1 /*
2 * Copyright (c) 2001, 2019, 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 "gc/shared/referenceProcessorPhaseTimes.hpp"
35 #include "logging/log.hpp"
36 #include "memory/allocation.inline.hpp"
37 #include "memory/resourceArea.hpp"
38 #include "memory/universe.hpp"
39 #include "oops/access.inline.hpp"
40 #include "oops/oop.inline.hpp"
41 #include "runtime/java.hpp"
42
43 ReferencePolicy* ReferenceProcessor::_always_clear_soft_ref_policy = NULL;
44 ReferencePolicy* ReferenceProcessor::_default_soft_ref_policy = NULL;
45 jlong ReferenceProcessor::_soft_ref_timestamp_clock = 0;
46
47 void referenceProcessor_init() {
48 ReferenceProcessor::init_statics();
49 }
50
51 void ReferenceProcessor::init_statics() {
52 // We need a monotonically non-decreasing time in ms but
53 // os::javaTimeMillis() does not guarantee monotonicity.
54 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
55
56 // Initialize the soft ref timestamp clock.
57 _soft_ref_timestamp_clock = now;
58 // Also update the soft ref clock in j.l.r.SoftReference
59 java_lang_ref_SoftReference::set_clock(_soft_ref_timestamp_clock);
60
61 _always_clear_soft_ref_policy = new AlwaysClearPolicy();
62 if (is_server_compilation_mode_vm()) {
63 _default_soft_ref_policy = new LRUMaxHeapPolicy();
64 } else {
65 _default_soft_ref_policy = new LRUCurrentHeapPolicy();
66 }
67 guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||
68 RefDiscoveryPolicy == ReferentBasedDiscovery,
69 "Unrecognized RefDiscoveryPolicy");
70 }
71
72 void ReferenceProcessor::enable_discovery(bool check_no_refs) {
73 #ifdef ASSERT
74 // Verify that we're not currently discovering refs
75 assert(!_discovering_refs, "nested call?");
76
77 if (check_no_refs) {
78 // Verify that the discovered lists are empty
79 verify_no_references_recorded();
80 }
81 #endif // ASSERT
82
83 // Someone could have modified the value of the static
84 // field in the j.l.r.SoftReference class that holds the
85 // soft reference timestamp clock using reflection or
86 // Unsafe between GCs. Unconditionally update the static
87 // field in ReferenceProcessor here so that we use the new
88 // value during reference discovery.
89
90 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
91 _discovering_refs = true;
92 }
93
94 ReferenceProcessor::ReferenceProcessor(BoolObjectClosure* is_subject_to_discovery,
95 bool mt_processing,
96 uint mt_processing_degree,
97 bool mt_discovery,
98 uint mt_discovery_degree,
99 bool atomic_discovery,
100 BoolObjectClosure* is_alive_non_header,
101 bool adjust_no_of_processing_threads) :
102 _is_subject_to_discovery(is_subject_to_discovery),
103 _discovering_refs(false),
104 _enqueuing_is_done(false),
105 _processing_is_mt(mt_processing),
106 _next_id(0),
107 _adjust_no_of_processing_threads(adjust_no_of_processing_threads),
108 _is_alive_non_header(is_alive_non_header)
109 {
110 assert(is_subject_to_discovery != NULL, "must be set");
111
112 _discovery_is_atomic = atomic_discovery;
113 _discovery_is_mt = mt_discovery;
114 _num_queues = MAX2(1U, mt_processing_degree);
115 _max_num_queues = MAX2(_num_queues, mt_discovery_degree);
116 _discovered_refs = NEW_C_HEAP_ARRAY(DiscoveredList,
117 _max_num_queues * number_of_subclasses_of_ref(), mtGC);
118
119 _discoveredSoftRefs = &_discovered_refs[0];
120 _discoveredWeakRefs = &_discoveredSoftRefs[_max_num_queues];
121 _discoveredFinalRefs = &_discoveredWeakRefs[_max_num_queues];
122 _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_queues];
123
124 // Initialize all entries to NULL
125 for (uint i = 0; i < _max_num_queues * number_of_subclasses_of_ref(); i++) {
126 _discovered_refs[i].clear();
127 }
128
129 setup_policy(false /* default soft ref policy */);
130 }
131
132 #ifndef PRODUCT
133 void ReferenceProcessor::verify_no_references_recorded() {
134 guarantee(!_discovering_refs, "Discovering refs?");
135 for (uint i = 0; i < _max_num_queues * number_of_subclasses_of_ref(); i++) {
136 guarantee(_discovered_refs[i].is_empty(),
137 "Found non-empty discovered list at %u", i);
138 }
139 }
140 #endif
141
142 void ReferenceProcessor::weak_oops_do(OopClosure* f) {
143 for (uint i = 0; i < _max_num_queues * number_of_subclasses_of_ref(); i++) {
144 if (UseCompressedOops) {
145 f->do_oop((narrowOop*)_discovered_refs[i].adr_head());
146 } else {
147 f->do_oop((oop*)_discovered_refs[i].adr_head());
148 }
149 }
150 }
151
152 void ReferenceProcessor::update_soft_ref_master_clock() {
153 // Update (advance) the soft ref master clock field. This must be done
154 // after processing the soft ref list.
155
156 // We need a monotonically non-decreasing time in ms but
157 // os::javaTimeMillis() does not guarantee monotonicity.
158 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
159 jlong soft_ref_clock = java_lang_ref_SoftReference::clock();
160 assert(soft_ref_clock == _soft_ref_timestamp_clock, "soft ref clocks out of sync");
161
162 NOT_PRODUCT(
163 if (now < _soft_ref_timestamp_clock) {
164 log_warning(gc)("time warp: " JLONG_FORMAT " to " JLONG_FORMAT,
165 _soft_ref_timestamp_clock, now);
166 }
167 )
168 // The values of now and _soft_ref_timestamp_clock are set using
169 // javaTimeNanos(), which is guaranteed to be monotonically
170 // non-decreasing provided the underlying platform provides such
171 // a time source (and it is bug free).
172 // In product mode, however, protect ourselves from non-monotonicity.
173 if (now > _soft_ref_timestamp_clock) {
174 _soft_ref_timestamp_clock = now;
175 java_lang_ref_SoftReference::set_clock(now);
176 }
177 // Else leave clock stalled at its old value until time progresses
178 // past clock value.
179 }
180
181 size_t ReferenceProcessor::total_count(DiscoveredList lists[]) const {
182 size_t total = 0;
183 for (uint i = 0; i < _max_num_queues; ++i) {
184 total += lists[i].length();
185 }
186 return total;
187 }
188
189 #ifdef ASSERT
190 void ReferenceProcessor::verify_total_count_zero(DiscoveredList lists[], const char* type) {
191 size_t count = total_count(lists);
192 assert(count == 0, "%ss must be empty but has " SIZE_FORMAT " elements", type, count);
193 }
194 #endif
195
196 ReferenceProcessorStats ReferenceProcessor::process_discovered_references(
197 BoolObjectClosure* is_alive,
198 OopClosure* keep_alive,
199 VoidClosure* complete_gc,
200 AbstractRefProcTaskExecutor* task_executor,
201 ReferenceProcessorPhaseTimes* phase_times) {
202
203 double start_time = os::elapsedTime();
204
205 assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
206 // Stop treating discovered references specially.
207 disable_discovery();
208
209 // If discovery was concurrent, someone could have modified
210 // the value of the static field in the j.l.r.SoftReference
211 // class that holds the soft reference timestamp clock using
212 // reflection or Unsafe between when discovery was enabled and
213 // now. Unconditionally update the static field in ReferenceProcessor
214 // here so that we use the new value during processing of the
215 // discovered soft refs.
216
217 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
218
219 ReferenceProcessorStats stats(total_count(_discoveredSoftRefs),
220 total_count(_discoveredWeakRefs),
221 total_count(_discoveredFinalRefs),
222 total_count(_discoveredPhantomRefs));
223
224 {
225 RefProcTotalPhaseTimesTracker tt(RefPhase1, phase_times, this);
226 process_soft_ref_reconsider(is_alive, keep_alive, complete_gc,
227 task_executor, phase_times);
228 }
229
230 update_soft_ref_master_clock();
231
232 {
233 RefProcTotalPhaseTimesTracker tt(RefPhase2, phase_times, this);
234 process_soft_weak_final_refs(is_alive, keep_alive, complete_gc, task_executor, phase_times);
235 }
236
237 {
238 RefProcTotalPhaseTimesTracker tt(RefPhase3, phase_times, this);
239 process_final_keep_alive(keep_alive, complete_gc, task_executor, phase_times);
240 }
241
242 {
243 RefProcTotalPhaseTimesTracker tt(RefPhase4, phase_times, this);
244 process_phantom_refs(is_alive, keep_alive, complete_gc, task_executor, phase_times);
245 }
246
247 if (task_executor != NULL) {
248 // Record the work done by the parallel workers.
249 task_executor->set_single_threaded_mode();
250 }
251
252 phase_times->set_total_time_ms((os::elapsedTime() - start_time) * 1000);
253
254 return stats;
255 }
256
257 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
258 _current_discovered_addr = java_lang_ref_Reference::discovered_addr_raw(_current_discovered);
259 oop discovered = java_lang_ref_Reference::discovered(_current_discovered);
260 assert(_current_discovered_addr && oopDesc::is_oop_or_null(discovered),
261 "Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered));
262 _next_discovered = discovered;
263
264 _referent_addr = java_lang_ref_Reference::referent_addr_raw(_current_discovered);
265 _referent = java_lang_ref_Reference::referent(_current_discovered);
266 assert(Universe::heap()->is_in_or_null(_referent),
267 "Wrong oop found in java.lang.Reference object");
268 assert(allow_null_referent ?
269 oopDesc::is_oop_or_null(_referent)
270 : oopDesc::is_oop(_referent),
271 "Expected an oop%s for referent field at " PTR_FORMAT,
272 (allow_null_referent ? " or NULL" : ""),
273 p2i(_referent));
274 }
275
276 void DiscoveredListIterator::remove() {
277 assert(oopDesc::is_oop(_current_discovered), "Dropping a bad reference");
278 RawAccess<>::oop_store(_current_discovered_addr, oop(NULL));
279
280 // First _prev_next ref actually points into DiscoveredList (gross).
281 oop new_next;
282 if (_next_discovered == _current_discovered) {
283 // At the end of the list, we should make _prev point to itself.
284 // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
285 // and _prev will be NULL.
286 new_next = _prev_discovered;
287 } else {
288 new_next = _next_discovered;
289 }
290 // Remove Reference object from discovered list. Note that G1 does not need a
291 // pre-barrier here because we know the Reference has already been found/marked,
292 // that's how it ended up in the discovered list in the first place.
293 RawAccess<>::oop_store(_prev_discovered_addr, new_next);
294 _removed++;
295 _refs_list.dec_length(1);
296 }
297
298 void DiscoveredListIterator::clear_referent() {
299 RawAccess<>::oop_store(_referent_addr, oop(NULL));
300 }
301
302 void DiscoveredListIterator::enqueue() {
303 HeapAccess<AS_NO_KEEPALIVE>::oop_store_at(_current_discovered,
304 java_lang_ref_Reference::discovered_offset,
305 _next_discovered);
306 }
307
308 void DiscoveredListIterator::complete_enqueue() {
309 if (_prev_discovered != NULL) {
310 // This is the last object.
311 // Swap refs_list into pending list and set obj's
312 // discovered to what we read from the pending list.
313 oop old = Universe::swap_reference_pending_list(_refs_list.head());
314 HeapAccess<AS_NO_KEEPALIVE>::oop_store_at(_prev_discovered, java_lang_ref_Reference::discovered_offset, old);
315 }
316 }
317
318 inline void log_dropped_ref(const DiscoveredListIterator& iter, const char* reason) {
319 if (log_develop_is_enabled(Trace, gc, ref)) {
320 ResourceMark rm;
321 log_develop_trace(gc, ref)("Dropping %s reference " PTR_FORMAT ": %s",
322 reason, p2i(iter.obj()),
323 iter.obj()->klass()->internal_name());
324 }
325 }
326
327 inline void log_enqueued_ref(const DiscoveredListIterator& iter, const char* reason) {
328 if (log_develop_is_enabled(Trace, gc, ref)) {
329 ResourceMark rm;
330 log_develop_trace(gc, ref)("Enqueue %s reference (" INTPTR_FORMAT ": %s)",
331 reason, p2i(iter.obj()), iter.obj()->klass()->internal_name());
332 }
333 assert(oopDesc::is_oop(iter.obj()), "Adding a bad reference");
334 }
335
336 size_t ReferenceProcessor::process_soft_ref_reconsider_work(DiscoveredList& refs_list,
337 ReferencePolicy* policy,
338 BoolObjectClosure* is_alive,
339 OopClosure* keep_alive,
340 VoidClosure* complete_gc) {
341 assert(policy != NULL, "Must have a non-NULL policy");
342 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
343 // Decide which softly reachable refs should be kept alive.
344 while (iter.has_next()) {
345 iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
346 bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
347 if (referent_is_dead &&
348 !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
349 log_dropped_ref(iter, "by policy");
350 // Remove Reference object from list
351 iter.remove();
352 // keep the referent around
353 iter.make_referent_alive();
354 iter.move_to_next();
355 } else {
356 iter.next();
357 }
358 }
359 // Close the reachable set
360 complete_gc->do_void();
361
362 log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " dead Refs out of " SIZE_FORMAT " discovered Refs by policy, from list " INTPTR_FORMAT,
363 iter.removed(), iter.processed(), p2i(&refs_list));
364 return iter.removed();
365 }
366
367 size_t ReferenceProcessor::process_soft_weak_final_refs_work(DiscoveredList& refs_list,
368 BoolObjectClosure* is_alive,
369 OopClosure* keep_alive,
370 bool do_enqueue_and_clear) {
371 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
372 while (iter.has_next()) {
373 iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
374 if (iter.referent() == NULL) {
375 // Reference has been cleared since discovery; only possible if
376 // discovery is not atomic (checked by load_ptrs). Remove
377 // reference from list.
378 log_dropped_ref(iter, "cleared");
379 iter.remove();
380 iter.move_to_next();
381 } else if (iter.is_referent_alive()) {
382 // The referent is reachable after all.
383 // Remove reference from list.
384 log_dropped_ref(iter, "reachable");
385 iter.remove();
386 // Update the referent pointer as necessary. Note that this
387 // should not entail any recursive marking because the
388 // referent must already have been traversed.
389 iter.make_referent_alive();
390 iter.move_to_next();
391 } else {
392 if (do_enqueue_and_clear) {
393 iter.clear_referent();
394 iter.enqueue();
395 log_enqueued_ref(iter, "cleared");
396 }
397 // Keep in discovered list
398 iter.next();
399 }
400 }
401 if (do_enqueue_and_clear) {
402 iter.complete_enqueue();
403 refs_list.clear();
404 }
405
406 log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT
407 " Refs in discovered list " INTPTR_FORMAT,
408 iter.removed(), iter.processed(), p2i(&refs_list));
409 return iter.removed();
410 }
411
412 size_t ReferenceProcessor::process_final_keep_alive_work(DiscoveredList& refs_list,
413 OopClosure* keep_alive,
414 VoidClosure* complete_gc) {
415 DiscoveredListIterator iter(refs_list, keep_alive, NULL);
416 while (iter.has_next()) {
417 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
418 // keep the referent and followers around
419 iter.make_referent_alive();
420
421 // Self-loop next, to mark the FinalReference not active.
422 assert(java_lang_ref_Reference::next(iter.obj()) == NULL, "enqueued FinalReference");
423 java_lang_ref_Reference::set_next_raw(iter.obj(), iter.obj());
424
425 iter.enqueue();
426 log_enqueued_ref(iter, "Final");
427 iter.next();
428 }
429 iter.complete_enqueue();
430 // Close the reachable set
431 complete_gc->do_void();
432 refs_list.clear();
433
434 assert(iter.removed() == 0, "This phase does not remove anything.");
435 return iter.removed();
436 }
437
438 size_t ReferenceProcessor::process_phantom_refs_work(DiscoveredList& refs_list,
439 BoolObjectClosure* is_alive,
440 OopClosure* keep_alive,
441 VoidClosure* complete_gc) {
442 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
443 while (iter.has_next()) {
444 iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
445
446 oop const referent = iter.referent();
447
448 if (referent == NULL || iter.is_referent_alive()) {
449 iter.make_referent_alive();
450 iter.remove();
451 iter.move_to_next();
452 } else {
453 iter.clear_referent();
454 iter.enqueue();
455 log_enqueued_ref(iter, "cleared Phantom");
456 iter.next();
457 }
458 }
459 iter.complete_enqueue();
460 // Close the reachable set; needed for collectors which keep_alive_closure do
461 // not immediately complete their work.
462 complete_gc->do_void();
463 refs_list.clear();
464
465 return iter.removed();
466 }
467
468 void
469 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
470 oop obj = NULL;
471 oop next = refs_list.head();
472 while (next != obj) {
473 obj = next;
474 next = java_lang_ref_Reference::discovered(obj);
475 java_lang_ref_Reference::set_discovered_raw(obj, NULL);
476 }
477 refs_list.clear();
478 }
479
480 void ReferenceProcessor::abandon_partial_discovery() {
481 // loop over the lists
482 for (uint i = 0; i < _max_num_queues * number_of_subclasses_of_ref(); i++) {
483 if ((i % _max_num_queues) == 0) {
484 log_develop_trace(gc, ref)("Abandoning %s discovered list", list_name(i));
485 }
486 clear_discovered_references(_discovered_refs[i]);
487 }
488 }
489
490 size_t ReferenceProcessor::total_reference_count(ReferenceType type) const {
491 DiscoveredList* list = NULL;
492
493 switch (type) {
494 case REF_SOFT:
495 list = _discoveredSoftRefs;
496 break;
497 case REF_WEAK:
498 list = _discoveredWeakRefs;
499 break;
500 case REF_FINAL:
501 list = _discoveredFinalRefs;
502 break;
503 case REF_PHANTOM:
504 list = _discoveredPhantomRefs;
505 break;
506 case REF_OTHER:
507 case REF_NONE:
508 default:
509 ShouldNotReachHere();
510 }
511 return total_count(list);
512 }
513
514 class RefProcPhase1Task : public AbstractRefProcTaskExecutor::ProcessTask {
515 public:
516 RefProcPhase1Task(ReferenceProcessor& ref_processor,
517 ReferenceProcessorPhaseTimes* phase_times,
518 ReferencePolicy* policy)
519 : ProcessTask(ref_processor, true /* marks_oops_alive */, phase_times),
520 _policy(policy) { }
521
522 virtual void work(uint worker_id,
523 BoolObjectClosure& is_alive,
524 OopClosure& keep_alive,
525 VoidClosure& complete_gc)
526 {
527 RefProcSubPhasesWorkerTimeTracker tt(ReferenceProcessor::SoftRefSubPhase1, _phase_times, worker_id);
528 size_t const removed = _ref_processor.process_soft_ref_reconsider_work(_ref_processor._discoveredSoftRefs[worker_id],
529 _policy,
530 &is_alive,
531 &keep_alive,
532 &complete_gc);
533 _phase_times->add_ref_cleared(REF_SOFT, removed);
534 }
535 private:
536 ReferencePolicy* _policy;
537 };
538
539 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
540 void run_phase2(uint worker_id,
541 DiscoveredList list[],
542 BoolObjectClosure& is_alive,
543 OopClosure& keep_alive,
544 bool do_enqueue_and_clear,
545 ReferenceType ref_type) {
546 size_t const removed = _ref_processor.process_soft_weak_final_refs_work(list[worker_id],
547 &is_alive,
548 &keep_alive,
549 do_enqueue_and_clear);
550 _phase_times->add_ref_cleared(ref_type, removed);
551 }
552
553 public:
554 RefProcPhase2Task(ReferenceProcessor& ref_processor,
555 ReferenceProcessorPhaseTimes* phase_times)
556 : ProcessTask(ref_processor, false /* marks_oops_alive */, phase_times) { }
557
558 virtual void work(uint worker_id,
559 BoolObjectClosure& is_alive,
560 OopClosure& keep_alive,
561 VoidClosure& complete_gc) {
562 RefProcWorkerTimeTracker t(_phase_times->phase2_worker_time_sec(), worker_id);
563 {
564 RefProcSubPhasesWorkerTimeTracker tt(ReferenceProcessor::SoftRefSubPhase2, _phase_times, worker_id);
565 run_phase2(worker_id, _ref_processor._discoveredSoftRefs, is_alive, keep_alive, true /* do_enqueue_and_clear */, REF_SOFT);
566 }
567 {
568 RefProcSubPhasesWorkerTimeTracker tt(ReferenceProcessor::WeakRefSubPhase2, _phase_times, worker_id);
569 run_phase2(worker_id, _ref_processor._discoveredWeakRefs, is_alive, keep_alive, true /* do_enqueue_and_clear */, REF_WEAK);
570 }
571 {
572 RefProcSubPhasesWorkerTimeTracker tt(ReferenceProcessor::FinalRefSubPhase2, _phase_times, worker_id);
573 run_phase2(worker_id, _ref_processor._discoveredFinalRefs, is_alive, keep_alive, false /* do_enqueue_and_clear */, REF_FINAL);
574 }
575 // Close the reachable set; needed for collectors which keep_alive_closure do
576 // not immediately complete their work.
577 complete_gc.do_void();
578 }
579 };
580
581 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
582 public:
583 RefProcPhase3Task(ReferenceProcessor& ref_processor,
584 ReferenceProcessorPhaseTimes* phase_times)
585 : ProcessTask(ref_processor, true /* marks_oops_alive */, phase_times) { }
586
587 virtual void work(uint worker_id,
588 BoolObjectClosure& is_alive,
589 OopClosure& keep_alive,
590 VoidClosure& complete_gc)
591 {
592 RefProcSubPhasesWorkerTimeTracker tt(ReferenceProcessor::FinalRefSubPhase3, _phase_times, worker_id);
593 _ref_processor.process_final_keep_alive_work(_ref_processor._discoveredFinalRefs[worker_id], &keep_alive, &complete_gc);
594 }
595 };
596
597 class RefProcPhase4Task: public AbstractRefProcTaskExecutor::ProcessTask {
598 public:
599 RefProcPhase4Task(ReferenceProcessor& ref_processor,
600 ReferenceProcessorPhaseTimes* phase_times)
601 : ProcessTask(ref_processor, false /* marks_oops_alive */, phase_times) { }
602
603 virtual void work(uint worker_id,
604 BoolObjectClosure& is_alive,
605 OopClosure& keep_alive,
606 VoidClosure& complete_gc)
607 {
608 RefProcSubPhasesWorkerTimeTracker tt(ReferenceProcessor::PhantomRefSubPhase4, _phase_times, worker_id);
609 size_t const removed = _ref_processor.process_phantom_refs_work(_ref_processor._discoveredPhantomRefs[worker_id],
610 &is_alive,
611 &keep_alive,
612 &complete_gc);
613 _phase_times->add_ref_cleared(REF_PHANTOM, removed);
614 }
615 };
616
617 void ReferenceProcessor::log_reflist(const char* prefix, DiscoveredList list[], uint num_active_queues) {
618 LogTarget(Trace, gc, ref) lt;
619
620 if (!lt.is_enabled()) {
621 return;
622 }
623
624 size_t total = 0;
625
626 LogStream ls(lt);
627 ls.print("%s", prefix);
628 for (uint i = 0; i < num_active_queues; i++) {
629 ls.print(SIZE_FORMAT " ", list[i].length());
630 total += list[i].length();
631 }
632 ls.print_cr("(" SIZE_FORMAT ")", total);
633 }
634
635 #ifndef PRODUCT
636 void ReferenceProcessor::log_reflist_counts(DiscoveredList ref_lists[], uint num_active_queues) {
637 if (!log_is_enabled(Trace, gc, ref)) {
638 return;
639 }
640
641 log_reflist("", ref_lists, num_active_queues);
642 #ifdef ASSERT
643 for (uint i = num_active_queues; i < _max_num_queues; i++) {
644 assert(ref_lists[i].length() == 0, SIZE_FORMAT " unexpected References in %u",
645 ref_lists[i].length(), i);
646 }
647 #endif
648 }
649 #endif
650
651 void ReferenceProcessor::set_active_mt_degree(uint v) {
652 _num_queues = v;
653 _next_id = 0;
654 }
655
656 bool ReferenceProcessor::need_balance_queues(DiscoveredList refs_lists[]) {
657 assert(_processing_is_mt, "why balance non-mt processing?");
658 // _num_queues is the processing degree. Only list entries up to
659 // _num_queues will be processed, so any non-empty lists beyond
660 // that must be redistributed to lists in that range. Even if not
661 // needed for that, balancing may be desirable to eliminate poor
662 // distribution of references among the lists.
663 if (ParallelRefProcBalancingEnabled) {
664 return true; // Configuration says do it.
665 } else {
666 // Configuration says don't balance, but if there are non-empty
667 // lists beyond the processing degree, then must ignore the
668 // configuration and balance anyway.
669 for (uint i = _num_queues; i < _max_num_queues; ++i) {
670 if (!refs_lists[i].is_empty()) {
671 return true; // Must balance despite configuration.
672 }
673 }
674 return false; // Safe to obey configuration and not balance.
675 }
676 }
677
678 void ReferenceProcessor::maybe_balance_queues(DiscoveredList refs_lists[]) {
679 assert(_processing_is_mt, "Should not call this otherwise");
680 if (need_balance_queues(refs_lists)) {
681 balance_queues(refs_lists);
682 }
683 }
684
685 // Balances reference queues.
686 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
687 // queues[0, 1, ..., _num_q-1] because only the first _num_q
688 // corresponding to the active workers will be processed.
689 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
690 {
691 // calculate total length
692 size_t total_refs = 0;
693 log_develop_trace(gc, ref)("Balance ref_lists ");
694
695 log_reflist_counts(ref_lists, _max_num_queues);
696
697 for (uint i = 0; i < _max_num_queues; ++i) {
698 total_refs += ref_lists[i].length();
699 }
700 size_t avg_refs = total_refs / _num_queues + 1;
701 uint to_idx = 0;
702 for (uint from_idx = 0; from_idx < _max_num_queues; from_idx++) {
703 bool move_all = false;
704 if (from_idx >= _num_queues) {
705 move_all = ref_lists[from_idx].length() > 0;
706 }
707 while ((ref_lists[from_idx].length() > avg_refs) ||
708 move_all) {
709 assert(to_idx < _num_queues, "Sanity Check!");
710 if (ref_lists[to_idx].length() < avg_refs) {
711 // move superfluous refs
712 size_t refs_to_move;
713 // Move all the Ref's if the from queue will not be processed.
714 if (move_all) {
715 refs_to_move = MIN2(ref_lists[from_idx].length(),
716 avg_refs - ref_lists[to_idx].length());
717 } else {
718 refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
719 avg_refs - ref_lists[to_idx].length());
720 }
721
722 assert(refs_to_move > 0, "otherwise the code below will fail");
723
724 oop move_head = ref_lists[from_idx].head();
725 oop move_tail = move_head;
726 oop new_head = move_head;
727 // find an element to split the list on
728 for (size_t j = 0; j < refs_to_move; ++j) {
729 move_tail = new_head;
730 new_head = java_lang_ref_Reference::discovered(new_head);
731 }
732
733 // Add the chain to the to list.
734 if (ref_lists[to_idx].head() == NULL) {
735 // to list is empty. Make a loop at the end.
736 java_lang_ref_Reference::set_discovered_raw(move_tail, move_tail);
737 } else {
738 java_lang_ref_Reference::set_discovered_raw(move_tail, ref_lists[to_idx].head());
739 }
740 ref_lists[to_idx].set_head(move_head);
741 ref_lists[to_idx].inc_length(refs_to_move);
742
743 // Remove the chain from the from list.
744 if (move_tail == new_head) {
745 // We found the end of the from list.
746 ref_lists[from_idx].set_head(NULL);
747 } else {
748 ref_lists[from_idx].set_head(new_head);
749 }
750 ref_lists[from_idx].dec_length(refs_to_move);
751 if (ref_lists[from_idx].length() == 0) {
752 break;
753 }
754 } else {
755 to_idx = (to_idx + 1) % _num_queues;
756 }
757 }
758 }
759 #ifdef ASSERT
760 log_reflist_counts(ref_lists, _num_queues);
761 size_t balanced_total_refs = 0;
762 for (uint i = 0; i < _num_queues; ++i) {
763 balanced_total_refs += ref_lists[i].length();
764 }
765 assert(total_refs == balanced_total_refs, "Balancing was incomplete");
766 #endif
767 }
768
769 bool ReferenceProcessor::is_mt_processing_set_up(AbstractRefProcTaskExecutor* task_executor) const {
770 return task_executor != NULL && _processing_is_mt;
771 }
772
773 void ReferenceProcessor::process_soft_ref_reconsider(BoolObjectClosure* is_alive,
774 OopClosure* keep_alive,
775 VoidClosure* complete_gc,
776 AbstractRefProcTaskExecutor* task_executor,
777 ReferenceProcessorPhaseTimes* phase_times) {
778 assert(!_processing_is_mt || task_executor != NULL, "Task executor must not be NULL when mt processing is set.");
779
780 size_t const num_soft_refs = total_count(_discoveredSoftRefs);
781 phase_times->set_ref_discovered(REF_SOFT, num_soft_refs);
782
783 phase_times->set_processing_is_mt(_processing_is_mt);
784
785 if (num_soft_refs == 0 || _current_soft_ref_policy == NULL) {
786 log_debug(gc, ref)("Skipped phase1 of Reference Processing due to unavailable references");
787 return;
788 }
789
790 RefProcMTDegreeAdjuster a(this, RefPhase1, num_soft_refs);
791
792 if (_processing_is_mt) {
793 RefProcBalanceQueuesTimeTracker tt(RefPhase1, phase_times);
794 maybe_balance_queues(_discoveredSoftRefs);
795 }
796
797 RefProcPhaseTimeTracker tt(RefPhase1, phase_times);
798
799 log_reflist("Phase1 Soft before", _discoveredSoftRefs, _max_num_queues);
800 if (_processing_is_mt) {
801 RefProcPhase1Task phase1(*this, phase_times, _current_soft_ref_policy);
802 task_executor->execute(phase1, num_queues());
803 } else {
804 size_t removed = 0;
805
806 RefProcSubPhasesWorkerTimeTracker tt2(SoftRefSubPhase1, phase_times, 0);
807 for (uint i = 0; i < _max_num_queues; i++) {
808 removed += process_soft_ref_reconsider_work(_discoveredSoftRefs[i], _current_soft_ref_policy,
809 is_alive, keep_alive, complete_gc);
810 }
811
812 phase_times->add_ref_cleared(REF_SOFT, removed);
813 }
814 log_reflist("Phase1 Soft after", _discoveredSoftRefs, _max_num_queues);
815 }
816
817 void ReferenceProcessor::process_soft_weak_final_refs(BoolObjectClosure* is_alive,
818 OopClosure* keep_alive,
819 VoidClosure* complete_gc,
820 AbstractRefProcTaskExecutor* task_executor,
821 ReferenceProcessorPhaseTimes* phase_times) {
822 assert(!_processing_is_mt || task_executor != NULL, "Task executor must not be NULL when mt processing is set.");
823
824 size_t const num_soft_refs = total_count(_discoveredSoftRefs);
825 size_t const num_weak_refs = total_count(_discoveredWeakRefs);
826 size_t const num_final_refs = total_count(_discoveredFinalRefs);
827 size_t const num_total_refs = num_soft_refs + num_weak_refs + num_final_refs;
828 phase_times->set_ref_discovered(REF_WEAK, num_weak_refs);
829 phase_times->set_ref_discovered(REF_FINAL, num_final_refs);
830
831 phase_times->set_processing_is_mt(_processing_is_mt);
832
833 if (num_total_refs == 0) {
834 log_debug(gc, ref)("Skipped phase2 of Reference Processing due to unavailable references");
835 return;
836 }
837
838 RefProcMTDegreeAdjuster a(this, RefPhase2, num_total_refs);
839
840 if (_processing_is_mt) {
841 RefProcBalanceQueuesTimeTracker tt(RefPhase2, phase_times);
842 maybe_balance_queues(_discoveredSoftRefs);
843 maybe_balance_queues(_discoveredWeakRefs);
844 maybe_balance_queues(_discoveredFinalRefs);
845 }
846
847 RefProcPhaseTimeTracker tt(RefPhase2, phase_times);
848
849 log_reflist("Phase2 Soft before", _discoveredSoftRefs, _max_num_queues);
850 log_reflist("Phase2 Weak before", _discoveredWeakRefs, _max_num_queues);
851 log_reflist("Phase2 Final before", _discoveredFinalRefs, _max_num_queues);
852 if (_processing_is_mt) {
853 RefProcPhase2Task phase2(*this, phase_times);
854 task_executor->execute(phase2, num_queues());
855 } else {
856 RefProcWorkerTimeTracker t(phase_times->phase2_worker_time_sec(), 0);
857 {
858 size_t removed = 0;
859
860 RefProcSubPhasesWorkerTimeTracker tt2(SoftRefSubPhase2, phase_times, 0);
861 for (uint i = 0; i < _max_num_queues; i++) {
862 removed += process_soft_weak_final_refs_work(_discoveredSoftRefs[i], is_alive, keep_alive, true /* do_enqueue */);
863 }
864
865 phase_times->add_ref_cleared(REF_SOFT, removed);
866 }
867 {
868 size_t removed = 0;
869
870 RefProcSubPhasesWorkerTimeTracker tt2(WeakRefSubPhase2, phase_times, 0);
871 for (uint i = 0; i < _max_num_queues; i++) {
872 removed += process_soft_weak_final_refs_work(_discoveredWeakRefs[i], is_alive, keep_alive, true /* do_enqueue */);
873 }
874
875 phase_times->add_ref_cleared(REF_WEAK, removed);
876 }
877 {
878 size_t removed = 0;
879
880 RefProcSubPhasesWorkerTimeTracker tt2(FinalRefSubPhase2, phase_times, 0);
881 for (uint i = 0; i < _max_num_queues; i++) {
882 removed += process_soft_weak_final_refs_work(_discoveredFinalRefs[i], is_alive, keep_alive, false /* do_enqueue */);
883 }
884
885 phase_times->add_ref_cleared(REF_FINAL, removed);
886 }
887 complete_gc->do_void();
888 }
889 verify_total_count_zero(_discoveredSoftRefs, "SoftReference");
890 verify_total_count_zero(_discoveredWeakRefs, "WeakReference");
891 log_reflist("Phase2 Final after", _discoveredFinalRefs, _max_num_queues);
892 }
893
894 void ReferenceProcessor::process_final_keep_alive(OopClosure* keep_alive,
895 VoidClosure* complete_gc,
896 AbstractRefProcTaskExecutor* task_executor,
897 ReferenceProcessorPhaseTimes* phase_times) {
898 assert(!_processing_is_mt || task_executor != NULL, "Task executor must not be NULL when mt processing is set.");
899
900 size_t const num_final_refs = total_count(_discoveredFinalRefs);
901
902 phase_times->set_processing_is_mt(_processing_is_mt);
903
904 if (num_final_refs == 0) {
905 log_debug(gc, ref)("Skipped phase3 of Reference Processing due to unavailable references");
906 return;
907 }
908
909 RefProcMTDegreeAdjuster a(this, RefPhase3, num_final_refs);
910
911 if (_processing_is_mt) {
912 RefProcBalanceQueuesTimeTracker tt(RefPhase3, phase_times);
913 maybe_balance_queues(_discoveredFinalRefs);
914 }
915
916 // Phase 3:
917 // . Traverse referents of final references and keep them and followers alive.
918 RefProcPhaseTimeTracker tt(RefPhase3, phase_times);
919
920 if (_processing_is_mt) {
921 RefProcPhase3Task phase3(*this, phase_times);
922 task_executor->execute(phase3, num_queues());
923 } else {
924 RefProcSubPhasesWorkerTimeTracker tt2(FinalRefSubPhase3, phase_times, 0);
925 for (uint i = 0; i < _max_num_queues; i++) {
926 process_final_keep_alive_work(_discoveredFinalRefs[i], keep_alive, complete_gc);
927 }
928 }
929 verify_total_count_zero(_discoveredFinalRefs, "FinalReference");
930 }
931
932 void ReferenceProcessor::process_phantom_refs(BoolObjectClosure* is_alive,
933 OopClosure* keep_alive,
934 VoidClosure* complete_gc,
935 AbstractRefProcTaskExecutor* task_executor,
936 ReferenceProcessorPhaseTimes* phase_times) {
937 assert(!_processing_is_mt || task_executor != NULL, "Task executor must not be NULL when mt processing is set.");
938
939 size_t const num_phantom_refs = total_count(_discoveredPhantomRefs);
940 phase_times->set_ref_discovered(REF_PHANTOM, num_phantom_refs);
941
942 phase_times->set_processing_is_mt(_processing_is_mt);
943
944 if (num_phantom_refs == 0) {
945 log_debug(gc, ref)("Skipped phase4 of Reference Processing due to unavailable references");
946 return;
947 }
948
949 RefProcMTDegreeAdjuster a(this, RefPhase4, num_phantom_refs);
950
951 if (_processing_is_mt) {
952 RefProcBalanceQueuesTimeTracker tt(RefPhase4, phase_times);
953 maybe_balance_queues(_discoveredPhantomRefs);
954 }
955
956 // Phase 4: Walk phantom references appropriately.
957 RefProcPhaseTimeTracker tt(RefPhase4, phase_times);
958
959 log_reflist("Phase4 Phantom before", _discoveredPhantomRefs, _max_num_queues);
960 if (_processing_is_mt) {
961 RefProcPhase4Task phase4(*this, phase_times);
962 task_executor->execute(phase4, num_queues());
963 } else {
964 size_t removed = 0;
965
966 RefProcSubPhasesWorkerTimeTracker tt(PhantomRefSubPhase4, phase_times, 0);
967 for (uint i = 0; i < _max_num_queues; i++) {
968 removed += process_phantom_refs_work(_discoveredPhantomRefs[i], is_alive, keep_alive, complete_gc);
969 }
970
971 phase_times->add_ref_cleared(REF_PHANTOM, removed);
972 }
973 verify_total_count_zero(_discoveredPhantomRefs, "PhantomReference");
974 }
975
976 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
977 uint id = 0;
978 // Determine the queue index to use for this object.
979 if (_discovery_is_mt) {
980 // During a multi-threaded discovery phase,
981 // each thread saves to its "own" list.
982 Thread* thr = Thread::current();
983 id = thr->as_Worker_thread()->id();
984 } else {
985 // single-threaded discovery, we save in round-robin
986 // fashion to each of the lists.
987 if (_processing_is_mt) {
988 id = next_id();
989 }
990 }
991 assert(id < _max_num_queues, "Id is out of bounds id %u and max id %u)", id, _max_num_queues);
992
993 // Get the discovered queue to which we will add
994 DiscoveredList* list = NULL;
995 switch (rt) {
996 case REF_OTHER:
997 // Unknown reference type, no special treatment
998 break;
999 case REF_SOFT:
1000 list = &_discoveredSoftRefs[id];
1001 break;
1002 case REF_WEAK:
1003 list = &_discoveredWeakRefs[id];
1004 break;
1005 case REF_FINAL:
1006 list = &_discoveredFinalRefs[id];
1007 break;
1008 case REF_PHANTOM:
1009 list = &_discoveredPhantomRefs[id];
1010 break;
1011 case REF_NONE:
1012 // we should not reach here if we are an InstanceRefKlass
1013 default:
1014 ShouldNotReachHere();
1015 }
1016 log_develop_trace(gc, ref)("Thread %d gets list " INTPTR_FORMAT, id, p2i(list));
1017 return list;
1018 }
1019
1020 inline void
1021 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
1022 oop obj,
1023 HeapWord* discovered_addr) {
1024 assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
1025 // First we must make sure this object is only enqueued once. CAS in a non null
1026 // discovered_addr.
1027 oop current_head = refs_list.head();
1028 // The last ref must have its discovered field pointing to itself.
1029 oop next_discovered = (current_head != NULL) ? current_head : obj;
1030
1031 oop retest = HeapAccess<AS_NO_KEEPALIVE>::oop_atomic_cmpxchg(discovered_addr, oop(NULL), next_discovered);
1032
1033 if (retest == NULL) {
1034 // This thread just won the right to enqueue the object.
1035 // We have separate lists for enqueueing, so no synchronization
1036 // is necessary.
1037 refs_list.set_head(obj);
1038 refs_list.inc_length(1);
1039
1040 log_develop_trace(gc, ref)("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
1041 p2i(obj), obj->klass()->internal_name());
1042 } else {
1043 // If retest was non NULL, another thread beat us to it:
1044 // The reference has already been discovered...
1045 log_develop_trace(gc, ref)("Already discovered reference (" INTPTR_FORMAT ": %s)",
1046 p2i(obj), obj->klass()->internal_name());
1047 }
1048 }
1049
1050 #ifndef PRODUCT
1051 // Non-atomic (i.e. concurrent) discovery might allow us
1052 // to observe j.l.References with NULL referents, being those
1053 // cleared concurrently by mutators during (or after) discovery.
1054 void ReferenceProcessor::verify_referent(oop obj) {
1055 bool da = discovery_is_atomic();
1056 oop referent = java_lang_ref_Reference::referent(obj);
1057 assert(da ? oopDesc::is_oop(referent) : oopDesc::is_oop_or_null(referent),
1058 "Bad referent " INTPTR_FORMAT " found in Reference "
1059 INTPTR_FORMAT " during %satomic discovery ",
1060 p2i(referent), p2i(obj), da ? "" : "non-");
1061 }
1062 #endif
1063
1064 bool ReferenceProcessor::is_subject_to_discovery(oop const obj) const {
1065 return _is_subject_to_discovery->do_object_b(obj);
1066 }
1067
1068 // We mention two of several possible choices here:
1069 // #0: if the reference object is not in the "originating generation"
1070 // (or part of the heap being collected, indicated by our "span"
1071 // we don't treat it specially (i.e. we scan it as we would
1072 // a normal oop, treating its references as strong references).
1073 // This means that references can't be discovered unless their
1074 // referent is also in the same span. This is the simplest,
1075 // most "local" and most conservative approach, albeit one
1076 // that may cause weak references to be enqueued least promptly.
1077 // We call this choice the "ReferenceBasedDiscovery" policy.
1078 // #1: the reference object may be in any generation (span), but if
1079 // the referent is in the generation (span) being currently collected
1080 // then we can discover the reference object, provided
1081 // the object has not already been discovered by
1082 // a different concurrently running collector (as may be the
1083 // case, for instance, if the reference object is in CMS and
1084 // the referent in DefNewGeneration), and provided the processing
1085 // of this reference object by the current collector will
1086 // appear atomic to every other collector in the system.
1087 // (Thus, for instance, a concurrent collector may not
1088 // discover references in other generations even if the
1089 // referent is in its own generation). This policy may,
1090 // in certain cases, enqueue references somewhat sooner than
1091 // might Policy #0 above, but at marginally increased cost
1092 // and complexity in processing these references.
1093 // We call this choice the "RefeferentBasedDiscovery" policy.
1094 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1095 // Make sure we are discovering refs (rather than processing discovered refs).
1096 if (!_discovering_refs || !RegisterReferences) {
1097 return false;
1098 }
1099
1100 if ((rt == REF_FINAL) && (java_lang_ref_Reference::next(obj) != NULL)) {
1101 // Don't rediscover non-active FinalReferences.
1102 return false;
1103 }
1104
1105 if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1106 !is_subject_to_discovery(obj)) {
1107 // Reference is not in the originating generation;
1108 // don't treat it specially (i.e. we want to scan it as a normal
1109 // object with strong references).
1110 return false;
1111 }
1112
1113 // We only discover references whose referents are not (yet)
1114 // known to be strongly reachable.
1115 if (is_alive_non_header() != NULL) {
1116 verify_referent(obj);
1117 if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
1118 return false; // referent is reachable
1119 }
1120 }
1121 if (rt == REF_SOFT) {
1122 // For soft refs we can decide now if these are not
1123 // current candidates for clearing, in which case we
1124 // can mark through them now, rather than delaying that
1125 // to the reference-processing phase. Since all current
1126 // time-stamp policies advance the soft-ref clock only
1127 // at a full collection cycle, this is always currently
1128 // accurate.
1129 if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
1130 return false;
1131 }
1132 }
1133
1134 ResourceMark rm; // Needed for tracing.
1135
1136 HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr_raw(obj);
1137 const oop discovered = java_lang_ref_Reference::discovered(obj);
1138 assert(oopDesc::is_oop_or_null(discovered), "Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered));
1139 if (discovered != NULL) {
1140 // The reference has already been discovered...
1141 log_develop_trace(gc, ref)("Already discovered reference (" INTPTR_FORMAT ": %s)",
1142 p2i(obj), obj->klass()->internal_name());
1143 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1144 // assumes that an object is not processed twice;
1145 // if it's been already discovered it must be on another
1146 // generation's discovered list; so we won't discover it.
1147 return false;
1148 } else {
1149 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1150 "Unrecognized policy");
1151 // Check assumption that an object is not potentially
1152 // discovered twice except by concurrent collectors that potentially
1153 // trace the same Reference object twice.
1154 assert(UseG1GC || UseShenandoahGC,
1155 "Only possible with a concurrent marking collector");
1156 return true;
1157 }
1158 }
1159
1160 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1161 verify_referent(obj);
1162 // Discover if and only if EITHER:
1163 // .. reference is in our span, OR
1164 // .. we are an atomic collector and referent is in our span
1165 if (is_subject_to_discovery(obj) ||
1166 (discovery_is_atomic() &&
1167 is_subject_to_discovery(java_lang_ref_Reference::referent(obj)))) {
1168 } else {
1169 return false;
1170 }
1171 } else {
1172 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1173 is_subject_to_discovery(obj), "code inconsistency");
1174 }
1175
1176 // Get the right type of discovered queue head.
1177 DiscoveredList* list = get_discovered_list(rt);
1178 if (list == NULL) {
1179 return false; // nothing special needs to be done
1180 }
1181
1182 if (_discovery_is_mt) {
1183 add_to_discovered_list_mt(*list, obj, discovered_addr);
1184 } else {
1185 // We do a raw store here: the field will be visited later when processing
1186 // the discovered references.
1187 oop current_head = list->head();
1188 // The last ref must have its discovered field pointing to itself.
1189 oop next_discovered = (current_head != NULL) ? current_head : obj;
1190
1191 assert(discovered == NULL, "control point invariant");
1192 RawAccess<>::oop_store(discovered_addr, next_discovered);
1193 list->set_head(obj);
1194 list->inc_length(1);
1195
1196 log_develop_trace(gc, ref)("Discovered reference (" INTPTR_FORMAT ": %s)", p2i(obj), obj->klass()->internal_name());
1197 }
1198 assert(oopDesc::is_oop(obj), "Discovered a bad reference");
1199 verify_referent(obj);
1200 return true;
1201 }
1202
1203 bool ReferenceProcessor::has_discovered_references() {
1204 for (uint i = 0; i < _max_num_queues * number_of_subclasses_of_ref(); i++) {
1205 if (!_discovered_refs[i].is_empty()) {
1206 return true;
1207 }
1208 }
1209 return false;
1210 }
1211
1212 void ReferenceProcessor::preclean_discovered_references(BoolObjectClosure* is_alive,
1213 OopClosure* keep_alive,
1214 VoidClosure* complete_gc,
1215 YieldClosure* yield,
1216 GCTimer* gc_timer) {
1217 // These lists can be handled here in any order and, indeed, concurrently.
1218
1219 // Soft references
1220 {
1221 GCTraceTime(Debug, gc, ref) tm("Preclean SoftReferences", gc_timer);
1222 log_reflist("SoftRef before: ", _discoveredSoftRefs, _max_num_queues);
1223 for (uint i = 0; i < _max_num_queues; i++) {
1224 if (yield->should_return()) {
1225 return;
1226 }
1227 if (preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1228 keep_alive, complete_gc, yield)) {
1229 log_reflist("SoftRef abort: ", _discoveredSoftRefs, _max_num_queues);
1230 return;
1231 }
1232 }
1233 log_reflist("SoftRef after: ", _discoveredSoftRefs, _max_num_queues);
1234 }
1235
1236 // Weak references
1237 {
1238 GCTraceTime(Debug, gc, ref) tm("Preclean WeakReferences", gc_timer);
1239 log_reflist("WeakRef before: ", _discoveredWeakRefs, _max_num_queues);
1240 for (uint i = 0; i < _max_num_queues; i++) {
1241 if (yield->should_return()) {
1242 return;
1243 }
1244 if (preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1245 keep_alive, complete_gc, yield)) {
1246 log_reflist("WeakRef abort: ", _discoveredWeakRefs, _max_num_queues);
1247 return;
1248 }
1249 }
1250 log_reflist("WeakRef after: ", _discoveredWeakRefs, _max_num_queues);
1251 }
1252
1253 // Final references
1254 {
1255 GCTraceTime(Debug, gc, ref) tm("Preclean FinalReferences", gc_timer);
1256 log_reflist("FinalRef before: ", _discoveredFinalRefs, _max_num_queues);
1257 for (uint i = 0; i < _max_num_queues; i++) {
1258 if (yield->should_return()) {
1259 return;
1260 }
1261 if (preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1262 keep_alive, complete_gc, yield)) {
1263 log_reflist("FinalRef abort: ", _discoveredFinalRefs, _max_num_queues);
1264 return;
1265 }
1266 }
1267 log_reflist("FinalRef after: ", _discoveredFinalRefs, _max_num_queues);
1268 }
1269
1270 // Phantom references
1271 {
1272 GCTraceTime(Debug, gc, ref) tm("Preclean PhantomReferences", gc_timer);
1273 log_reflist("PhantomRef before: ", _discoveredPhantomRefs, _max_num_queues);
1274 for (uint i = 0; i < _max_num_queues; i++) {
1275 if (yield->should_return()) {
1276 return;
1277 }
1278 if (preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1279 keep_alive, complete_gc, yield)) {
1280 log_reflist("PhantomRef abort: ", _discoveredPhantomRefs, _max_num_queues);
1281 return;
1282 }
1283 }
1284 log_reflist("PhantomRef after: ", _discoveredPhantomRefs, _max_num_queues);
1285 }
1286 }
1287
1288 // Walk the given discovered ref list, and remove all reference objects
1289 // whose referents are still alive, whose referents are NULL or which
1290 // are not active (have a non-NULL next field). NOTE: When we are
1291 // thus precleaning the ref lists (which happens single-threaded today),
1292 // we do not disable refs discovery to honor the correct semantics of
1293 // java.lang.Reference. As a result, we need to be careful below
1294 // that ref removal steps interleave safely with ref discovery steps
1295 // (in this thread).
1296 bool ReferenceProcessor::preclean_discovered_reflist(DiscoveredList& refs_list,
1297 BoolObjectClosure* is_alive,
1298 OopClosure* keep_alive,
1299 VoidClosure* complete_gc,
1300 YieldClosure* yield) {
1301 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1302 while (iter.has_next()) {
1303 if (yield->should_return_fine_grain()) {
1304 return true;
1305 }
1306 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1307 if (iter.referent() == NULL || iter.is_referent_alive()) {
1308 // The referent has been cleared, or is alive; we need to trace
1309 // and mark its cohort.
1310 log_develop_trace(gc, ref)("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1311 p2i(iter.obj()), iter.obj()->klass()->internal_name());
1312 // Remove Reference object from list
1313 iter.remove();
1314 // Keep alive its cohort.
1315 iter.make_referent_alive();
1316 iter.move_to_next();
1317 } else {
1318 iter.next();
1319 }
1320 }
1321 // Close the reachable set
1322 complete_gc->do_void();
1323
1324 if (iter.processed() > 0) {
1325 log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " Refs out of " SIZE_FORMAT " Refs in discovered list " INTPTR_FORMAT,
1326 iter.removed(), iter.processed(), p2i(&refs_list));
1327 }
1328 return false;
1329 }
1330
1331 const char* ReferenceProcessor::list_name(uint i) {
1332 assert(i <= _max_num_queues * number_of_subclasses_of_ref(),
1333 "Out of bounds index");
1334
1335 int j = i / _max_num_queues;
1336 switch (j) {
1337 case 0: return "SoftRef";
1338 case 1: return "WeakRef";
1339 case 2: return "FinalRef";
1340 case 3: return "PhantomRef";
1341 }
1342 ShouldNotReachHere();
1343 return NULL;
1344 }
1345
1346 uint RefProcMTDegreeAdjuster::ergo_proc_thread_count(size_t ref_count,
1347 uint max_threads,
1348 RefProcPhases phase) const {
1349 assert(0 < max_threads, "must allow at least one thread");
1350
1351 if (use_max_threads(phase) || (ReferencesPerThread == 0)) {
1352 return max_threads;
1353 }
1354
1355 size_t thread_count = 1 + (ref_count / ReferencesPerThread);
1356 return (uint)MIN3(thread_count,
1357 static_cast<size_t>(max_threads),
1358 (size_t)os::active_processor_count());
1359 }
1360
1361 bool RefProcMTDegreeAdjuster::use_max_threads(RefProcPhases phase) const {
1362 // Even a small number of references in either of those cases could produce large amounts of work.
1363 return (phase == ReferenceProcessor::RefPhase1 || phase == ReferenceProcessor::RefPhase3);
1364 }
1365
1366 RefProcMTDegreeAdjuster::RefProcMTDegreeAdjuster(ReferenceProcessor* rp,
1367 RefProcPhases phase,
1368 size_t ref_count):
1369 _rp(rp),
1370 _saved_mt_processing(_rp->processing_is_mt()),
1371 _saved_num_queues(_rp->num_queues()) {
1372 if (!_rp->processing_is_mt() || !_rp->adjust_no_of_processing_threads() || (ReferencesPerThread == 0)) {
1373 return;
1374 }
1375
1376 uint workers = ergo_proc_thread_count(ref_count, _rp->num_queues(), phase);
1377
1378 _rp->set_mt_processing(workers > 1);
1379 _rp->set_active_mt_degree(workers);
1380 }
1381
1382 RefProcMTDegreeAdjuster::~RefProcMTDegreeAdjuster() {
1383 // Revert to previous status.
1384 _rp->set_mt_processing(_saved_mt_processing);
1385 _rp->set_active_mt_degree(_saved_num_queues);
1386 }