1 /* 2 * Copyright (c) 2001, 2018, Oracle and/or its affiliates. All rights reserved. 3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 4 * 5 * This code is free software; you can redistribute it and/or modify it 6 * under the terms of the GNU General Public License version 2 only, as 7 * published by the Free Software Foundation. 8 * 9 * This code is distributed in the hope that it will be useful, but WITHOUT 10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 12 * version 2 for more details (a copy is included in the LICENSE file that 13 * accompanied this code). 14 * 15 * You should have received a copy of the GNU General Public License version 16 * 2 along with this work; if not, write to the Free Software Foundation, 17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 18 * 19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 20 * or visit www.oracle.com if you need additional information or have any 21 * questions. 22 * 23 */ 24 25 #include "precompiled.hpp" 26 #include "classfile/javaClasses.inline.hpp" 27 #include "classfile/systemDictionary.hpp" 28 #include "gc/shared/collectedHeap.hpp" 29 #include "gc/shared/collectedHeap.inline.hpp" 30 #include "gc/shared/gcTimer.hpp" 31 #include "gc/shared/gcTraceTime.inline.hpp" 32 #include "gc/shared/referencePolicy.hpp" 33 #include "gc/shared/referenceProcessor.inline.hpp" 34 #include "logging/log.hpp" 35 #include "memory/allocation.inline.hpp" 36 #include "memory/resourceArea.hpp" 37 #include "oops/access.inline.hpp" 38 #include "oops/oop.inline.hpp" 39 #include "runtime/java.hpp" 40 41 ReferencePolicy* ReferenceProcessor::_always_clear_soft_ref_policy = NULL; 42 ReferencePolicy* ReferenceProcessor::_default_soft_ref_policy = NULL; 43 jlong ReferenceProcessor::_soft_ref_timestamp_clock = 0; 44 45 void referenceProcessor_init() { 46 ReferenceProcessor::init_statics(); 47 } 48 49 void ReferenceProcessor::init_statics() { 50 // We need a monotonically non-decreasing time in ms but 51 // os::javaTimeMillis() does not guarantee monotonicity. 52 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC; 53 54 // Initialize the soft ref timestamp clock. 55 _soft_ref_timestamp_clock = now; 56 // Also update the soft ref clock in j.l.r.SoftReference 57 java_lang_ref_SoftReference::set_clock(_soft_ref_timestamp_clock); 58 59 _always_clear_soft_ref_policy = new AlwaysClearPolicy(); 60 if (is_server_compilation_mode_vm()) { 61 _default_soft_ref_policy = new LRUMaxHeapPolicy(); 62 } else { 63 _default_soft_ref_policy = new LRUCurrentHeapPolicy(); 64 } 65 if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) { 66 vm_exit_during_initialization("Could not allocate reference policy object"); 67 } 68 guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery || 69 RefDiscoveryPolicy == ReferentBasedDiscovery, 70 "Unrecognized RefDiscoveryPolicy"); 71 } 72 73 void ReferenceProcessor::enable_discovery(bool check_no_refs) { 74 #ifdef ASSERT 75 // Verify that we're not currently discovering refs 76 assert(!_discovering_refs, "nested call?"); 77 78 if (check_no_refs) { 79 // Verify that the discovered lists are empty 80 verify_no_references_recorded(); 81 } 82 #endif // ASSERT 83 84 // Someone could have modified the value of the static 85 // field in the j.l.r.SoftReference class that holds the 86 // soft reference timestamp clock using reflection or 87 // Unsafe between GCs. Unconditionally update the static 88 // field in ReferenceProcessor here so that we use the new 89 // value during reference discovery. 90 91 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock(); 92 _discovering_refs = true; 93 } 94 95 ReferenceProcessor::ReferenceProcessor(BoolObjectClosure* is_subject_to_discovery, 96 bool mt_processing, 97 uint mt_processing_degree, 98 bool mt_discovery, 99 uint mt_discovery_degree, 100 bool atomic_discovery, 101 BoolObjectClosure* is_alive_non_header) : 102 _is_subject_to_discovery(is_subject_to_discovery), 103 _discovering_refs(false), 104 _enqueuing_is_done(false), 105 _is_alive_non_header(is_alive_non_header), 106 _processing_is_mt(mt_processing), 107 _next_id(0) 108 { 109 assert(is_subject_to_discovery != NULL, "must be set"); 110 111 _discovery_is_atomic = atomic_discovery; 112 _discovery_is_mt = mt_discovery; 113 _num_queues = MAX2(1U, mt_processing_degree); 114 _max_num_queues = MAX2(_num_queues, mt_discovery_degree); 115 _discovered_refs = NEW_C_HEAP_ARRAY(DiscoveredList, 116 _max_num_queues * number_of_subclasses_of_ref(), mtGC); 117 118 if (_discovered_refs == NULL) { 119 vm_exit_during_initialization("Could not allocated RefProc Array"); 120 } 121 _discoveredSoftRefs = &_discovered_refs[0]; 122 _discoveredWeakRefs = &_discoveredSoftRefs[_max_num_queues]; 123 _discoveredFinalRefs = &_discoveredWeakRefs[_max_num_queues]; 124 _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_queues]; 125 126 // Initialize all entries to NULL 127 for (uint i = 0; i < _max_num_queues * number_of_subclasses_of_ref(); i++) { 128 _discovered_refs[i].set_head(NULL); 129 _discovered_refs[i].set_length(0); 130 } 131 132 setup_policy(false /* default soft ref policy */); 133 } 134 135 SpanReferenceProcessor::SpanReferenceProcessor(MemRegion span, 136 bool mt_processing, 137 uint mt_processing_degree, 138 bool mt_discovery, 139 uint mt_discovery_degree, 140 bool atomic_discovery, 141 BoolObjectClosure* is_alive_non_header) : 142 ReferenceProcessor(&_span_based_discoverer, 143 mt_processing, 144 mt_processing_degree, 145 mt_discovery, 146 mt_discovery_degree, 147 atomic_discovery, 148 is_alive_non_header), 149 _span_based_discoverer(span) { 150 151 } 152 153 #ifndef PRODUCT 154 void ReferenceProcessor::verify_no_references_recorded() { 155 guarantee(!_discovering_refs, "Discovering refs?"); 156 for (uint i = 0; i < _max_num_queues * number_of_subclasses_of_ref(); i++) { 157 guarantee(_discovered_refs[i].is_empty(), 158 "Found non-empty discovered list at %u", i); 159 } 160 } 161 #endif 162 163 void ReferenceProcessor::weak_oops_do(OopClosure* f) { 164 for (uint i = 0; i < _max_num_queues * number_of_subclasses_of_ref(); i++) { 165 if (UseCompressedOops) { 166 f->do_oop((narrowOop*)_discovered_refs[i].adr_head()); 167 } else { 168 f->do_oop((oop*)_discovered_refs[i].adr_head()); 169 } 170 } 171 } 172 173 void ReferenceProcessor::update_soft_ref_master_clock() { 174 // Update (advance) the soft ref master clock field. This must be done 175 // after processing the soft ref list. 176 177 // We need a monotonically non-decreasing time in ms but 178 // os::javaTimeMillis() does not guarantee monotonicity. 179 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC; 180 jlong soft_ref_clock = java_lang_ref_SoftReference::clock(); 181 assert(soft_ref_clock == _soft_ref_timestamp_clock, "soft ref clocks out of sync"); 182 183 NOT_PRODUCT( 184 if (now < _soft_ref_timestamp_clock) { 185 log_warning(gc)("time warp: " JLONG_FORMAT " to " JLONG_FORMAT, 186 _soft_ref_timestamp_clock, now); 187 } 188 ) 189 // The values of now and _soft_ref_timestamp_clock are set using 190 // javaTimeNanos(), which is guaranteed to be monotonically 191 // non-decreasing provided the underlying platform provides such 192 // a time source (and it is bug free). 193 // In product mode, however, protect ourselves from non-monotonicity. 194 if (now > _soft_ref_timestamp_clock) { 195 _soft_ref_timestamp_clock = now; 196 java_lang_ref_SoftReference::set_clock(now); 197 } 198 // Else leave clock stalled at its old value until time progresses 199 // past clock value. 200 } 201 202 size_t ReferenceProcessor::total_count(DiscoveredList lists[]) const { 203 size_t total = 0; 204 for (uint i = 0; i < _max_num_queues; ++i) { 205 total += lists[i].length(); 206 } 207 return total; 208 } 209 210 ReferenceProcessorStats ReferenceProcessor::process_discovered_references( 211 BoolObjectClosure* is_alive, 212 OopClosure* keep_alive, 213 VoidClosure* complete_gc, 214 AbstractRefProcTaskExecutor* task_executor, 215 ReferenceProcessorPhaseTimes* phase_times) { 216 217 double start_time = os::elapsedTime(); 218 219 assert(!enqueuing_is_done(), "If here enqueuing should not be complete"); 220 // Stop treating discovered references specially. 221 disable_discovery(); 222 223 // If discovery was concurrent, someone could have modified 224 // the value of the static field in the j.l.r.SoftReference 225 // class that holds the soft reference timestamp clock using 226 // reflection or Unsafe between when discovery was enabled and 227 // now. Unconditionally update the static field in ReferenceProcessor 228 // here so that we use the new value during processing of the 229 // discovered soft refs. 230 231 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock(); 232 233 ReferenceProcessorStats stats(total_count(_discoveredSoftRefs), 234 total_count(_discoveredWeakRefs), 235 total_count(_discoveredFinalRefs), 236 total_count(_discoveredPhantomRefs)); 237 238 // Soft references 239 { 240 RefProcPhaseTimesTracker tt(REF_SOFT, phase_times, this); 241 process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true, 242 is_alive, keep_alive, complete_gc, task_executor, phase_times); 243 } 244 245 update_soft_ref_master_clock(); 246 247 // Weak references 248 { 249 RefProcPhaseTimesTracker tt(REF_WEAK, phase_times, this); 250 process_discovered_reflist(_discoveredWeakRefs, NULL, true, 251 is_alive, keep_alive, complete_gc, task_executor, phase_times); 252 } 253 254 // Final references 255 { 256 RefProcPhaseTimesTracker tt(REF_FINAL, phase_times, this); 257 process_discovered_reflist(_discoveredFinalRefs, NULL, false, 258 is_alive, keep_alive, complete_gc, task_executor, phase_times); 259 } 260 261 // Phantom references 262 { 263 RefProcPhaseTimesTracker tt(REF_PHANTOM, phase_times, this); 264 process_discovered_reflist(_discoveredPhantomRefs, NULL, true, 265 is_alive, keep_alive, complete_gc, task_executor, phase_times); 266 } 267 268 if (task_executor != NULL) { 269 // Record the work done by the parallel workers. 270 task_executor->set_single_threaded_mode(); 271 } 272 273 phase_times->set_total_time_ms((os::elapsedTime() - start_time) * 1000); 274 275 return stats; 276 } 277 278 void ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor, 279 ReferenceProcessorPhaseTimes* phase_times) { 280 // Enqueue references that are not made active again, and 281 // clear the decks for the next collection (cycle). 282 enqueue_discovered_reflists(task_executor, phase_times); 283 284 // Stop treating discovered references specially. 285 disable_discovery(); 286 } 287 288 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list) { 289 // Given a list of refs linked through the "discovered" field 290 // (java.lang.ref.Reference.discovered), self-loop their "next" field 291 // thus distinguishing them from active References, then 292 // prepend them to the pending list. 293 // 294 // The Java threads will see the Reference objects linked together through 295 // the discovered field. Instead of trying to do the write barrier updates 296 // in all places in the reference processor where we manipulate the discovered 297 // field we make sure to do the barrier here where we anyway iterate through 298 // all linked Reference objects. Note that it is important to not dirty any 299 // cards during reference processing since this will cause card table 300 // verification to fail for G1. 301 log_develop_trace(gc, ref)("ReferenceProcessor::enqueue_discovered_reflist list " INTPTR_FORMAT, p2i(&refs_list)); 302 303 oop obj = NULL; 304 oop next_discovered = refs_list.head(); 305 // Walk down the list, self-looping the next field 306 // so that the References are not considered active. 307 while (obj != next_discovered) { 308 obj = next_discovered; 309 assert(obj->is_instance(), "should be an instance object"); 310 assert(InstanceKlass::cast(obj->klass())->is_reference_instance_klass(), "should be reference object"); 311 next_discovered = java_lang_ref_Reference::discovered(obj); 312 log_develop_trace(gc, ref)(" obj " INTPTR_FORMAT "/next_discovered " INTPTR_FORMAT, p2i(obj), p2i(next_discovered)); 313 assert(java_lang_ref_Reference::next(obj) == NULL, 314 "Reference not active; should not be discovered"); 315 // Self-loop next, so as to make Ref not active. 316 java_lang_ref_Reference::set_next_raw(obj, obj); 317 if (next_discovered != obj) { 318 HeapAccess<AS_NO_KEEPALIVE>::oop_store_at(obj, java_lang_ref_Reference::discovered_offset, next_discovered); 319 } else { 320 // This is the last object. 321 // Swap refs_list into pending list and set obj's 322 // discovered to what we read from the pending list. 323 oop old = Universe::swap_reference_pending_list(refs_list.head()); 324 HeapAccess<AS_NO_KEEPALIVE>::oop_store_at(obj, java_lang_ref_Reference::discovered_offset, old); 325 } 326 } 327 } 328 329 // Parallel enqueue task 330 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask { 331 public: 332 RefProcEnqueueTask(ReferenceProcessor& ref_processor, 333 DiscoveredList discovered_refs[], 334 int n_queues, 335 ReferenceProcessorPhaseTimes* phase_times) 336 : EnqueueTask(ref_processor, discovered_refs, n_queues, phase_times) 337 { } 338 339 virtual void work(unsigned int work_id) { 340 RefProcWorkerTimeTracker tt(ReferenceProcessorPhaseTimes::RefEnqueue, _phase_times, work_id); 341 342 assert(work_id < (unsigned int)_ref_processor.max_num_queues(), "Index out-of-bounds"); 343 // Simplest first cut: static partitioning. 344 int index = work_id; 345 // The increment on "index" must correspond to the maximum number of queues 346 // (n_queues) with which that ReferenceProcessor was created. That 347 // is because of the "clever" way the discovered references lists were 348 // allocated and are indexed into. 349 assert(_n_queues == (int) _ref_processor.max_num_queues(), "Different number not expected"); 350 for (int j = 0; 351 j < ReferenceProcessor::number_of_subclasses_of_ref(); 352 j++, index += _n_queues) { 353 _ref_processor.enqueue_discovered_reflist(_refs_lists[index]); 354 _refs_lists[index].set_head(NULL); 355 _refs_lists[index].set_length(0); 356 } 357 } 358 }; 359 360 // Enqueue references that are not made active again 361 void ReferenceProcessor::enqueue_discovered_reflists(AbstractRefProcTaskExecutor* task_executor, 362 ReferenceProcessorPhaseTimes* phase_times) { 363 364 ReferenceProcessorStats stats(total_count(_discoveredSoftRefs), 365 total_count(_discoveredWeakRefs), 366 total_count(_discoveredFinalRefs), 367 total_count(_discoveredPhantomRefs)); 368 369 RefProcEnqueueTimeTracker tt(phase_times, stats); 370 371 if (_processing_is_mt && task_executor != NULL) { 372 // Parallel code 373 RefProcEnqueueTask tsk(*this, _discovered_refs, _max_num_queues, phase_times); 374 task_executor->execute(tsk); 375 } else { 376 // Serial code: call the parent class's implementation 377 for (uint i = 0; i < _max_num_queues * number_of_subclasses_of_ref(); i++) { 378 enqueue_discovered_reflist(_discovered_refs[i]); 379 _discovered_refs[i].set_head(NULL); 380 _discovered_refs[i].set_length(0); 381 } 382 } 383 } 384 385 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) { 386 _discovered_addr = java_lang_ref_Reference::discovered_addr_raw(_ref); 387 oop discovered = java_lang_ref_Reference::discovered(_ref); 388 assert(_discovered_addr && oopDesc::is_oop_or_null(discovered), 389 "Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered)); 390 _next_discovered = discovered; 391 _referent_addr = java_lang_ref_Reference::referent_addr_raw(_ref); 392 _referent = java_lang_ref_Reference::referent(_ref); 393 assert(Universe::heap()->is_in_reserved_or_null(_referent), 394 "Wrong oop found in java.lang.Reference object"); 395 assert(allow_null_referent ? 396 oopDesc::is_oop_or_null(_referent) 397 : oopDesc::is_oop(_referent), 398 "Expected an oop%s for referent field at " PTR_FORMAT, 399 (allow_null_referent ? " or NULL" : ""), 400 p2i(_referent)); 401 } 402 403 void DiscoveredListIterator::remove() { 404 assert(oopDesc::is_oop(_ref), "Dropping a bad reference"); 405 RawAccess<>::oop_store(_discovered_addr, oop(NULL)); 406 407 // First _prev_next ref actually points into DiscoveredList (gross). 408 oop new_next; 409 if (_next_discovered == _ref) { 410 // At the end of the list, we should make _prev point to itself. 411 // If _ref is the first ref, then _prev_next will be in the DiscoveredList, 412 // and _prev will be NULL. 413 new_next = _prev_discovered; 414 } else { 415 new_next = _next_discovered; 416 } 417 // Remove Reference object from discovered list. Note that G1 does not need a 418 // pre-barrier here because we know the Reference has already been found/marked, 419 // that's how it ended up in the discovered list in the first place. 420 RawAccess<>::oop_store(_prev_discovered_addr, new_next); 421 NOT_PRODUCT(_removed++); 422 _refs_list.dec_length(1); 423 } 424 425 void DiscoveredListIterator::clear_referent() { 426 RawAccess<>::oop_store(_referent_addr, oop(NULL)); 427 } 428 429 // NOTE: process_phase*() are largely similar, and at a high level 430 // merely iterate over the extant list applying a predicate to 431 // each of its elements and possibly removing that element from the 432 // list and applying some further closures to that element. 433 // We should consider the possibility of replacing these 434 // process_phase*() methods by abstracting them into 435 // a single general iterator invocation that receives appropriate 436 // closures that accomplish this work. 437 438 // (SoftReferences only) Traverse the list and remove any SoftReferences whose 439 // referents are not alive, but that should be kept alive for policy reasons. 440 // Keep alive the transitive closure of all such referents. 441 void 442 ReferenceProcessor::process_phase1(DiscoveredList& refs_list, 443 ReferencePolicy* policy, 444 BoolObjectClosure* is_alive, 445 OopClosure* keep_alive, 446 VoidClosure* complete_gc) { 447 assert(policy != NULL, "Must have a non-NULL policy"); 448 DiscoveredListIterator iter(refs_list, keep_alive, is_alive); 449 // Decide which softly reachable refs should be kept alive. 450 while (iter.has_next()) { 451 iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */)); 452 bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive(); 453 if (referent_is_dead && 454 !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) { 455 log_develop_trace(gc, ref)("Dropping reference (" INTPTR_FORMAT ": %s" ") by policy", 456 p2i(iter.obj()), iter.obj()->klass()->internal_name()); 457 // Remove Reference object from list 458 iter.remove(); 459 // keep the referent around 460 iter.make_referent_alive(); 461 iter.move_to_next(); 462 } else { 463 iter.next(); 464 } 465 } 466 // Close the reachable set 467 complete_gc->do_void(); 468 log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " dead Refs out of " SIZE_FORMAT " discovered Refs by policy, from list " INTPTR_FORMAT, 469 iter.removed(), iter.processed(), p2i(&refs_list)); 470 } 471 472 void ReferenceProcessor::process_phase2(DiscoveredList& refs_list, 473 BoolObjectClosure* is_alive, 474 OopClosure* keep_alive, 475 VoidClosure* complete_gc) { 476 if (discovery_is_atomic()) { 477 // complete_gc is ignored in this case for this phase 478 pp2_work(refs_list, is_alive, keep_alive); 479 } else { 480 assert(complete_gc != NULL, "Error"); 481 pp2_work_concurrent_discovery(refs_list, is_alive, 482 keep_alive, complete_gc); 483 } 484 } 485 // Traverse the list and remove any Refs that are not active, or 486 // whose referents are either alive or NULL. 487 void 488 ReferenceProcessor::pp2_work(DiscoveredList& refs_list, 489 BoolObjectClosure* is_alive, 490 OopClosure* keep_alive) { 491 assert(discovery_is_atomic(), "Error"); 492 DiscoveredListIterator iter(refs_list, keep_alive, is_alive); 493 while (iter.has_next()) { 494 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */)); 495 DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());) 496 assert(next == NULL, "Should not discover inactive Reference"); 497 if (iter.is_referent_alive()) { 498 log_develop_trace(gc, ref)("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)", 499 p2i(iter.obj()), iter.obj()->klass()->internal_name()); 500 // The referent is reachable after all. 501 // Remove Reference object from list. 502 iter.remove(); 503 // Update the referent pointer as necessary: Note that this 504 // should not entail any recursive marking because the 505 // referent must already have been traversed. 506 iter.make_referent_alive(); 507 iter.move_to_next(); 508 } else { 509 iter.next(); 510 } 511 } 512 NOT_PRODUCT( 513 if (iter.processed() > 0) { 514 log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT 515 " Refs in discovered list " INTPTR_FORMAT, 516 iter.removed(), iter.processed(), p2i(&refs_list)); 517 } 518 ) 519 } 520 521 void 522 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList& refs_list, 523 BoolObjectClosure* is_alive, 524 OopClosure* keep_alive, 525 VoidClosure* complete_gc) { 526 assert(!discovery_is_atomic(), "Error"); 527 DiscoveredListIterator iter(refs_list, keep_alive, is_alive); 528 while (iter.has_next()) { 529 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */)); 530 HeapWord* next_addr = java_lang_ref_Reference::next_addr_raw(iter.obj()); 531 oop next = java_lang_ref_Reference::next(iter.obj()); 532 if ((iter.referent() == NULL || iter.is_referent_alive() || 533 next != NULL)) { 534 assert(oopDesc::is_oop_or_null(next), "Expected an oop or NULL for next field at " PTR_FORMAT, p2i(next)); 535 // Remove Reference object from list 536 iter.remove(); 537 // Trace the cohorts 538 iter.make_referent_alive(); 539 if (UseCompressedOops) { 540 keep_alive->do_oop((narrowOop*)next_addr); 541 } else { 542 keep_alive->do_oop((oop*)next_addr); 543 } 544 iter.move_to_next(); 545 } else { 546 iter.next(); 547 } 548 } 549 // Now close the newly reachable set 550 complete_gc->do_void(); 551 NOT_PRODUCT( 552 if (iter.processed() > 0) { 553 log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT 554 " Refs in discovered list " INTPTR_FORMAT, 555 iter.removed(), iter.processed(), p2i(&refs_list)); 556 } 557 ) 558 } 559 560 void ReferenceProcessor::process_phase3(DiscoveredList& refs_list, 561 bool clear_referent, 562 BoolObjectClosure* is_alive, 563 OopClosure* keep_alive, 564 VoidClosure* complete_gc) { 565 ResourceMark rm; 566 DiscoveredListIterator iter(refs_list, keep_alive, is_alive); 567 while (iter.has_next()) { 568 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */)); 569 if (clear_referent) { 570 // NULL out referent pointer 571 iter.clear_referent(); 572 } else { 573 // keep the referent around 574 iter.make_referent_alive(); 575 } 576 log_develop_trace(gc, ref)("Adding %sreference (" INTPTR_FORMAT ": %s) as pending", 577 clear_referent ? "cleared " : "", p2i(iter.obj()), iter.obj()->klass()->internal_name()); 578 assert(oopDesc::is_oop(iter.obj(), UseConcMarkSweepGC), "Adding a bad reference"); 579 iter.next(); 580 } 581 // Close the reachable set 582 complete_gc->do_void(); 583 } 584 585 void 586 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) { 587 oop obj = NULL; 588 oop next = refs_list.head(); 589 while (next != obj) { 590 obj = next; 591 next = java_lang_ref_Reference::discovered(obj); 592 java_lang_ref_Reference::set_discovered_raw(obj, NULL); 593 } 594 refs_list.set_head(NULL); 595 refs_list.set_length(0); 596 } 597 598 void ReferenceProcessor::abandon_partial_discovery() { 599 // loop over the lists 600 for (uint i = 0; i < _max_num_queues * number_of_subclasses_of_ref(); i++) { 601 if ((i % _max_num_queues) == 0) { 602 log_develop_trace(gc, ref)("Abandoning %s discovered list", list_name(i)); 603 } 604 clear_discovered_references(_discovered_refs[i]); 605 } 606 } 607 608 size_t ReferenceProcessor::total_reference_count(ReferenceType type) const { 609 DiscoveredList* list = NULL; 610 611 switch (type) { 612 case REF_SOFT: 613 list = _discoveredSoftRefs; 614 break; 615 case REF_WEAK: 616 list = _discoveredWeakRefs; 617 break; 618 case REF_FINAL: 619 list = _discoveredFinalRefs; 620 break; 621 case REF_PHANTOM: 622 list = _discoveredPhantomRefs; 623 break; 624 case REF_OTHER: 625 case REF_NONE: 626 default: 627 ShouldNotReachHere(); 628 } 629 return total_count(list); 630 } 631 632 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask { 633 public: 634 RefProcPhase1Task(ReferenceProcessor& ref_processor, 635 DiscoveredList refs_lists[], 636 ReferencePolicy* policy, 637 bool marks_oops_alive, 638 ReferenceProcessorPhaseTimes* phase_times) 639 : ProcessTask(ref_processor, refs_lists, marks_oops_alive, phase_times), 640 _policy(policy) 641 { } 642 virtual void work(unsigned int i, BoolObjectClosure& is_alive, 643 OopClosure& keep_alive, 644 VoidClosure& complete_gc) 645 { 646 RefProcWorkerTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase1, _phase_times, i); 647 648 _ref_processor.process_phase1(_refs_lists[i], _policy, 649 &is_alive, &keep_alive, &complete_gc); 650 } 651 private: 652 ReferencePolicy* _policy; 653 }; 654 655 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask { 656 public: 657 RefProcPhase2Task(ReferenceProcessor& ref_processor, 658 DiscoveredList refs_lists[], 659 bool marks_oops_alive, 660 ReferenceProcessorPhaseTimes* phase_times) 661 : ProcessTask(ref_processor, refs_lists, marks_oops_alive, phase_times) 662 { } 663 virtual void work(unsigned int i, BoolObjectClosure& is_alive, 664 OopClosure& keep_alive, 665 VoidClosure& complete_gc) 666 { 667 RefProcWorkerTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase2, _phase_times, i); 668 669 _ref_processor.process_phase2(_refs_lists[i], 670 &is_alive, &keep_alive, &complete_gc); 671 } 672 }; 673 674 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask { 675 public: 676 RefProcPhase3Task(ReferenceProcessor& ref_processor, 677 DiscoveredList refs_lists[], 678 bool clear_referent, 679 bool marks_oops_alive, 680 ReferenceProcessorPhaseTimes* phase_times) 681 : ProcessTask(ref_processor, refs_lists, marks_oops_alive, phase_times), 682 _clear_referent(clear_referent) 683 { } 684 virtual void work(unsigned int i, BoolObjectClosure& is_alive, 685 OopClosure& keep_alive, 686 VoidClosure& complete_gc) 687 { 688 RefProcWorkerTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase3, _phase_times, i); 689 690 _ref_processor.process_phase3(_refs_lists[i], _clear_referent, 691 &is_alive, &keep_alive, &complete_gc); 692 } 693 private: 694 bool _clear_referent; 695 }; 696 697 #ifndef PRODUCT 698 void ReferenceProcessor::log_reflist_counts(DiscoveredList ref_lists[], uint active_length, size_t total_refs) { 699 if (!log_is_enabled(Trace, gc, ref)) { 700 return; 701 } 702 703 stringStream st; 704 for (uint i = 0; i < active_length; ++i) { 705 st.print(SIZE_FORMAT " ", ref_lists[i].length()); 706 } 707 log_develop_trace(gc, ref)("%s= " SIZE_FORMAT, st.as_string(), total_refs); 708 #ifdef ASSERT 709 for (uint i = active_length; i < _max_num_queues; i++) { 710 assert(ref_lists[i].length() == 0, SIZE_FORMAT " unexpected References in %u", 711 ref_lists[i].length(), i); 712 } 713 #endif 714 } 715 #endif 716 717 void ReferenceProcessor::set_active_mt_degree(uint v) { 718 _num_queues = v; 719 _next_id = 0; 720 } 721 722 // Balances reference queues. 723 // Move entries from all queues[0, 1, ..., _max_num_q-1] to 724 // queues[0, 1, ..., _num_q-1] because only the first _num_q 725 // corresponding to the active workers will be processed. 726 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[]) 727 { 728 // calculate total length 729 size_t total_refs = 0; 730 log_develop_trace(gc, ref)("Balance ref_lists "); 731 732 for (uint i = 0; i < _max_num_queues; ++i) { 733 total_refs += ref_lists[i].length(); 734 } 735 log_reflist_counts(ref_lists, _max_num_queues, total_refs); 736 size_t avg_refs = total_refs / _num_queues + 1; 737 uint to_idx = 0; 738 for (uint from_idx = 0; from_idx < _max_num_queues; from_idx++) { 739 bool move_all = false; 740 if (from_idx >= _num_queues) { 741 move_all = ref_lists[from_idx].length() > 0; 742 } 743 while ((ref_lists[from_idx].length() > avg_refs) || 744 move_all) { 745 assert(to_idx < _num_queues, "Sanity Check!"); 746 if (ref_lists[to_idx].length() < avg_refs) { 747 // move superfluous refs 748 size_t refs_to_move; 749 // Move all the Ref's if the from queue will not be processed. 750 if (move_all) { 751 refs_to_move = MIN2(ref_lists[from_idx].length(), 752 avg_refs - ref_lists[to_idx].length()); 753 } else { 754 refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs, 755 avg_refs - ref_lists[to_idx].length()); 756 } 757 758 assert(refs_to_move > 0, "otherwise the code below will fail"); 759 760 oop move_head = ref_lists[from_idx].head(); 761 oop move_tail = move_head; 762 oop new_head = move_head; 763 // find an element to split the list on 764 for (size_t j = 0; j < refs_to_move; ++j) { 765 move_tail = new_head; 766 new_head = java_lang_ref_Reference::discovered(new_head); 767 } 768 769 // Add the chain to the to list. 770 if (ref_lists[to_idx].head() == NULL) { 771 // to list is empty. Make a loop at the end. 772 java_lang_ref_Reference::set_discovered_raw(move_tail, move_tail); 773 } else { 774 java_lang_ref_Reference::set_discovered_raw(move_tail, ref_lists[to_idx].head()); 775 } 776 ref_lists[to_idx].set_head(move_head); 777 ref_lists[to_idx].inc_length(refs_to_move); 778 779 // Remove the chain from the from list. 780 if (move_tail == new_head) { 781 // We found the end of the from list. 782 ref_lists[from_idx].set_head(NULL); 783 } else { 784 ref_lists[from_idx].set_head(new_head); 785 } 786 ref_lists[from_idx].dec_length(refs_to_move); 787 if (ref_lists[from_idx].length() == 0) { 788 break; 789 } 790 } else { 791 to_idx = (to_idx + 1) % _num_queues; 792 } 793 } 794 } 795 #ifdef ASSERT 796 size_t balanced_total_refs = 0; 797 for (uint i = 0; i < _num_queues; ++i) { 798 balanced_total_refs += ref_lists[i].length(); 799 } 800 log_reflist_counts(ref_lists, _num_queues, balanced_total_refs); 801 assert(total_refs == balanced_total_refs, "Balancing was incomplete"); 802 #endif 803 } 804 805 void ReferenceProcessor::balance_all_queues() { 806 balance_queues(_discoveredSoftRefs); 807 balance_queues(_discoveredWeakRefs); 808 balance_queues(_discoveredFinalRefs); 809 balance_queues(_discoveredPhantomRefs); 810 } 811 812 void ReferenceProcessor::process_discovered_reflist( 813 DiscoveredList refs_lists[], 814 ReferencePolicy* policy, 815 bool clear_referent, 816 BoolObjectClosure* is_alive, 817 OopClosure* keep_alive, 818 VoidClosure* complete_gc, 819 AbstractRefProcTaskExecutor* task_executor, 820 ReferenceProcessorPhaseTimes* phase_times) 821 { 822 bool mt_processing = task_executor != NULL && _processing_is_mt; 823 824 phase_times->set_processing_is_mt(mt_processing); 825 826 if (mt_processing && ParallelRefProcBalancingEnabled) { 827 RefProcBalanceQueuesTimeTracker tt(phase_times); 828 balance_queues(refs_lists); 829 } 830 831 // Phase 1 (soft refs only): 832 // . Traverse the list and remove any SoftReferences whose 833 // referents are not alive, but that should be kept alive for 834 // policy reasons. Keep alive the transitive closure of all 835 // such referents. 836 if (policy != NULL) { 837 RefProcParPhaseTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase1, phase_times); 838 839 if (mt_processing) { 840 RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/, phase_times); 841 task_executor->execute(phase1); 842 } else { 843 for (uint i = 0; i < _max_num_queues; i++) { 844 process_phase1(refs_lists[i], policy, 845 is_alive, keep_alive, complete_gc); 846 } 847 } 848 } else { // policy == NULL 849 assert(refs_lists != _discoveredSoftRefs, 850 "Policy must be specified for soft references."); 851 } 852 853 // Phase 2: 854 // . Traverse the list and remove any refs whose referents are alive. 855 { 856 RefProcParPhaseTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase2, phase_times); 857 858 if (mt_processing) { 859 RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/, phase_times); 860 task_executor->execute(phase2); 861 } else { 862 for (uint i = 0; i < _max_num_queues; i++) { 863 process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc); 864 } 865 } 866 } 867 868 // Phase 3: 869 // . Traverse the list and process referents as appropriate. 870 { 871 RefProcParPhaseTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase3, phase_times); 872 873 if (mt_processing) { 874 RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/, phase_times); 875 task_executor->execute(phase3); 876 } else { 877 for (uint i = 0; i < _max_num_queues; i++) { 878 process_phase3(refs_lists[i], clear_referent, 879 is_alive, keep_alive, complete_gc); 880 } 881 } 882 } 883 } 884 885 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) { 886 uint id = 0; 887 // Determine the queue index to use for this object. 888 if (_discovery_is_mt) { 889 // During a multi-threaded discovery phase, 890 // each thread saves to its "own" list. 891 Thread* thr = Thread::current(); 892 id = thr->as_Worker_thread()->id(); 893 } else { 894 // single-threaded discovery, we save in round-robin 895 // fashion to each of the lists. 896 if (_processing_is_mt) { 897 id = next_id(); 898 } 899 } 900 assert(id < _max_num_queues, "Id is out-of-bounds id %u and max id %u)", id, _max_num_queues); 901 902 // Get the discovered queue to which we will add 903 DiscoveredList* list = NULL; 904 switch (rt) { 905 case REF_OTHER: 906 // Unknown reference type, no special treatment 907 break; 908 case REF_SOFT: 909 list = &_discoveredSoftRefs[id]; 910 break; 911 case REF_WEAK: 912 list = &_discoveredWeakRefs[id]; 913 break; 914 case REF_FINAL: 915 list = &_discoveredFinalRefs[id]; 916 break; 917 case REF_PHANTOM: 918 list = &_discoveredPhantomRefs[id]; 919 break; 920 case REF_NONE: 921 // we should not reach here if we are an InstanceRefKlass 922 default: 923 ShouldNotReachHere(); 924 } 925 log_develop_trace(gc, ref)("Thread %d gets list " INTPTR_FORMAT, id, p2i(list)); 926 return list; 927 } 928 929 inline void 930 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list, 931 oop obj, 932 HeapWord* discovered_addr) { 933 assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller"); 934 // First we must make sure this object is only enqueued once. CAS in a non null 935 // discovered_addr. 936 oop current_head = refs_list.head(); 937 // The last ref must have its discovered field pointing to itself. 938 oop next_discovered = (current_head != NULL) ? current_head : obj; 939 940 oop retest = RawAccess<>::oop_atomic_cmpxchg(next_discovered, discovered_addr, oop(NULL)); 941 942 if (retest == NULL) { 943 // This thread just won the right to enqueue the object. 944 // We have separate lists for enqueueing, so no synchronization 945 // is necessary. 946 refs_list.set_head(obj); 947 refs_list.inc_length(1); 948 949 log_develop_trace(gc, ref)("Discovered reference (mt) (" INTPTR_FORMAT ": %s)", 950 p2i(obj), obj->klass()->internal_name()); 951 } else { 952 // If retest was non NULL, another thread beat us to it: 953 // The reference has already been discovered... 954 log_develop_trace(gc, ref)("Already discovered reference (" INTPTR_FORMAT ": %s)", 955 p2i(obj), obj->klass()->internal_name()); 956 } 957 } 958 959 #ifndef PRODUCT 960 // Non-atomic (i.e. concurrent) discovery might allow us 961 // to observe j.l.References with NULL referents, being those 962 // cleared concurrently by mutators during (or after) discovery. 963 void ReferenceProcessor::verify_referent(oop obj) { 964 bool da = discovery_is_atomic(); 965 oop referent = java_lang_ref_Reference::referent(obj); 966 assert(da ? oopDesc::is_oop(referent) : oopDesc::is_oop_or_null(referent), 967 "Bad referent " INTPTR_FORMAT " found in Reference " 968 INTPTR_FORMAT " during %satomic discovery ", 969 p2i(referent), p2i(obj), da ? "" : "non-"); 970 } 971 #endif 972 973 template <class T> 974 bool ReferenceProcessor::is_subject_to_discovery(T const obj) const { 975 return _is_subject_to_discovery->do_object_b(obj); 976 } 977 978 // We mention two of several possible choices here: 979 // #0: if the reference object is not in the "originating generation" 980 // (or part of the heap being collected, indicated by our "span" 981 // we don't treat it specially (i.e. we scan it as we would 982 // a normal oop, treating its references as strong references). 983 // This means that references can't be discovered unless their 984 // referent is also in the same span. This is the simplest, 985 // most "local" and most conservative approach, albeit one 986 // that may cause weak references to be enqueued least promptly. 987 // We call this choice the "ReferenceBasedDiscovery" policy. 988 // #1: the reference object may be in any generation (span), but if 989 // the referent is in the generation (span) being currently collected 990 // then we can discover the reference object, provided 991 // the object has not already been discovered by 992 // a different concurrently running collector (as may be the 993 // case, for instance, if the reference object is in CMS and 994 // the referent in DefNewGeneration), and provided the processing 995 // of this reference object by the current collector will 996 // appear atomic to every other collector in the system. 997 // (Thus, for instance, a concurrent collector may not 998 // discover references in other generations even if the 999 // referent is in its own generation). This policy may, 1000 // in certain cases, enqueue references somewhat sooner than 1001 // might Policy #0 above, but at marginally increased cost 1002 // and complexity in processing these references. 1003 // We call this choice the "RefeferentBasedDiscovery" policy. 1004 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) { 1005 // Make sure we are discovering refs (rather than processing discovered refs). 1006 if (!_discovering_refs || !RegisterReferences) { 1007 return false; 1008 } 1009 // We only discover active references. 1010 oop next = java_lang_ref_Reference::next(obj); 1011 if (next != NULL) { // Ref is no longer active 1012 return false; 1013 } 1014 1015 if (RefDiscoveryPolicy == ReferenceBasedDiscovery && 1016 !is_subject_to_discovery(obj)) { 1017 // Reference is not in the originating generation; 1018 // don't treat it specially (i.e. we want to scan it as a normal 1019 // object with strong references). 1020 return false; 1021 } 1022 1023 // We only discover references whose referents are not (yet) 1024 // known to be strongly reachable. 1025 if (is_alive_non_header() != NULL) { 1026 verify_referent(obj); 1027 if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) { 1028 return false; // referent is reachable 1029 } 1030 } 1031 if (rt == REF_SOFT) { 1032 // For soft refs we can decide now if these are not 1033 // current candidates for clearing, in which case we 1034 // can mark through them now, rather than delaying that 1035 // to the reference-processing phase. Since all current 1036 // time-stamp policies advance the soft-ref clock only 1037 // at a full collection cycle, this is always currently 1038 // accurate. 1039 if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) { 1040 return false; 1041 } 1042 } 1043 1044 ResourceMark rm; // Needed for tracing. 1045 1046 HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr_raw(obj); 1047 const oop discovered = java_lang_ref_Reference::discovered(obj); 1048 assert(oopDesc::is_oop_or_null(discovered), "Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered)); 1049 if (discovered != NULL) { 1050 // The reference has already been discovered... 1051 log_develop_trace(gc, ref)("Already discovered reference (" INTPTR_FORMAT ": %s)", 1052 p2i(obj), obj->klass()->internal_name()); 1053 if (RefDiscoveryPolicy == ReferentBasedDiscovery) { 1054 // assumes that an object is not processed twice; 1055 // if it's been already discovered it must be on another 1056 // generation's discovered list; so we won't discover it. 1057 return false; 1058 } else { 1059 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery, 1060 "Unrecognized policy"); 1061 // Check assumption that an object is not potentially 1062 // discovered twice except by concurrent collectors that potentially 1063 // trace the same Reference object twice. 1064 assert(UseConcMarkSweepGC || UseG1GC, 1065 "Only possible with a concurrent marking collector"); 1066 return true; 1067 } 1068 } 1069 1070 if (RefDiscoveryPolicy == ReferentBasedDiscovery) { 1071 verify_referent(obj); 1072 // Discover if and only if EITHER: 1073 // .. reference is in our span, OR 1074 // .. we are an atomic collector and referent is in our span 1075 if (is_subject_to_discovery(obj) || 1076 (discovery_is_atomic() && 1077 is_subject_to_discovery(java_lang_ref_Reference::referent(obj)))) { 1078 } else { 1079 return false; 1080 } 1081 } else { 1082 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery && 1083 is_subject_to_discovery(obj), "code inconsistency"); 1084 } 1085 1086 // Get the right type of discovered queue head. 1087 DiscoveredList* list = get_discovered_list(rt); 1088 if (list == NULL) { 1089 return false; // nothing special needs to be done 1090 } 1091 1092 if (_discovery_is_mt) { 1093 add_to_discovered_list_mt(*list, obj, discovered_addr); 1094 } else { 1095 // We do a raw store here: the field will be visited later when processing 1096 // the discovered references. 1097 oop current_head = list->head(); 1098 // The last ref must have its discovered field pointing to itself. 1099 oop next_discovered = (current_head != NULL) ? current_head : obj; 1100 1101 assert(discovered == NULL, "control point invariant"); 1102 RawAccess<>::oop_store(discovered_addr, next_discovered); 1103 list->set_head(obj); 1104 list->inc_length(1); 1105 1106 log_develop_trace(gc, ref)("Discovered reference (" INTPTR_FORMAT ": %s)", p2i(obj), obj->klass()->internal_name()); 1107 } 1108 assert(oopDesc::is_oop(obj), "Discovered a bad reference"); 1109 verify_referent(obj); 1110 return true; 1111 } 1112 1113 bool ReferenceProcessor::has_discovered_references() { 1114 for (uint i = 0; i < _max_num_queues * number_of_subclasses_of_ref(); i++) { 1115 if (!_discovered_refs[i].is_empty()) { 1116 return true; 1117 } 1118 } 1119 return false; 1120 } 1121 1122 // Preclean the discovered references by removing those 1123 // whose referents are alive, and by marking from those that 1124 // are not active. These lists can be handled here 1125 // in any order and, indeed, concurrently. 1126 void ReferenceProcessor::preclean_discovered_references( 1127 BoolObjectClosure* is_alive, 1128 OopClosure* keep_alive, 1129 VoidClosure* complete_gc, 1130 YieldClosure* yield, 1131 GCTimer* gc_timer) { 1132 1133 // Soft references 1134 { 1135 GCTraceTime(Debug, gc, ref) tm("Preclean SoftReferences", gc_timer); 1136 for (uint i = 0; i < _max_num_queues; i++) { 1137 if (yield->should_return()) { 1138 return; 1139 } 1140 preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive, 1141 keep_alive, complete_gc, yield); 1142 } 1143 } 1144 1145 // Weak references 1146 { 1147 GCTraceTime(Debug, gc, ref) tm("Preclean WeakReferences", gc_timer); 1148 for (uint i = 0; i < _max_num_queues; i++) { 1149 if (yield->should_return()) { 1150 return; 1151 } 1152 preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive, 1153 keep_alive, complete_gc, yield); 1154 } 1155 } 1156 1157 // Final references 1158 { 1159 GCTraceTime(Debug, gc, ref) tm("Preclean FinalReferences", gc_timer); 1160 for (uint i = 0; i < _max_num_queues; i++) { 1161 if (yield->should_return()) { 1162 return; 1163 } 1164 preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive, 1165 keep_alive, complete_gc, yield); 1166 } 1167 } 1168 1169 // Phantom references 1170 { 1171 GCTraceTime(Debug, gc, ref) tm("Preclean PhantomReferences", gc_timer); 1172 for (uint i = 0; i < _max_num_queues; i++) { 1173 if (yield->should_return()) { 1174 return; 1175 } 1176 preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive, 1177 keep_alive, complete_gc, yield); 1178 } 1179 } 1180 } 1181 1182 // Walk the given discovered ref list, and remove all reference objects 1183 // whose referents are still alive, whose referents are NULL or which 1184 // are not active (have a non-NULL next field). NOTE: When we are 1185 // thus precleaning the ref lists (which happens single-threaded today), 1186 // we do not disable refs discovery to honor the correct semantics of 1187 // java.lang.Reference. As a result, we need to be careful below 1188 // that ref removal steps interleave safely with ref discovery steps 1189 // (in this thread). 1190 void 1191 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList& refs_list, 1192 BoolObjectClosure* is_alive, 1193 OopClosure* keep_alive, 1194 VoidClosure* complete_gc, 1195 YieldClosure* yield) { 1196 DiscoveredListIterator iter(refs_list, keep_alive, is_alive); 1197 while (iter.has_next()) { 1198 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */)); 1199 oop obj = iter.obj(); 1200 oop next = java_lang_ref_Reference::next(obj); 1201 if (iter.referent() == NULL || iter.is_referent_alive() || 1202 next != NULL) { 1203 // The referent has been cleared, or is alive, or the Reference is not 1204 // active; we need to trace and mark its cohort. 1205 log_develop_trace(gc, ref)("Precleaning Reference (" INTPTR_FORMAT ": %s)", 1206 p2i(iter.obj()), iter.obj()->klass()->internal_name()); 1207 // Remove Reference object from list 1208 iter.remove(); 1209 // Keep alive its cohort. 1210 iter.make_referent_alive(); 1211 if (UseCompressedOops) { 1212 narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr_raw(obj); 1213 keep_alive->do_oop(next_addr); 1214 } else { 1215 oop* next_addr = (oop*)java_lang_ref_Reference::next_addr_raw(obj); 1216 keep_alive->do_oop(next_addr); 1217 } 1218 iter.move_to_next(); 1219 } else { 1220 iter.next(); 1221 } 1222 } 1223 // Close the reachable set 1224 complete_gc->do_void(); 1225 1226 NOT_PRODUCT( 1227 if (iter.processed() > 0) { 1228 log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " Refs out of " SIZE_FORMAT " Refs in discovered list " INTPTR_FORMAT, 1229 iter.removed(), iter.processed(), p2i(&refs_list)); 1230 } 1231 ) 1232 } 1233 1234 const char* ReferenceProcessor::list_name(uint i) { 1235 assert(i <= _max_num_queues * number_of_subclasses_of_ref(), 1236 "Out of bounds index"); 1237 1238 int j = i / _max_num_queues; 1239 switch (j) { 1240 case 0: return "SoftRef"; 1241 case 1: return "WeakRef"; 1242 case 2: return "FinalRef"; 1243 case 3: return "PhantomRef"; 1244 } 1245 ShouldNotReachHere(); 1246 return NULL; 1247 }