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_queues = MAX2(1U, mt_processing_degree);
114 _max_num_queues = MAX2(_num_queues, mt_discovery_degree);
115 _discovered_refs = NEW_C_HEAP_ARRAY(DiscoveredList,
116 _max_num_queues * 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_queues];
123 _discoveredFinalRefs = &_discoveredWeakRefs[_max_num_queues];
124 _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_queues];
125
126 // Initialize all entries to NULL
127 for (uint i = 0; i < _max_num_queues * 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_queues * 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_queues * 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_queues; ++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 DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
279 _current_discovered_addr = java_lang_ref_Reference::discovered_addr_raw(_current_discovered);
280 oop discovered = java_lang_ref_Reference::discovered(_current_discovered);
281 assert(_current_discovered_addr && oopDesc::is_oop_or_null(discovered),
282 "Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered));
283 _next_discovered = discovered;
284
285 _referent_addr = java_lang_ref_Reference::referent_addr_raw(_current_discovered);
286 _referent = java_lang_ref_Reference::referent(_current_discovered);
287 assert(Universe::heap()->is_in_reserved_or_null(_referent),
288 "Wrong oop found in java.lang.Reference object");
289 assert(allow_null_referent ?
290 oopDesc::is_oop_or_null(_referent)
291 : oopDesc::is_oop(_referent),
292 "Expected an oop%s for referent field at " PTR_FORMAT,
293 (allow_null_referent ? " or NULL" : ""),
294 p2i(_referent));
295 }
296
297 void DiscoveredListIterator::remove() {
298 assert(oopDesc::is_oop(_current_discovered), "Dropping a bad reference");
299 RawAccess<>::oop_store(_current_discovered_addr, oop(NULL));
300
301 // First _prev_next ref actually points into DiscoveredList (gross).
302 oop new_next;
303 if (_next_discovered == _current_discovered) {
304 // At the end of the list, we should make _prev point to itself.
305 // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
306 // and _prev will be NULL.
307 new_next = _prev_discovered;
308 } else {
309 new_next = _next_discovered;
310 }
311 // Remove Reference object from discovered list. Note that G1 does not need a
312 // pre-barrier here because we know the Reference has already been found/marked,
313 // that's how it ended up in the discovered list in the first place.
314 RawAccess<>::oop_store(_prev_discovered_addr, new_next);
315 NOT_PRODUCT(_removed++);
316 _refs_list.dec_length(1);
317 }
318
319 void DiscoveredListIterator::clear_referent() {
320 RawAccess<>::oop_store(_referent_addr, oop(NULL));
321 }
322
323 void DiscoveredListIterator::enqueue() {
324 // Self-loop next, so as to make Ref not active.
325 java_lang_ref_Reference::set_next_raw(_current_discovered, _current_discovered);
326
327 HeapAccess<AS_NO_KEEPALIVE>::oop_store_at(_current_discovered,
328 java_lang_ref_Reference::discovered_offset,
329 _next_discovered);
330 }
331
332 void DiscoveredListIterator::complete_enqeue() {
333 if (_prev_discovered != NULL) {
334 // This is the last object.
335 // Swap refs_list into pending list and set obj's
336 // discovered to what we read from the pending list.
337 oop old = Universe::swap_reference_pending_list(_refs_list.head());
338 HeapAccess<AS_NO_KEEPALIVE>::oop_store_at(_prev_discovered, java_lang_ref_Reference::discovered_offset, old);
339 }
340 }
341
342 // NOTE: process_phase*() are largely similar, and at a high level
343 // merely iterate over the extant list applying a predicate to
344 // each of its elements and possibly removing that element from the
345 // list and applying some further closures to that element.
346 // We should consider the possibility of replacing these
347 // process_phase*() methods by abstracting them into
348 // a single general iterator invocation that receives appropriate
349 // closures that accomplish this work.
350
351 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
352 // referents are not alive, but that should be kept alive for policy reasons.
353 // Keep alive the transitive closure of all such referents.
354 void
355 ReferenceProcessor::process_phase1(DiscoveredList& refs_list,
356 ReferencePolicy* policy,
357 BoolObjectClosure* is_alive,
358 OopClosure* keep_alive,
359 VoidClosure* complete_gc) {
360 assert(policy != NULL, "Must have a non-NULL policy");
361 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
362 // Decide which softly reachable refs should be kept alive.
363 while (iter.has_next()) {
364 iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
365 bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
366 if (referent_is_dead &&
367 !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
368 log_develop_trace(gc, ref)("Dropping reference (" INTPTR_FORMAT ": %s" ") by policy",
369 p2i(iter.obj()), iter.obj()->klass()->internal_name());
370 // Remove Reference object from list
371 iter.remove();
372 // keep the referent around
373 iter.make_referent_alive();
374 iter.move_to_next();
375 } else {
376 iter.next();
377 }
378 }
379 // Close the reachable set
380 complete_gc->do_void();
381 log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " dead Refs out of " SIZE_FORMAT " discovered Refs by policy, from list " INTPTR_FORMAT,
382 iter.removed(), iter.processed(), p2i(&refs_list));
383 }
384
385 void ReferenceProcessor::process_phase2(DiscoveredList& refs_list,
386 BoolObjectClosure* is_alive,
387 OopClosure* keep_alive,
388 VoidClosure* complete_gc) {
389 if (discovery_is_atomic()) {
390 // complete_gc is ignored in this case for this phase
391 pp2_work(refs_list, is_alive, keep_alive);
392 } else {
393 assert(complete_gc != NULL, "Error");
394 pp2_work_concurrent_discovery(refs_list, is_alive,
395 keep_alive, complete_gc);
396 }
397 }
398 // Traverse the list and remove any Refs that are not active, or
399 // whose referents are either alive or NULL.
400 void
401 ReferenceProcessor::pp2_work(DiscoveredList& refs_list,
402 BoolObjectClosure* is_alive,
403 OopClosure* keep_alive) {
404 assert(discovery_is_atomic(), "Error");
405 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
406 while (iter.has_next()) {
407 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
408 DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
409 assert(next == NULL, "Should not discover inactive Reference");
410 if (iter.is_referent_alive()) {
411 log_develop_trace(gc, ref)("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
412 p2i(iter.obj()), iter.obj()->klass()->internal_name());
413 // The referent is reachable after all.
414 // Remove Reference object from list.
415 iter.remove();
416 // Update the referent pointer as necessary: Note that this
417 // should not entail any recursive marking because the
418 // referent must already have been traversed.
419 iter.make_referent_alive();
420 iter.move_to_next();
421 } else {
422 iter.next();
423 }
424 }
425 NOT_PRODUCT(
426 if (iter.processed() > 0) {
427 log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT
428 " Refs in discovered list " INTPTR_FORMAT,
429 iter.removed(), iter.processed(), p2i(&refs_list));
430 }
431 )
432 }
433
434 void
435 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList& refs_list,
436 BoolObjectClosure* is_alive,
437 OopClosure* keep_alive,
438 VoidClosure* complete_gc) {
439 assert(!discovery_is_atomic(), "Error");
440 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
441 while (iter.has_next()) {
442 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
443 HeapWord* next_addr = java_lang_ref_Reference::next_addr_raw(iter.obj());
444 oop next = java_lang_ref_Reference::next(iter.obj());
445 if ((iter.referent() == NULL || iter.is_referent_alive() ||
446 next != NULL)) {
447 assert(oopDesc::is_oop_or_null(next), "Expected an oop or NULL for next field at " PTR_FORMAT, p2i(next));
448 // Remove Reference object from list
449 iter.remove();
450 // Trace the cohorts
451 iter.make_referent_alive();
452 if (UseCompressedOops) {
453 keep_alive->do_oop((narrowOop*)next_addr);
454 } else {
455 keep_alive->do_oop((oop*)next_addr);
456 }
457 iter.move_to_next();
458 } else {
459 iter.next();
460 }
461 }
462 // Now close the newly reachable set
463 complete_gc->do_void();
464 NOT_PRODUCT(
465 if (iter.processed() > 0) {
466 log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT
467 " Refs in discovered list " INTPTR_FORMAT,
468 iter.removed(), iter.processed(), p2i(&refs_list));
469 }
470 )
471 }
472
473 void ReferenceProcessor::process_phase3(DiscoveredList& refs_list,
474 bool clear_referent,
475 BoolObjectClosure* is_alive,
476 OopClosure* keep_alive,
477 VoidClosure* complete_gc) {
478 ResourceMark rm;
479 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
480 while (iter.has_next()) {
481 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
482 if (clear_referent) {
483 // NULL out referent pointer
484 iter.clear_referent();
485 } else {
486 // keep the referent around
487 iter.make_referent_alive();
488 }
489 iter.enqueue();
490 log_develop_trace(gc, ref)("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
491 clear_referent ? "cleared " : "", p2i(iter.obj()), iter.obj()->klass()->internal_name());
492 assert(oopDesc::is_oop(iter.obj(), UseConcMarkSweepGC), "Adding a bad reference");
493 iter.next();
494 }
495 iter.complete_enqeue();
496 // Close the reachable set
497 complete_gc->do_void();
498 // Clear the list.
499 refs_list.set_head(NULL);
500 refs_list.set_length(0);
501 }
502
503 void
504 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
505 oop obj = NULL;
506 oop next = refs_list.head();
507 while (next != obj) {
508 obj = next;
509 next = java_lang_ref_Reference::discovered(obj);
510 java_lang_ref_Reference::set_discovered_raw(obj, NULL);
511 }
512 refs_list.set_head(NULL);
513 refs_list.set_length(0);
514 }
515
516 void ReferenceProcessor::abandon_partial_discovery() {
517 // loop over the lists
518 for (uint i = 0; i < _max_num_queues * number_of_subclasses_of_ref(); i++) {
519 if ((i % _max_num_queues) == 0) {
520 log_develop_trace(gc, ref)("Abandoning %s discovered list", list_name(i));
521 }
522 clear_discovered_references(_discovered_refs[i]);
523 }
524 }
525
526 size_t ReferenceProcessor::total_reference_count(ReferenceType type) const {
527 DiscoveredList* list = NULL;
528
529 switch (type) {
530 case REF_SOFT:
531 list = _discoveredSoftRefs;
532 break;
533 case REF_WEAK:
534 list = _discoveredWeakRefs;
535 break;
536 case REF_FINAL:
537 list = _discoveredFinalRefs;
538 break;
539 case REF_PHANTOM:
540 list = _discoveredPhantomRefs;
541 break;
542 case REF_OTHER:
543 case REF_NONE:
544 default:
545 ShouldNotReachHere();
546 }
547 return total_count(list);
548 }
549
550 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
551 public:
552 RefProcPhase1Task(ReferenceProcessor& ref_processor,
553 DiscoveredList refs_lists[],
554 ReferencePolicy* policy,
555 bool marks_oops_alive,
556 ReferenceProcessorPhaseTimes* phase_times)
557 : ProcessTask(ref_processor, refs_lists, marks_oops_alive, phase_times),
558 _policy(policy)
559 { }
560 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
561 OopClosure& keep_alive,
562 VoidClosure& complete_gc)
563 {
564 RefProcWorkerTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase1, _phase_times, i);
565
566 _ref_processor.process_phase1(_refs_lists[i], _policy,
567 &is_alive, &keep_alive, &complete_gc);
568 }
569 private:
570 ReferencePolicy* _policy;
571 };
572
573 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
574 public:
575 RefProcPhase2Task(ReferenceProcessor& ref_processor,
576 DiscoveredList refs_lists[],
577 bool marks_oops_alive,
578 ReferenceProcessorPhaseTimes* phase_times)
579 : ProcessTask(ref_processor, refs_lists, marks_oops_alive, phase_times)
580 { }
581 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
582 OopClosure& keep_alive,
583 VoidClosure& complete_gc)
584 {
585 RefProcWorkerTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase2, _phase_times, i);
586
587 _ref_processor.process_phase2(_refs_lists[i],
588 &is_alive, &keep_alive, &complete_gc);
589 }
590 };
591
592 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
593 public:
594 RefProcPhase3Task(ReferenceProcessor& ref_processor,
595 DiscoveredList refs_lists[],
596 bool clear_referent,
597 bool marks_oops_alive,
598 ReferenceProcessorPhaseTimes* phase_times)
599 : ProcessTask(ref_processor, refs_lists, marks_oops_alive, phase_times),
600 _clear_referent(clear_referent)
601 { }
602 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
603 OopClosure& keep_alive,
604 VoidClosure& complete_gc)
605 {
606 RefProcWorkerTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase3, _phase_times, i);
607
608 _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
609 &is_alive, &keep_alive, &complete_gc);
610 }
611 private:
612 bool _clear_referent;
613 };
614
615 #ifndef PRODUCT
616 void ReferenceProcessor::log_reflist_counts(DiscoveredList ref_lists[], uint active_length, size_t total_refs) {
617 if (!log_is_enabled(Trace, gc, ref)) {
618 return;
619 }
620
621 stringStream st;
622 for (uint i = 0; i < active_length; ++i) {
623 st.print(SIZE_FORMAT " ", ref_lists[i].length());
624 }
625 log_develop_trace(gc, ref)("%s= " SIZE_FORMAT, st.as_string(), total_refs);
626 #ifdef ASSERT
627 for (uint i = active_length; i < _max_num_queues; i++) {
628 assert(ref_lists[i].length() == 0, SIZE_FORMAT " unexpected References in %u",
629 ref_lists[i].length(), i);
630 }
631 #endif
632 }
633 #endif
634
635 void ReferenceProcessor::set_active_mt_degree(uint v) {
636 _num_queues = v;
637 _next_id = 0;
638 }
639
640 // Balances reference queues.
641 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
642 // queues[0, 1, ..., _num_q-1] because only the first _num_q
643 // corresponding to the active workers will be processed.
644 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
645 {
646 // calculate total length
647 size_t total_refs = 0;
648 log_develop_trace(gc, ref)("Balance ref_lists ");
649
650 for (uint i = 0; i < _max_num_queues; ++i) {
651 total_refs += ref_lists[i].length();
652 }
653 log_reflist_counts(ref_lists, _max_num_queues, total_refs);
654 size_t avg_refs = total_refs / _num_queues + 1;
655 uint to_idx = 0;
656 for (uint from_idx = 0; from_idx < _max_num_queues; from_idx++) {
657 bool move_all = false;
658 if (from_idx >= _num_queues) {
659 move_all = ref_lists[from_idx].length() > 0;
660 }
661 while ((ref_lists[from_idx].length() > avg_refs) ||
662 move_all) {
663 assert(to_idx < _num_queues, "Sanity Check!");
664 if (ref_lists[to_idx].length() < avg_refs) {
665 // move superfluous refs
666 size_t refs_to_move;
667 // Move all the Ref's if the from queue will not be processed.
668 if (move_all) {
669 refs_to_move = MIN2(ref_lists[from_idx].length(),
670 avg_refs - ref_lists[to_idx].length());
671 } else {
672 refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
673 avg_refs - ref_lists[to_idx].length());
674 }
675
676 assert(refs_to_move > 0, "otherwise the code below will fail");
677
678 oop move_head = ref_lists[from_idx].head();
679 oop move_tail = move_head;
680 oop new_head = move_head;
681 // find an element to split the list on
682 for (size_t j = 0; j < refs_to_move; ++j) {
683 move_tail = new_head;
684 new_head = java_lang_ref_Reference::discovered(new_head);
685 }
686
687 // Add the chain to the to list.
688 if (ref_lists[to_idx].head() == NULL) {
689 // to list is empty. Make a loop at the end.
690 java_lang_ref_Reference::set_discovered_raw(move_tail, move_tail);
691 } else {
692 java_lang_ref_Reference::set_discovered_raw(move_tail, ref_lists[to_idx].head());
693 }
694 ref_lists[to_idx].set_head(move_head);
695 ref_lists[to_idx].inc_length(refs_to_move);
696
697 // Remove the chain from the from list.
698 if (move_tail == new_head) {
699 // We found the end of the from list.
700 ref_lists[from_idx].set_head(NULL);
701 } else {
702 ref_lists[from_idx].set_head(new_head);
703 }
704 ref_lists[from_idx].dec_length(refs_to_move);
705 if (ref_lists[from_idx].length() == 0) {
706 break;
707 }
708 } else {
709 to_idx = (to_idx + 1) % _num_queues;
710 }
711 }
712 }
713 #ifdef ASSERT
714 size_t balanced_total_refs = 0;
715 for (uint i = 0; i < _num_queues; ++i) {
716 balanced_total_refs += ref_lists[i].length();
717 }
718 log_reflist_counts(ref_lists, _num_queues, balanced_total_refs);
719 assert(total_refs == balanced_total_refs, "Balancing was incomplete");
720 #endif
721 }
722
723 void ReferenceProcessor::process_discovered_reflist(
724 DiscoveredList refs_lists[],
725 ReferencePolicy* policy,
726 bool clear_referent,
727 BoolObjectClosure* is_alive,
728 OopClosure* keep_alive,
729 VoidClosure* complete_gc,
730 AbstractRefProcTaskExecutor* task_executor,
731 ReferenceProcessorPhaseTimes* phase_times)
732 {
733 bool mt_processing = task_executor != NULL && _processing_is_mt;
734
735 phase_times->set_processing_is_mt(mt_processing);
736
737 if (mt_processing && ParallelRefProcBalancingEnabled) {
738 RefProcBalanceQueuesTimeTracker tt(phase_times);
739 balance_queues(refs_lists);
740 }
741
742 // Phase 1 (soft refs only):
743 // . Traverse the list and remove any SoftReferences whose
744 // referents are not alive, but that should be kept alive for
745 // policy reasons. Keep alive the transitive closure of all
746 // such referents.
747 if (policy != NULL) {
748 RefProcParPhaseTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase1, phase_times);
749
750 if (mt_processing) {
751 RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/, phase_times);
752 task_executor->execute(phase1);
753 } else {
754 for (uint i = 0; i < _max_num_queues; i++) {
755 process_phase1(refs_lists[i], policy,
756 is_alive, keep_alive, complete_gc);
757 }
758 }
759 } else { // policy == NULL
760 assert(refs_lists != _discoveredSoftRefs,
761 "Policy must be specified for soft references.");
762 }
763
764 // Phase 2:
765 // . Traverse the list and remove any refs whose referents are alive.
766 {
767 RefProcParPhaseTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase2, phase_times);
768
769 if (mt_processing) {
770 RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/, phase_times);
771 task_executor->execute(phase2);
772 } else {
773 for (uint i = 0; i < _max_num_queues; i++) {
774 process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
775 }
776 }
777 }
778
779 // Phase 3:
780 // . Traverse the list and process referents as appropriate.
781 {
782 RefProcParPhaseTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase3, phase_times);
783
784 if (mt_processing) {
785 RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/, phase_times);
786 task_executor->execute(phase3);
787 } else {
788 for (uint i = 0; i < _max_num_queues; i++) {
789 process_phase3(refs_lists[i], clear_referent,
790 is_alive, keep_alive, complete_gc);
791 }
792 }
793 }
794 }
795
796 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
797 uint id = 0;
798 // Determine the queue index to use for this object.
799 if (_discovery_is_mt) {
800 // During a multi-threaded discovery phase,
801 // each thread saves to its "own" list.
802 Thread* thr = Thread::current();
803 id = thr->as_Worker_thread()->id();
804 } else {
805 // single-threaded discovery, we save in round-robin
806 // fashion to each of the lists.
807 if (_processing_is_mt) {
808 id = next_id();
809 }
810 }
811 assert(id < _max_num_queues, "Id is out-of-bounds id %u and max id %u)", id, _max_num_queues);
812
813 // Get the discovered queue to which we will add
814 DiscoveredList* list = NULL;
815 switch (rt) {
816 case REF_OTHER:
817 // Unknown reference type, no special treatment
818 break;
819 case REF_SOFT:
820 list = &_discoveredSoftRefs[id];
821 break;
822 case REF_WEAK:
823 list = &_discoveredWeakRefs[id];
824 break;
825 case REF_FINAL:
826 list = &_discoveredFinalRefs[id];
827 break;
828 case REF_PHANTOM:
829 list = &_discoveredPhantomRefs[id];
830 break;
831 case REF_NONE:
832 // we should not reach here if we are an InstanceRefKlass
833 default:
834 ShouldNotReachHere();
835 }
836 log_develop_trace(gc, ref)("Thread %d gets list " INTPTR_FORMAT, id, p2i(list));
837 return list;
838 }
839
840 inline void
841 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
842 oop obj,
843 HeapWord* discovered_addr) {
844 assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
845 // First we must make sure this object is only enqueued once. CAS in a non null
846 // discovered_addr.
847 oop current_head = refs_list.head();
848 // The last ref must have its discovered field pointing to itself.
849 oop next_discovered = (current_head != NULL) ? current_head : obj;
850
851 oop retest = RawAccess<>::oop_atomic_cmpxchg(next_discovered, discovered_addr, oop(NULL));
852
853 if (retest == NULL) {
854 // This thread just won the right to enqueue the object.
855 // We have separate lists for enqueueing, so no synchronization
856 // is necessary.
857 refs_list.set_head(obj);
858 refs_list.inc_length(1);
859
860 log_develop_trace(gc, ref)("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
861 p2i(obj), obj->klass()->internal_name());
862 } else {
863 // If retest was non NULL, another thread beat us to it:
864 // The reference has already been discovered...
865 log_develop_trace(gc, ref)("Already discovered reference (" INTPTR_FORMAT ": %s)",
866 p2i(obj), obj->klass()->internal_name());
867 }
868 }
869
870 #ifndef PRODUCT
871 // Non-atomic (i.e. concurrent) discovery might allow us
872 // to observe j.l.References with NULL referents, being those
873 // cleared concurrently by mutators during (or after) discovery.
874 void ReferenceProcessor::verify_referent(oop obj) {
875 bool da = discovery_is_atomic();
876 oop referent = java_lang_ref_Reference::referent(obj);
877 assert(da ? oopDesc::is_oop(referent) : oopDesc::is_oop_or_null(referent),
878 "Bad referent " INTPTR_FORMAT " found in Reference "
879 INTPTR_FORMAT " during %satomic discovery ",
880 p2i(referent), p2i(obj), da ? "" : "non-");
881 }
882 #endif
883
884 template <class T>
885 bool ReferenceProcessor::is_subject_to_discovery(T const obj) const {
886 return _is_subject_to_discovery->do_object_b(obj);
887 }
888
889 // We mention two of several possible choices here:
890 // #0: if the reference object is not in the "originating generation"
891 // (or part of the heap being collected, indicated by our "span"
892 // we don't treat it specially (i.e. we scan it as we would
893 // a normal oop, treating its references as strong references).
894 // This means that references can't be discovered unless their
895 // referent is also in the same span. This is the simplest,
896 // most "local" and most conservative approach, albeit one
897 // that may cause weak references to be enqueued least promptly.
898 // We call this choice the "ReferenceBasedDiscovery" policy.
899 // #1: the reference object may be in any generation (span), but if
900 // the referent is in the generation (span) being currently collected
901 // then we can discover the reference object, provided
902 // the object has not already been discovered by
903 // a different concurrently running collector (as may be the
904 // case, for instance, if the reference object is in CMS and
905 // the referent in DefNewGeneration), and provided the processing
906 // of this reference object by the current collector will
907 // appear atomic to every other collector in the system.
908 // (Thus, for instance, a concurrent collector may not
909 // discover references in other generations even if the
910 // referent is in its own generation). This policy may,
911 // in certain cases, enqueue references somewhat sooner than
912 // might Policy #0 above, but at marginally increased cost
913 // and complexity in processing these references.
914 // We call this choice the "RefeferentBasedDiscovery" policy.
915 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
916 // Make sure we are discovering refs (rather than processing discovered refs).
917 if (!_discovering_refs || !RegisterReferences) {
918 return false;
919 }
920 // We only discover active references.
921 oop next = java_lang_ref_Reference::next(obj);
922 if (next != NULL) { // Ref is no longer active
923 return false;
924 }
925
926 if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
927 !is_subject_to_discovery(obj)) {
928 // Reference is not in the originating generation;
929 // don't treat it specially (i.e. we want to scan it as a normal
930 // object with strong references).
931 return false;
932 }
933
934 // We only discover references whose referents are not (yet)
935 // known to be strongly reachable.
936 if (is_alive_non_header() != NULL) {
937 verify_referent(obj);
938 if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
939 return false; // referent is reachable
940 }
941 }
942 if (rt == REF_SOFT) {
943 // For soft refs we can decide now if these are not
944 // current candidates for clearing, in which case we
945 // can mark through them now, rather than delaying that
946 // to the reference-processing phase. Since all current
947 // time-stamp policies advance the soft-ref clock only
948 // at a full collection cycle, this is always currently
949 // accurate.
950 if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
951 return false;
952 }
953 }
954
955 ResourceMark rm; // Needed for tracing.
956
957 HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr_raw(obj);
958 const oop discovered = java_lang_ref_Reference::discovered(obj);
959 assert(oopDesc::is_oop_or_null(discovered), "Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered));
960 if (discovered != NULL) {
961 // The reference has already been discovered...
962 log_develop_trace(gc, ref)("Already discovered reference (" INTPTR_FORMAT ": %s)",
963 p2i(obj), obj->klass()->internal_name());
964 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
965 // assumes that an object is not processed twice;
966 // if it's been already discovered it must be on another
967 // generation's discovered list; so we won't discover it.
968 return false;
969 } else {
970 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
971 "Unrecognized policy");
972 // Check assumption that an object is not potentially
973 // discovered twice except by concurrent collectors that potentially
974 // trace the same Reference object twice.
975 assert(UseConcMarkSweepGC || UseG1GC,
976 "Only possible with a concurrent marking collector");
977 return true;
978 }
979 }
980
981 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
982 verify_referent(obj);
983 // Discover if and only if EITHER:
984 // .. reference is in our span, OR
985 // .. we are an atomic collector and referent is in our span
986 if (is_subject_to_discovery(obj) ||
987 (discovery_is_atomic() &&
988 is_subject_to_discovery(java_lang_ref_Reference::referent(obj)))) {
989 } else {
990 return false;
991 }
992 } else {
993 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
994 is_subject_to_discovery(obj), "code inconsistency");
995 }
996
997 // Get the right type of discovered queue head.
998 DiscoveredList* list = get_discovered_list(rt);
999 if (list == NULL) {
1000 return false; // nothing special needs to be done
1001 }
1002
1003 if (_discovery_is_mt) {
1004 add_to_discovered_list_mt(*list, obj, discovered_addr);
1005 } else {
1006 // We do a raw store here: the field will be visited later when processing
1007 // the discovered references.
1008 oop current_head = list->head();
1009 // The last ref must have its discovered field pointing to itself.
1010 oop next_discovered = (current_head != NULL) ? current_head : obj;
1011
1012 assert(discovered == NULL, "control point invariant");
1013 RawAccess<>::oop_store(discovered_addr, next_discovered);
1014 list->set_head(obj);
1015 list->inc_length(1);
1016
1017 log_develop_trace(gc, ref)("Discovered reference (" INTPTR_FORMAT ": %s)", p2i(obj), obj->klass()->internal_name());
1018 }
1019 assert(oopDesc::is_oop(obj), "Discovered a bad reference");
1020 verify_referent(obj);
1021 return true;
1022 }
1023
1024 bool ReferenceProcessor::has_discovered_references() {
1025 for (uint i = 0; i < _max_num_queues * number_of_subclasses_of_ref(); i++) {
1026 if (!_discovered_refs[i].is_empty()) {
1027 return true;
1028 }
1029 }
1030 return false;
1031 }
1032
1033 // Preclean the discovered references by removing those
1034 // whose referents are alive, and by marking from those that
1035 // are not active. These lists can be handled here
1036 // in any order and, indeed, concurrently.
1037 void ReferenceProcessor::preclean_discovered_references(
1038 BoolObjectClosure* is_alive,
1039 OopClosure* keep_alive,
1040 VoidClosure* complete_gc,
1041 YieldClosure* yield,
1042 GCTimer* gc_timer) {
1043
1044 // Soft references
1045 {
1046 GCTraceTime(Debug, gc, ref) tm("Preclean SoftReferences", gc_timer);
1047 for (uint i = 0; i < _max_num_queues; i++) {
1048 if (yield->should_return()) {
1049 return;
1050 }
1051 preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1052 keep_alive, complete_gc, yield);
1053 }
1054 }
1055
1056 // Weak references
1057 {
1058 GCTraceTime(Debug, gc, ref) tm("Preclean WeakReferences", gc_timer);
1059 for (uint i = 0; i < _max_num_queues; i++) {
1060 if (yield->should_return()) {
1061 return;
1062 }
1063 preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1064 keep_alive, complete_gc, yield);
1065 }
1066 }
1067
1068 // Final references
1069 {
1070 GCTraceTime(Debug, gc, ref) tm("Preclean FinalReferences", gc_timer);
1071 for (uint i = 0; i < _max_num_queues; i++) {
1072 if (yield->should_return()) {
1073 return;
1074 }
1075 preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1076 keep_alive, complete_gc, yield);
1077 }
1078 }
1079
1080 // Phantom references
1081 {
1082 GCTraceTime(Debug, gc, ref) tm("Preclean PhantomReferences", gc_timer);
1083 for (uint i = 0; i < _max_num_queues; i++) {
1084 if (yield->should_return()) {
1085 return;
1086 }
1087 preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1088 keep_alive, complete_gc, yield);
1089 }
1090 }
1091 }
1092
1093 // Walk the given discovered ref list, and remove all reference objects
1094 // whose referents are still alive, whose referents are NULL or which
1095 // are not active (have a non-NULL next field). NOTE: When we are
1096 // thus precleaning the ref lists (which happens single-threaded today),
1097 // we do not disable refs discovery to honor the correct semantics of
1098 // java.lang.Reference. As a result, we need to be careful below
1099 // that ref removal steps interleave safely with ref discovery steps
1100 // (in this thread).
1101 void
1102 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList& refs_list,
1103 BoolObjectClosure* is_alive,
1104 OopClosure* keep_alive,
1105 VoidClosure* complete_gc,
1106 YieldClosure* yield) {
1107 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1108 while (iter.has_next()) {
1109 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1110 oop obj = iter.obj();
1111 oop next = java_lang_ref_Reference::next(obj);
1112 if (iter.referent() == NULL || iter.is_referent_alive() ||
1113 next != NULL) {
1114 // The referent has been cleared, or is alive, or the Reference is not
1115 // active; we need to trace and mark its cohort.
1116 log_develop_trace(gc, ref)("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1117 p2i(iter.obj()), iter.obj()->klass()->internal_name());
1118 // Remove Reference object from list
1119 iter.remove();
1120 // Keep alive its cohort.
1121 iter.make_referent_alive();
1122 if (UseCompressedOops) {
1123 narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr_raw(obj);
1124 keep_alive->do_oop(next_addr);
1125 } else {
1126 oop* next_addr = (oop*)java_lang_ref_Reference::next_addr_raw(obj);
1127 keep_alive->do_oop(next_addr);
1128 }
1129 iter.move_to_next();
1130 } else {
1131 iter.next();
1132 }
1133 }
1134 // Close the reachable set
1135 complete_gc->do_void();
1136
1137 NOT_PRODUCT(
1138 if (iter.processed() > 0) {
1139 log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " Refs out of " SIZE_FORMAT " Refs in discovered list " INTPTR_FORMAT,
1140 iter.removed(), iter.processed(), p2i(&refs_list));
1141 }
1142 )
1143 }
1144
1145 const char* ReferenceProcessor::list_name(uint i) {
1146 assert(i <= _max_num_queues * number_of_subclasses_of_ref(),
1147 "Out of bounds index");
1148
1149 int j = i / _max_num_queues;
1150 switch (j) {
1151 case 0: return "SoftRef";
1152 case 1: return "WeakRef";
1153 case 2: return "FinalRef";
1154 case 3: return "PhantomRef";
1155 }
1156 ShouldNotReachHere();
1157 return NULL;
1158 }