1 /*
2 * Copyright (c) 2001, 2018, 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.
22 *
23 */
24
25 #include "precompiled.hpp"
26 #include "classfile/javaClasses.inline.hpp"
27 #include "classfile/systemDictionary.hpp"
28 #include "gc/shared/collectedHeap.hpp"
29 #include "gc/shared/collectedHeap.inline.hpp"
30 #include "gc/shared/gcTimer.hpp"
31 #include "gc/shared/gcTraceTime.inline.hpp"
32 #include "gc/shared/referencePolicy.hpp"
33 #include "gc/shared/referenceProcessor.inline.hpp"
34 #include "logging/log.hpp"
35 #include "memory/allocation.inline.hpp"
36 #include "memory/resourceArea.hpp"
37 #include "oops/access.inline.hpp"
38 #include "oops/oop.inline.hpp"
39 #include "runtime/java.hpp"
40
41 ReferencePolicy* ReferenceProcessor::_always_clear_soft_ref_policy = NULL;
42 ReferencePolicy* ReferenceProcessor::_default_soft_ref_policy = NULL;
43 jlong ReferenceProcessor::_soft_ref_timestamp_clock = 0;
44
45 void referenceProcessor_init() {
46 ReferenceProcessor::init_statics();
47 }
48
49 void ReferenceProcessor::init_statics() {
50 // We need a monotonically non-decreasing time in ms but
51 // os::javaTimeMillis() does not guarantee monotonicity.
52 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
53
54 // Initialize the soft ref timestamp clock.
55 _soft_ref_timestamp_clock = now;
56 // Also update the soft ref clock in j.l.r.SoftReference
57 java_lang_ref_SoftReference::set_clock(_soft_ref_timestamp_clock);
58
59 _always_clear_soft_ref_policy = new AlwaysClearPolicy();
60 if (is_server_compilation_mode_vm()) {
61 _default_soft_ref_policy = new LRUMaxHeapPolicy();
62 } else {
63 _default_soft_ref_policy = new LRUCurrentHeapPolicy();
64 }
65 if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) {
66 vm_exit_during_initialization("Could not allocate reference policy object");
67 }
68 guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||
69 RefDiscoveryPolicy == ReferentBasedDiscovery,
70 "Unrecognized RefDiscoveryPolicy");
71 }
72
73 void ReferenceProcessor::enable_discovery(bool check_no_refs) {
74 #ifdef ASSERT
75 // Verify that we're not currently discovering refs
76 assert(!_discovering_refs, "nested call?");
77
78 if (check_no_refs) {
79 // Verify that the discovered lists are empty
80 verify_no_references_recorded();
81 }
82 #endif // ASSERT
83
84 // Someone could have modified the value of the static
85 // field in the j.l.r.SoftReference class that holds the
86 // soft reference timestamp clock using reflection or
87 // Unsafe between GCs. Unconditionally update the static
88 // field in ReferenceProcessor here so that we use the new
89 // value during reference discovery.
90
91 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
92 _discovering_refs = true;
93 }
94
95 ReferenceProcessor::ReferenceProcessor(BoolObjectClosure* is_subject_to_discovery,
96 bool mt_processing,
97 uint mt_processing_degree,
98 bool mt_discovery,
99 uint mt_discovery_degree,
100 bool atomic_discovery,
101 BoolObjectClosure* is_alive_non_header) :
102 _is_subject_to_discovery(is_subject_to_discovery),
103 _discovering_refs(false),
104 _enqueuing_is_done(false),
105 _is_alive_non_header(is_alive_non_header),
106 _processing_is_mt(mt_processing),
107 _next_id(0)
108 {
109 assert(is_subject_to_discovery != NULL, "must be set");
110
111 _discovery_is_atomic = atomic_discovery;
112 _discovery_is_mt = mt_discovery;
113 _num_q = MAX2(1U, mt_processing_degree);
114 _max_num_q = MAX2(_num_q, mt_discovery_degree);
115 _discovered_refs = NEW_C_HEAP_ARRAY(DiscoveredList,
116 _max_num_q * number_of_subclasses_of_ref(), mtGC);
117
118 if (_discovered_refs == NULL) {
119 vm_exit_during_initialization("Could not allocated RefProc Array");
120 }
121 _discoveredSoftRefs = &_discovered_refs[0];
122 _discoveredWeakRefs = &_discoveredSoftRefs[_max_num_q];
123 _discoveredFinalRefs = &_discoveredWeakRefs[_max_num_q];
124 _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_q];
125
126 // Initialize all entries to NULL
127 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
128 _discovered_refs[i].set_head(NULL);
129 _discovered_refs[i].set_length(0);
130 }
131
132 setup_policy(false /* default soft ref policy */);
133 }
134
135 SpanReferenceProcessor::SpanReferenceProcessor(MemRegion span,
136 bool mt_processing,
137 uint mt_processing_degree,
138 bool mt_discovery,
139 uint mt_discovery_degree,
140 bool atomic_discovery,
141 BoolObjectClosure* is_alive_non_header) :
142 ReferenceProcessor(&_span_based_discoverer,
143 mt_processing,
144 mt_processing_degree,
145 mt_discovery,
146 mt_discovery_degree,
147 atomic_discovery,
148 is_alive_non_header),
149 _span_based_discoverer(span) {
150
151 }
152
153 #ifndef PRODUCT
154 void ReferenceProcessor::verify_no_references_recorded() {
155 guarantee(!_discovering_refs, "Discovering refs?");
156 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
157 guarantee(_discovered_refs[i].is_empty(),
158 "Found non-empty discovered list at %u", i);
159 }
160 }
161 #endif
162
163 void ReferenceProcessor::weak_oops_do(OopClosure* f) {
164 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
165 if (UseCompressedOops) {
166 f->do_oop((narrowOop*)_discovered_refs[i].adr_head());
167 } else {
168 f->do_oop((oop*)_discovered_refs[i].adr_head());
169 }
170 }
171 }
172
173 void ReferenceProcessor::update_soft_ref_master_clock() {
174 // Update (advance) the soft ref master clock field. This must be done
175 // after processing the soft ref list.
176
177 // We need a monotonically non-decreasing time in ms but
178 // os::javaTimeMillis() does not guarantee monotonicity.
179 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
180 jlong soft_ref_clock = java_lang_ref_SoftReference::clock();
181 assert(soft_ref_clock == _soft_ref_timestamp_clock, "soft ref clocks out of sync");
182
183 NOT_PRODUCT(
184 if (now < _soft_ref_timestamp_clock) {
185 log_warning(gc)("time warp: " JLONG_FORMAT " to " JLONG_FORMAT,
186 _soft_ref_timestamp_clock, now);
187 }
188 )
189 // The values of now and _soft_ref_timestamp_clock are set using
190 // javaTimeNanos(), which is guaranteed to be monotonically
191 // non-decreasing provided the underlying platform provides such
192 // a time source (and it is bug free).
193 // In product mode, however, protect ourselves from non-monotonicity.
194 if (now > _soft_ref_timestamp_clock) {
195 _soft_ref_timestamp_clock = now;
196 java_lang_ref_SoftReference::set_clock(now);
197 }
198 // Else leave clock stalled at its old value until time progresses
199 // past clock value.
200 }
201
202 size_t ReferenceProcessor::total_count(DiscoveredList lists[]) const {
203 size_t total = 0;
204 for (uint i = 0; i < _max_num_q; ++i) {
205 total += lists[i].length();
206 }
207 return total;
208 }
209
210 ReferenceProcessorStats ReferenceProcessor::process_discovered_references(
211 BoolObjectClosure* is_alive,
212 OopClosure* keep_alive,
213 VoidClosure* complete_gc,
214 AbstractRefProcTaskExecutor* task_executor,
215 ReferenceProcessorPhaseTimes* phase_times) {
216
217 double start_time = os::elapsedTime();
218
219 assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
220 // Stop treating discovered references specially.
221 disable_discovery();
222
223 // If discovery was concurrent, someone could have modified
224 // the value of the static field in the j.l.r.SoftReference
225 // class that holds the soft reference timestamp clock using
226 // reflection or Unsafe between when discovery was enabled and
227 // now. Unconditionally update the static field in ReferenceProcessor
228 // here so that we use the new value during processing of the
229 // discovered soft refs.
230
231 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
232
233 ReferenceProcessorStats stats(total_count(_discoveredSoftRefs),
234 total_count(_discoveredWeakRefs),
235 total_count(_discoveredFinalRefs),
236 total_count(_discoveredPhantomRefs));
237
238 // Soft references
239 {
240 RefProcPhaseTimesTracker tt(REF_SOFT, phase_times, this);
241 process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
242 is_alive, keep_alive, complete_gc, task_executor, phase_times);
243 }
244
245 update_soft_ref_master_clock();
246
247 // Weak references
248 {
249 RefProcPhaseTimesTracker tt(REF_WEAK, phase_times, this);
250 process_discovered_reflist(_discoveredWeakRefs, NULL, true,
251 is_alive, keep_alive, complete_gc, task_executor, phase_times);
252 }
253
254 // Final references
255 {
256 RefProcPhaseTimesTracker tt(REF_FINAL, phase_times, this);
257 process_discovered_reflist(_discoveredFinalRefs, NULL, false,
258 is_alive, keep_alive, complete_gc, task_executor, phase_times);
259 }
260
261 // Phantom references
262 {
263 RefProcPhaseTimesTracker tt(REF_PHANTOM, phase_times, this);
264 process_discovered_reflist(_discoveredPhantomRefs, NULL, true,
265 is_alive, keep_alive, complete_gc, task_executor, phase_times);
266 }
267
268 if (task_executor != NULL) {
269 // Record the work done by the parallel workers.
270 task_executor->set_single_threaded_mode();
271 }
272
273 phase_times->set_total_time_ms((os::elapsedTime() - start_time) * 1000);
274
275 return stats;
276 }
277
278 void ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor,
279 ReferenceProcessorPhaseTimes* phase_times) {
280 // Enqueue references that are not made active again, and
281 // clear the decks for the next collection (cycle).
282 enqueue_discovered_reflists(task_executor, phase_times);
283
284 // Stop treating discovered references specially.
285 disable_discovery();
286 }
287
288 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list) {
289 // Given a list of refs linked through the "discovered" field
290 // (java.lang.ref.Reference.discovered), self-loop their "next" field
291 // thus distinguishing them from active References, then
292 // prepend them to the pending list.
293 //
294 // The Java threads will see the Reference objects linked together through
295 // the discovered field. Instead of trying to do the write barrier updates
296 // in all places in the reference processor where we manipulate the discovered
297 // field we make sure to do the barrier here where we anyway iterate through
298 // all linked Reference objects. Note that it is important to not dirty any
299 // cards during reference processing since this will cause card table
300 // verification to fail for G1.
301 log_develop_trace(gc, ref)("ReferenceProcessor::enqueue_discovered_reflist list " INTPTR_FORMAT, p2i(&refs_list));
302
303 oop obj = NULL;
304 oop next_d = refs_list.head();
305 // Walk down the list, self-looping the next field
306 // so that the References are not considered active.
307 while (obj != next_d) {
308 obj = next_d;
309 assert(obj->is_instance(), "should be an instance object");
310 assert(InstanceKlass::cast(obj->klass())->is_reference_instance_klass(), "should be reference object");
311 next_d = java_lang_ref_Reference::discovered(obj);
312 log_develop_trace(gc, ref)(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT, p2i(obj), p2i(next_d));
313 assert(java_lang_ref_Reference::next(obj) == NULL,
314 "Reference not active; should not be discovered");
315 // Self-loop next, so as to make Ref not active.
316 java_lang_ref_Reference::set_next_raw(obj, obj);
317 if (next_d != obj) {
318 HeapAccess<AS_NO_KEEPALIVE>::oop_store_at(obj, java_lang_ref_Reference::discovered_offset, next_d);
319 } else {
320 // This is the last object.
321 // Swap refs_list into pending list and set obj's
322 // discovered to what we read from the pending list.
323 oop old = Universe::swap_reference_pending_list(refs_list.head());
324 HeapAccess<AS_NO_KEEPALIVE>::oop_store_at(obj, java_lang_ref_Reference::discovered_offset, old);
325 }
326 }
327 }
328
329 // Parallel enqueue task
330 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
331 public:
332 RefProcEnqueueTask(ReferenceProcessor& ref_processor,
333 DiscoveredList discovered_refs[],
334 int n_queues,
335 ReferenceProcessorPhaseTimes* phase_times)
336 : EnqueueTask(ref_processor, discovered_refs, n_queues, phase_times)
337 { }
338
339 virtual void work(unsigned int work_id) {
340 RefProcWorkerTimeTracker tt(ReferenceProcessorPhaseTimes::RefEnqueue, _phase_times, work_id);
341
342 assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds");
343 // Simplest first cut: static partitioning.
344 int index = work_id;
345 // The increment on "index" must correspond to the maximum number of queues
346 // (n_queues) with which that ReferenceProcessor was created. That
347 // is because of the "clever" way the discovered references lists were
348 // allocated and are indexed into.
349 assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected");
350 for (int j = 0;
351 j < ReferenceProcessor::number_of_subclasses_of_ref();
352 j++, index += _n_queues) {
353 _ref_processor.enqueue_discovered_reflist(_refs_lists[index]);
354 _refs_lists[index].set_head(NULL);
355 _refs_lists[index].set_length(0);
356 }
357 }
358 };
359
360 // Enqueue references that are not made active again
361 void ReferenceProcessor::enqueue_discovered_reflists(AbstractRefProcTaskExecutor* task_executor,
362 ReferenceProcessorPhaseTimes* phase_times) {
363
364 ReferenceProcessorStats stats(total_count(_discoveredSoftRefs),
365 total_count(_discoveredWeakRefs),
366 total_count(_discoveredFinalRefs),
367 total_count(_discoveredPhantomRefs));
368
369 RefProcEnqueueTimeTracker tt(phase_times, stats);
370
371 if (_processing_is_mt && task_executor != NULL) {
372 // Parallel code
373 RefProcEnqueueTask tsk(*this, _discovered_refs, _max_num_q, phase_times);
374 task_executor->execute(tsk);
375 } else {
376 // Serial code: call the parent class's implementation
377 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
378 enqueue_discovered_reflist(_discovered_refs[i]);
379 _discovered_refs[i].set_head(NULL);
380 _discovered_refs[i].set_length(0);
381 }
382 }
383 }
384
385 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
386 _discovered_addr = java_lang_ref_Reference::discovered_addr_raw(_ref);
387 oop discovered = java_lang_ref_Reference::discovered(_ref);
388 assert(_discovered_addr && oopDesc::is_oop_or_null(discovered),
389 "Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered));
390 _next = discovered;
391 _referent_addr = java_lang_ref_Reference::referent_addr_raw(_ref);
392 _referent = java_lang_ref_Reference::referent(_ref);
393 assert(Universe::heap()->is_in_reserved_or_null(_referent),
394 "Wrong oop found in java.lang.Reference object");
395 assert(allow_null_referent ?
396 oopDesc::is_oop_or_null(_referent)
397 : oopDesc::is_oop(_referent),
398 "Expected an oop%s for referent field at " PTR_FORMAT,
399 (allow_null_referent ? " or NULL" : ""),
400 p2i(_referent));
401 }
402
403 void DiscoveredListIterator::remove() {
404 assert(oopDesc::is_oop(_ref), "Dropping a bad reference");
405 RawAccess<>::oop_store(_discovered_addr, oop(NULL));
406
407 // First _prev_next ref actually points into DiscoveredList (gross).
408 oop new_next;
409 if (_next == _ref) {
410 // At the end of the list, we should make _prev point to itself.
411 // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
412 // and _prev will be NULL.
413 new_next = _prev;
414 } else {
415 new_next = _next;
416 }
417 // Remove Reference object from discovered list. Note that G1 does not need a
418 // pre-barrier here because we know the Reference has already been found/marked,
419 // that's how it ended up in the discovered list in the first place.
420 RawAccess<>::oop_store(_prev_next, new_next);
421 NOT_PRODUCT(_removed++);
422 _refs_list.dec_length(1);
423 }
424
425 void DiscoveredListIterator::clear_referent() {
426 RawAccess<>::oop_store(_referent_addr, oop(NULL));
427 }
428
429 // NOTE: process_phase*() are largely similar, and at a high level
430 // merely iterate over the extant list applying a predicate to
431 // each of its elements and possibly removing that element from the
432 // list and applying some further closures to that element.
433 // We should consider the possibility of replacing these
434 // process_phase*() methods by abstracting them into
435 // a single general iterator invocation that receives appropriate
436 // closures that accomplish this work.
437
438 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
439 // referents are not alive, but that should be kept alive for policy reasons.
440 // Keep alive the transitive closure of all such referents.
441 void
442 ReferenceProcessor::process_phase1(DiscoveredList& refs_list,
443 ReferencePolicy* policy,
444 BoolObjectClosure* is_alive,
445 OopClosure* keep_alive,
446 VoidClosure* complete_gc) {
447 assert(policy != NULL, "Must have a non-NULL policy");
448 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
449 // Decide which softly reachable refs should be kept alive.
450 while (iter.has_next()) {
451 iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
452 bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
453 if (referent_is_dead &&
454 !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
455 log_develop_trace(gc, ref)("Dropping reference (" INTPTR_FORMAT ": %s" ") by policy",
456 p2i(iter.obj()), iter.obj()->klass()->internal_name());
457 // Remove Reference object from list
458 iter.remove();
459 // keep the referent around
460 iter.make_referent_alive();
461 iter.move_to_next();
462 } else {
463 iter.next();
464 }
465 }
466 // Close the reachable set
467 complete_gc->do_void();
468 log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " dead Refs out of " SIZE_FORMAT " discovered Refs by policy, from list " INTPTR_FORMAT,
469 iter.removed(), iter.processed(), p2i(&refs_list));
470 }
471
472 void ReferenceProcessor::process_phase2(DiscoveredList& refs_list,
473 BoolObjectClosure* is_alive,
474 OopClosure* keep_alive,
475 VoidClosure* complete_gc) {
476 if (discovery_is_atomic()) {
477 // complete_gc is ignored in this case for this phase
478 pp2_work(refs_list, is_alive, keep_alive);
479 } else {
480 assert(complete_gc != NULL, "Error");
481 pp2_work_concurrent_discovery(refs_list, is_alive,
482 keep_alive, complete_gc);
483 }
484 }
485 // Traverse the list and remove any Refs that are not active, or
486 // whose referents are either alive or NULL.
487 void
488 ReferenceProcessor::pp2_work(DiscoveredList& refs_list,
489 BoolObjectClosure* is_alive,
490 OopClosure* keep_alive) {
491 assert(discovery_is_atomic(), "Error");
492 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
493 while (iter.has_next()) {
494 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
495 DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
496 assert(next == NULL, "Should not discover inactive Reference");
497 if (iter.is_referent_alive()) {
498 log_develop_trace(gc, ref)("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
499 p2i(iter.obj()), iter.obj()->klass()->internal_name());
500 // The referent is reachable after all.
501 // Remove Reference object from list.
502 iter.remove();
503 // Update the referent pointer as necessary: Note that this
504 // should not entail any recursive marking because the
505 // referent must already have been traversed.
506 iter.make_referent_alive();
507 iter.move_to_next();
508 } else {
509 iter.next();
510 }
511 }
512 NOT_PRODUCT(
513 if (iter.processed() > 0) {
514 log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT
515 " Refs in discovered list " INTPTR_FORMAT,
516 iter.removed(), iter.processed(), p2i(&refs_list));
517 }
518 )
519 }
520
521 void
522 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList& refs_list,
523 BoolObjectClosure* is_alive,
524 OopClosure* keep_alive,
525 VoidClosure* complete_gc) {
526 assert(!discovery_is_atomic(), "Error");
527 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
528 while (iter.has_next()) {
529 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
530 HeapWord* next_addr = java_lang_ref_Reference::next_addr_raw(iter.obj());
531 oop next = java_lang_ref_Reference::next(iter.obj());
532 if ((iter.referent() == NULL || iter.is_referent_alive() ||
533 next != NULL)) {
534 assert(oopDesc::is_oop_or_null(next), "Expected an oop or NULL for next field at " PTR_FORMAT, p2i(next));
535 // Remove Reference object from list
536 iter.remove();
537 // Trace the cohorts
538 iter.make_referent_alive();
539 if (UseCompressedOops) {
540 keep_alive->do_oop((narrowOop*)next_addr);
541 } else {
542 keep_alive->do_oop((oop*)next_addr);
543 }
544 iter.move_to_next();
545 } else {
546 iter.next();
547 }
548 }
549 // Now close the newly reachable set
550 complete_gc->do_void();
551 NOT_PRODUCT(
552 if (iter.processed() > 0) {
553 log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT
554 " Refs in discovered list " INTPTR_FORMAT,
555 iter.removed(), iter.processed(), p2i(&refs_list));
556 }
557 )
558 }
559
560 // Traverse the list and process the referents, by either
561 // clearing them or keeping them (and their reachable
562 // closure) alive.
563 void
564 ReferenceProcessor::process_phase3(DiscoveredList& refs_list,
565 bool clear_referent,
566 BoolObjectClosure* is_alive,
567 OopClosure* keep_alive,
568 VoidClosure* complete_gc) {
569 ResourceMark rm;
570 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
571 while (iter.has_next()) {
572 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
573 if (clear_referent) {
574 // NULL out referent pointer
575 iter.clear_referent();
576 } else {
577 // keep the referent around
578 iter.make_referent_alive();
579 }
580 log_develop_trace(gc, ref)("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
581 clear_referent ? "cleared " : "", p2i(iter.obj()), iter.obj()->klass()->internal_name());
582 assert(oopDesc::is_oop(iter.obj(), UseConcMarkSweepGC), "Adding a bad reference");
583 iter.next();
584 }
585 // Close the reachable set
586 complete_gc->do_void();
587 }
588
589 void
590 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
591 oop obj = NULL;
592 oop next = refs_list.head();
593 while (next != obj) {
594 obj = next;
595 next = java_lang_ref_Reference::discovered(obj);
596 java_lang_ref_Reference::set_discovered_raw(obj, NULL);
597 }
598 refs_list.set_head(NULL);
599 refs_list.set_length(0);
600 }
601
602 void ReferenceProcessor::abandon_partial_discovery() {
603 // loop over the lists
604 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
605 if ((i % _max_num_q) == 0) {
606 log_develop_trace(gc, ref)("Abandoning %s discovered list", list_name(i));
607 }
608 clear_discovered_references(_discovered_refs[i]);
609 }
610 }
611
612 size_t ReferenceProcessor::total_reference_count(ReferenceType type) const {
613 DiscoveredList* list = NULL;
614
615 switch (type) {
616 case REF_SOFT:
617 list = _discoveredSoftRefs;
618 break;
619 case REF_WEAK:
620 list = _discoveredWeakRefs;
621 break;
622 case REF_FINAL:
623 list = _discoveredFinalRefs;
624 break;
625 case REF_PHANTOM:
626 list = _discoveredPhantomRefs;
627 break;
628 case REF_OTHER:
629 case REF_NONE:
630 default:
631 ShouldNotReachHere();
632 }
633 return total_count(list);
634 }
635
636 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
637 public:
638 RefProcPhase1Task(ReferenceProcessor& ref_processor,
639 DiscoveredList refs_lists[],
640 ReferencePolicy* policy,
641 bool marks_oops_alive,
642 ReferenceProcessorPhaseTimes* phase_times)
643 : ProcessTask(ref_processor, refs_lists, marks_oops_alive, phase_times),
644 _policy(policy)
645 { }
646 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
647 OopClosure& keep_alive,
648 VoidClosure& complete_gc)
649 {
650 RefProcWorkerTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase1, _phase_times, i);
651
652 _ref_processor.process_phase1(_refs_lists[i], _policy,
653 &is_alive, &keep_alive, &complete_gc);
654 }
655 private:
656 ReferencePolicy* _policy;
657 };
658
659 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
660 public:
661 RefProcPhase2Task(ReferenceProcessor& ref_processor,
662 DiscoveredList refs_lists[],
663 bool marks_oops_alive,
664 ReferenceProcessorPhaseTimes* phase_times)
665 : ProcessTask(ref_processor, refs_lists, marks_oops_alive, phase_times)
666 { }
667 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
668 OopClosure& keep_alive,
669 VoidClosure& complete_gc)
670 {
671 RefProcWorkerTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase2, _phase_times, i);
672
673 _ref_processor.process_phase2(_refs_lists[i],
674 &is_alive, &keep_alive, &complete_gc);
675 }
676 };
677
678 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
679 public:
680 RefProcPhase3Task(ReferenceProcessor& ref_processor,
681 DiscoveredList refs_lists[],
682 bool clear_referent,
683 bool marks_oops_alive,
684 ReferenceProcessorPhaseTimes* phase_times)
685 : ProcessTask(ref_processor, refs_lists, marks_oops_alive, phase_times),
686 _clear_referent(clear_referent)
687 { }
688 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
689 OopClosure& keep_alive,
690 VoidClosure& complete_gc)
691 {
692 RefProcWorkerTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase3, _phase_times, i);
693
694 _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
695 &is_alive, &keep_alive, &complete_gc);
696 }
697 private:
698 bool _clear_referent;
699 };
700
701 #ifndef PRODUCT
702 void ReferenceProcessor::log_reflist_counts(DiscoveredList ref_lists[], uint active_length, size_t total_refs) {
703 if (!log_is_enabled(Trace, gc, ref)) {
704 return;
705 }
706
707 stringStream st;
708 for (uint i = 0; i < active_length; ++i) {
709 st.print(SIZE_FORMAT " ", ref_lists[i].length());
710 }
711 log_develop_trace(gc, ref)("%s= " SIZE_FORMAT, st.as_string(), total_refs);
712 #ifdef ASSERT
713 for (uint i = active_length; i < _max_num_q; i++) {
714 assert(ref_lists[i].length() == 0, SIZE_FORMAT " unexpected References in %u",
715 ref_lists[i].length(), i);
716 }
717 #endif
718 }
719 #endif
720
721 void ReferenceProcessor::set_active_mt_degree(uint v) {
722 _num_q = v;
723 _next_id = 0;
724 }
725
726 // Balances reference queues.
727 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
728 // queues[0, 1, ..., _num_q-1] because only the first _num_q
729 // corresponding to the active workers will be processed.
730 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
731 {
732 // calculate total length
733 size_t total_refs = 0;
734 log_develop_trace(gc, ref)("Balance ref_lists ");
735
736 for (uint i = 0; i < _max_num_q; ++i) {
737 total_refs += ref_lists[i].length();
738 }
739 log_reflist_counts(ref_lists, _max_num_q, total_refs);
740 size_t avg_refs = total_refs / _num_q + 1;
741 uint to_idx = 0;
742 for (uint from_idx = 0; from_idx < _max_num_q; from_idx++) {
743 bool move_all = false;
744 if (from_idx >= _num_q) {
745 move_all = ref_lists[from_idx].length() > 0;
746 }
747 while ((ref_lists[from_idx].length() > avg_refs) ||
748 move_all) {
749 assert(to_idx < _num_q, "Sanity Check!");
750 if (ref_lists[to_idx].length() < avg_refs) {
751 // move superfluous refs
752 size_t refs_to_move;
753 // Move all the Ref's if the from queue will not be processed.
754 if (move_all) {
755 refs_to_move = MIN2(ref_lists[from_idx].length(),
756 avg_refs - ref_lists[to_idx].length());
757 } else {
758 refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
759 avg_refs - ref_lists[to_idx].length());
760 }
761
762 assert(refs_to_move > 0, "otherwise the code below will fail");
763
764 oop move_head = ref_lists[from_idx].head();
765 oop move_tail = move_head;
766 oop new_head = move_head;
767 // find an element to split the list on
768 for (size_t j = 0; j < refs_to_move; ++j) {
769 move_tail = new_head;
770 new_head = java_lang_ref_Reference::discovered(new_head);
771 }
772
773 // Add the chain to the to list.
774 if (ref_lists[to_idx].head() == NULL) {
775 // to list is empty. Make a loop at the end.
776 java_lang_ref_Reference::set_discovered_raw(move_tail, move_tail);
777 } else {
778 java_lang_ref_Reference::set_discovered_raw(move_tail, ref_lists[to_idx].head());
779 }
780 ref_lists[to_idx].set_head(move_head);
781 ref_lists[to_idx].inc_length(refs_to_move);
782
783 // Remove the chain from the from list.
784 if (move_tail == new_head) {
785 // We found the end of the from list.
786 ref_lists[from_idx].set_head(NULL);
787 } else {
788 ref_lists[from_idx].set_head(new_head);
789 }
790 ref_lists[from_idx].dec_length(refs_to_move);
791 if (ref_lists[from_idx].length() == 0) {
792 break;
793 }
794 } else {
795 to_idx = (to_idx + 1) % _num_q;
796 }
797 }
798 }
799 #ifdef ASSERT
800 size_t balanced_total_refs = 0;
801 for (uint i = 0; i < _num_q; ++i) {
802 balanced_total_refs += ref_lists[i].length();
803 }
804 log_reflist_counts(ref_lists, _num_q, balanced_total_refs);
805 assert(total_refs == balanced_total_refs, "Balancing was incomplete");
806 #endif
807 }
808
809 void ReferenceProcessor::balance_all_queues() {
810 balance_queues(_discoveredSoftRefs);
811 balance_queues(_discoveredWeakRefs);
812 balance_queues(_discoveredFinalRefs);
813 balance_queues(_discoveredPhantomRefs);
814 }
815
816 void ReferenceProcessor::process_discovered_reflist(
817 DiscoveredList refs_lists[],
818 ReferencePolicy* policy,
819 bool clear_referent,
820 BoolObjectClosure* is_alive,
821 OopClosure* keep_alive,
822 VoidClosure* complete_gc,
823 AbstractRefProcTaskExecutor* task_executor,
824 ReferenceProcessorPhaseTimes* phase_times)
825 {
826 bool mt_processing = task_executor != NULL && _processing_is_mt;
827
828 phase_times->set_processing_is_mt(mt_processing);
829
830 if (mt_processing && ParallelRefProcBalancingEnabled) {
831 RefProcBalanceQueuesTimeTracker tt(phase_times);
832 balance_queues(refs_lists);
833 }
834
835 // Phase 1 (soft refs only):
836 // . Traverse the list and remove any SoftReferences whose
837 // referents are not alive, but that should be kept alive for
838 // policy reasons. Keep alive the transitive closure of all
839 // such referents.
840 if (policy != NULL) {
841 RefProcParPhaseTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase1, phase_times);
842
843 if (mt_processing) {
844 RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/, phase_times);
845 task_executor->execute(phase1);
846 } else {
847 for (uint i = 0; i < _max_num_q; i++) {
848 process_phase1(refs_lists[i], policy,
849 is_alive, keep_alive, complete_gc);
850 }
851 }
852 } else { // policy == NULL
853 assert(refs_lists != _discoveredSoftRefs,
854 "Policy must be specified for soft references.");
855 }
856
857 // Phase 2:
858 // . Traverse the list and remove any refs whose referents are alive.
859 {
860 RefProcParPhaseTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase2, phase_times);
861
862 if (mt_processing) {
863 RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/, phase_times);
864 task_executor->execute(phase2);
865 } else {
866 for (uint i = 0; i < _max_num_q; i++) {
867 process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
868 }
869 }
870 }
871
872 // Phase 3:
873 // . Traverse the list and process referents as appropriate.
874 {
875 RefProcParPhaseTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase3, phase_times);
876
877 if (mt_processing) {
878 RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/, phase_times);
879 task_executor->execute(phase3);
880 } else {
881 for (uint i = 0; i < _max_num_q; i++) {
882 process_phase3(refs_lists[i], clear_referent,
883 is_alive, keep_alive, complete_gc);
884 }
885 }
886 }
887 }
888
889 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
890 uint id = 0;
891 // Determine the queue index to use for this object.
892 if (_discovery_is_mt) {
893 // During a multi-threaded discovery phase,
894 // each thread saves to its "own" list.
895 Thread* thr = Thread::current();
896 id = thr->as_Worker_thread()->id();
897 } else {
898 // single-threaded discovery, we save in round-robin
899 // fashion to each of the lists.
900 if (_processing_is_mt) {
901 id = next_id();
902 }
903 }
904 assert(id < _max_num_q, "Id is out-of-bounds id %u and max id %u)", id, _max_num_q);
905
906 // Get the discovered queue to which we will add
907 DiscoveredList* list = NULL;
908 switch (rt) {
909 case REF_OTHER:
910 // Unknown reference type, no special treatment
911 break;
912 case REF_SOFT:
913 list = &_discoveredSoftRefs[id];
914 break;
915 case REF_WEAK:
916 list = &_discoveredWeakRefs[id];
917 break;
918 case REF_FINAL:
919 list = &_discoveredFinalRefs[id];
920 break;
921 case REF_PHANTOM:
922 list = &_discoveredPhantomRefs[id];
923 break;
924 case REF_NONE:
925 // we should not reach here if we are an InstanceRefKlass
926 default:
927 ShouldNotReachHere();
928 }
929 log_develop_trace(gc, ref)("Thread %d gets list " INTPTR_FORMAT, id, p2i(list));
930 return list;
931 }
932
933 inline void
934 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
935 oop obj,
936 HeapWord* discovered_addr) {
937 assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
938 // First we must make sure this object is only enqueued once. CAS in a non null
939 // discovered_addr.
940 oop current_head = refs_list.head();
941 // The last ref must have its discovered field pointing to itself.
942 oop next_discovered = (current_head != NULL) ? current_head : obj;
943
944 oop retest = RawAccess<>::oop_atomic_cmpxchg(next_discovered, discovered_addr, oop(NULL));
945
946 if (retest == NULL) {
947 // This thread just won the right to enqueue the object.
948 // We have separate lists for enqueueing, so no synchronization
949 // is necessary.
950 refs_list.set_head(obj);
951 refs_list.inc_length(1);
952
953 log_develop_trace(gc, ref)("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
954 p2i(obj), obj->klass()->internal_name());
955 } else {
956 // If retest was non NULL, another thread beat us to it:
957 // The reference has already been discovered...
958 log_develop_trace(gc, ref)("Already discovered reference (" INTPTR_FORMAT ": %s)",
959 p2i(obj), obj->klass()->internal_name());
960 }
961 }
962
963 #ifndef PRODUCT
964 // Non-atomic (i.e. concurrent) discovery might allow us
965 // to observe j.l.References with NULL referents, being those
966 // cleared concurrently by mutators during (or after) discovery.
967 void ReferenceProcessor::verify_referent(oop obj) {
968 bool da = discovery_is_atomic();
969 oop referent = java_lang_ref_Reference::referent(obj);
970 assert(da ? oopDesc::is_oop(referent) : oopDesc::is_oop_or_null(referent),
971 "Bad referent " INTPTR_FORMAT " found in Reference "
972 INTPTR_FORMAT " during %satomic discovery ",
973 p2i(referent), p2i(obj), da ? "" : "non-");
974 }
975 #endif
976
977 template <class T>
978 bool ReferenceProcessor::is_subject_to_discovery(T const obj) const {
979 return _is_subject_to_discovery->do_object_b(obj);
980 }
981
982 // We mention two of several possible choices here:
983 // #0: if the reference object is not in the "originating generation"
984 // (or part of the heap being collected, indicated by our "span"
985 // we don't treat it specially (i.e. we scan it as we would
986 // a normal oop, treating its references as strong references).
987 // This means that references can't be discovered unless their
988 // referent is also in the same span. This is the simplest,
989 // most "local" and most conservative approach, albeit one
990 // that may cause weak references to be enqueued least promptly.
991 // We call this choice the "ReferenceBasedDiscovery" policy.
992 // #1: the reference object may be in any generation (span), but if
993 // the referent is in the generation (span) being currently collected
994 // then we can discover the reference object, provided
995 // the object has not already been discovered by
996 // a different concurrently running collector (as may be the
997 // case, for instance, if the reference object is in CMS and
998 // the referent in DefNewGeneration), and provided the processing
999 // of this reference object by the current collector will
1000 // appear atomic to every other collector in the system.
1001 // (Thus, for instance, a concurrent collector may not
1002 // discover references in other generations even if the
1003 // referent is in its own generation). This policy may,
1004 // in certain cases, enqueue references somewhat sooner than
1005 // might Policy #0 above, but at marginally increased cost
1006 // and complexity in processing these references.
1007 // We call this choice the "RefeferentBasedDiscovery" policy.
1008 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1009 // Make sure we are discovering refs (rather than processing discovered refs).
1010 if (!_discovering_refs || !RegisterReferences) {
1011 return false;
1012 }
1013 // We only discover active references.
1014 oop next = java_lang_ref_Reference::next(obj);
1015 if (next != NULL) { // Ref is no longer active
1016 return false;
1017 }
1018
1019 if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1020 !is_subject_to_discovery(obj)) {
1021 // Reference is not in the originating generation;
1022 // don't treat it specially (i.e. we want to scan it as a normal
1023 // object with strong references).
1024 return false;
1025 }
1026
1027 // We only discover references whose referents are not (yet)
1028 // known to be strongly reachable.
1029 if (is_alive_non_header() != NULL) {
1030 verify_referent(obj);
1031 if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
1032 return false; // referent is reachable
1033 }
1034 }
1035 if (rt == REF_SOFT) {
1036 // For soft refs we can decide now if these are not
1037 // current candidates for clearing, in which case we
1038 // can mark through them now, rather than delaying that
1039 // to the reference-processing phase. Since all current
1040 // time-stamp policies advance the soft-ref clock only
1041 // at a full collection cycle, this is always currently
1042 // accurate.
1043 if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
1044 return false;
1045 }
1046 }
1047
1048 ResourceMark rm; // Needed for tracing.
1049
1050 HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr_raw(obj);
1051 const oop discovered = java_lang_ref_Reference::discovered(obj);
1052 assert(oopDesc::is_oop_or_null(discovered), "Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered));
1053 if (discovered != NULL) {
1054 // The reference has already been discovered...
1055 log_develop_trace(gc, ref)("Already discovered reference (" INTPTR_FORMAT ": %s)",
1056 p2i(obj), obj->klass()->internal_name());
1057 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1058 // assumes that an object is not processed twice;
1059 // if it's been already discovered it must be on another
1060 // generation's discovered list; so we won't discover it.
1061 return false;
1062 } else {
1063 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1064 "Unrecognized policy");
1065 // Check assumption that an object is not potentially
1066 // discovered twice except by concurrent collectors that potentially
1067 // trace the same Reference object twice.
1068 assert(UseConcMarkSweepGC || UseG1GC,
1069 "Only possible with a concurrent marking collector");
1070 return true;
1071 }
1072 }
1073
1074 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1075 verify_referent(obj);
1076 // Discover if and only if EITHER:
1077 // .. reference is in our span, OR
1078 // .. we are an atomic collector and referent is in our span
1079 if (is_subject_to_discovery(obj) ||
1080 (discovery_is_atomic() &&
1081 is_subject_to_discovery(java_lang_ref_Reference::referent(obj)))) {
1082 } else {
1083 return false;
1084 }
1085 } else {
1086 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1087 is_subject_to_discovery(obj), "code inconsistency");
1088 }
1089
1090 // Get the right type of discovered queue head.
1091 DiscoveredList* list = get_discovered_list(rt);
1092 if (list == NULL) {
1093 return false; // nothing special needs to be done
1094 }
1095
1096 if (_discovery_is_mt) {
1097 add_to_discovered_list_mt(*list, obj, discovered_addr);
1098 } else {
1099 // We do a raw store here: the field will be visited later when processing
1100 // the discovered references.
1101 oop current_head = list->head();
1102 // The last ref must have its discovered field pointing to itself.
1103 oop next_discovered = (current_head != NULL) ? current_head : obj;
1104
1105 assert(discovered == NULL, "control point invariant");
1106 RawAccess<>::oop_store(discovered_addr, next_discovered);
1107 list->set_head(obj);
1108 list->inc_length(1);
1109
1110 log_develop_trace(gc, ref)("Discovered reference (" INTPTR_FORMAT ": %s)", p2i(obj), obj->klass()->internal_name());
1111 }
1112 assert(oopDesc::is_oop(obj), "Discovered a bad reference");
1113 verify_referent(obj);
1114 return true;
1115 }
1116
1117 bool ReferenceProcessor::has_discovered_references() {
1118 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
1119 if (!_discovered_refs[i].is_empty()) {
1120 return true;
1121 }
1122 }
1123 return false;
1124 }
1125
1126 // Preclean the discovered references by removing those
1127 // whose referents are alive, and by marking from those that
1128 // are not active. These lists can be handled here
1129 // in any order and, indeed, concurrently.
1130 void ReferenceProcessor::preclean_discovered_references(
1131 BoolObjectClosure* is_alive,
1132 OopClosure* keep_alive,
1133 VoidClosure* complete_gc,
1134 YieldClosure* yield,
1135 GCTimer* gc_timer) {
1136
1137 // Soft references
1138 {
1139 GCTraceTime(Debug, gc, ref) tm("Preclean SoftReferences", gc_timer);
1140 for (uint i = 0; i < _max_num_q; i++) {
1141 if (yield->should_return()) {
1142 return;
1143 }
1144 preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1145 keep_alive, complete_gc, yield);
1146 }
1147 }
1148
1149 // Weak references
1150 {
1151 GCTraceTime(Debug, gc, ref) tm("Preclean WeakReferences", gc_timer);
1152 for (uint i = 0; i < _max_num_q; i++) {
1153 if (yield->should_return()) {
1154 return;
1155 }
1156 preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1157 keep_alive, complete_gc, yield);
1158 }
1159 }
1160
1161 // Final references
1162 {
1163 GCTraceTime(Debug, gc, ref) tm("Preclean FinalReferences", gc_timer);
1164 for (uint i = 0; i < _max_num_q; i++) {
1165 if (yield->should_return()) {
1166 return;
1167 }
1168 preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1169 keep_alive, complete_gc, yield);
1170 }
1171 }
1172
1173 // Phantom references
1174 {
1175 GCTraceTime(Debug, gc, ref) tm("Preclean PhantomReferences", gc_timer);
1176 for (uint i = 0; i < _max_num_q; i++) {
1177 if (yield->should_return()) {
1178 return;
1179 }
1180 preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1181 keep_alive, complete_gc, yield);
1182 }
1183 }
1184 }
1185
1186 // Walk the given discovered ref list, and remove all reference objects
1187 // whose referents are still alive, whose referents are NULL or which
1188 // are not active (have a non-NULL next field). NOTE: When we are
1189 // thus precleaning the ref lists (which happens single-threaded today),
1190 // we do not disable refs discovery to honor the correct semantics of
1191 // java.lang.Reference. As a result, we need to be careful below
1192 // that ref removal steps interleave safely with ref discovery steps
1193 // (in this thread).
1194 void
1195 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList& refs_list,
1196 BoolObjectClosure* is_alive,
1197 OopClosure* keep_alive,
1198 VoidClosure* complete_gc,
1199 YieldClosure* yield) {
1200 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1201 while (iter.has_next()) {
1202 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1203 oop obj = iter.obj();
1204 oop next = java_lang_ref_Reference::next(obj);
1205 if (iter.referent() == NULL || iter.is_referent_alive() ||
1206 next != NULL) {
1207 // The referent has been cleared, or is alive, or the Reference is not
1208 // active; we need to trace and mark its cohort.
1209 log_develop_trace(gc, ref)("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1210 p2i(iter.obj()), iter.obj()->klass()->internal_name());
1211 // Remove Reference object from list
1212 iter.remove();
1213 // Keep alive its cohort.
1214 iter.make_referent_alive();
1215 if (UseCompressedOops) {
1216 narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr_raw(obj);
1217 keep_alive->do_oop(next_addr);
1218 } else {
1219 oop* next_addr = (oop*)java_lang_ref_Reference::next_addr_raw(obj);
1220 keep_alive->do_oop(next_addr);
1221 }
1222 iter.move_to_next();
1223 } else {
1224 iter.next();
1225 }
1226 }
1227 // Close the reachable set
1228 complete_gc->do_void();
1229
1230 NOT_PRODUCT(
1231 if (iter.processed() > 0) {
1232 log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " Refs out of " SIZE_FORMAT " Refs in discovered list " INTPTR_FORMAT,
1233 iter.removed(), iter.processed(), p2i(&refs_list));
1234 }
1235 )
1236 }
1237
1238 const char* ReferenceProcessor::list_name(uint i) {
1239 assert(i <= _max_num_q * number_of_subclasses_of_ref(),
1240 "Out of bounds index");
1241
1242 int j = i / _max_num_q;
1243 switch (j) {
1244 case 0: return "SoftRef";
1245 case 1: return "WeakRef";
1246 case 2: return "FinalRef";
1247 case 3: return "PhantomRef";
1248 }
1249 ShouldNotReachHere();
1250 return NULL;
1251 }