Tue, 08 Aug 2017 15:57:29 +0800
merge
1 /*
2 * Copyright (c) 2001, 2014, 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 */
25 #include "precompiled.hpp"
26 #include "classfile/javaClasses.hpp"
27 #include "classfile/systemDictionary.hpp"
28 #include "gc_implementation/shared/gcTimer.hpp"
29 #include "gc_implementation/shared/gcTraceTime.hpp"
30 #include "gc_interface/collectedHeap.hpp"
31 #include "gc_interface/collectedHeap.inline.hpp"
32 #include "memory/referencePolicy.hpp"
33 #include "memory/referenceProcessor.hpp"
34 #include "oops/oop.inline.hpp"
35 #include "runtime/java.hpp"
36 #include "runtime/jniHandles.hpp"
38 PRAGMA_FORMAT_MUTE_WARNINGS_FOR_GCC
40 ReferencePolicy* ReferenceProcessor::_always_clear_soft_ref_policy = NULL;
41 ReferencePolicy* ReferenceProcessor::_default_soft_ref_policy = NULL;
42 bool ReferenceProcessor::_pending_list_uses_discovered_field = false;
43 jlong ReferenceProcessor::_soft_ref_timestamp_clock = 0;
45 void referenceProcessor_init() {
46 ReferenceProcessor::init_statics();
47 }
49 void ReferenceProcessor::init_statics() {
50 // We need a monotonically non-deccreasing time in ms but
51 // os::javaTimeMillis() does not guarantee monotonicity.
52 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
54 // Initialize the soft ref timestamp clock.
55 _soft_ref_timestamp_clock = now;
56 // Also update the soft ref clock in j.l.r.SoftReference
57 java_lang_ref_SoftReference::set_clock(_soft_ref_timestamp_clock);
59 _always_clear_soft_ref_policy = new AlwaysClearPolicy();
60 _default_soft_ref_policy = new COMPILER2_PRESENT(LRUMaxHeapPolicy())
61 NOT_COMPILER2(LRUCurrentHeapPolicy());
62 if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) {
63 vm_exit_during_initialization("Could not allocate reference policy object");
64 }
65 guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||
66 RefDiscoveryPolicy == ReferentBasedDiscovery,
67 "Unrecongnized RefDiscoveryPolicy");
68 _pending_list_uses_discovered_field = JDK_Version::current().pending_list_uses_discovered_field();
69 }
71 void ReferenceProcessor::enable_discovery(bool verify_disabled, bool check_no_refs) {
72 #ifdef ASSERT
73 // Verify that we're not currently discovering refs
74 assert(!verify_disabled || !_discovering_refs, "nested call?");
76 if (check_no_refs) {
77 // Verify that the discovered lists are empty
78 verify_no_references_recorded();
79 }
80 #endif // ASSERT
82 // Someone could have modified the value of the static
83 // field in the j.l.r.SoftReference class that holds the
84 // soft reference timestamp clock using reflection or
85 // Unsafe between GCs. Unconditionally update the static
86 // field in ReferenceProcessor here so that we use the new
87 // value during reference discovery.
89 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
90 _discovering_refs = true;
91 }
93 ReferenceProcessor::ReferenceProcessor(MemRegion span,
94 bool mt_processing,
95 uint mt_processing_degree,
96 bool mt_discovery,
97 uint mt_discovery_degree,
98 bool atomic_discovery,
99 BoolObjectClosure* is_alive_non_header) :
100 _discovering_refs(false),
101 _enqueuing_is_done(false),
102 _is_alive_non_header(is_alive_non_header),
103 _processing_is_mt(mt_processing),
104 _next_id(0)
105 {
106 _span = span;
107 _discovery_is_atomic = atomic_discovery;
108 _discovery_is_mt = mt_discovery;
109 _num_q = MAX2(1U, mt_processing_degree);
110 _max_num_q = MAX2(_num_q, mt_discovery_degree);
111 _discovered_refs = NEW_C_HEAP_ARRAY(DiscoveredList,
112 _max_num_q * number_of_subclasses_of_ref(), mtGC);
114 if (_discovered_refs == NULL) {
115 vm_exit_during_initialization("Could not allocated RefProc Array");
116 }
117 _discoveredSoftRefs = &_discovered_refs[0];
118 _discoveredWeakRefs = &_discoveredSoftRefs[_max_num_q];
119 _discoveredFinalRefs = &_discoveredWeakRefs[_max_num_q];
120 _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_q];
122 // Initialize all entries to NULL
123 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
124 _discovered_refs[i].set_head(NULL);
125 _discovered_refs[i].set_length(0);
126 }
128 setup_policy(false /* default soft ref policy */);
129 }
131 #ifndef PRODUCT
132 void ReferenceProcessor::verify_no_references_recorded() {
133 guarantee(!_discovering_refs, "Discovering refs?");
134 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
135 guarantee(_discovered_refs[i].is_empty(),
136 "Found non-empty discovered list");
137 }
138 }
139 #endif
141 void ReferenceProcessor::weak_oops_do(OopClosure* f) {
142 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
143 if (UseCompressedOops) {
144 f->do_oop((narrowOop*)_discovered_refs[i].adr_head());
145 } else {
146 f->do_oop((oop*)_discovered_refs[i].adr_head());
147 }
148 }
149 }
151 void ReferenceProcessor::update_soft_ref_master_clock() {
152 // Update (advance) the soft ref master clock field. This must be done
153 // after processing the soft ref list.
155 // We need a monotonically non-deccreasing time in ms but
156 // os::javaTimeMillis() does not guarantee monotonicity.
157 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
158 jlong soft_ref_clock = java_lang_ref_SoftReference::clock();
159 assert(soft_ref_clock == _soft_ref_timestamp_clock, "soft ref clocks out of sync");
161 NOT_PRODUCT(
162 if (now < _soft_ref_timestamp_clock) {
163 warning("time warp: "INT64_FORMAT" to "INT64_FORMAT,
164 _soft_ref_timestamp_clock, now);
165 }
166 )
167 // The values of now and _soft_ref_timestamp_clock are set using
168 // javaTimeNanos(), which is guaranteed to be monotonically
169 // non-decreasing provided the underlying platform provides such
170 // a time source (and it is bug free).
171 // In product mode, however, protect ourselves from non-monotonicty.
172 if (now > _soft_ref_timestamp_clock) {
173 _soft_ref_timestamp_clock = now;
174 java_lang_ref_SoftReference::set_clock(now);
175 }
176 // Else leave clock stalled at its old value until time progresses
177 // past clock value.
178 }
180 size_t ReferenceProcessor::total_count(DiscoveredList lists[]) {
181 size_t total = 0;
182 for (uint i = 0; i < _max_num_q; ++i) {
183 total += lists[i].length();
184 }
185 return total;
186 }
188 ReferenceProcessorStats ReferenceProcessor::process_discovered_references(
189 BoolObjectClosure* is_alive,
190 OopClosure* keep_alive,
191 VoidClosure* complete_gc,
192 AbstractRefProcTaskExecutor* task_executor,
193 GCTimer* gc_timer) {
194 NOT_PRODUCT(verify_ok_to_handle_reflists());
196 assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
197 // Stop treating discovered references specially.
198 disable_discovery();
200 // If discovery was concurrent, someone could have modified
201 // the value of the static field in the j.l.r.SoftReference
202 // class that holds the soft reference timestamp clock using
203 // reflection or Unsafe between when discovery was enabled and
204 // now. Unconditionally update the static field in ReferenceProcessor
205 // here so that we use the new value during processing of the
206 // discovered soft refs.
208 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
210 bool trace_time = PrintGCDetails && PrintReferenceGC;
212 // Soft references
213 size_t soft_count = 0;
214 {
215 GCTraceTime tt("SoftReference", trace_time, false, gc_timer);
216 soft_count =
217 process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
218 is_alive, keep_alive, complete_gc, task_executor);
219 }
221 update_soft_ref_master_clock();
223 // Weak references
224 size_t weak_count = 0;
225 {
226 GCTraceTime tt("WeakReference", trace_time, false, gc_timer);
227 weak_count =
228 process_discovered_reflist(_discoveredWeakRefs, NULL, true,
229 is_alive, keep_alive, complete_gc, task_executor);
230 }
232 // Final references
233 size_t final_count = 0;
234 {
235 GCTraceTime tt("FinalReference", trace_time, false, gc_timer);
236 final_count =
237 process_discovered_reflist(_discoveredFinalRefs, NULL, false,
238 is_alive, keep_alive, complete_gc, task_executor);
239 }
241 // Phantom references
242 size_t phantom_count = 0;
243 {
244 GCTraceTime tt("PhantomReference", trace_time, false, gc_timer);
245 phantom_count =
246 process_discovered_reflist(_discoveredPhantomRefs, NULL, false,
247 is_alive, keep_alive, complete_gc, task_executor);
248 }
250 // Weak global JNI references. It would make more sense (semantically) to
251 // traverse these simultaneously with the regular weak references above, but
252 // that is not how the JDK1.2 specification is. See #4126360. Native code can
253 // thus use JNI weak references to circumvent the phantom references and
254 // resurrect a "post-mortem" object.
255 {
256 GCTraceTime tt("JNI Weak Reference", trace_time, false, gc_timer);
257 if (task_executor != NULL) {
258 task_executor->set_single_threaded_mode();
259 }
260 process_phaseJNI(is_alive, keep_alive, complete_gc);
261 }
263 return ReferenceProcessorStats(soft_count, weak_count, final_count, phantom_count);
264 }
266 #ifndef PRODUCT
267 // Calculate the number of jni handles.
268 uint ReferenceProcessor::count_jni_refs() {
269 class AlwaysAliveClosure: public BoolObjectClosure {
270 public:
271 virtual bool do_object_b(oop obj) { return true; }
272 };
274 class CountHandleClosure: public OopClosure {
275 private:
276 int _count;
277 public:
278 CountHandleClosure(): _count(0) {}
279 void do_oop(oop* unused) { _count++; }
280 void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
281 int count() { return _count; }
282 };
283 CountHandleClosure global_handle_count;
284 AlwaysAliveClosure always_alive;
285 JNIHandles::weak_oops_do(&always_alive, &global_handle_count);
286 return global_handle_count.count();
287 }
288 #endif
290 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,
291 OopClosure* keep_alive,
292 VoidClosure* complete_gc) {
293 #ifndef PRODUCT
294 if (PrintGCDetails && PrintReferenceGC) {
295 unsigned int count = count_jni_refs();
296 gclog_or_tty->print(", %u refs", count);
297 }
298 #endif
299 JNIHandles::weak_oops_do(is_alive, keep_alive);
300 complete_gc->do_void();
301 }
304 template <class T>
305 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref,
306 AbstractRefProcTaskExecutor* task_executor) {
308 // Remember old value of pending references list
309 T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr();
310 T old_pending_list_value = *pending_list_addr;
312 // Enqueue references that are not made active again, and
313 // clear the decks for the next collection (cycle).
314 ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor);
315 // Do the post-barrier on pending_list_addr missed in
316 // enqueue_discovered_reflist.
317 oopDesc::bs()->write_ref_field(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr));
319 // Stop treating discovered references specially.
320 ref->disable_discovery();
322 // Return true if new pending references were added
323 return old_pending_list_value != *pending_list_addr;
324 }
326 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) {
327 NOT_PRODUCT(verify_ok_to_handle_reflists());
328 if (UseCompressedOops) {
329 return enqueue_discovered_ref_helper<narrowOop>(this, task_executor);
330 } else {
331 return enqueue_discovered_ref_helper<oop>(this, task_executor);
332 }
333 }
335 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,
336 HeapWord* pending_list_addr) {
337 // Given a list of refs linked through the "discovered" field
338 // (java.lang.ref.Reference.discovered), self-loop their "next" field
339 // thus distinguishing them from active References, then
340 // prepend them to the pending list.
341 //
342 // The Java threads will see the Reference objects linked together through
343 // the discovered field. Instead of trying to do the write barrier updates
344 // in all places in the reference processor where we manipulate the discovered
345 // field we make sure to do the barrier here where we anyway iterate through
346 // all linked Reference objects. Note that it is important to not dirty any
347 // cards during reference processing since this will cause card table
348 // verification to fail for G1.
349 //
350 // BKWRD COMPATIBILITY NOTE: For older JDKs (prior to the fix for 4956777),
351 // the "next" field is used to chain the pending list, not the discovered
352 // field.
353 if (TraceReferenceGC && PrintGCDetails) {
354 gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list "
355 INTPTR_FORMAT, (address)refs_list.head());
356 }
358 oop obj = NULL;
359 oop next_d = refs_list.head();
360 if (pending_list_uses_discovered_field()) { // New behavior
361 // Walk down the list, self-looping the next field
362 // so that the References are not considered active.
363 while (obj != next_d) {
364 obj = next_d;
365 assert(obj->is_instanceRef(), "should be reference object");
366 next_d = java_lang_ref_Reference::discovered(obj);
367 if (TraceReferenceGC && PrintGCDetails) {
368 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
369 (void *)obj, (void *)next_d);
370 }
371 assert(java_lang_ref_Reference::next(obj) == NULL,
372 "Reference not active; should not be discovered");
373 // Self-loop next, so as to make Ref not active.
374 java_lang_ref_Reference::set_next_raw(obj, obj);
375 if (next_d != obj) {
376 oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), next_d);
377 } else {
378 // This is the last object.
379 // Swap refs_list into pending_list_addr and
380 // set obj's discovered to what we read from pending_list_addr.
381 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
382 // Need post-barrier on pending_list_addr. See enqueue_discovered_ref_helper() above.
383 java_lang_ref_Reference::set_discovered_raw(obj, old); // old may be NULL
384 oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), old);
385 }
386 }
387 } else { // Old behaviour
388 // Walk down the list, copying the discovered field into
389 // the next field and clearing the discovered field.
390 while (obj != next_d) {
391 obj = next_d;
392 assert(obj->is_instanceRef(), "should be reference object");
393 next_d = java_lang_ref_Reference::discovered(obj);
394 if (TraceReferenceGC && PrintGCDetails) {
395 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
396 (void *)obj, (void *)next_d);
397 }
398 assert(java_lang_ref_Reference::next(obj) == NULL,
399 "The reference should not be enqueued");
400 if (next_d == obj) { // obj is last
401 // Swap refs_list into pendling_list_addr and
402 // set obj's next to what we read from pending_list_addr.
403 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
404 // Need oop_check on pending_list_addr above;
405 // see special oop-check code at the end of
406 // enqueue_discovered_reflists() further below.
407 if (old == NULL) {
408 // obj should be made to point to itself, since
409 // pending list was empty.
410 java_lang_ref_Reference::set_next(obj, obj);
411 } else {
412 java_lang_ref_Reference::set_next(obj, old);
413 }
414 } else {
415 java_lang_ref_Reference::set_next(obj, next_d);
416 }
417 java_lang_ref_Reference::set_discovered(obj, (oop) NULL);
418 }
419 }
420 }
422 // Parallel enqueue task
423 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
424 public:
425 RefProcEnqueueTask(ReferenceProcessor& ref_processor,
426 DiscoveredList discovered_refs[],
427 HeapWord* pending_list_addr,
428 int n_queues)
429 : EnqueueTask(ref_processor, discovered_refs,
430 pending_list_addr, n_queues)
431 { }
433 virtual void work(unsigned int work_id) {
434 assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds");
435 // Simplest first cut: static partitioning.
436 int index = work_id;
437 // The increment on "index" must correspond to the maximum number of queues
438 // (n_queues) with which that ReferenceProcessor was created. That
439 // is because of the "clever" way the discovered references lists were
440 // allocated and are indexed into.
441 assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected");
442 for (int j = 0;
443 j < ReferenceProcessor::number_of_subclasses_of_ref();
444 j++, index += _n_queues) {
445 _ref_processor.enqueue_discovered_reflist(
446 _refs_lists[index], _pending_list_addr);
447 _refs_lists[index].set_head(NULL);
448 _refs_lists[index].set_length(0);
449 }
450 }
451 };
453 // Enqueue references that are not made active again
454 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr,
455 AbstractRefProcTaskExecutor* task_executor) {
456 if (_processing_is_mt && task_executor != NULL) {
457 // Parallel code
458 RefProcEnqueueTask tsk(*this, _discovered_refs,
459 pending_list_addr, _max_num_q);
460 task_executor->execute(tsk);
461 } else {
462 // Serial code: call the parent class's implementation
463 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
464 enqueue_discovered_reflist(_discovered_refs[i], pending_list_addr);
465 _discovered_refs[i].set_head(NULL);
466 _discovered_refs[i].set_length(0);
467 }
468 }
469 }
471 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
472 _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);
473 oop discovered = java_lang_ref_Reference::discovered(_ref);
474 assert(_discovered_addr && discovered->is_oop_or_null(),
475 "discovered field is bad");
476 _next = discovered;
477 _referent_addr = java_lang_ref_Reference::referent_addr(_ref);
478 _referent = java_lang_ref_Reference::referent(_ref);
479 assert(Universe::heap()->is_in_reserved_or_null(_referent),
480 "Wrong oop found in java.lang.Reference object");
481 assert(allow_null_referent ?
482 _referent->is_oop_or_null()
483 : _referent->is_oop(),
484 "bad referent");
485 }
487 void DiscoveredListIterator::remove() {
488 assert(_ref->is_oop(), "Dropping a bad reference");
489 oop_store_raw(_discovered_addr, NULL);
491 // First _prev_next ref actually points into DiscoveredList (gross).
492 oop new_next;
493 if (_next == _ref) {
494 // At the end of the list, we should make _prev point to itself.
495 // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
496 // and _prev will be NULL.
497 new_next = _prev;
498 } else {
499 new_next = _next;
500 }
501 // Remove Reference object from discovered list. Note that G1 does not need a
502 // pre-barrier here because we know the Reference has already been found/marked,
503 // that's how it ended up in the discovered list in the first place.
504 oop_store_raw(_prev_next, new_next);
505 NOT_PRODUCT(_removed++);
506 _refs_list.dec_length(1);
507 }
509 // Make the Reference object active again.
510 void DiscoveredListIterator::make_active() {
511 // The pre barrier for G1 is probably just needed for the old
512 // reference processing behavior. Should we guard this with
513 // ReferenceProcessor::pending_list_uses_discovered_field() ?
514 if (UseG1GC) {
515 HeapWord* next_addr = java_lang_ref_Reference::next_addr(_ref);
516 if (UseCompressedOops) {
517 oopDesc::bs()->write_ref_field_pre((narrowOop*)next_addr, NULL);
518 } else {
519 oopDesc::bs()->write_ref_field_pre((oop*)next_addr, NULL);
520 }
521 }
522 java_lang_ref_Reference::set_next_raw(_ref, NULL);
523 }
525 void DiscoveredListIterator::clear_referent() {
526 oop_store_raw(_referent_addr, NULL);
527 }
529 // NOTE: process_phase*() are largely similar, and at a high level
530 // merely iterate over the extant list applying a predicate to
531 // each of its elements and possibly removing that element from the
532 // list and applying some further closures to that element.
533 // We should consider the possibility of replacing these
534 // process_phase*() methods by abstracting them into
535 // a single general iterator invocation that receives appropriate
536 // closures that accomplish this work.
538 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
539 // referents are not alive, but that should be kept alive for policy reasons.
540 // Keep alive the transitive closure of all such referents.
541 void
542 ReferenceProcessor::process_phase1(DiscoveredList& refs_list,
543 ReferencePolicy* policy,
544 BoolObjectClosure* is_alive,
545 OopClosure* keep_alive,
546 VoidClosure* complete_gc) {
547 assert(policy != NULL, "Must have a non-NULL policy");
548 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
549 // Decide which softly reachable refs should be kept alive.
550 while (iter.has_next()) {
551 iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
552 bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
553 if (referent_is_dead &&
554 !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
555 if (TraceReferenceGC) {
556 gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s" ") by policy",
557 (void *)iter.obj(), iter.obj()->klass()->internal_name());
558 }
559 // Remove Reference object from list
560 iter.remove();
561 // Make the Reference object active again
562 iter.make_active();
563 // keep the referent around
564 iter.make_referent_alive();
565 iter.move_to_next();
566 } else {
567 iter.next();
568 }
569 }
570 // Close the reachable set
571 complete_gc->do_void();
572 NOT_PRODUCT(
573 if (PrintGCDetails && TraceReferenceGC) {
574 gclog_or_tty->print_cr(" Dropped %d dead Refs out of %d "
575 "discovered Refs by policy, from list " INTPTR_FORMAT,
576 iter.removed(), iter.processed(), (address)refs_list.head());
577 }
578 )
579 }
581 // Traverse the list and remove any Refs that are not active, or
582 // whose referents are either alive or NULL.
583 void
584 ReferenceProcessor::pp2_work(DiscoveredList& refs_list,
585 BoolObjectClosure* is_alive,
586 OopClosure* keep_alive) {
587 assert(discovery_is_atomic(), "Error");
588 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
589 while (iter.has_next()) {
590 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
591 DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
592 assert(next == NULL, "Should not discover inactive Reference");
593 if (iter.is_referent_alive()) {
594 if (TraceReferenceGC) {
595 gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
596 (void *)iter.obj(), iter.obj()->klass()->internal_name());
597 }
598 // The referent is reachable after all.
599 // Remove Reference object from list.
600 iter.remove();
601 // Update the referent pointer as necessary: Note that this
602 // should not entail any recursive marking because the
603 // referent must already have been traversed.
604 iter.make_referent_alive();
605 iter.move_to_next();
606 } else {
607 iter.next();
608 }
609 }
610 NOT_PRODUCT(
611 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
612 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
613 "Refs in discovered list " INTPTR_FORMAT,
614 iter.removed(), iter.processed(), (address)refs_list.head());
615 }
616 )
617 }
619 void
620 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList& refs_list,
621 BoolObjectClosure* is_alive,
622 OopClosure* keep_alive,
623 VoidClosure* complete_gc) {
624 assert(!discovery_is_atomic(), "Error");
625 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
626 while (iter.has_next()) {
627 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
628 HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
629 oop next = java_lang_ref_Reference::next(iter.obj());
630 if ((iter.referent() == NULL || iter.is_referent_alive() ||
631 next != NULL)) {
632 assert(next->is_oop_or_null(), "bad next field");
633 // Remove Reference object from list
634 iter.remove();
635 // Trace the cohorts
636 iter.make_referent_alive();
637 if (UseCompressedOops) {
638 keep_alive->do_oop((narrowOop*)next_addr);
639 } else {
640 keep_alive->do_oop((oop*)next_addr);
641 }
642 iter.move_to_next();
643 } else {
644 iter.next();
645 }
646 }
647 // Now close the newly reachable set
648 complete_gc->do_void();
649 NOT_PRODUCT(
650 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
651 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
652 "Refs in discovered list " INTPTR_FORMAT,
653 iter.removed(), iter.processed(), (address)refs_list.head());
654 }
655 )
656 }
658 // Traverse the list and process the referents, by either
659 // clearing them or keeping them (and their reachable
660 // closure) alive.
661 void
662 ReferenceProcessor::process_phase3(DiscoveredList& refs_list,
663 bool clear_referent,
664 BoolObjectClosure* is_alive,
665 OopClosure* keep_alive,
666 VoidClosure* complete_gc) {
667 ResourceMark rm;
668 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
669 while (iter.has_next()) {
670 iter.update_discovered();
671 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
672 if (clear_referent) {
673 // NULL out referent pointer
674 iter.clear_referent();
675 } else {
676 // keep the referent around
677 iter.make_referent_alive();
678 }
679 if (TraceReferenceGC) {
680 gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
681 clear_referent ? "cleared " : "",
682 (void *)iter.obj(), iter.obj()->klass()->internal_name());
683 }
684 assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");
685 iter.next();
686 }
687 // Remember to update the next pointer of the last ref.
688 iter.update_discovered();
689 // Close the reachable set
690 complete_gc->do_void();
691 }
693 void
694 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
695 oop obj = NULL;
696 oop next = refs_list.head();
697 while (next != obj) {
698 obj = next;
699 next = java_lang_ref_Reference::discovered(obj);
700 java_lang_ref_Reference::set_discovered_raw(obj, NULL);
701 }
702 refs_list.set_head(NULL);
703 refs_list.set_length(0);
704 }
706 void
707 ReferenceProcessor::abandon_partial_discovered_list(DiscoveredList& refs_list) {
708 clear_discovered_references(refs_list);
709 }
711 void ReferenceProcessor::abandon_partial_discovery() {
712 // loop over the lists
713 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
714 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
715 gclog_or_tty->print_cr("\nAbandoning %s discovered list", list_name(i));
716 }
717 abandon_partial_discovered_list(_discovered_refs[i]);
718 }
719 }
721 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
722 public:
723 RefProcPhase1Task(ReferenceProcessor& ref_processor,
724 DiscoveredList refs_lists[],
725 ReferencePolicy* policy,
726 bool marks_oops_alive)
727 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
728 _policy(policy)
729 { }
730 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
731 OopClosure& keep_alive,
732 VoidClosure& complete_gc)
733 {
734 Thread* thr = Thread::current();
735 int refs_list_index = ((WorkerThread*)thr)->id();
736 _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy,
737 &is_alive, &keep_alive, &complete_gc);
738 }
739 private:
740 ReferencePolicy* _policy;
741 };
743 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
744 public:
745 RefProcPhase2Task(ReferenceProcessor& ref_processor,
746 DiscoveredList refs_lists[],
747 bool marks_oops_alive)
748 : ProcessTask(ref_processor, refs_lists, marks_oops_alive)
749 { }
750 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
751 OopClosure& keep_alive,
752 VoidClosure& complete_gc)
753 {
754 _ref_processor.process_phase2(_refs_lists[i],
755 &is_alive, &keep_alive, &complete_gc);
756 }
757 };
759 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
760 public:
761 RefProcPhase3Task(ReferenceProcessor& ref_processor,
762 DiscoveredList refs_lists[],
763 bool clear_referent,
764 bool marks_oops_alive)
765 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
766 _clear_referent(clear_referent)
767 { }
768 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
769 OopClosure& keep_alive,
770 VoidClosure& complete_gc)
771 {
772 // Don't use "refs_list_index" calculated in this way because
773 // balance_queues() has moved the Ref's into the first n queues.
774 // Thread* thr = Thread::current();
775 // int refs_list_index = ((WorkerThread*)thr)->id();
776 // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent,
777 _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
778 &is_alive, &keep_alive, &complete_gc);
779 }
780 private:
781 bool _clear_referent;
782 };
784 // Balances reference queues.
785 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
786 // queues[0, 1, ..., _num_q-1] because only the first _num_q
787 // corresponding to the active workers will be processed.
788 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
789 {
790 // calculate total length
791 size_t total_refs = 0;
792 if (TraceReferenceGC && PrintGCDetails) {
793 gclog_or_tty->print_cr("\nBalance ref_lists ");
794 }
796 for (uint i = 0; i < _max_num_q; ++i) {
797 total_refs += ref_lists[i].length();
798 if (TraceReferenceGC && PrintGCDetails) {
799 gclog_or_tty->print("%d ", ref_lists[i].length());
800 }
801 }
802 if (TraceReferenceGC && PrintGCDetails) {
803 gclog_or_tty->print_cr(" = %d", total_refs);
804 }
805 size_t avg_refs = total_refs / _num_q + 1;
806 uint to_idx = 0;
807 for (uint from_idx = 0; from_idx < _max_num_q; from_idx++) {
808 bool move_all = false;
809 if (from_idx >= _num_q) {
810 move_all = ref_lists[from_idx].length() > 0;
811 }
812 while ((ref_lists[from_idx].length() > avg_refs) ||
813 move_all) {
814 assert(to_idx < _num_q, "Sanity Check!");
815 if (ref_lists[to_idx].length() < avg_refs) {
816 // move superfluous refs
817 size_t refs_to_move;
818 // Move all the Ref's if the from queue will not be processed.
819 if (move_all) {
820 refs_to_move = MIN2(ref_lists[from_idx].length(),
821 avg_refs - ref_lists[to_idx].length());
822 } else {
823 refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
824 avg_refs - ref_lists[to_idx].length());
825 }
827 assert(refs_to_move > 0, "otherwise the code below will fail");
829 oop move_head = ref_lists[from_idx].head();
830 oop move_tail = move_head;
831 oop new_head = move_head;
832 // find an element to split the list on
833 for (size_t j = 0; j < refs_to_move; ++j) {
834 move_tail = new_head;
835 new_head = java_lang_ref_Reference::discovered(new_head);
836 }
838 // Add the chain to the to list.
839 if (ref_lists[to_idx].head() == NULL) {
840 // to list is empty. Make a loop at the end.
841 java_lang_ref_Reference::set_discovered_raw(move_tail, move_tail);
842 } else {
843 java_lang_ref_Reference::set_discovered_raw(move_tail, ref_lists[to_idx].head());
844 }
845 ref_lists[to_idx].set_head(move_head);
846 ref_lists[to_idx].inc_length(refs_to_move);
848 // Remove the chain from the from list.
849 if (move_tail == new_head) {
850 // We found the end of the from list.
851 ref_lists[from_idx].set_head(NULL);
852 } else {
853 ref_lists[from_idx].set_head(new_head);
854 }
855 ref_lists[from_idx].dec_length(refs_to_move);
856 if (ref_lists[from_idx].length() == 0) {
857 break;
858 }
859 } else {
860 to_idx = (to_idx + 1) % _num_q;
861 }
862 }
863 }
864 #ifdef ASSERT
865 size_t balanced_total_refs = 0;
866 for (uint i = 0; i < _max_num_q; ++i) {
867 balanced_total_refs += ref_lists[i].length();
868 if (TraceReferenceGC && PrintGCDetails) {
869 gclog_or_tty->print("%d ", ref_lists[i].length());
870 }
871 }
872 if (TraceReferenceGC && PrintGCDetails) {
873 gclog_or_tty->print_cr(" = %d", balanced_total_refs);
874 gclog_or_tty->flush();
875 }
876 assert(total_refs == balanced_total_refs, "Balancing was incomplete");
877 #endif
878 }
880 void ReferenceProcessor::balance_all_queues() {
881 balance_queues(_discoveredSoftRefs);
882 balance_queues(_discoveredWeakRefs);
883 balance_queues(_discoveredFinalRefs);
884 balance_queues(_discoveredPhantomRefs);
885 }
887 size_t
888 ReferenceProcessor::process_discovered_reflist(
889 DiscoveredList refs_lists[],
890 ReferencePolicy* policy,
891 bool clear_referent,
892 BoolObjectClosure* is_alive,
893 OopClosure* keep_alive,
894 VoidClosure* complete_gc,
895 AbstractRefProcTaskExecutor* task_executor)
896 {
897 bool mt_processing = task_executor != NULL && _processing_is_mt;
898 // If discovery used MT and a dynamic number of GC threads, then
899 // the queues must be balanced for correctness if fewer than the
900 // maximum number of queues were used. The number of queue used
901 // during discovery may be different than the number to be used
902 // for processing so don't depend of _num_q < _max_num_q as part
903 // of the test.
904 bool must_balance = _discovery_is_mt;
906 if ((mt_processing && ParallelRefProcBalancingEnabled) ||
907 must_balance) {
908 balance_queues(refs_lists);
909 }
911 size_t total_list_count = total_count(refs_lists);
913 if (PrintReferenceGC && PrintGCDetails) {
914 gclog_or_tty->print(", %u refs", total_list_count);
915 }
917 // Phase 1 (soft refs only):
918 // . Traverse the list and remove any SoftReferences whose
919 // referents are not alive, but that should be kept alive for
920 // policy reasons. Keep alive the transitive closure of all
921 // such referents.
922 if (policy != NULL) {
923 if (mt_processing) {
924 RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/);
925 task_executor->execute(phase1);
926 } else {
927 for (uint i = 0; i < _max_num_q; i++) {
928 process_phase1(refs_lists[i], policy,
929 is_alive, keep_alive, complete_gc);
930 }
931 }
932 } else { // policy == NULL
933 assert(refs_lists != _discoveredSoftRefs,
934 "Policy must be specified for soft references.");
935 }
937 // Phase 2:
938 // . Traverse the list and remove any refs whose referents are alive.
939 if (mt_processing) {
940 RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/);
941 task_executor->execute(phase2);
942 } else {
943 for (uint i = 0; i < _max_num_q; i++) {
944 process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
945 }
946 }
948 // Phase 3:
949 // . Traverse the list and process referents as appropriate.
950 if (mt_processing) {
951 RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/);
952 task_executor->execute(phase3);
953 } else {
954 for (uint i = 0; i < _max_num_q; i++) {
955 process_phase3(refs_lists[i], clear_referent,
956 is_alive, keep_alive, complete_gc);
957 }
958 }
960 return total_list_count;
961 }
963 void ReferenceProcessor::clean_up_discovered_references() {
964 // loop over the lists
965 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
966 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
967 gclog_or_tty->print_cr(
968 "\nScrubbing %s discovered list of Null referents",
969 list_name(i));
970 }
971 clean_up_discovered_reflist(_discovered_refs[i]);
972 }
973 }
975 void ReferenceProcessor::clean_up_discovered_reflist(DiscoveredList& refs_list) {
976 assert(!discovery_is_atomic(), "Else why call this method?");
977 DiscoveredListIterator iter(refs_list, NULL, NULL);
978 while (iter.has_next()) {
979 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
980 oop next = java_lang_ref_Reference::next(iter.obj());
981 assert(next->is_oop_or_null(), "bad next field");
982 // If referent has been cleared or Reference is not active,
983 // drop it.
984 if (iter.referent() == NULL || next != NULL) {
985 debug_only(
986 if (PrintGCDetails && TraceReferenceGC) {
987 gclog_or_tty->print_cr("clean_up_discovered_list: Dropping Reference: "
988 INTPTR_FORMAT " with next field: " INTPTR_FORMAT
989 " and referent: " INTPTR_FORMAT,
990 (void *)iter.obj(), (void *)next, (void *)iter.referent());
991 }
992 )
993 // Remove Reference object from list
994 iter.remove();
995 iter.move_to_next();
996 } else {
997 iter.next();
998 }
999 }
1000 NOT_PRODUCT(
1001 if (PrintGCDetails && TraceReferenceGC) {
1002 gclog_or_tty->print(
1003 " Removed %d Refs with NULL referents out of %d discovered Refs",
1004 iter.removed(), iter.processed());
1005 }
1006 )
1007 }
1009 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
1010 uint id = 0;
1011 // Determine the queue index to use for this object.
1012 if (_discovery_is_mt) {
1013 // During a multi-threaded discovery phase,
1014 // each thread saves to its "own" list.
1015 Thread* thr = Thread::current();
1016 id = thr->as_Worker_thread()->id();
1017 } else {
1018 // single-threaded discovery, we save in round-robin
1019 // fashion to each of the lists.
1020 if (_processing_is_mt) {
1021 id = next_id();
1022 }
1023 }
1024 assert(0 <= id && id < _max_num_q, "Id is out-of-bounds (call Freud?)");
1026 // Get the discovered queue to which we will add
1027 DiscoveredList* list = NULL;
1028 switch (rt) {
1029 case REF_OTHER:
1030 // Unknown reference type, no special treatment
1031 break;
1032 case REF_SOFT:
1033 list = &_discoveredSoftRefs[id];
1034 break;
1035 case REF_WEAK:
1036 list = &_discoveredWeakRefs[id];
1037 break;
1038 case REF_FINAL:
1039 list = &_discoveredFinalRefs[id];
1040 break;
1041 case REF_PHANTOM:
1042 list = &_discoveredPhantomRefs[id];
1043 break;
1044 case REF_NONE:
1045 // we should not reach here if we are an InstanceRefKlass
1046 default:
1047 ShouldNotReachHere();
1048 }
1049 if (TraceReferenceGC && PrintGCDetails) {
1050 gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, list);
1051 }
1052 return list;
1053 }
1055 inline void
1056 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
1057 oop obj,
1058 HeapWord* discovered_addr) {
1059 assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
1060 // First we must make sure this object is only enqueued once. CAS in a non null
1061 // discovered_addr.
1062 oop current_head = refs_list.head();
1063 // The last ref must have its discovered field pointing to itself.
1064 oop next_discovered = (current_head != NULL) ? current_head : obj;
1066 oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr,
1067 NULL);
1068 if (retest == NULL) {
1069 // This thread just won the right to enqueue the object.
1070 // We have separate lists for enqueueing, so no synchronization
1071 // is necessary.
1072 refs_list.set_head(obj);
1073 refs_list.inc_length(1);
1075 if (TraceReferenceGC) {
1076 gclog_or_tty->print_cr("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
1077 (void *)obj, obj->klass()->internal_name());
1078 }
1079 } else {
1080 // If retest was non NULL, another thread beat us to it:
1081 // The reference has already been discovered...
1082 if (TraceReferenceGC) {
1083 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1084 (void *)obj, obj->klass()->internal_name());
1085 }
1086 }
1087 }
1089 #ifndef PRODUCT
1090 // Non-atomic (i.e. concurrent) discovery might allow us
1091 // to observe j.l.References with NULL referents, being those
1092 // cleared concurrently by mutators during (or after) discovery.
1093 void ReferenceProcessor::verify_referent(oop obj) {
1094 bool da = discovery_is_atomic();
1095 oop referent = java_lang_ref_Reference::referent(obj);
1096 assert(da ? referent->is_oop() : referent->is_oop_or_null(),
1097 err_msg("Bad referent " INTPTR_FORMAT " found in Reference "
1098 INTPTR_FORMAT " during %satomic discovery ",
1099 (void *)referent, (void *)obj, da ? "" : "non-"));
1100 }
1101 #endif
1103 // We mention two of several possible choices here:
1104 // #0: if the reference object is not in the "originating generation"
1105 // (or part of the heap being collected, indicated by our "span"
1106 // we don't treat it specially (i.e. we scan it as we would
1107 // a normal oop, treating its references as strong references).
1108 // This means that references can't be discovered unless their
1109 // referent is also in the same span. This is the simplest,
1110 // most "local" and most conservative approach, albeit one
1111 // that may cause weak references to be enqueued least promptly.
1112 // We call this choice the "ReferenceBasedDiscovery" policy.
1113 // #1: the reference object may be in any generation (span), but if
1114 // the referent is in the generation (span) being currently collected
1115 // then we can discover the reference object, provided
1116 // the object has not already been discovered by
1117 // a different concurrently running collector (as may be the
1118 // case, for instance, if the reference object is in CMS and
1119 // the referent in DefNewGeneration), and provided the processing
1120 // of this reference object by the current collector will
1121 // appear atomic to every other collector in the system.
1122 // (Thus, for instance, a concurrent collector may not
1123 // discover references in other generations even if the
1124 // referent is in its own generation). This policy may,
1125 // in certain cases, enqueue references somewhat sooner than
1126 // might Policy #0 above, but at marginally increased cost
1127 // and complexity in processing these references.
1128 // We call this choice the "RefeferentBasedDiscovery" policy.
1129 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1130 // Make sure we are discovering refs (rather than processing discovered refs).
1131 if (!_discovering_refs || !RegisterReferences) {
1132 return false;
1133 }
1134 // We only discover active references.
1135 oop next = java_lang_ref_Reference::next(obj);
1136 if (next != NULL) { // Ref is no longer active
1137 return false;
1138 }
1140 HeapWord* obj_addr = (HeapWord*)obj;
1141 if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1142 !_span.contains(obj_addr)) {
1143 // Reference is not in the originating generation;
1144 // don't treat it specially (i.e. we want to scan it as a normal
1145 // object with strong references).
1146 return false;
1147 }
1149 // We only discover references whose referents are not (yet)
1150 // known to be strongly reachable.
1151 if (is_alive_non_header() != NULL) {
1152 verify_referent(obj);
1153 if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
1154 return false; // referent is reachable
1155 }
1156 }
1157 if (rt == REF_SOFT) {
1158 // For soft refs we can decide now if these are not
1159 // current candidates for clearing, in which case we
1160 // can mark through them now, rather than delaying that
1161 // to the reference-processing phase. Since all current
1162 // time-stamp policies advance the soft-ref clock only
1163 // at a major collection cycle, this is always currently
1164 // accurate.
1165 if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
1166 return false;
1167 }
1168 }
1170 ResourceMark rm; // Needed for tracing.
1172 HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1173 const oop discovered = java_lang_ref_Reference::discovered(obj);
1174 assert(discovered->is_oop_or_null(), "bad discovered field");
1175 if (discovered != NULL) {
1176 // The reference has already been discovered...
1177 if (TraceReferenceGC) {
1178 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1179 (void *)obj, obj->klass()->internal_name());
1180 }
1181 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1182 // assumes that an object is not processed twice;
1183 // if it's been already discovered it must be on another
1184 // generation's discovered list; so we won't discover it.
1185 return false;
1186 } else {
1187 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1188 "Unrecognized policy");
1189 // Check assumption that an object is not potentially
1190 // discovered twice except by concurrent collectors that potentially
1191 // trace the same Reference object twice.
1192 assert(UseConcMarkSweepGC || UseG1GC,
1193 "Only possible with a concurrent marking collector");
1194 return true;
1195 }
1196 }
1198 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1199 verify_referent(obj);
1200 // Discover if and only if EITHER:
1201 // .. reference is in our span, OR
1202 // .. we are an atomic collector and referent is in our span
1203 if (_span.contains(obj_addr) ||
1204 (discovery_is_atomic() &&
1205 _span.contains(java_lang_ref_Reference::referent(obj)))) {
1206 // should_enqueue = true;
1207 } else {
1208 return false;
1209 }
1210 } else {
1211 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1212 _span.contains(obj_addr), "code inconsistency");
1213 }
1215 // Get the right type of discovered queue head.
1216 DiscoveredList* list = get_discovered_list(rt);
1217 if (list == NULL) {
1218 return false; // nothing special needs to be done
1219 }
1221 if (_discovery_is_mt) {
1222 add_to_discovered_list_mt(*list, obj, discovered_addr);
1223 } else {
1224 // We do a raw store here: the field will be visited later when processing
1225 // the discovered references.
1226 oop current_head = list->head();
1227 // The last ref must have its discovered field pointing to itself.
1228 oop next_discovered = (current_head != NULL) ? current_head : obj;
1230 assert(discovered == NULL, "control point invariant");
1231 oop_store_raw(discovered_addr, next_discovered);
1232 list->set_head(obj);
1233 list->inc_length(1);
1235 if (TraceReferenceGC) {
1236 gclog_or_tty->print_cr("Discovered reference (" INTPTR_FORMAT ": %s)",
1237 (void *)obj, obj->klass()->internal_name());
1238 }
1239 }
1240 assert(obj->is_oop(), "Discovered a bad reference");
1241 verify_referent(obj);
1242 return true;
1243 }
1245 // Preclean the discovered references by removing those
1246 // whose referents are alive, and by marking from those that
1247 // are not active. These lists can be handled here
1248 // in any order and, indeed, concurrently.
1249 void ReferenceProcessor::preclean_discovered_references(
1250 BoolObjectClosure* is_alive,
1251 OopClosure* keep_alive,
1252 VoidClosure* complete_gc,
1253 YieldClosure* yield,
1254 GCTimer* gc_timer) {
1256 NOT_PRODUCT(verify_ok_to_handle_reflists());
1258 // Soft references
1259 {
1260 GCTraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC,
1261 false, gc_timer);
1262 for (uint i = 0; i < _max_num_q; i++) {
1263 if (yield->should_return()) {
1264 return;
1265 }
1266 preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1267 keep_alive, complete_gc, yield);
1268 }
1269 }
1271 // Weak references
1272 {
1273 GCTraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC,
1274 false, gc_timer);
1275 for (uint i = 0; i < _max_num_q; i++) {
1276 if (yield->should_return()) {
1277 return;
1278 }
1279 preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1280 keep_alive, complete_gc, yield);
1281 }
1282 }
1284 // Final references
1285 {
1286 GCTraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC,
1287 false, gc_timer);
1288 for (uint i = 0; i < _max_num_q; i++) {
1289 if (yield->should_return()) {
1290 return;
1291 }
1292 preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1293 keep_alive, complete_gc, yield);
1294 }
1295 }
1297 // Phantom references
1298 {
1299 GCTraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC,
1300 false, gc_timer);
1301 for (uint i = 0; i < _max_num_q; i++) {
1302 if (yield->should_return()) {
1303 return;
1304 }
1305 preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1306 keep_alive, complete_gc, yield);
1307 }
1308 }
1309 }
1311 // Walk the given discovered ref list, and remove all reference objects
1312 // whose referents are still alive, whose referents are NULL or which
1313 // are not active (have a non-NULL next field). NOTE: When we are
1314 // thus precleaning the ref lists (which happens single-threaded today),
1315 // we do not disable refs discovery to honour the correct semantics of
1316 // java.lang.Reference. As a result, we need to be careful below
1317 // that ref removal steps interleave safely with ref discovery steps
1318 // (in this thread).
1319 void
1320 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList& refs_list,
1321 BoolObjectClosure* is_alive,
1322 OopClosure* keep_alive,
1323 VoidClosure* complete_gc,
1324 YieldClosure* yield) {
1325 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1326 while (iter.has_next()) {
1327 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1328 oop obj = iter.obj();
1329 oop next = java_lang_ref_Reference::next(obj);
1330 if (iter.referent() == NULL || iter.is_referent_alive() ||
1331 next != NULL) {
1332 // The referent has been cleared, or is alive, or the Reference is not
1333 // active; we need to trace and mark its cohort.
1334 if (TraceReferenceGC) {
1335 gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1336 (void *)iter.obj(), iter.obj()->klass()->internal_name());
1337 }
1338 // Remove Reference object from list
1339 iter.remove();
1340 // Keep alive its cohort.
1341 iter.make_referent_alive();
1342 if (UseCompressedOops) {
1343 narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1344 keep_alive->do_oop(next_addr);
1345 } else {
1346 oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1347 keep_alive->do_oop(next_addr);
1348 }
1349 iter.move_to_next();
1350 } else {
1351 iter.next();
1352 }
1353 }
1354 // Close the reachable set
1355 complete_gc->do_void();
1357 NOT_PRODUCT(
1358 if (PrintGCDetails && PrintReferenceGC && (iter.processed() > 0)) {
1359 gclog_or_tty->print_cr(" Dropped %d Refs out of %d "
1360 "Refs in discovered list " INTPTR_FORMAT,
1361 iter.removed(), iter.processed(), (address)refs_list.head());
1362 }
1363 )
1364 }
1366 const char* ReferenceProcessor::list_name(uint i) {
1367 assert(i >= 0 && i <= _max_num_q * number_of_subclasses_of_ref(),
1368 "Out of bounds index");
1370 int j = i / _max_num_q;
1371 switch (j) {
1372 case 0: return "SoftRef";
1373 case 1: return "WeakRef";
1374 case 2: return "FinalRef";
1375 case 3: return "PhantomRef";
1376 }
1377 ShouldNotReachHere();
1378 return NULL;
1379 }
1381 #ifndef PRODUCT
1382 void ReferenceProcessor::verify_ok_to_handle_reflists() {
1383 // empty for now
1384 }
1385 #endif
1387 #ifndef PRODUCT
1388 void ReferenceProcessor::clear_discovered_references() {
1389 guarantee(!_discovering_refs, "Discovering refs?");
1390 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
1391 clear_discovered_references(_discovered_refs[i]);
1392 }
1393 }
1395 #endif // PRODUCT