 }
1065 if (TraceReferenceGC && PrintGCDetails) {
1066 gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, list);
1067 }
1068 return list;
1069 }
1071 inline void
1072 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
1073 oop obj,
1074 HeapWord* discovered_addr) {
1075 assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
1076 // First we must make sure this object is only enqueued once. CAS in a non null
1077 // discovered_addr.
1078 oop current_head = refs_list.head();
1079 // The last ref must have its discovered field pointing to itself.
1080 oop next_discovered = (current_head != NULL) ? current_head : obj;
1082 // Note: In the case of G1, this specific pre-barrier is strictly
1083 // not necessary because the only case we are interested in
1084 // here is when *discovered_addr is NULL (see the CAS further below),
1085 // so this will expand to nothing. As a result, we have manually
1086 // elided this out for G1, but left in the test for some future
1087 // collector that might have need for a pre-barrier here, e.g.:-
1088 // _bs->write_ref_field_pre((oop* or narrowOop*)discovered_addr, next_discovered);
1089 assert(!_discovered_list_needs_barrier || UseG1GC,
1090 "Need to check non-G1 collector: "
1091 "may need a pre-write-barrier for CAS from NULL below");
1092 oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr,
1093 NULL);
1094 if (retest == NULL) {
1095 // This thread just won the right to enqueue the object.
1096 // We have separate lists for enqueueing, so no synchronization
1097 // is necessary.
1098 refs_list.set_head(obj);
1099 refs_list.inc_length(1);
1100 if (_discovered_list_needs_barrier) {
1101 _bs->write_ref_field((void*)discovered_addr, next_discovered);
1102 }
1104 if (TraceReferenceGC) {
1105 gclog_or_tty->print_cr("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
1106 obj, obj->klass()->internal_name());
1107 }
1108 } else {
1109 // If retest was non NULL, another thread beat us to it:
1110 // The reference has already been discovered...
1111 if (TraceReferenceGC) {
1112 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1113 obj, obj->klass()->internal_name());
1114 }
1115 }
1116 }
1118 #ifndef PRODUCT
1119 // Non-atomic (i.e. concurrent) discovery might allow us
1120 // to observe j.l.References with NULL referents, being those
1121 // cleared concurrently by mutators during (or after) discovery.
1122 void ReferenceProcessor::verify_referent(oop obj) {
1123 bool da = discovery_is_atomic();
1124 oop referent = java_lang_ref_Reference::referent(obj);
1125 assert(da ? referent->is_oop() : referent->is_oop_or_null(),
1126 err_msg("Bad referent " INTPTR_FORMAT " found in Reference "
1127 INTPTR_FORMAT " during %satomic discovery ",
1128 (intptr_t)referent, (intptr_t)obj, da ? "" : "non-"));
1129 }
1130 #endif
1132 // We mention two of several possible choices here:
1133 // #0: if the reference object is not in the "originating generation"
1134 // (or part of the heap being collected, indicated by our "span"
1135 // we don't treat it specially (i.e. we scan it as we would
1136 // a normal oop, treating its references as strong references).
1137 // This means that references can't be discovered unless their
1138 // referent is also in the same span. This is the simplest,
1139 // most "local" and most conservative approach, albeit one
1140 // that may cause weak references to be enqueued least promptly.
1141 // We call this choice the "ReferenceBasedDiscovery" policy.
1142 // #1: the reference object may be in any generation (span), but if
1143 // the referent is in the generation (span) being currently collected
1144 // then we can discover the reference object, provided
1145 // the object has not already been discovered by
1146 // a different concurrently running collector (as may be the
1147 // case, for instance, if the reference object is in CMS and
1148 // the referent in DefNewGeneration), and provided the processing
1149 // of this reference object by the current collector will
1150 // appear atomic to every other collector in the system.
1151 // (Thus, for instance, a concurrent collector may not
1152 // discover references in other generations even if the
1153 // referent is in its own generation). This policy may,
1154 // in certain cases, enqueue references somewhat sooner than
1155 // might Policy #0 above, but at marginally increased cost
1156 // and complexity in processing these references.
1157 // We call this choice the "RefeferentBasedDiscovery" policy.
1158 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1159 // Make sure we are discovering refs (rather than processing discovered refs).
1160 if (!_discovering_refs || !RegisterReferences) {
1161 return false;
1162 }
1163 // We only discover active references.
1164 oop next = java_lang_ref_Reference::next(obj);
1165 if (next != NULL) { // Ref is no longer active
1166 return false;
1167 }
1169 HeapWord* obj_addr = (HeapWord*)obj;
1170 if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1171 !_span.contains(obj_addr)) {
1172 // Reference is not in the originating generation;
1173 // don't treat it specially (i.e. we want to scan it as a normal
1174 // object with strong references).
1175 return false;
1176 }
1178 // We only discover references whose referents are not (yet)
1179 // known to be strongly reachable.
1180 if (is_alive_non_header() != NULL) {
1181 verify_referent(obj);
1182 if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
1183 return false; // referent is reachable
1184 }
1185 }
1186 if (rt == REF_SOFT) {
1187 // For soft refs we can decide now if these are not
1188 // current candidates for clearing, in which case we
1189 // can mark through them now, rather than delaying that
1190 // to the reference-processing phase. Since all current
1191 // time-stamp policies advance the soft-ref clock only
1192 // at a major collection cycle, this is always currently
1193 // accurate.
1194 if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
1195 return false;
1196 }
1197 }
1199 ResourceMark rm; // Needed for tracing.
1201 HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1202 const oop discovered = java_lang_ref_Reference::discovered(obj);
1203 assert(discovered->is_oop_or_null(), "bad discovered field");
1204 if (discovered != NULL) {
1205 // The reference has already been discovered...
1206 if (TraceReferenceGC) {
1207 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1208 obj, obj->klass()->internal_name());
1209 }
1210 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1211 // assumes that an object is not processed twice;
1212 // if it's been already discovered it must be on another
1213 // generation's discovered list; so we won't discover it.
1214 return false;
1215 } else {
1216 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1217 "Unrecognized policy");
1218 // Check assumption that an object is not potentially
1219 // discovered twice except by concurrent collectors that potentially
1220 // trace the same Reference object twice.
1221 assert(UseConcMarkSweepGC || UseG1GC,
1222 "Only possible with a concurrent marking collector");
1223 return true;
1224 }
1225 }
1227 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1228 verify_referent(obj);
1229 // Discover if and only if EITHER:
1230 // .. reference is in our span, OR
1231 // .. we are an atomic collector and referent is in our span
1232 if (_span.contains(obj_addr) ||
1233 (discovery_is_atomic() &&
1234 _span.contains(java_lang_ref_Reference::referent(obj)))) {
1235 // should_enqueue = true;
1236 } else {
1237 return false;
1238 }
1239 } else {
1240 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1241 _span.contains(obj_addr), "code inconsistency");
1242 }
1244 // Get the right type of discovered queue head.
1245 DiscoveredList* list = get_discovered_list(rt);
1246 if (list == NULL) {
1247 return false; // nothing special needs to be done
1248 }
1250 if (_discovery_is_mt) {
1251 add_to_discovered_list_mt(*list, obj, discovered_addr);
1252 } else {
1253 // If "_discovered_list_needs_barrier", we do write barriers when
1254 // updating the discovered reference list. Otherwise, we do a raw store
1255 // here: the field will be visited later when processing the discovered
1256 // references.
1257 oop current_head = list->head();
1258 // The last ref must have its discovered field pointing to itself.
1259 oop next_discovered = (current_head != NULL) ? current_head : obj;
1261 // As in the case further above, since we are over-writing a NULL
1262 // pre-value, we can safely elide the pre-barrier here for the case of G1.
1263 // e.g.:- _bs->write_ref_field_pre((oop* or narrowOop*)discovered_addr, next_discovered);
1264 assert(discovered == NULL, "control point invariant");
1265 assert(!_discovered_list_needs_barrier || UseG1GC,
1266 "For non-G1 collector, may need a pre-write-barrier for CAS from NULL below");
1267 oop_store_raw(discovered_addr, next_discovered);
1268 if (_discovered_list_needs_barrier) {
1269 _bs->write_ref_field((void*)discovered_addr, next_discovered);
1270 }
1271 list->set_head(obj);
1272 list->inc_length(1);
1274 if (TraceReferenceGC) {
1275 gclog_or_tty->print_cr("Discovered reference (" INTPTR_FORMAT ": %s)",
1276 obj, obj->klass()->internal_name());
1277 }
1278 }
1279 assert(obj->is_oop(), "Discovered a bad reference");
1280 verify_referent(obj);
1281 return true;
1282 }
1284 // Preclean the discovered references by removing those
1285 // whose referents are alive, and by marking from those that
1286 // are not active. These lists can be handled here
1287 // in any order and, indeed, concurrently.
1288 void ReferenceProcessor::preclean_discovered_references(
1289 BoolObjectClosure* is_alive,
1290 OopClosure* keep_alive,
1291 VoidClosure* complete_gc,
1292 YieldClosure* yield,
1293 GCTimer* gc_timer) {
1295 NOT_PRODUCT(verify_ok_to_handle_reflists());
1297 // Soft references
1298 {
1299 GCTraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC,
1300 false, gc_timer);
1301 for (uint i = 0; i < _max_num_q; i++) {
1302 if (yield->should_return()) {
1303 return;
1304 }
1305 preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1306 keep_alive, complete_gc, yield);
1307 }
1308 }
1310 // Weak references
1311 {
1312 GCTraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC,
1313 false, gc_timer);
1314 for (uint i = 0; i < _max_num_q; i++) {
1315 if (yield->should_return()) {
1316 return;
1317 }
1318 preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1319 keep_alive, complete_gc, yield);
1320 }
1321 }
1323 // Final references
1324 {
1325 GCTraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC,
1326 false, gc_timer);
1327 for (uint i = 0; i < _max_num_q; i++) {
1328 if (yield->should_return()) {
1329 return;
1330 }
1331 preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1332 keep_alive, complete_gc, yield);
1333 }
1334 }
1336 // Phantom references
1337 {
1338 GCTraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC,
1339 false, gc_timer);
1340 for (uint i = 0; i < _max_num_q; i++) {
1341 if (yield->should_return()) {
1342 return;
1343 }
1344 preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1345 keep_alive, complete_gc, yield);
1346 }
1347 }
1348 }
1350 // Walk the given discovered ref list, and remove all reference objects
1351 // whose referents are still alive, whose referents are NULL or which
1352 // are not active (have a non-NULL next field). NOTE: When we are
1353 // thus precleaning the ref lists (which happens single-threaded today),
1354 // we do not disable refs discovery to honour the correct semantics of
1355 // java.lang.Reference. As a result, we need to be careful below
1356 // that ref removal steps interleave safely with ref discovery steps
1357 // (in this thread).
1358 void
1359 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList& refs_list,
1360 BoolObjectClosure* is_alive,
1361 OopClosure* keep_alive,
1362 VoidClosure* complete_gc,
1363 YieldClosure* yield) {
1364 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1365 while (iter.has_next()) {
1366 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1367 oop obj = iter.obj();
1368 oop next = java_lang_ref_Reference::next(obj);
1369 if (iter.referent() == NULL || iter.is_referent_alive() ||
1370 next != NULL) {
1371 // The referent has been cleared, or is alive, or the Reference is not
1372 // active; we need to trace and mark its cohort.
1373 if (TraceReferenceGC) {
1374 gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1375 iter.obj(), iter.obj()->klass()->internal_name());
1376 }
1377 // Remove Reference object from list
1378 iter.remove();
1379 // Keep alive its cohort.
1380 iter.make_referent_alive();
1381 if (UseCompressedOops) {
1382 narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1383 keep_alive->do_oop(next_addr);
1384 } else {
1385 oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1386 keep_alive->do_oop(next_addr);
1387 }
1388 iter.move_to_next();
1389 } else {
1390 iter.next();
1391 }
1392 }
1393 // Close the reachable set
1394 complete_gc->do_void();
1396 NOT_PRODUCT(
1397 if (PrintGCDetails && PrintReferenceGC && (iter.processed() > 0)) {
1398 gclog_or_tty->print_cr(" Dropped %d Refs out of %d "
1399 "Refs in discovered list " INTPTR_FORMAT,
1400 iter.removed(), iter.processed(), (address)refs_list.head());
1401 }
1402 )
1403 }
1405 const char* ReferenceProcessor::list_name(uint i) {
1406 assert(i >= 0 && i <= _max_num_q * number_of_subclasses_of_ref(),
1407 "Out of bounds index");
1409 int j = i / _max_num_q;
1410 switch (j) {
1411 case 0: return "SoftRef";
1412 case 1: return "WeakRef";
1413 case 2: return "FinalRef";
1414 case 3: return "PhantomRef";
1415 }
1416 ShouldNotReachHere();
1417 return NULL;
1418 }
1420 #ifndef PRODUCT
1421 void ReferenceProcessor::verify_ok_to_handle_reflists() {
1422 // empty for now
1423 }
1424 #endif
1426 #ifndef PRODUCT
1427 void ReferenceProcessor::clear_discovered_references() {
1428 guarantee(!_discovering_refs, "Discovering refs?");
1429 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
1430 clear_discovered_references(_discovered_refs[i]);
1431 }
1432 }
1434 #endif // PRODUCT