Thu, 21 Aug 2014 13:57:51 -0700
8046070: Class Data Sharing clean up and refactoring
Summary: Cleaned up CDS to be more configurable, maintainable and extensible
Reviewed-by: dholmes, coleenp, acorn, mchung
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.
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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.
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19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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21 * questions.
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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 GCId gc_id) {
195 NOT_PRODUCT(verify_ok_to_handle_reflists());
197 assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
198 // Stop treating discovered references specially.
199 disable_discovery();
201 // If discovery was concurrent, someone could have modified
202 // the value of the static field in the j.l.r.SoftReference
203 // class that holds the soft reference timestamp clock using
204 // reflection or Unsafe between when discovery was enabled and
205 // now. Unconditionally update the static field in ReferenceProcessor
206 // here so that we use the new value during processing of the
207 // discovered soft refs.
209 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
211 bool trace_time = PrintGCDetails && PrintReferenceGC;
213 // Soft references
214 size_t soft_count = 0;
215 {
216 GCTraceTime tt("SoftReference", trace_time, false, gc_timer, gc_id);
217 soft_count =
218 process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
219 is_alive, keep_alive, complete_gc, task_executor);
220 }
222 update_soft_ref_master_clock();
224 // Weak references
225 size_t weak_count = 0;
226 {
227 GCTraceTime tt("WeakReference", trace_time, false, gc_timer, gc_id);
228 weak_count =
229 process_discovered_reflist(_discoveredWeakRefs, NULL, true,
230 is_alive, keep_alive, complete_gc, task_executor);
231 }
233 // Final references
234 size_t final_count = 0;
235 {
236 GCTraceTime tt("FinalReference", trace_time, false, gc_timer, gc_id);
237 final_count =
238 process_discovered_reflist(_discoveredFinalRefs, NULL, false,
239 is_alive, keep_alive, complete_gc, task_executor);
240 }
242 // Phantom references
243 size_t phantom_count = 0;
244 {
245 GCTraceTime tt("PhantomReference", trace_time, false, gc_timer, gc_id);
246 phantom_count =
247 process_discovered_reflist(_discoveredPhantomRefs, NULL, false,
248 is_alive, keep_alive, complete_gc, task_executor);
249 }
251 // Weak global JNI references. It would make more sense (semantically) to
252 // traverse these simultaneously with the regular weak references above, but
253 // that is not how the JDK1.2 specification is. See #4126360. Native code can
254 // thus use JNI weak references to circumvent the phantom references and
255 // resurrect a "post-mortem" object.
256 {
257 GCTraceTime tt("JNI Weak Reference", trace_time, false, gc_timer, gc_id);
258 if (task_executor != NULL) {
259 task_executor->set_single_threaded_mode();
260 }
261 process_phaseJNI(is_alive, keep_alive, complete_gc);
262 }
264 return ReferenceProcessorStats(soft_count, weak_count, final_count, phantom_count);
265 }
267 #ifndef PRODUCT
268 // Calculate the number of jni handles.
269 uint ReferenceProcessor::count_jni_refs() {
270 class AlwaysAliveClosure: public BoolObjectClosure {
271 public:
272 virtual bool do_object_b(oop obj) { return true; }
273 };
275 class CountHandleClosure: public OopClosure {
276 private:
277 int _count;
278 public:
279 CountHandleClosure(): _count(0) {}
280 void do_oop(oop* unused) { _count++; }
281 void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
282 int count() { return _count; }
283 };
284 CountHandleClosure global_handle_count;
285 AlwaysAliveClosure always_alive;
286 JNIHandles::weak_oops_do(&always_alive, &global_handle_count);
287 return global_handle_count.count();
288 }
289 #endif
291 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,
292 OopClosure* keep_alive,
293 VoidClosure* complete_gc) {
294 #ifndef PRODUCT
295 if (PrintGCDetails && PrintReferenceGC) {
296 unsigned int count = count_jni_refs();
297 gclog_or_tty->print(", %u refs", count);
298 }
299 #endif
300 JNIHandles::weak_oops_do(is_alive, keep_alive);
301 complete_gc->do_void();
302 }
305 template <class T>
306 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref,
307 AbstractRefProcTaskExecutor* task_executor) {
309 // Remember old value of pending references list
310 T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr();
311 T old_pending_list_value = *pending_list_addr;
313 // Enqueue references that are not made active again, and
314 // clear the decks for the next collection (cycle).
315 ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor);
316 // Do the post-barrier on pending_list_addr missed in
317 // enqueue_discovered_reflist.
318 oopDesc::bs()->write_ref_field(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr));
320 // Stop treating discovered references specially.
321 ref->disable_discovery();
323 // Return true if new pending references were added
324 return old_pending_list_value != *pending_list_addr;
325 }
327 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) {
328 NOT_PRODUCT(verify_ok_to_handle_reflists());
329 if (UseCompressedOops) {
330 return enqueue_discovered_ref_helper<narrowOop>(this, task_executor);
331 } else {
332 return enqueue_discovered_ref_helper<oop>(this, task_executor);
333 }
334 }
336 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,
337 HeapWord* pending_list_addr) {
338 // Given a list of refs linked through the "discovered" field
339 // (java.lang.ref.Reference.discovered), self-loop their "next" field
340 // thus distinguishing them from active References, then
341 // prepend them to the pending list.
342 //
343 // The Java threads will see the Reference objects linked together through
344 // the discovered field. Instead of trying to do the write barrier updates
345 // in all places in the reference processor where we manipulate the discovered
346 // field we make sure to do the barrier here where we anyway iterate through
347 // all linked Reference objects. Note that it is important to not dirty any
348 // cards during reference processing since this will cause card table
349 // verification to fail for G1.
350 //
351 // BKWRD COMPATIBILITY NOTE: For older JDKs (prior to the fix for 4956777),
352 // the "next" field is used to chain the pending list, not the discovered
353 // field.
354 if (TraceReferenceGC && PrintGCDetails) {
355 gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list "
356 INTPTR_FORMAT, (address)refs_list.head());
357 }
359 oop obj = NULL;
360 oop next_d = refs_list.head();
361 if (pending_list_uses_discovered_field()) { // New behavior
362 // Walk down the list, self-looping the next field
363 // so that the References are not considered active.
364 while (obj != next_d) {
365 obj = next_d;
366 assert(obj->is_instanceRef(), "should be reference object");
367 next_d = java_lang_ref_Reference::discovered(obj);
368 if (TraceReferenceGC && PrintGCDetails) {
369 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
370 (void *)obj, (void *)next_d);
371 }
372 assert(java_lang_ref_Reference::next(obj) == NULL,
373 "Reference not active; should not be discovered");
374 // Self-loop next, so as to make Ref not active.
375 java_lang_ref_Reference::set_next_raw(obj, obj);
376 if (next_d != obj) {
377 oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), next_d);
378 } else {
379 // This is the last object.
380 // Swap refs_list into pending_list_addr and
381 // set obj's discovered to what we read from pending_list_addr.
382 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
383 // Need post-barrier on pending_list_addr. See enqueue_discovered_ref_helper() above.
384 java_lang_ref_Reference::set_discovered_raw(obj, old); // old may be NULL
385 oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), old);
386 }
387 }
388 } else { // Old behaviour
389 // Walk down the list, copying the discovered field into
390 // the next field and clearing the discovered field.
391 while (obj != next_d) {
392 obj = next_d;
393 assert(obj->is_instanceRef(), "should be reference object");
394 next_d = java_lang_ref_Reference::discovered(obj);
395 if (TraceReferenceGC && PrintGCDetails) {
396 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
397 (void *)obj, (void *)next_d);
398 }
399 assert(java_lang_ref_Reference::next(obj) == NULL,
400 "The reference should not be enqueued");
401 if (next_d == obj) { // obj is last
402 // Swap refs_list into pendling_list_addr and
403 // set obj's next to what we read from pending_list_addr.
404 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
405 // Need oop_check on pending_list_addr above;
406 // see special oop-check code at the end of
407 // enqueue_discovered_reflists() further below.
408 if (old == NULL) {
409 // obj should be made to point to itself, since
410 // pending list was empty.
411 java_lang_ref_Reference::set_next(obj, obj);
412 } else {
413 java_lang_ref_Reference::set_next(obj, old);
414 }
415 } else {
416 java_lang_ref_Reference::set_next(obj, next_d);
417 }
418 java_lang_ref_Reference::set_discovered(obj, (oop) NULL);
419 }
420 }
421 }
423 // Parallel enqueue task
424 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
425 public:
426 RefProcEnqueueTask(ReferenceProcessor& ref_processor,
427 DiscoveredList discovered_refs[],
428 HeapWord* pending_list_addr,
429 int n_queues)
430 : EnqueueTask(ref_processor, discovered_refs,
431 pending_list_addr, n_queues)
432 { }
434 virtual void work(unsigned int work_id) {
435 assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds");
436 // Simplest first cut: static partitioning.
437 int index = work_id;
438 // The increment on "index" must correspond to the maximum number of queues
439 // (n_queues) with which that ReferenceProcessor was created. That
440 // is because of the "clever" way the discovered references lists were
441 // allocated and are indexed into.
442 assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected");
443 for (int j = 0;
444 j < ReferenceProcessor::number_of_subclasses_of_ref();
445 j++, index += _n_queues) {
446 _ref_processor.enqueue_discovered_reflist(
447 _refs_lists[index], _pending_list_addr);
448 _refs_lists[index].set_head(NULL);
449 _refs_lists[index].set_length(0);
450 }
451 }
452 };
454 // Enqueue references that are not made active again
455 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr,
456 AbstractRefProcTaskExecutor* task_executor) {
457 if (_processing_is_mt && task_executor != NULL) {
458 // Parallel code
459 RefProcEnqueueTask tsk(*this, _discovered_refs,
460 pending_list_addr, _max_num_q);
461 task_executor->execute(tsk);
462 } else {
463 // Serial code: call the parent class's implementation
464 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
465 enqueue_discovered_reflist(_discovered_refs[i], pending_list_addr);
466 _discovered_refs[i].set_head(NULL);
467 _discovered_refs[i].set_length(0);
468 }
469 }
470 }
472 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
473 _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);
474 oop discovered = java_lang_ref_Reference::discovered(_ref);
475 assert(_discovered_addr && discovered->is_oop_or_null(),
476 "discovered field is bad");
477 _next = discovered;
478 _referent_addr = java_lang_ref_Reference::referent_addr(_ref);
479 _referent = java_lang_ref_Reference::referent(_ref);
480 assert(Universe::heap()->is_in_reserved_or_null(_referent),
481 "Wrong oop found in java.lang.Reference object");
482 assert(allow_null_referent ?
483 _referent->is_oop_or_null()
484 : _referent->is_oop(),
485 "bad referent");
486 }
488 void DiscoveredListIterator::remove() {
489 assert(_ref->is_oop(), "Dropping a bad reference");
490 oop_store_raw(_discovered_addr, NULL);
492 // First _prev_next ref actually points into DiscoveredList (gross).
493 oop new_next;
494 if (_next == _ref) {
495 // At the end of the list, we should make _prev point to itself.
496 // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
497 // and _prev will be NULL.
498 new_next = _prev;
499 } else {
500 new_next = _next;
501 }
502 // Remove Reference object from discovered list. Note that G1 does not need a
503 // pre-barrier here because we know the Reference has already been found/marked,
504 // that's how it ended up in the discovered list in the first place.
505 oop_store_raw(_prev_next, new_next);
506 NOT_PRODUCT(_removed++);
507 _refs_list.dec_length(1);
508 }
510 // Make the Reference object active again.
511 void DiscoveredListIterator::make_active() {
512 // The pre barrier for G1 is probably just needed for the old
513 // reference processing behavior. Should we guard this with
514 // ReferenceProcessor::pending_list_uses_discovered_field() ?
515 if (UseG1GC) {
516 HeapWord* next_addr = java_lang_ref_Reference::next_addr(_ref);
517 if (UseCompressedOops) {
518 oopDesc::bs()->write_ref_field_pre((narrowOop*)next_addr, NULL);
519 } else {
520 oopDesc::bs()->write_ref_field_pre((oop*)next_addr, NULL);
521 }
522 }
523 java_lang_ref_Reference::set_next_raw(_ref, NULL);
524 }
526 void DiscoveredListIterator::clear_referent() {
527 oop_store_raw(_referent_addr, NULL);
528 }
530 // NOTE: process_phase*() are largely similar, and at a high level
531 // merely iterate over the extant list applying a predicate to
532 // each of its elements and possibly removing that element from the
533 // list and applying some further closures to that element.
534 // We should consider the possibility of replacing these
535 // process_phase*() methods by abstracting them into
536 // a single general iterator invocation that receives appropriate
537 // closures that accomplish this work.
539 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
540 // referents are not alive, but that should be kept alive for policy reasons.
541 // Keep alive the transitive closure of all such referents.
542 void
543 ReferenceProcessor::process_phase1(DiscoveredList& refs_list,
544 ReferencePolicy* policy,
545 BoolObjectClosure* is_alive,
546 OopClosure* keep_alive,
547 VoidClosure* complete_gc) {
548 assert(policy != NULL, "Must have a non-NULL policy");
549 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
550 // Decide which softly reachable refs should be kept alive.
551 while (iter.has_next()) {
552 iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
553 bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
554 if (referent_is_dead &&
555 !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
556 if (TraceReferenceGC) {
557 gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s" ") by policy",
558 (void *)iter.obj(), iter.obj()->klass()->internal_name());
559 }
560 // Remove Reference object from list
561 iter.remove();
562 // Make the Reference object active again
563 iter.make_active();
564 // keep the referent around
565 iter.make_referent_alive();
566 iter.move_to_next();
567 } else {
568 iter.next();
569 }
570 }
571 // Close the reachable set
572 complete_gc->do_void();
573 NOT_PRODUCT(
574 if (PrintGCDetails && TraceReferenceGC) {
575 gclog_or_tty->print_cr(" Dropped %d dead Refs out of %d "
576 "discovered Refs by policy, from list " INTPTR_FORMAT,
577 iter.removed(), iter.processed(), (address)refs_list.head());
578 }
579 )
580 }
582 // Traverse the list and remove any Refs that are not active, or
583 // whose referents are either alive or NULL.
584 void
585 ReferenceProcessor::pp2_work(DiscoveredList& refs_list,
586 BoolObjectClosure* is_alive,
587 OopClosure* keep_alive) {
588 assert(discovery_is_atomic(), "Error");
589 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
590 while (iter.has_next()) {
591 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
592 DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
593 assert(next == NULL, "Should not discover inactive Reference");
594 if (iter.is_referent_alive()) {
595 if (TraceReferenceGC) {
596 gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
597 (void *)iter.obj(), iter.obj()->klass()->internal_name());
598 }
599 // The referent is reachable after all.
600 // Remove Reference object from list.
601 iter.remove();
602 // Update the referent pointer as necessary: Note that this
603 // should not entail any recursive marking because the
604 // referent must already have been traversed.
605 iter.make_referent_alive();
606 iter.move_to_next();
607 } else {
608 iter.next();
609 }
610 }
611 NOT_PRODUCT(
612 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
613 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
614 "Refs in discovered list " INTPTR_FORMAT,
615 iter.removed(), iter.processed(), (address)refs_list.head());
616 }
617 )
618 }
620 void
621 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList& refs_list,
622 BoolObjectClosure* is_alive,
623 OopClosure* keep_alive,
624 VoidClosure* complete_gc) {
625 assert(!discovery_is_atomic(), "Error");
626 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
627 while (iter.has_next()) {
628 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
629 HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
630 oop next = java_lang_ref_Reference::next(iter.obj());
631 if ((iter.referent() == NULL || iter.is_referent_alive() ||
632 next != NULL)) {
633 assert(next->is_oop_or_null(), "bad next field");
634 // Remove Reference object from list
635 iter.remove();
636 // Trace the cohorts
637 iter.make_referent_alive();
638 if (UseCompressedOops) {
639 keep_alive->do_oop((narrowOop*)next_addr);
640 } else {
641 keep_alive->do_oop((oop*)next_addr);
642 }
643 iter.move_to_next();
644 } else {
645 iter.next();
646 }
647 }
648 // Now close the newly reachable set
649 complete_gc->do_void();
650 NOT_PRODUCT(
651 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
652 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
653 "Refs in discovered list " INTPTR_FORMAT,
654 iter.removed(), iter.processed(), (address)refs_list.head());
655 }
656 )
657 }
659 // Traverse the list and process the referents, by either
660 // clearing them or keeping them (and their reachable
661 // closure) alive.
662 void
663 ReferenceProcessor::process_phase3(DiscoveredList& refs_list,
664 bool clear_referent,
665 BoolObjectClosure* is_alive,
666 OopClosure* keep_alive,
667 VoidClosure* complete_gc) {
668 ResourceMark rm;
669 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
670 while (iter.has_next()) {
671 iter.update_discovered();
672 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
673 if (clear_referent) {
674 // NULL out referent pointer
675 iter.clear_referent();
676 } else {
677 // keep the referent around
678 iter.make_referent_alive();
679 }
680 if (TraceReferenceGC) {
681 gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
682 clear_referent ? "cleared " : "",
683 (void *)iter.obj(), iter.obj()->klass()->internal_name());
684 }
685 assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");
686 iter.next();
687 }
688 // Remember to update the next pointer of the last ref.
689 iter.update_discovered();
690 // Close the reachable set
691 complete_gc->do_void();
692 }
694 void
695 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
696 oop obj = NULL;
697 oop next = refs_list.head();
698 while (next != obj) {
699 obj = next;
700 next = java_lang_ref_Reference::discovered(obj);
701 java_lang_ref_Reference::set_discovered_raw(obj, NULL);
702 }
703 refs_list.set_head(NULL);
704 refs_list.set_length(0);
705 }
707 void
708 ReferenceProcessor::abandon_partial_discovered_list(DiscoveredList& refs_list) {
709 clear_discovered_references(refs_list);
710 }
712 void ReferenceProcessor::abandon_partial_discovery() {
713 // loop over the lists
714 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
715 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
716 gclog_or_tty->print_cr("\nAbandoning %s discovered list", list_name(i));
717 }
718 abandon_partial_discovered_list(_discovered_refs[i]);
719 }
720 }
722 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
723 public:
724 RefProcPhase1Task(ReferenceProcessor& ref_processor,
725 DiscoveredList refs_lists[],
726 ReferencePolicy* policy,
727 bool marks_oops_alive)
728 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
729 _policy(policy)
730 { }
731 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
732 OopClosure& keep_alive,
733 VoidClosure& complete_gc)
734 {
735 Thread* thr = Thread::current();
736 int refs_list_index = ((WorkerThread*)thr)->id();
737 _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy,
738 &is_alive, &keep_alive, &complete_gc);
739 }
740 private:
741 ReferencePolicy* _policy;
742 };
744 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
745 public:
746 RefProcPhase2Task(ReferenceProcessor& ref_processor,
747 DiscoveredList refs_lists[],
748 bool marks_oops_alive)
749 : ProcessTask(ref_processor, refs_lists, marks_oops_alive)
750 { }
751 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
752 OopClosure& keep_alive,
753 VoidClosure& complete_gc)
754 {
755 _ref_processor.process_phase2(_refs_lists[i],
756 &is_alive, &keep_alive, &complete_gc);
757 }
758 };
760 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
761 public:
762 RefProcPhase3Task(ReferenceProcessor& ref_processor,
763 DiscoveredList refs_lists[],
764 bool clear_referent,
765 bool marks_oops_alive)
766 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
767 _clear_referent(clear_referent)
768 { }
769 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
770 OopClosure& keep_alive,
771 VoidClosure& complete_gc)
772 {
773 // Don't use "refs_list_index" calculated in this way because
774 // balance_queues() has moved the Ref's into the first n queues.
775 // Thread* thr = Thread::current();
776 // int refs_list_index = ((WorkerThread*)thr)->id();
777 // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent,
778 _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
779 &is_alive, &keep_alive, &complete_gc);
780 }
781 private:
782 bool _clear_referent;
783 };
785 // Balances reference queues.
786 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
787 // queues[0, 1, ..., _num_q-1] because only the first _num_q
788 // corresponding to the active workers will be processed.
789 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
790 {
791 // calculate total length
792 size_t total_refs = 0;
793 if (TraceReferenceGC && PrintGCDetails) {
794 gclog_or_tty->print_cr("\nBalance ref_lists ");
795 }
797 for (uint i = 0; i < _max_num_q; ++i) {
798 total_refs += ref_lists[i].length();
799 if (TraceReferenceGC && PrintGCDetails) {
800 gclog_or_tty->print("%d ", ref_lists[i].length());
801 }
802 }
803 if (TraceReferenceGC && PrintGCDetails) {
804 gclog_or_tty->print_cr(" = %d", total_refs);
805 }
806 size_t avg_refs = total_refs / _num_q + 1;
807 uint to_idx = 0;
808 for (uint from_idx = 0; from_idx < _max_num_q; from_idx++) {
809 bool move_all = false;
810 if (from_idx >= _num_q) {
811 move_all = ref_lists[from_idx].length() > 0;
812 }
813 while ((ref_lists[from_idx].length() > avg_refs) ||
814 move_all) {
815 assert(to_idx < _num_q, "Sanity Check!");
816 if (ref_lists[to_idx].length() < avg_refs) {
817 // move superfluous refs
818 size_t refs_to_move;
819 // Move all the Ref's if the from queue will not be processed.
820 if (move_all) {
821 refs_to_move = MIN2(ref_lists[from_idx].length(),
822 avg_refs - ref_lists[to_idx].length());
823 } else {
824 refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
825 avg_refs - ref_lists[to_idx].length());
826 }
828 assert(refs_to_move > 0, "otherwise the code below will fail");
830 oop move_head = ref_lists[from_idx].head();
831 oop move_tail = move_head;
832 oop new_head = move_head;
833 // find an element to split the list on
834 for (size_t j = 0; j < refs_to_move; ++j) {
835 move_tail = new_head;
836 new_head = java_lang_ref_Reference::discovered(new_head);
837 }
839 // Add the chain to the to list.
840 if (ref_lists[to_idx].head() == NULL) {
841 // to list is empty. Make a loop at the end.
842 java_lang_ref_Reference::set_discovered_raw(move_tail, move_tail);
843 } else {
844 java_lang_ref_Reference::set_discovered_raw(move_tail, ref_lists[to_idx].head());
845 }
846 ref_lists[to_idx].set_head(move_head);
847 ref_lists[to_idx].inc_length(refs_to_move);
849 // Remove the chain from the from list.
850 if (move_tail == new_head) {
851 // We found the end of the from list.
852 ref_lists[from_idx].set_head(NULL);
853 } else {
854 ref_lists[from_idx].set_head(new_head);
855 }
856 ref_lists[from_idx].dec_length(refs_to_move);
857 if (ref_lists[from_idx].length() == 0) {
858 break;
859 }
860 } else {
861 to_idx = (to_idx + 1) % _num_q;
862 }
863 }
864 }
865 #ifdef ASSERT
866 size_t balanced_total_refs = 0;
867 for (uint i = 0; i < _max_num_q; ++i) {
868 balanced_total_refs += ref_lists[i].length();
869 if (TraceReferenceGC && PrintGCDetails) {
870 gclog_or_tty->print("%d ", ref_lists[i].length());
871 }
872 }
873 if (TraceReferenceGC && PrintGCDetails) {
874 gclog_or_tty->print_cr(" = %d", balanced_total_refs);
875 gclog_or_tty->flush();
876 }
877 assert(total_refs == balanced_total_refs, "Balancing was incomplete");
878 #endif
879 }
881 void ReferenceProcessor::balance_all_queues() {
882 balance_queues(_discoveredSoftRefs);
883 balance_queues(_discoveredWeakRefs);
884 balance_queues(_discoveredFinalRefs);
885 balance_queues(_discoveredPhantomRefs);
886 }
888 size_t
889 ReferenceProcessor::process_discovered_reflist(
890 DiscoveredList refs_lists[],
891 ReferencePolicy* policy,
892 bool clear_referent,
893 BoolObjectClosure* is_alive,
894 OopClosure* keep_alive,
895 VoidClosure* complete_gc,
896 AbstractRefProcTaskExecutor* task_executor)
897 {
898 bool mt_processing = task_executor != NULL && _processing_is_mt;
899 // If discovery used MT and a dynamic number of GC threads, then
900 // the queues must be balanced for correctness if fewer than the
901 // maximum number of queues were used. The number of queue used
902 // during discovery may be different than the number to be used
903 // for processing so don't depend of _num_q < _max_num_q as part
904 // of the test.
905 bool must_balance = _discovery_is_mt;
907 if ((mt_processing && ParallelRefProcBalancingEnabled) ||
908 must_balance) {
909 balance_queues(refs_lists);
910 }
912 size_t total_list_count = total_count(refs_lists);
914 if (PrintReferenceGC && PrintGCDetails) {
915 gclog_or_tty->print(", %u refs", total_list_count);
916 }
918 // Phase 1 (soft refs only):
919 // . Traverse the list and remove any SoftReferences whose
920 // referents are not alive, but that should be kept alive for
921 // policy reasons. Keep alive the transitive closure of all
922 // such referents.
923 if (policy != NULL) {
924 if (mt_processing) {
925 RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/);
926 task_executor->execute(phase1);
927 } else {
928 for (uint i = 0; i < _max_num_q; i++) {
929 process_phase1(refs_lists[i], policy,
930 is_alive, keep_alive, complete_gc);
931 }
932 }
933 } else { // policy == NULL
934 assert(refs_lists != _discoveredSoftRefs,
935 "Policy must be specified for soft references.");
936 }
938 // Phase 2:
939 // . Traverse the list and remove any refs whose referents are alive.
940 if (mt_processing) {
941 RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/);
942 task_executor->execute(phase2);
943 } else {
944 for (uint i = 0; i < _max_num_q; i++) {
945 process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
946 }
947 }
949 // Phase 3:
950 // . Traverse the list and process referents as appropriate.
951 if (mt_processing) {
952 RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/);
953 task_executor->execute(phase3);
954 } else {
955 for (uint i = 0; i < _max_num_q; i++) {
956 process_phase3(refs_lists[i], clear_referent,
957 is_alive, keep_alive, complete_gc);
958 }
959 }
961 return total_list_count;
962 }
964 void ReferenceProcessor::clean_up_discovered_references() {
965 // loop over the lists
966 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
967 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
968 gclog_or_tty->print_cr(
969 "\nScrubbing %s discovered list of Null referents",
970 list_name(i));
971 }
972 clean_up_discovered_reflist(_discovered_refs[i]);
973 }
974 }
976 void ReferenceProcessor::clean_up_discovered_reflist(DiscoveredList& refs_list) {
977 assert(!discovery_is_atomic(), "Else why call this method?");
978 DiscoveredListIterator iter(refs_list, NULL, NULL);
979 while (iter.has_next()) {
980 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
981 oop next = java_lang_ref_Reference::next(iter.obj());
982 assert(next->is_oop_or_null(), "bad next field");
983 // If referent has been cleared or Reference is not active,
984 // drop it.
985 if (iter.referent() == NULL || next != NULL) {
986 debug_only(
987 if (PrintGCDetails && TraceReferenceGC) {
988 gclog_or_tty->print_cr("clean_up_discovered_list: Dropping Reference: "
989 INTPTR_FORMAT " with next field: " INTPTR_FORMAT
990 " and referent: " INTPTR_FORMAT,
991 (void *)iter.obj(), (void *)next, (void *)iter.referent());
992 }
993 )
994 // Remove Reference object from list
995 iter.remove();
996 iter.move_to_next();
997 } else {
998 iter.next();
999 }
1000 }
1001 NOT_PRODUCT(
1002 if (PrintGCDetails && TraceReferenceGC) {
1003 gclog_or_tty->print(
1004 " Removed %d Refs with NULL referents out of %d discovered Refs",
1005 iter.removed(), iter.processed());
1006 }
1007 )
1008 }
1010 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
1011 uint id = 0;
1012 // Determine the queue index to use for this object.
1013 if (_discovery_is_mt) {
1014 // During a multi-threaded discovery phase,
1015 // each thread saves to its "own" list.
1016 Thread* thr = Thread::current();
1017 id = thr->as_Worker_thread()->id();
1018 } else {
1019 // single-threaded discovery, we save in round-robin
1020 // fashion to each of the lists.
1021 if (_processing_is_mt) {
1022 id = next_id();
1023 }
1024 }
1025 assert(0 <= id && id < _max_num_q, "Id is out-of-bounds (call Freud?)");
1027 // Get the discovered queue to which we will add
1028 DiscoveredList* list = NULL;
1029 switch (rt) {
1030 case REF_OTHER:
1031 // Unknown reference type, no special treatment
1032 break;
1033 case REF_SOFT:
1034 list = &_discoveredSoftRefs[id];
1035 break;
1036 case REF_WEAK:
1037 list = &_discoveredWeakRefs[id];
1038 break;
1039 case REF_FINAL:
1040 list = &_discoveredFinalRefs[id];
1041 break;
1042 case REF_PHANTOM:
1043 list = &_discoveredPhantomRefs[id];
1044 break;
1045 case REF_NONE:
1046 // we should not reach here if we are an InstanceRefKlass
1047 default:
1048 ShouldNotReachHere();
1049 }
1050 if (TraceReferenceGC && PrintGCDetails) {
1051 gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, list);
1052 }
1053 return list;
1054 }
1056 inline void
1057 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
1058 oop obj,
1059 HeapWord* discovered_addr) {
1060 assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
1061 // First we must make sure this object is only enqueued once. CAS in a non null
1062 // discovered_addr.
1063 oop current_head = refs_list.head();
1064 // The last ref must have its discovered field pointing to itself.
1065 oop next_discovered = (current_head != NULL) ? current_head : obj;
1067 oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr,
1068 NULL);
1069 if (retest == NULL) {
1070 // This thread just won the right to enqueue the object.
1071 // We have separate lists for enqueueing, so no synchronization
1072 // is necessary.
1073 refs_list.set_head(obj);
1074 refs_list.inc_length(1);
1076 if (TraceReferenceGC) {
1077 gclog_or_tty->print_cr("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
1078 (void *)obj, obj->klass()->internal_name());
1079 }
1080 } else {
1081 // If retest was non NULL, another thread beat us to it:
1082 // The reference has already been discovered...
1083 if (TraceReferenceGC) {
1084 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1085 (void *)obj, obj->klass()->internal_name());
1086 }
1087 }
1088 }
1090 #ifndef PRODUCT
1091 // Non-atomic (i.e. concurrent) discovery might allow us
1092 // to observe j.l.References with NULL referents, being those
1093 // cleared concurrently by mutators during (or after) discovery.
1094 void ReferenceProcessor::verify_referent(oop obj) {
1095 bool da = discovery_is_atomic();
1096 oop referent = java_lang_ref_Reference::referent(obj);
1097 assert(da ? referent->is_oop() : referent->is_oop_or_null(),
1098 err_msg("Bad referent " INTPTR_FORMAT " found in Reference "
1099 INTPTR_FORMAT " during %satomic discovery ",
1100 (void *)referent, (void *)obj, da ? "" : "non-"));
1101 }
1102 #endif
1104 // We mention two of several possible choices here:
1105 // #0: if the reference object is not in the "originating generation"
1106 // (or part of the heap being collected, indicated by our "span"
1107 // we don't treat it specially (i.e. we scan it as we would
1108 // a normal oop, treating its references as strong references).
1109 // This means that references can't be discovered unless their
1110 // referent is also in the same span. This is the simplest,
1111 // most "local" and most conservative approach, albeit one
1112 // that may cause weak references to be enqueued least promptly.
1113 // We call this choice the "ReferenceBasedDiscovery" policy.
1114 // #1: the reference object may be in any generation (span), but if
1115 // the referent is in the generation (span) being currently collected
1116 // then we can discover the reference object, provided
1117 // the object has not already been discovered by
1118 // a different concurrently running collector (as may be the
1119 // case, for instance, if the reference object is in CMS and
1120 // the referent in DefNewGeneration), and provided the processing
1121 // of this reference object by the current collector will
1122 // appear atomic to every other collector in the system.
1123 // (Thus, for instance, a concurrent collector may not
1124 // discover references in other generations even if the
1125 // referent is in its own generation). This policy may,
1126 // in certain cases, enqueue references somewhat sooner than
1127 // might Policy #0 above, but at marginally increased cost
1128 // and complexity in processing these references.
1129 // We call this choice the "RefeferentBasedDiscovery" policy.
1130 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1131 // Make sure we are discovering refs (rather than processing discovered refs).
1132 if (!_discovering_refs || !RegisterReferences) {
1133 return false;
1134 }
1135 // We only discover active references.
1136 oop next = java_lang_ref_Reference::next(obj);
1137 if (next != NULL) { // Ref is no longer active
1138 return false;
1139 }
1141 HeapWord* obj_addr = (HeapWord*)obj;
1142 if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1143 !_span.contains(obj_addr)) {
1144 // Reference is not in the originating generation;
1145 // don't treat it specially (i.e. we want to scan it as a normal
1146 // object with strong references).
1147 return false;
1148 }
1150 // We only discover references whose referents are not (yet)
1151 // known to be strongly reachable.
1152 if (is_alive_non_header() != NULL) {
1153 verify_referent(obj);
1154 if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
1155 return false; // referent is reachable
1156 }
1157 }
1158 if (rt == REF_SOFT) {
1159 // For soft refs we can decide now if these are not
1160 // current candidates for clearing, in which case we
1161 // can mark through them now, rather than delaying that
1162 // to the reference-processing phase. Since all current
1163 // time-stamp policies advance the soft-ref clock only
1164 // at a major collection cycle, this is always currently
1165 // accurate.
1166 if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
1167 return false;
1168 }
1169 }
1171 ResourceMark rm; // Needed for tracing.
1173 HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1174 const oop discovered = java_lang_ref_Reference::discovered(obj);
1175 assert(discovered->is_oop_or_null(), "bad discovered field");
1176 if (discovered != NULL) {
1177 // The reference has already been discovered...
1178 if (TraceReferenceGC) {
1179 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1180 (void *)obj, obj->klass()->internal_name());
1181 }
1182 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1183 // assumes that an object is not processed twice;
1184 // if it's been already discovered it must be on another
1185 // generation's discovered list; so we won't discover it.
1186 return false;
1187 } else {
1188 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1189 "Unrecognized policy");
1190 // Check assumption that an object is not potentially
1191 // discovered twice except by concurrent collectors that potentially
1192 // trace the same Reference object twice.
1193 assert(UseConcMarkSweepGC || UseG1GC,
1194 "Only possible with a concurrent marking collector");
1195 return true;
1196 }
1197 }
1199 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1200 verify_referent(obj);
1201 // Discover if and only if EITHER:
1202 // .. reference is in our span, OR
1203 // .. we are an atomic collector and referent is in our span
1204 if (_span.contains(obj_addr) ||
1205 (discovery_is_atomic() &&
1206 _span.contains(java_lang_ref_Reference::referent(obj)))) {
1207 // should_enqueue = true;
1208 } else {
1209 return false;
1210 }
1211 } else {
1212 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1213 _span.contains(obj_addr), "code inconsistency");
1214 }
1216 // Get the right type of discovered queue head.
1217 DiscoveredList* list = get_discovered_list(rt);
1218 if (list == NULL) {
1219 return false; // nothing special needs to be done
1220 }
1222 if (_discovery_is_mt) {
1223 add_to_discovered_list_mt(*list, obj, discovered_addr);
1224 } else {
1225 // We do a raw store here: the field will be visited later when processing
1226 // the discovered references.
1227 oop current_head = list->head();
1228 // The last ref must have its discovered field pointing to itself.
1229 oop next_discovered = (current_head != NULL) ? current_head : obj;
1231 assert(discovered == NULL, "control point invariant");
1232 oop_store_raw(discovered_addr, next_discovered);
1233 list->set_head(obj);
1234 list->inc_length(1);
1236 if (TraceReferenceGC) {
1237 gclog_or_tty->print_cr("Discovered reference (" INTPTR_FORMAT ": %s)",
1238 (void *)obj, obj->klass()->internal_name());
1239 }
1240 }
1241 assert(obj->is_oop(), "Discovered a bad reference");
1242 verify_referent(obj);
1243 return true;
1244 }
1246 // Preclean the discovered references by removing those
1247 // whose referents are alive, and by marking from those that
1248 // are not active. These lists can be handled here
1249 // in any order and, indeed, concurrently.
1250 void ReferenceProcessor::preclean_discovered_references(
1251 BoolObjectClosure* is_alive,
1252 OopClosure* keep_alive,
1253 VoidClosure* complete_gc,
1254 YieldClosure* yield,
1255 GCTimer* gc_timer,
1256 GCId gc_id) {
1258 NOT_PRODUCT(verify_ok_to_handle_reflists());
1260 // Soft references
1261 {
1262 GCTraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC,
1263 false, gc_timer, gc_id);
1264 for (uint i = 0; i < _max_num_q; i++) {
1265 if (yield->should_return()) {
1266 return;
1267 }
1268 preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1269 keep_alive, complete_gc, yield);
1270 }
1271 }
1273 // Weak references
1274 {
1275 GCTraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC,
1276 false, gc_timer, gc_id);
1277 for (uint i = 0; i < _max_num_q; i++) {
1278 if (yield->should_return()) {
1279 return;
1280 }
1281 preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1282 keep_alive, complete_gc, yield);
1283 }
1284 }
1286 // Final references
1287 {
1288 GCTraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC,
1289 false, gc_timer, gc_id);
1290 for (uint i = 0; i < _max_num_q; i++) {
1291 if (yield->should_return()) {
1292 return;
1293 }
1294 preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1295 keep_alive, complete_gc, yield);
1296 }
1297 }
1299 // Phantom references
1300 {
1301 GCTraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC,
1302 false, gc_timer, gc_id);
1303 for (uint i = 0; i < _max_num_q; i++) {
1304 if (yield->should_return()) {
1305 return;
1306 }
1307 preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1308 keep_alive, complete_gc, yield);
1309 }
1310 }
1311 }
1313 // Walk the given discovered ref list, and remove all reference objects
1314 // whose referents are still alive, whose referents are NULL or which
1315 // are not active (have a non-NULL next field). NOTE: When we are
1316 // thus precleaning the ref lists (which happens single-threaded today),
1317 // we do not disable refs discovery to honour the correct semantics of
1318 // java.lang.Reference. As a result, we need to be careful below
1319 // that ref removal steps interleave safely with ref discovery steps
1320 // (in this thread).
1321 void
1322 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList& refs_list,
1323 BoolObjectClosure* is_alive,
1324 OopClosure* keep_alive,
1325 VoidClosure* complete_gc,
1326 YieldClosure* yield) {
1327 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1328 while (iter.has_next()) {
1329 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1330 oop obj = iter.obj();
1331 oop next = java_lang_ref_Reference::next(obj);
1332 if (iter.referent() == NULL || iter.is_referent_alive() ||
1333 next != NULL) {
1334 // The referent has been cleared, or is alive, or the Reference is not
1335 // active; we need to trace and mark its cohort.
1336 if (TraceReferenceGC) {
1337 gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1338 (void *)iter.obj(), iter.obj()->klass()->internal_name());
1339 }
1340 // Remove Reference object from list
1341 iter.remove();
1342 // Keep alive its cohort.
1343 iter.make_referent_alive();
1344 if (UseCompressedOops) {
1345 narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1346 keep_alive->do_oop(next_addr);
1347 } else {
1348 oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1349 keep_alive->do_oop(next_addr);
1350 }
1351 iter.move_to_next();
1352 } else {
1353 iter.next();
1354 }
1355 }
1356 // Close the reachable set
1357 complete_gc->do_void();
1359 NOT_PRODUCT(
1360 if (PrintGCDetails && PrintReferenceGC && (iter.processed() > 0)) {
1361 gclog_or_tty->print_cr(" Dropped %d Refs out of %d "
1362 "Refs in discovered list " INTPTR_FORMAT,
1363 iter.removed(), iter.processed(), (address)refs_list.head());
1364 }
1365 )
1366 }
1368 const char* ReferenceProcessor::list_name(uint i) {
1369 assert(i >= 0 && i <= _max_num_q * number_of_subclasses_of_ref(),
1370 "Out of bounds index");
1372 int j = i / _max_num_q;
1373 switch (j) {
1374 case 0: return "SoftRef";
1375 case 1: return "WeakRef";
1376 case 2: return "FinalRef";
1377 case 3: return "PhantomRef";
1378 }
1379 ShouldNotReachHere();
1380 return NULL;
1381 }
1383 #ifndef PRODUCT
1384 void ReferenceProcessor::verify_ok_to_handle_reflists() {
1385 // empty for now
1386 }
1387 #endif
1389 #ifndef PRODUCT
1390 void ReferenceProcessor::clear_discovered_references() {
1391 guarantee(!_discovering_refs, "Discovering refs?");
1392 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
1393 clear_discovered_references(_discovered_refs[i]);
1394 }
1395 }
1397 #endif // PRODUCT