1 /*
2 * Copyright (c) 2001, 2015, 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.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.hpp"
32 #include "gc/shared/referencePolicy.hpp"
33 #include "gc/shared/referenceProcessor.hpp"
34 #include "oops/oop.inline.hpp"
35 #include "runtime/java.hpp"
36 #include "runtime/jniHandles.hpp"
37
38 ReferencePolicy* ReferenceProcessor::_always_clear_soft_ref_policy = NULL;
39 ReferencePolicy* ReferenceProcessor::_default_soft_ref_policy = NULL;
40 jlong ReferenceProcessor::_soft_ref_timestamp_clock = 0;
41
42 void referenceProcessor_init() {
43 ReferenceProcessor::init_statics();
44 }
45
46 void ReferenceProcessor::init_statics() {
47 // We need a monotonically non-decreasing time in ms but
48 // os::javaTimeMillis() does not guarantee monotonicity.
49 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
50
51 // Initialize the soft ref timestamp clock.
52 _soft_ref_timestamp_clock = now;
53 // Also update the soft ref clock in j.l.r.SoftReference
54 java_lang_ref_SoftReference::set_clock(_soft_ref_timestamp_clock);
55
56 _always_clear_soft_ref_policy = new AlwaysClearPolicy();
57 _default_soft_ref_policy = new COMPILER2_PRESENT(LRUMaxHeapPolicy())
58 NOT_COMPILER2(LRUCurrentHeapPolicy());
59 if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) {
60 vm_exit_during_initialization("Could not allocate reference policy object");
61 }
62 guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||
63 RefDiscoveryPolicy == ReferentBasedDiscovery,
64 "Unrecognized RefDiscoveryPolicy");
65 }
66
67 void ReferenceProcessor::enable_discovery(bool check_no_refs) {
68 #ifdef ASSERT
69 // Verify that we're not currently discovering refs
70 assert(!_discovering_refs, "nested call?");
71
72 if (check_no_refs) {
73 // Verify that the discovered lists are empty
74 verify_no_references_recorded();
75 }
76 #endif // ASSERT
77
78 // Someone could have modified the value of the static
79 // field in the j.l.r.SoftReference class that holds the
80 // soft reference timestamp clock using reflection or
81 // Unsafe between GCs. Unconditionally update the static
82 // field in ReferenceProcessor here so that we use the new
83 // value during reference discovery.
84
85 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
86 _discovering_refs = true;
87 }
88
89 ReferenceProcessor::ReferenceProcessor(MemRegion span,
90 bool mt_processing,
91 uint mt_processing_degree,
92 bool mt_discovery,
93 uint mt_discovery_degree,
94 bool atomic_discovery,
95 BoolObjectClosure* is_alive_non_header) :
96 _discovering_refs(false),
97 _enqueuing_is_done(false),
98 _is_alive_non_header(is_alive_non_header),
99 _processing_is_mt(mt_processing),
100 _next_id(0)
101 {
102 _span = span;
103 _discovery_is_atomic = atomic_discovery;
104 _discovery_is_mt = mt_discovery;
105 _num_q = MAX2(1U, mt_processing_degree);
106 _max_num_q = MAX2(_num_q, mt_discovery_degree);
107 _discovered_refs = NEW_C_HEAP_ARRAY(DiscoveredList,
108 _max_num_q * number_of_subclasses_of_ref(), mtGC);
109
110 if (_discovered_refs == NULL) {
111 vm_exit_during_initialization("Could not allocated RefProc Array");
112 }
113 _discoveredSoftRefs = &_discovered_refs[0];
114 _discoveredWeakRefs = &_discoveredSoftRefs[_max_num_q];
115 _discoveredFinalRefs = &_discoveredWeakRefs[_max_num_q];
116 _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_q];
117 _discoveredCleanerRefs = &_discoveredPhantomRefs[_max_num_q];
118
119 // Initialize all entries to NULL
120 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
121 _discovered_refs[i].set_head(NULL);
122 _discovered_refs[i].set_length(0);
123 }
124
125 setup_policy(false /* default soft ref policy */);
126 }
127
128 #ifndef PRODUCT
129 void ReferenceProcessor::verify_no_references_recorded() {
130 guarantee(!_discovering_refs, "Discovering refs?");
131 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
132 guarantee(_discovered_refs[i].is_empty(),
133 "Found non-empty discovered list");
134 }
135 }
136 #endif
137
138 void ReferenceProcessor::weak_oops_do(OopClosure* f) {
139 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
140 if (UseCompressedOops) {
141 f->do_oop((narrowOop*)_discovered_refs[i].adr_head());
142 } else {
143 f->do_oop((oop*)_discovered_refs[i].adr_head());
144 }
145 }
146 }
147
148 void ReferenceProcessor::update_soft_ref_master_clock() {
149 // Update (advance) the soft ref master clock field. This must be done
150 // after processing the soft ref list.
151
152 // We need a monotonically non-decreasing time in ms but
153 // os::javaTimeMillis() does not guarantee monotonicity.
154 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
155 jlong soft_ref_clock = java_lang_ref_SoftReference::clock();
156 assert(soft_ref_clock == _soft_ref_timestamp_clock, "soft ref clocks out of sync");
157
158 NOT_PRODUCT(
159 if (now < _soft_ref_timestamp_clock) {
160 warning("time warp: " JLONG_FORMAT " to " JLONG_FORMAT,
161 _soft_ref_timestamp_clock, now);
162 }
163 )
164 // The values of now and _soft_ref_timestamp_clock are set using
165 // javaTimeNanos(), which is guaranteed to be monotonically
166 // non-decreasing provided the underlying platform provides such
167 // a time source (and it is bug free).
168 // In product mode, however, protect ourselves from non-monotonicity.
169 if (now > _soft_ref_timestamp_clock) {
170 _soft_ref_timestamp_clock = now;
171 java_lang_ref_SoftReference::set_clock(now);
172 }
173 // Else leave clock stalled at its old value until time progresses
174 // past clock value.
175 }
176
177 size_t ReferenceProcessor::total_count(DiscoveredList lists[]) {
178 size_t total = 0;
179 for (uint i = 0; i < _max_num_q; ++i) {
180 total += lists[i].length();
181 }
182 return total;
183 }
184
185 ReferenceProcessorStats ReferenceProcessor::process_discovered_references(
186 BoolObjectClosure* is_alive,
187 OopClosure* keep_alive,
188 VoidClosure* complete_gc,
189 AbstractRefProcTaskExecutor* task_executor,
190 GCTimer* gc_timer) {
191
192 assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
193 // Stop treating discovered references specially.
194 disable_discovery();
195
196 // If discovery was concurrent, someone could have modified
197 // the value of the static field in the j.l.r.SoftReference
198 // class that holds the soft reference timestamp clock using
199 // reflection or Unsafe between when discovery was enabled and
200 // now. Unconditionally update the static field in ReferenceProcessor
201 // here so that we use the new value during processing of the
202 // discovered soft refs.
203
204 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
205
206 bool trace_time = PrintGCDetails && PrintReferenceGC;
207
208 // Soft references
209 size_t soft_count = 0;
210 {
211 GCTraceTime tt("SoftReference", trace_time, false, gc_timer);
212 soft_count =
213 process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
214 is_alive, keep_alive, complete_gc, task_executor);
215 }
216
217 update_soft_ref_master_clock();
218
219 // Weak references
220 size_t weak_count = 0;
221 {
222 GCTraceTime tt("WeakReference", trace_time, false, gc_timer);
223 weak_count =
224 process_discovered_reflist(_discoveredWeakRefs, NULL, true,
225 is_alive, keep_alive, complete_gc, task_executor);
226 }
227
228 // Final references
229 size_t final_count = 0;
230 {
231 GCTraceTime tt("FinalReference", trace_time, false, gc_timer);
232 final_count =
233 process_discovered_reflist(_discoveredFinalRefs, NULL, false,
234 is_alive, keep_alive, complete_gc, task_executor);
235 }
236
237 // Phantom references
238 size_t phantom_count = 0;
239 {
240 GCTraceTime tt("PhantomReference", trace_time, false, gc_timer);
241 phantom_count =
242 process_discovered_reflist(_discoveredPhantomRefs, NULL, false,
243 is_alive, keep_alive, complete_gc, task_executor);
244
245 }
246
247 // Cleaners
248 size_t cleaner_count = 0;
249 {
250 GCTraceTime tt("Cleaners", trace_time, false, gc_timer, gc_id);
251 cleaner_count =
252 process_discovered_reflist(_discoveredCleanerRefs, NULL, true,
253 is_alive, keep_alive, complete_gc, task_executor);
254 }
255
256 // Weak global JNI references. It would make more sense (semantically) to
257 // traverse these simultaneously with the regular weak references above, but
258 // that is not how the JDK1.2 specification is. See #4126360. Native code can
259 // thus use JNI weak references to circumvent the phantom references and
260 // resurrect a "post-mortem" object.
261 size_t jni_weak_ref_count = 0;
262 {
263 GCTraceTime tt("JNI Weak Reference", trace_time, false, gc_timer);
264 if (task_executor != NULL) {
265 task_executor->set_single_threaded_mode();
266 }
267 jni_weak_ref_count =
268 process_phaseJNI(is_alive, keep_alive, complete_gc);
269 }
270
271 return ReferenceProcessorStats(soft_count, weak_count, final_count, phantom_count, cleaner_count, jni_weak_ref_count);
272 }
273
274 #ifndef PRODUCT
275 // Calculate the number of jni handles.
276 uint ReferenceProcessor::count_jni_refs() {
277 class AlwaysAliveClosure: public BoolObjectClosure {
278 public:
279 virtual bool do_object_b(oop obj) { return true; }
280 };
281
282 class CountHandleClosure: public OopClosure {
283 private:
284 int _count;
285 public:
286 CountHandleClosure(): _count(0) {}
287 void do_oop(oop* unused) { _count++; }
288 void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
289 int count() { return _count; }
290 };
291 CountHandleClosure global_handle_count;
292 AlwaysAliveClosure always_alive;
293 JNIHandles::weak_oops_do(&always_alive, &global_handle_count);
294 return global_handle_count.count();
295 }
296 #endif
297
298 size_t ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,
299 OopClosure* keep_alive,
300 VoidClosure* complete_gc) {
301 DEBUG_ONLY(size_t check_count = count_jni_refs();)
302 size_t count = JNIHandles::weak_oops_do(is_alive, keep_alive);
303 assert(count == check_count, "Counts didn't match");
304 complete_gc->do_void();
305 if (PrintGCDetails && PrintReferenceGC) {
306 gclog_or_tty->print(", " SIZE_FORMAT " refs", count);
307 }
308 return count;
309 }
310
311
312 template <class T>
313 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref,
314 AbstractRefProcTaskExecutor* task_executor) {
315
316 // Remember old value of pending references list
317 T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr();
318 T old_pending_list_value = *pending_list_addr;
319
320 // Enqueue references that are not made active again, and
321 // clear the decks for the next collection (cycle).
322 ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor);
323 // Do the post-barrier on pending_list_addr missed in
324 // enqueue_discovered_reflist.
325 oopDesc::bs()->write_ref_field(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr));
326
327 // Stop treating discovered references specially.
328 ref->disable_discovery();
329
330 // Return true if new pending references were added
331 return old_pending_list_value != *pending_list_addr;
332 }
333
334 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) {
335 if (UseCompressedOops) {
336 return enqueue_discovered_ref_helper<narrowOop>(this, task_executor);
337 } else {
338 return enqueue_discovered_ref_helper<oop>(this, task_executor);
339 }
340 }
341
342 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,
343 HeapWord* pending_list_addr) {
344 // Given a list of refs linked through the "discovered" field
345 // (java.lang.ref.Reference.discovered), self-loop their "next" field
346 // thus distinguishing them from active References, then
347 // prepend them to the pending list.
348 //
349 // The Java threads will see the Reference objects linked together through
350 // the discovered field. Instead of trying to do the write barrier updates
351 // in all places in the reference processor where we manipulate the discovered
352 // field we make sure to do the barrier here where we anyway iterate through
353 // all linked Reference objects. Note that it is important to not dirty any
354 // cards during reference processing since this will cause card table
355 // verification to fail for G1.
356 if (TraceReferenceGC && PrintGCDetails) {
357 gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list "
358 INTPTR_FORMAT, p2i(refs_list.head()));
359 }
360
361 oop obj = NULL;
362 oop next_d = refs_list.head();
363 // Walk down the list, self-looping the next field
364 // so that the References are not considered active.
365 while (obj != next_d) {
366 obj = next_d;
367 assert(obj->is_instanceRef(), "should be reference object");
368 next_d = java_lang_ref_Reference::discovered(obj);
369 if (TraceReferenceGC && PrintGCDetails) {
370 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
371 p2i(obj), p2i(next_d));
372 }
373 assert(java_lang_ref_Reference::next(obj) == NULL,
374 "Reference not active; should not be discovered");
375 // Self-loop next, so as to make Ref not active.
376 java_lang_ref_Reference::set_next_raw(obj, obj);
377 if (next_d != obj) {
378 oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), next_d);
379 } else {
380 // This is the last object.
381 // Swap refs_list into pending_list_addr and
382 // set obj's discovered to what we read from pending_list_addr.
383 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
384 // Need post-barrier on pending_list_addr. See enqueue_discovered_ref_helper() above.
385 java_lang_ref_Reference::set_discovered_raw(obj, old); // old may be NULL
386 oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), old);
387 }
388 }
389 }
390
391 // Parallel enqueue task
392 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
393 public:
394 RefProcEnqueueTask(ReferenceProcessor& ref_processor,
395 DiscoveredList discovered_refs[],
396 HeapWord* pending_list_addr,
397 int n_queues)
398 : EnqueueTask(ref_processor, discovered_refs,
399 pending_list_addr, n_queues)
400 { }
401
402 virtual void work(unsigned int work_id) {
403 assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds");
404 // Simplest first cut: static partitioning.
405 int index = work_id;
406 // The increment on "index" must correspond to the maximum number of queues
407 // (n_queues) with which that ReferenceProcessor was created. That
408 // is because of the "clever" way the discovered references lists were
409 // allocated and are indexed into.
410 assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected");
411 for (int j = 0;
412 j < ReferenceProcessor::number_of_subclasses_of_ref();
413 j++, index += _n_queues) {
414 _ref_processor.enqueue_discovered_reflist(
415 _refs_lists[index], _pending_list_addr);
416 _refs_lists[index].set_head(NULL);
417 _refs_lists[index].set_length(0);
418 }
419 }
420 };
421
422 // Enqueue references that are not made active again
423 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr,
424 AbstractRefProcTaskExecutor* task_executor) {
425 if (_processing_is_mt && task_executor != NULL) {
426 // Parallel code
427 RefProcEnqueueTask tsk(*this, _discovered_refs,
428 pending_list_addr, _max_num_q);
429 task_executor->execute(tsk);
430 } else {
431 // Serial code: call the parent class's implementation
432 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
433 enqueue_discovered_reflist(_discovered_refs[i], pending_list_addr);
434 _discovered_refs[i].set_head(NULL);
435 _discovered_refs[i].set_length(0);
436 }
437 }
438 }
439
440 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
441 _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);
442 oop discovered = java_lang_ref_Reference::discovered(_ref);
443 assert(_discovered_addr && discovered->is_oop_or_null(),
444 err_msg("Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered)));
445 _next = discovered;
446 _referent_addr = java_lang_ref_Reference::referent_addr(_ref);
447 _referent = java_lang_ref_Reference::referent(_ref);
448 assert(Universe::heap()->is_in_reserved_or_null(_referent),
449 "Wrong oop found in java.lang.Reference object");
450 assert(allow_null_referent ?
451 _referent->is_oop_or_null()
452 : _referent->is_oop(),
453 err_msg("Expected an oop%s for referent field at " PTR_FORMAT,
454 (allow_null_referent ? " or NULL" : ""),
455 p2i(_referent)));
456 }
457
458 void DiscoveredListIterator::remove() {
459 assert(_ref->is_oop(), "Dropping a bad reference");
460 oop_store_raw(_discovered_addr, NULL);
461
462 // First _prev_next ref actually points into DiscoveredList (gross).
463 oop new_next;
464 if (_next == _ref) {
465 // At the end of the list, we should make _prev point to itself.
466 // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
467 // and _prev will be NULL.
468 new_next = _prev;
469 } else {
470 new_next = _next;
471 }
472 // Remove Reference object from discovered list. Note that G1 does not need a
473 // pre-barrier here because we know the Reference has already been found/marked,
474 // that's how it ended up in the discovered list in the first place.
475 oop_store_raw(_prev_next, new_next);
476 NOT_PRODUCT(_removed++);
477 _refs_list.dec_length(1);
478 }
479
480 void DiscoveredListIterator::clear_referent() {
481 oop_store_raw(_referent_addr, NULL);
482 }
483
484 // NOTE: process_phase*() are largely similar, and at a high level
485 // merely iterate over the extant list applying a predicate to
486 // each of its elements and possibly removing that element from the
487 // list and applying some further closures to that element.
488 // We should consider the possibility of replacing these
489 // process_phase*() methods by abstracting them into
490 // a single general iterator invocation that receives appropriate
491 // closures that accomplish this work.
492
493 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
494 // referents are not alive, but that should be kept alive for policy reasons.
495 // Keep alive the transitive closure of all such referents.
496 void
497 ReferenceProcessor::process_phase1(DiscoveredList& refs_list,
498 ReferencePolicy* policy,
499 BoolObjectClosure* is_alive,
500 OopClosure* keep_alive,
501 VoidClosure* complete_gc) {
502 assert(policy != NULL, "Must have a non-NULL policy");
503 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
504 // Decide which softly reachable refs should be kept alive.
505 while (iter.has_next()) {
506 iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
507 bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
508 if (referent_is_dead &&
509 !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
510 if (TraceReferenceGC) {
511 gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s" ") by policy",
512 p2i(iter.obj()), iter.obj()->klass()->internal_name());
513 }
514 // Remove Reference object from list
515 iter.remove();
516 // keep the referent around
517 iter.make_referent_alive();
518 iter.move_to_next();
519 } else {
520 iter.next();
521 }
522 }
523 // Close the reachable set
524 complete_gc->do_void();
525 NOT_PRODUCT(
526 if (PrintGCDetails && TraceReferenceGC) {
527 gclog_or_tty->print_cr(" Dropped " SIZE_FORMAT " dead Refs out of " SIZE_FORMAT
528 " discovered Refs by policy, from list " INTPTR_FORMAT,
529 iter.removed(), iter.processed(), p2i(refs_list.head()));
530 }
531 )
532 }
533
534 // Traverse the list and remove any Refs that are not active, or
535 // whose referents are either alive or NULL.
536 void
537 ReferenceProcessor::pp2_work(DiscoveredList& refs_list,
538 BoolObjectClosure* is_alive,
539 OopClosure* keep_alive) {
540 assert(discovery_is_atomic(), "Error");
541 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
542 while (iter.has_next()) {
543 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
544 DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
545 assert(next == NULL, "Should not discover inactive Reference");
546 if (iter.is_referent_alive()) {
547 if (TraceReferenceGC) {
548 gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
549 p2i(iter.obj()), iter.obj()->klass()->internal_name());
550 }
551 // The referent is reachable after all.
552 // Remove Reference object from list.
553 iter.remove();
554 // Update the referent pointer as necessary: Note that this
555 // should not entail any recursive marking because the
556 // referent must already have been traversed.
557 iter.make_referent_alive();
558 iter.move_to_next();
559 } else {
560 iter.next();
561 }
562 }
563 NOT_PRODUCT(
564 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
565 gclog_or_tty->print_cr(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT
566 " Refs in discovered list " INTPTR_FORMAT,
567 iter.removed(), iter.processed(), p2i(refs_list.head()));
568 }
569 )
570 }
571
572 void
573 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList& refs_list,
574 BoolObjectClosure* is_alive,
575 OopClosure* keep_alive,
576 VoidClosure* complete_gc) {
577 assert(!discovery_is_atomic(), "Error");
578 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
579 while (iter.has_next()) {
580 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
581 HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
582 oop next = java_lang_ref_Reference::next(iter.obj());
583 if ((iter.referent() == NULL || iter.is_referent_alive() ||
584 next != NULL)) {
585 assert(next->is_oop_or_null(), err_msg("Expected an oop or NULL for next field at " PTR_FORMAT, p2i(next)));
586 // Remove Reference object from list
587 iter.remove();
588 // Trace the cohorts
589 iter.make_referent_alive();
590 if (UseCompressedOops) {
591 keep_alive->do_oop((narrowOop*)next_addr);
592 } else {
593 keep_alive->do_oop((oop*)next_addr);
594 }
595 iter.move_to_next();
596 } else {
597 iter.next();
598 }
599 }
600 // Now close the newly reachable set
601 complete_gc->do_void();
602 NOT_PRODUCT(
603 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
604 gclog_or_tty->print_cr(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT
605 " Refs in discovered list " INTPTR_FORMAT,
606 iter.removed(), iter.processed(), p2i(refs_list.head()));
607 }
608 )
609 }
610
611 // Traverse the list and process the referents, by either
612 // clearing them or keeping them (and their reachable
613 // closure) alive.
614 void
615 ReferenceProcessor::process_phase3(DiscoveredList& refs_list,
616 bool clear_referent,
617 BoolObjectClosure* is_alive,
618 OopClosure* keep_alive,
619 VoidClosure* complete_gc) {
620 ResourceMark rm;
621 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
622 while (iter.has_next()) {
623 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
624 if (clear_referent) {
625 // NULL out referent pointer
626 iter.clear_referent();
627 } else {
628 // keep the referent around
629 iter.make_referent_alive();
630 }
631 if (TraceReferenceGC) {
632 gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
633 clear_referent ? "cleared " : "",
634 p2i(iter.obj()), iter.obj()->klass()->internal_name());
635 }
636 assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");
637 iter.next();
638 }
639 // Close the reachable set
640 complete_gc->do_void();
641 }
642
643 void
644 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
645 oop obj = NULL;
646 oop next = refs_list.head();
647 while (next != obj) {
648 obj = next;
649 next = java_lang_ref_Reference::discovered(obj);
650 java_lang_ref_Reference::set_discovered_raw(obj, NULL);
651 }
652 refs_list.set_head(NULL);
653 refs_list.set_length(0);
654 }
655
656 void ReferenceProcessor::abandon_partial_discovery() {
657 // loop over the lists
658 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
659 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
660 gclog_or_tty->print_cr("\nAbandoning %s discovered list", list_name(i));
661 }
662 clear_discovered_references(_discovered_refs[i]);
663 }
664 }
665
666 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
667 public:
668 RefProcPhase1Task(ReferenceProcessor& ref_processor,
669 DiscoveredList refs_lists[],
670 ReferencePolicy* policy,
671 bool marks_oops_alive)
672 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
673 _policy(policy)
674 { }
675 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
676 OopClosure& keep_alive,
677 VoidClosure& complete_gc)
678 {
679 Thread* thr = Thread::current();
680 int refs_list_index = ((WorkerThread*)thr)->id();
681 _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy,
682 &is_alive, &keep_alive, &complete_gc);
683 }
684 private:
685 ReferencePolicy* _policy;
686 };
687
688 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
689 public:
690 RefProcPhase2Task(ReferenceProcessor& ref_processor,
691 DiscoveredList refs_lists[],
692 bool marks_oops_alive)
693 : ProcessTask(ref_processor, refs_lists, marks_oops_alive)
694 { }
695 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
696 OopClosure& keep_alive,
697 VoidClosure& complete_gc)
698 {
699 _ref_processor.process_phase2(_refs_lists[i],
700 &is_alive, &keep_alive, &complete_gc);
701 }
702 };
703
704 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
705 public:
706 RefProcPhase3Task(ReferenceProcessor& ref_processor,
707 DiscoveredList refs_lists[],
708 bool clear_referent,
709 bool marks_oops_alive)
710 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
711 _clear_referent(clear_referent)
712 { }
713 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
714 OopClosure& keep_alive,
715 VoidClosure& complete_gc)
716 {
717 // Don't use "refs_list_index" calculated in this way because
718 // balance_queues() has moved the Ref's into the first n queues.
719 // Thread* thr = Thread::current();
720 // int refs_list_index = ((WorkerThread*)thr)->id();
721 // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent,
722 _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
723 &is_alive, &keep_alive, &complete_gc);
724 }
725 private:
726 bool _clear_referent;
727 };
728
729 // Balances reference queues.
730 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
731 // queues[0, 1, ..., _num_q-1] because only the first _num_q
732 // corresponding to the active workers will be processed.
733 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
734 {
735 // calculate total length
736 size_t total_refs = 0;
737 if (TraceReferenceGC && PrintGCDetails) {
738 gclog_or_tty->print_cr("\nBalance ref_lists ");
739 }
740
741 for (uint i = 0; i < _max_num_q; ++i) {
742 total_refs += ref_lists[i].length();
743 if (TraceReferenceGC && PrintGCDetails) {
744 gclog_or_tty->print(SIZE_FORMAT " ", ref_lists[i].length());
745 }
746 }
747 if (TraceReferenceGC && PrintGCDetails) {
748 gclog_or_tty->print_cr(" = " SIZE_FORMAT, total_refs);
749 }
750 size_t avg_refs = total_refs / _num_q + 1;
751 uint to_idx = 0;
752 for (uint from_idx = 0; from_idx < _max_num_q; from_idx++) {
753 bool move_all = false;
754 if (from_idx >= _num_q) {
755 move_all = ref_lists[from_idx].length() > 0;
756 }
757 while ((ref_lists[from_idx].length() > avg_refs) ||
758 move_all) {
759 assert(to_idx < _num_q, "Sanity Check!");
760 if (ref_lists[to_idx].length() < avg_refs) {
761 // move superfluous refs
762 size_t refs_to_move;
763 // Move all the Ref's if the from queue will not be processed.
764 if (move_all) {
765 refs_to_move = MIN2(ref_lists[from_idx].length(),
766 avg_refs - ref_lists[to_idx].length());
767 } else {
768 refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
769 avg_refs - ref_lists[to_idx].length());
770 }
771
772 assert(refs_to_move > 0, "otherwise the code below will fail");
773
774 oop move_head = ref_lists[from_idx].head();
775 oop move_tail = move_head;
776 oop new_head = move_head;
777 // find an element to split the list on
778 for (size_t j = 0; j < refs_to_move; ++j) {
779 move_tail = new_head;
780 new_head = java_lang_ref_Reference::discovered(new_head);
781 }
782
783 // Add the chain to the to list.
784 if (ref_lists[to_idx].head() == NULL) {
785 // to list is empty. Make a loop at the end.
786 java_lang_ref_Reference::set_discovered_raw(move_tail, move_tail);
787 } else {
788 java_lang_ref_Reference::set_discovered_raw(move_tail, ref_lists[to_idx].head());
789 }
790 ref_lists[to_idx].set_head(move_head);
791 ref_lists[to_idx].inc_length(refs_to_move);
792
793 // Remove the chain from the from list.
794 if (move_tail == new_head) {
795 // We found the end of the from list.
796 ref_lists[from_idx].set_head(NULL);
797 } else {
798 ref_lists[from_idx].set_head(new_head);
799 }
800 ref_lists[from_idx].dec_length(refs_to_move);
801 if (ref_lists[from_idx].length() == 0) {
802 break;
803 }
804 } else {
805 to_idx = (to_idx + 1) % _num_q;
806 }
807 }
808 }
809 #ifdef ASSERT
810 size_t balanced_total_refs = 0;
811 for (uint i = 0; i < _max_num_q; ++i) {
812 balanced_total_refs += ref_lists[i].length();
813 if (TraceReferenceGC && PrintGCDetails) {
814 gclog_or_tty->print(SIZE_FORMAT " ", ref_lists[i].length());
815 }
816 }
817 if (TraceReferenceGC && PrintGCDetails) {
818 gclog_or_tty->print_cr(" = " SIZE_FORMAT, balanced_total_refs);
819 gclog_or_tty->flush();
820 }
821 assert(total_refs == balanced_total_refs, "Balancing was incomplete");
822 #endif
823 }
824
825 void ReferenceProcessor::balance_all_queues() {
826 balance_queues(_discoveredSoftRefs);
827 balance_queues(_discoveredWeakRefs);
828 balance_queues(_discoveredFinalRefs);
829 balance_queues(_discoveredPhantomRefs);
830 balance_queues(_discoveredCleanerRefs);
831 }
832
833 size_t
834 ReferenceProcessor::process_discovered_reflist(
835 DiscoveredList refs_lists[],
836 ReferencePolicy* policy,
837 bool clear_referent,
838 BoolObjectClosure* is_alive,
839 OopClosure* keep_alive,
840 VoidClosure* complete_gc,
841 AbstractRefProcTaskExecutor* task_executor)
842 {
843 bool mt_processing = task_executor != NULL && _processing_is_mt;
844 // If discovery used MT and a dynamic number of GC threads, then
845 // the queues must be balanced for correctness if fewer than the
846 // maximum number of queues were used. The number of queue used
847 // during discovery may be different than the number to be used
848 // for processing so don't depend of _num_q < _max_num_q as part
849 // of the test.
850 bool must_balance = _discovery_is_mt;
851
852 if ((mt_processing && ParallelRefProcBalancingEnabled) ||
853 must_balance) {
854 balance_queues(refs_lists);
855 }
856
857 size_t total_list_count = total_count(refs_lists);
858
859 if (PrintReferenceGC && PrintGCDetails) {
860 gclog_or_tty->print(", " SIZE_FORMAT " refs", total_list_count);
861 }
862
863 // Phase 1 (soft refs only):
864 // . Traverse the list and remove any SoftReferences whose
865 // referents are not alive, but that should be kept alive for
866 // policy reasons. Keep alive the transitive closure of all
867 // such referents.
868 if (policy != NULL) {
869 if (mt_processing) {
870 RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/);
871 task_executor->execute(phase1);
872 } else {
873 for (uint i = 0; i < _max_num_q; i++) {
874 process_phase1(refs_lists[i], policy,
875 is_alive, keep_alive, complete_gc);
876 }
877 }
878 } else { // policy == NULL
879 assert(refs_lists != _discoveredSoftRefs,
880 "Policy must be specified for soft references.");
881 }
882
883 // Phase 2:
884 // . Traverse the list and remove any refs whose referents are alive.
885 if (mt_processing) {
886 RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/);
887 task_executor->execute(phase2);
888 } else {
889 for (uint i = 0; i < _max_num_q; i++) {
890 process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
891 }
892 }
893
894 // Phase 3:
895 // . Traverse the list and process referents as appropriate.
896 if (mt_processing) {
897 RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/);
898 task_executor->execute(phase3);
899 } else {
900 for (uint i = 0; i < _max_num_q; i++) {
901 process_phase3(refs_lists[i], clear_referent,
902 is_alive, keep_alive, complete_gc);
903 }
904 }
905
906 return total_list_count;
907 }
908
909 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
910 uint id = 0;
911 // Determine the queue index to use for this object.
912 if (_discovery_is_mt) {
913 // During a multi-threaded discovery phase,
914 // each thread saves to its "own" list.
915 Thread* thr = Thread::current();
916 id = thr->as_Worker_thread()->id();
917 } else {
918 // single-threaded discovery, we save in round-robin
919 // fashion to each of the lists.
920 if (_processing_is_mt) {
921 id = next_id();
922 }
923 }
924 assert(id < _max_num_q, "Id is out-of-bounds (call Freud?)");
925
926 // Get the discovered queue to which we will add
927 DiscoveredList* list = NULL;
928 switch (rt) {
929 case REF_OTHER:
930 // Unknown reference type, no special treatment
931 break;
932 case REF_SOFT:
933 list = &_discoveredSoftRefs[id];
934 break;
935 case REF_WEAK:
936 list = &_discoveredWeakRefs[id];
937 break;
938 case REF_FINAL:
939 list = &_discoveredFinalRefs[id];
940 break;
941 case REF_PHANTOM:
942 list = &_discoveredPhantomRefs[id];
943 break;
944 case REF_CLEANER:
945 list = &_discoveredCleanerRefs[id];
946 break;
947 case REF_NONE:
948 case REF_JNI:
949 // we should not reach here if we are an InstanceRefKlass
950 default:
951 guarantee(false, err_msg("rt should not be %d", rt));
952 }
953 if (TraceReferenceGC && PrintGCDetails) {
954 gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, p2i(list));
955 }
956 return list;
957 }
958
959 inline void
960 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
961 oop obj,
962 HeapWord* discovered_addr) {
963 assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
964 // First we must make sure this object is only enqueued once. CAS in a non null
965 // discovered_addr.
966 oop current_head = refs_list.head();
967 // The last ref must have its discovered field pointing to itself.
968 oop next_discovered = (current_head != NULL) ? current_head : obj;
969
970 oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr,
971 NULL);
972 if (retest == NULL) {
973 // This thread just won the right to enqueue the object.
974 // We have separate lists for enqueueing, so no synchronization
975 // is necessary.
976 refs_list.set_head(obj);
977 refs_list.inc_length(1);
978
979 if (TraceReferenceGC) {
980 gclog_or_tty->print_cr("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
981 p2i(obj), obj->klass()->internal_name());
982 }
983 } else {
984 // If retest was non NULL, another thread beat us to it:
985 // The reference has already been discovered...
986 if (TraceReferenceGC) {
987 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
988 p2i(obj), obj->klass()->internal_name());
989 }
990 }
991 }
992
993 #ifndef PRODUCT
994 // Non-atomic (i.e. concurrent) discovery might allow us
995 // to observe j.l.References with NULL referents, being those
996 // cleared concurrently by mutators during (or after) discovery.
997 void ReferenceProcessor::verify_referent(oop obj) {
998 bool da = discovery_is_atomic();
999 oop referent = java_lang_ref_Reference::referent(obj);
1000 assert(da ? referent->is_oop() : referent->is_oop_or_null(),
1001 err_msg("Bad referent " INTPTR_FORMAT " found in Reference "
1002 INTPTR_FORMAT " during %satomic discovery ",
1003 p2i(referent), p2i(obj), da ? "" : "non-"));
1004 }
1005 #endif
1006
1007 // We mention two of several possible choices here:
1008 // #0: if the reference object is not in the "originating generation"
1009 // (or part of the heap being collected, indicated by our "span"
1010 // we don't treat it specially (i.e. we scan it as we would
1011 // a normal oop, treating its references as strong references).
1012 // This means that references can't be discovered unless their
1013 // referent is also in the same span. This is the simplest,
1014 // most "local" and most conservative approach, albeit one
1015 // that may cause weak references to be enqueued least promptly.
1016 // We call this choice the "ReferenceBasedDiscovery" policy.
1017 // #1: the reference object may be in any generation (span), but if
1018 // the referent is in the generation (span) being currently collected
1019 // then we can discover the reference object, provided
1020 // the object has not already been discovered by
1021 // a different concurrently running collector (as may be the
1022 // case, for instance, if the reference object is in CMS and
1023 // the referent in DefNewGeneration), and provided the processing
1024 // of this reference object by the current collector will
1025 // appear atomic to every other collector in the system.
1026 // (Thus, for instance, a concurrent collector may not
1027 // discover references in other generations even if the
1028 // referent is in its own generation). This policy may,
1029 // in certain cases, enqueue references somewhat sooner than
1030 // might Policy #0 above, but at marginally increased cost
1031 // and complexity in processing these references.
1032 // We call this choice the "RefeferentBasedDiscovery" policy.
1033 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1034 // Make sure we are discovering refs (rather than processing discovered refs).
1035 if (!_discovering_refs || !RegisterReferences) {
1036 return false;
1037 }
1038 // We only discover active references.
1039 oop next = java_lang_ref_Reference::next(obj);
1040 if (next != NULL) { // Ref is no longer active
1041 return false;
1042 }
1043
1044 HeapWord* obj_addr = (HeapWord*)obj;
1045 if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1046 !_span.contains(obj_addr)) {
1047 // Reference is not in the originating generation;
1048 // don't treat it specially (i.e. we want to scan it as a normal
1049 // object with strong references).
1050 return false;
1051 }
1052
1053 // We only discover references whose referents are not (yet)
1054 // known to be strongly reachable.
1055 if (is_alive_non_header() != NULL) {
1056 verify_referent(obj);
1057 if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
1058 return false; // referent is reachable
1059 }
1060 }
1061 if (rt == REF_SOFT) {
1062 // For soft refs we can decide now if these are not
1063 // current candidates for clearing, in which case we
1064 // can mark through them now, rather than delaying that
1065 // to the reference-processing phase. Since all current
1066 // time-stamp policies advance the soft-ref clock only
1067 // at a full collection cycle, this is always currently
1068 // accurate.
1069 if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
1070 return false;
1071 }
1072 }
1073
1074 ResourceMark rm; // Needed for tracing.
1075
1076 HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1077 const oop discovered = java_lang_ref_Reference::discovered(obj);
1078 assert(discovered->is_oop_or_null(), err_msg("Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered)));
1079 if (discovered != NULL) {
1080 // The reference has already been discovered...
1081 if (TraceReferenceGC) {
1082 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1083 p2i(obj), obj->klass()->internal_name());
1084 }
1085 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1086 // assumes that an object is not processed twice;
1087 // if it's been already discovered it must be on another
1088 // generation's discovered list; so we won't discover it.
1089 return false;
1090 } else {
1091 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1092 "Unrecognized policy");
1093 // Check assumption that an object is not potentially
1094 // discovered twice except by concurrent collectors that potentially
1095 // trace the same Reference object twice.
1096 assert(UseConcMarkSweepGC || UseG1GC,
1097 "Only possible with a concurrent marking collector");
1098 return true;
1099 }
1100 }
1101
1102 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1103 verify_referent(obj);
1104 // Discover if and only if EITHER:
1105 // .. reference is in our span, OR
1106 // .. we are an atomic collector and referent is in our span
1107 if (_span.contains(obj_addr) ||
1108 (discovery_is_atomic() &&
1109 _span.contains(java_lang_ref_Reference::referent(obj)))) {
1110 // should_enqueue = true;
1111 } else {
1112 return false;
1113 }
1114 } else {
1115 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1116 _span.contains(obj_addr), "code inconsistency");
1117 }
1118
1119 // Get the right type of discovered queue head.
1120 DiscoveredList* list = get_discovered_list(rt);
1121 if (list == NULL) {
1122 return false; // nothing special needs to be done
1123 }
1124
1125 if (_discovery_is_mt) {
1126 add_to_discovered_list_mt(*list, obj, discovered_addr);
1127 } else {
1128 // We do a raw store here: the field will be visited later when processing
1129 // the discovered references.
1130 oop current_head = list->head();
1131 // The last ref must have its discovered field pointing to itself.
1132 oop next_discovered = (current_head != NULL) ? current_head : obj;
1133
1134 assert(discovered == NULL, "control point invariant");
1135 oop_store_raw(discovered_addr, next_discovered);
1136 list->set_head(obj);
1137 list->inc_length(1);
1138
1139 if (TraceReferenceGC) {
1140 gclog_or_tty->print_cr("Discovered reference (" INTPTR_FORMAT ": %s)",
1141 p2i(obj), obj->klass()->internal_name());
1142 }
1143 }
1144 assert(obj->is_oop(), "Discovered a bad reference");
1145 verify_referent(obj);
1146 return true;
1147 }
1148
1149 // Preclean the discovered references by removing those
1150 // whose referents are alive, and by marking from those that
1151 // are not active. These lists can be handled here
1152 // in any order and, indeed, concurrently.
1153 void ReferenceProcessor::preclean_discovered_references(
1154 BoolObjectClosure* is_alive,
1155 OopClosure* keep_alive,
1156 VoidClosure* complete_gc,
1157 YieldClosure* yield,
1158 GCTimer* gc_timer) {
1159
1160 // Soft references
1161 {
1162 GCTraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC,
1163 false, gc_timer);
1164 for (uint i = 0; i < _max_num_q; i++) {
1165 if (yield->should_return()) {
1166 return;
1167 }
1168 preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1169 keep_alive, complete_gc, yield);
1170 }
1171 }
1172
1173 // Weak references
1174 {
1175 GCTraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC,
1176 false, gc_timer);
1177 for (uint i = 0; i < _max_num_q; i++) {
1178 if (yield->should_return()) {
1179 return;
1180 }
1181 preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1182 keep_alive, complete_gc, yield);
1183 }
1184 }
1185
1186 // Final references
1187 {
1188 GCTraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC,
1189 false, gc_timer);
1190 for (uint i = 0; i < _max_num_q; i++) {
1191 if (yield->should_return()) {
1192 return;
1193 }
1194 preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1195 keep_alive, complete_gc, yield);
1196 }
1197 }
1198
1199 // Phantom references
1200 {
1201 GCTraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC,
1202 false, gc_timer);
1203 for (uint i = 0; i < _max_num_q; i++) {
1204 if (yield->should_return()) {
1205 return;
1206 }
1207 preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1208 keep_alive, complete_gc, yield);
1209 }
1210
1211 // Cleaner references. Included in timing for phantom references. We
1212 // expect Cleaner references to be temporary, and don't want to deal with
1213 // possible incompatibilities arising from making it more visible.
1214 for (uint i = 0; i < _max_num_q; i++) {
1215 if (yield->should_return()) {
1216 return;
1217 }
1218 preclean_discovered_reflist(_discoveredCleanerRefs[i], is_alive,
1219 keep_alive, complete_gc, yield);
1220 }
1221 }
1222 }
1223
1224 // Walk the given discovered ref list, and remove all reference objects
1225 // whose referents are still alive, whose referents are NULL or which
1226 // are not active (have a non-NULL next field). NOTE: When we are
1227 // thus precleaning the ref lists (which happens single-threaded today),
1228 // we do not disable refs discovery to honor the correct semantics of
1229 // java.lang.Reference. As a result, we need to be careful below
1230 // that ref removal steps interleave safely with ref discovery steps
1231 // (in this thread).
1232 void
1233 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList& refs_list,
1234 BoolObjectClosure* is_alive,
1235 OopClosure* keep_alive,
1236 VoidClosure* complete_gc,
1237 YieldClosure* yield) {
1238 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1239 while (iter.has_next()) {
1240 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1241 oop obj = iter.obj();
1242 oop next = java_lang_ref_Reference::next(obj);
1243 if (iter.referent() == NULL || iter.is_referent_alive() ||
1244 next != NULL) {
1245 // The referent has been cleared, or is alive, or the Reference is not
1246 // active; we need to trace and mark its cohort.
1247 if (TraceReferenceGC) {
1248 gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1249 p2i(iter.obj()), iter.obj()->klass()->internal_name());
1250 }
1251 // Remove Reference object from list
1252 iter.remove();
1253 // Keep alive its cohort.
1254 iter.make_referent_alive();
1255 if (UseCompressedOops) {
1256 narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1257 keep_alive->do_oop(next_addr);
1258 } else {
1259 oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1260 keep_alive->do_oop(next_addr);
1261 }
1262 iter.move_to_next();
1263 } else {
1264 iter.next();
1265 }
1266 }
1267 // Close the reachable set
1268 complete_gc->do_void();
1269
1270 NOT_PRODUCT(
1271 if (PrintGCDetails && PrintReferenceGC && (iter.processed() > 0)) {
1272 gclog_or_tty->print_cr(" Dropped " SIZE_FORMAT " Refs out of " SIZE_FORMAT
1273 " Refs in discovered list " INTPTR_FORMAT,
1274 iter.removed(), iter.processed(), p2i(refs_list.head()));
1275 }
1276 )
1277 }
1278
1279 const char* ReferenceProcessor::list_name(uint i) {
1280 assert(i <= _max_num_q * number_of_subclasses_of_ref(),
1281 "Out of bounds index");
1282
1283 int j = i / _max_num_q;
1284 switch (j) {
1285 case 0: return "SoftRef";
1286 case 1: return "WeakRef";
1287 case 2: return "FinalRef";
1288 case 3: return "PhantomRef";
1289 case 4: return "CleanerRef";
1290 }
1291 ShouldNotReachHere();
1292 return NULL;
1293 }
1294