1 /* 2 * Copyright (c) 2001, 2015, Oracle and/or its affiliates. All rights reserved. 3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 4 * 5 * This code is free software; you can redistribute it and/or modify it 6 * under the terms of the GNU General Public License version 2 only, as 7 * published by the Free Software Foundation. 8 * 9 * This code is distributed in the hope that it will be useful, but WITHOUT 10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 12 * version 2 for more details (a copy is included in the LICENSE file that 13 * accompanied this code). 14 * 15 * You should have received a copy of the GNU General Public License version 16 * 2 along with this work; if not, write to the Free Software Foundation, 17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 18 * 19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 20 * or visit www.oracle.com if you need additional information or have any 21 * questions. 22 * 23 */ 24 25 #include "precompiled.hpp" 26 #include "classfile/javaClasses.hpp" 27 #include "classfile/systemDictionary.hpp" 28 #include "gc/shared/collectedHeap.hpp" 29 #include "gc/shared/collectedHeap.inline.hpp" 30 #include "gc/shared/gcTimer.hpp" 31 #include "gc/shared/gcTraceTime.hpp" 32 #include "gc/shared/referencePolicy.hpp" 33 #include "gc/shared/referenceProcessor.hpp" 34 #include "oops/oop.inline.hpp" 35 #include "runtime/java.hpp" 36 #include "runtime/jniHandles.hpp" 37 38 ReferencePolicy* ReferenceProcessor::_always_clear_soft_ref_policy = NULL; 39 ReferencePolicy* ReferenceProcessor::_default_soft_ref_policy = NULL; 40 jlong ReferenceProcessor::_soft_ref_timestamp_clock = 0; 41 42 void referenceProcessor_init() { 43 ReferenceProcessor::init_statics(); 44 } 45 46 void ReferenceProcessor::init_statics() { 47 // We need a monotonically non-decreasing time in ms but 48 // os::javaTimeMillis() does not guarantee monotonicity. 49 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC; 50 51 // Initialize the soft ref timestamp clock. 52 _soft_ref_timestamp_clock = now; 53 // Also update the soft ref clock in j.l.r.SoftReference 54 java_lang_ref_SoftReference::set_clock(_soft_ref_timestamp_clock); 55 56 _always_clear_soft_ref_policy = new AlwaysClearPolicy(); 57 _default_soft_ref_policy = new COMPILER2_PRESENT(LRUMaxHeapPolicy()) 58 NOT_COMPILER2(LRUCurrentHeapPolicy()); 59 if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) { 60 vm_exit_during_initialization("Could not allocate reference policy object"); 61 } 62 guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery || 63 RefDiscoveryPolicy == ReferentBasedDiscovery, 64 "Unrecognized RefDiscoveryPolicy"); 65 } 66 67 void ReferenceProcessor::enable_discovery(bool check_no_refs) { 68 #ifdef ASSERT 69 // Verify that we're not currently discovering refs 70 assert(!_discovering_refs, "nested call?"); 71 72 if (check_no_refs) { 73 // Verify that the discovered lists are empty 74 verify_no_references_recorded(); 75 } 76 #endif // ASSERT 77 78 // Someone could have modified the value of the static 79 // field in the j.l.r.SoftReference class that holds the 80 // soft reference timestamp clock using reflection or 81 // Unsafe between GCs. Unconditionally update the static 82 // field in ReferenceProcessor here so that we use the new 83 // value during reference discovery. 84 85 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock(); 86 _discovering_refs = true; 87 } 88 89 ReferenceProcessor::ReferenceProcessor(MemRegion span, 90 bool mt_processing, 91 uint mt_processing_degree, 92 bool mt_discovery, 93 uint mt_discovery_degree, 94 bool atomic_discovery, 95 BoolObjectClosure* is_alive_non_header) : 96 _discovering_refs(false), 97 _enqueuing_is_done(false), 98 _is_alive_non_header(is_alive_non_header), 99 _processing_is_mt(mt_processing), 100 _next_id(0) 101 { 102 _span = span; 103 _discovery_is_atomic = atomic_discovery; 104 _discovery_is_mt = mt_discovery; 105 _num_q = MAX2(1U, mt_processing_degree); 106 _max_num_q = MAX2(_num_q, mt_discovery_degree); 107 _discovered_refs = NEW_C_HEAP_ARRAY(DiscoveredList, 108 _max_num_q * number_of_subclasses_of_ref(), mtGC); 109 110 if (_discovered_refs == NULL) { 111 vm_exit_during_initialization("Could not allocated RefProc Array"); 112 } 113 _discoveredSoftRefs = &_discovered_refs[0]; 114 _discoveredWeakRefs = &_discoveredSoftRefs[_max_num_q]; 115 _discoveredFinalRefs = &_discoveredWeakRefs[_max_num_q]; 116 _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_q]; 117 _discoveredCleanerRefs = &_discoveredPhantomRefs[_max_num_q]; 118 119 // Initialize all entries to NULL 120 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) { 121 _discovered_refs[i].set_head(NULL); 122 _discovered_refs[i].set_length(0); 123 } 124 125 setup_policy(false /* default soft ref policy */); 126 } 127 128 #ifndef PRODUCT 129 void ReferenceProcessor::verify_no_references_recorded() { 130 guarantee(!_discovering_refs, "Discovering refs?"); 131 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) { 132 guarantee(_discovered_refs[i].is_empty(), 133 "Found non-empty discovered list"); 134 } 135 } 136 #endif 137 138 void ReferenceProcessor::weak_oops_do(OopClosure* f) { 139 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) { 140 if (UseCompressedOops) { 141 f->do_oop((narrowOop*)_discovered_refs[i].adr_head()); 142 } else { 143 f->do_oop((oop*)_discovered_refs[i].adr_head()); 144 } 145 } 146 } 147 148 void ReferenceProcessor::update_soft_ref_master_clock() { 149 // Update (advance) the soft ref master clock field. This must be done 150 // after processing the soft ref list. 151 152 // We need a monotonically non-decreasing time in ms but 153 // os::javaTimeMillis() does not guarantee monotonicity. 154 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC; 155 jlong soft_ref_clock = java_lang_ref_SoftReference::clock(); 156 assert(soft_ref_clock == _soft_ref_timestamp_clock, "soft ref clocks out of sync"); 157 158 NOT_PRODUCT( 159 if (now < _soft_ref_timestamp_clock) { 160 warning("time warp: " JLONG_FORMAT " to " JLONG_FORMAT, 161 _soft_ref_timestamp_clock, now); 162 } 163 ) 164 // The values of now and _soft_ref_timestamp_clock are set using 165 // javaTimeNanos(), which is guaranteed to be monotonically 166 // non-decreasing provided the underlying platform provides such 167 // a time source (and it is bug free). 168 // In product mode, however, protect ourselves from non-monotonicity. 169 if (now > _soft_ref_timestamp_clock) { 170 _soft_ref_timestamp_clock = now; 171 java_lang_ref_SoftReference::set_clock(now); 172 } 173 // Else leave clock stalled at its old value until time progresses 174 // past clock value. 175 } 176 177 size_t ReferenceProcessor::total_count(DiscoveredList lists[]) { 178 size_t total = 0; 179 for (uint i = 0; i < _max_num_q; ++i) { 180 total += lists[i].length(); 181 } 182 return total; 183 } 184 185 ReferenceProcessorStats ReferenceProcessor::process_discovered_references( 186 BoolObjectClosure* is_alive, 187 OopClosure* keep_alive, 188 VoidClosure* complete_gc, 189 AbstractRefProcTaskExecutor* task_executor, 190 GCTimer* gc_timer, 191 GCId gc_id) { 192 193 assert(!enqueuing_is_done(), "If here enqueuing should not be complete"); 194 // Stop treating discovered references specially. 195 disable_discovery(); 196 197 // If discovery was concurrent, someone could have modified 198 // the value of the static field in the j.l.r.SoftReference 199 // class that holds the soft reference timestamp clock using 200 // reflection or Unsafe between when discovery was enabled and 201 // now. Unconditionally update the static field in ReferenceProcessor 202 // here so that we use the new value during processing of the 203 // discovered soft refs. 204 205 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock(); 206 207 bool trace_time = PrintGCDetails && PrintReferenceGC; 208 209 // Soft references 210 size_t soft_count = 0; 211 { 212 GCTraceTime tt("SoftReference", trace_time, false, gc_timer, gc_id); 213 soft_count = 214 process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true, 215 is_alive, keep_alive, complete_gc, task_executor); 216 } 217 218 update_soft_ref_master_clock(); 219 220 // Weak references 221 size_t weak_count = 0; 222 { 223 GCTraceTime tt("WeakReference", trace_time, false, gc_timer, gc_id); 224 weak_count = 225 process_discovered_reflist(_discoveredWeakRefs, NULL, true, 226 is_alive, keep_alive, complete_gc, task_executor); 227 } 228 229 // Final references 230 size_t final_count = 0; 231 { 232 GCTraceTime tt("FinalReference", trace_time, false, gc_timer, gc_id); 233 final_count = 234 process_discovered_reflist(_discoveredFinalRefs, NULL, false, 235 is_alive, keep_alive, complete_gc, task_executor); 236 } 237 238 // Phantom references 239 size_t phantom_count = 0; 240 { 241 GCTraceTime tt("PhantomReference", trace_time, false, gc_timer, gc_id); 242 phantom_count = 243 process_discovered_reflist(_discoveredPhantomRefs, NULL, false, 244 is_alive, keep_alive, complete_gc, task_executor); 245 246 // Process cleaners, but include them in phantom statistics. We expect 247 // Cleaner references to be temporary, and don't want to deal with 248 // possible incompatibilities arising from making it more visible. 249 phantom_count += 250 process_discovered_reflist(_discoveredCleanerRefs, NULL, true, 251 is_alive, keep_alive, complete_gc, task_executor); 252 } 253 254 // Weak global JNI references. It would make more sense (semantically) to 255 // traverse these simultaneously with the regular weak references above, but 256 // that is not how the JDK1.2 specification is. See #4126360. Native code can 257 // thus use JNI weak references to circumvent the phantom references and 258 // resurrect a "post-mortem" object. 259 { 260 GCTraceTime tt("JNI Weak Reference", trace_time, false, gc_timer, gc_id); 261 if (task_executor != NULL) { 262 task_executor->set_single_threaded_mode(); 263 } 264 process_phaseJNI(is_alive, keep_alive, complete_gc); 265 } 266 267 return ReferenceProcessorStats(soft_count, weak_count, final_count, phantom_count); 268 } 269 270 #ifndef PRODUCT 271 // Calculate the number of jni handles. 272 uint ReferenceProcessor::count_jni_refs() { 273 class AlwaysAliveClosure: public BoolObjectClosure { 274 public: 275 virtual bool do_object_b(oop obj) { return true; } 276 }; 277 278 class CountHandleClosure: public OopClosure { 279 private: 280 int _count; 281 public: 282 CountHandleClosure(): _count(0) {} 283 void do_oop(oop* unused) { _count++; } 284 void do_oop(narrowOop* unused) { ShouldNotReachHere(); } 285 int count() { return _count; } 286 }; 287 CountHandleClosure global_handle_count; 288 AlwaysAliveClosure always_alive; 289 JNIHandles::weak_oops_do(&always_alive, &global_handle_count); 290 return global_handle_count.count(); 291 } 292 #endif 293 294 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive, 295 OopClosure* keep_alive, 296 VoidClosure* complete_gc) { 297 #ifndef PRODUCT 298 if (PrintGCDetails && PrintReferenceGC) { 299 unsigned int count = count_jni_refs(); 300 gclog_or_tty->print(", %u refs", count); 301 } 302 #endif 303 JNIHandles::weak_oops_do(is_alive, keep_alive); 304 complete_gc->do_void(); 305 } 306 307 308 template <class T> 309 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref, 310 AbstractRefProcTaskExecutor* task_executor) { 311 312 // Remember old value of pending references list 313 T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr(); 314 T old_pending_list_value = *pending_list_addr; 315 316 // Enqueue references that are not made active again, and 317 // clear the decks for the next collection (cycle). 318 ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor); 319 // Do the post-barrier on pending_list_addr missed in 320 // enqueue_discovered_reflist. 321 oopDesc::bs()->write_ref_field(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr)); 322 323 // Stop treating discovered references specially. 324 ref->disable_discovery(); 325 326 // Return true if new pending references were added 327 return old_pending_list_value != *pending_list_addr; 328 } 329 330 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) { 331 if (UseCompressedOops) { 332 return enqueue_discovered_ref_helper<narrowOop>(this, task_executor); 333 } else { 334 return enqueue_discovered_ref_helper<oop>(this, task_executor); 335 } 336 } 337 338 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list, 339 HeapWord* pending_list_addr) { 340 // Given a list of refs linked through the "discovered" field 341 // (java.lang.ref.Reference.discovered), self-loop their "next" field 342 // thus distinguishing them from active References, then 343 // prepend them to the pending list. 344 // 345 // The Java threads will see the Reference objects linked together through 346 // the discovered field. Instead of trying to do the write barrier updates 347 // in all places in the reference processor where we manipulate the discovered 348 // field we make sure to do the barrier here where we anyway iterate through 349 // all linked Reference objects. Note that it is important to not dirty any 350 // cards during reference processing since this will cause card table 351 // verification to fail for G1. 352 if (TraceReferenceGC && PrintGCDetails) { 353 gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list " 354 INTPTR_FORMAT, p2i(refs_list.head())); 355 } 356 357 oop obj = NULL; 358 oop next_d = refs_list.head(); 359 // Walk down the list, self-looping the next field 360 // so that the References are not considered active. 361 while (obj != next_d) { 362 obj = next_d; 363 assert(obj->is_instanceRef(), "should be reference object"); 364 next_d = java_lang_ref_Reference::discovered(obj); 365 if (TraceReferenceGC && PrintGCDetails) { 366 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT, 367 p2i(obj), p2i(next_d)); 368 } 369 assert(java_lang_ref_Reference::next(obj) == NULL, 370 "Reference not active; should not be discovered"); 371 // Self-loop next, so as to make Ref not active. 372 java_lang_ref_Reference::set_next_raw(obj, obj); 373 if (next_d != obj) { 374 oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), next_d); 375 } else { 376 // This is the last object. 377 // Swap refs_list into pending_list_addr and 378 // set obj's discovered to what we read from pending_list_addr. 379 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr); 380 // Need post-barrier on pending_list_addr. See enqueue_discovered_ref_helper() above. 381 java_lang_ref_Reference::set_discovered_raw(obj, old); // old may be NULL 382 oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), old); 383 } 384 } 385 } 386 387 // Parallel enqueue task 388 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask { 389 public: 390 RefProcEnqueueTask(ReferenceProcessor& ref_processor, 391 DiscoveredList discovered_refs[], 392 HeapWord* pending_list_addr, 393 int n_queues) 394 : EnqueueTask(ref_processor, discovered_refs, 395 pending_list_addr, n_queues) 396 { } 397 398 virtual void work(unsigned int work_id) { 399 assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds"); 400 // Simplest first cut: static partitioning. 401 int index = work_id; 402 // The increment on "index" must correspond to the maximum number of queues 403 // (n_queues) with which that ReferenceProcessor was created. That 404 // is because of the "clever" way the discovered references lists were 405 // allocated and are indexed into. 406 assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected"); 407 for (int j = 0; 408 j < ReferenceProcessor::number_of_subclasses_of_ref(); 409 j++, index += _n_queues) { 410 _ref_processor.enqueue_discovered_reflist( 411 _refs_lists[index], _pending_list_addr); 412 _refs_lists[index].set_head(NULL); 413 _refs_lists[index].set_length(0); 414 } 415 } 416 }; 417 418 // Enqueue references that are not made active again 419 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr, 420 AbstractRefProcTaskExecutor* task_executor) { 421 if (_processing_is_mt && task_executor != NULL) { 422 // Parallel code 423 RefProcEnqueueTask tsk(*this, _discovered_refs, 424 pending_list_addr, _max_num_q); 425 task_executor->execute(tsk); 426 } else { 427 // Serial code: call the parent class's implementation 428 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) { 429 enqueue_discovered_reflist(_discovered_refs[i], pending_list_addr); 430 _discovered_refs[i].set_head(NULL); 431 _discovered_refs[i].set_length(0); 432 } 433 } 434 } 435 436 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) { 437 _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref); 438 oop discovered = java_lang_ref_Reference::discovered(_ref); 439 assert(_discovered_addr && discovered->is_oop_or_null(), 440 err_msg("Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered))); 441 _next = discovered; 442 _referent_addr = java_lang_ref_Reference::referent_addr(_ref); 443 _referent = java_lang_ref_Reference::referent(_ref); 444 assert(Universe::heap()->is_in_reserved_or_null(_referent), 445 "Wrong oop found in java.lang.Reference object"); 446 assert(allow_null_referent ? 447 _referent->is_oop_or_null() 448 : _referent->is_oop(), 449 err_msg("Expected an oop%s for referent field at " PTR_FORMAT, 450 (allow_null_referent ? " or NULL" : ""), 451 p2i(_referent))); 452 } 453 454 void DiscoveredListIterator::remove() { 455 assert(_ref->is_oop(), "Dropping a bad reference"); 456 oop_store_raw(_discovered_addr, NULL); 457 458 // First _prev_next ref actually points into DiscoveredList (gross). 459 oop new_next; 460 if (_next == _ref) { 461 // At the end of the list, we should make _prev point to itself. 462 // If _ref is the first ref, then _prev_next will be in the DiscoveredList, 463 // and _prev will be NULL. 464 new_next = _prev; 465 } else { 466 new_next = _next; 467 } 468 // Remove Reference object from discovered list. Note that G1 does not need a 469 // pre-barrier here because we know the Reference has already been found/marked, 470 // that's how it ended up in the discovered list in the first place. 471 oop_store_raw(_prev_next, new_next); 472 NOT_PRODUCT(_removed++); 473 _refs_list.dec_length(1); 474 } 475 476 void DiscoveredListIterator::clear_referent() { 477 oop_store_raw(_referent_addr, NULL); 478 } 479 480 // NOTE: process_phase*() are largely similar, and at a high level 481 // merely iterate over the extant list applying a predicate to 482 // each of its elements and possibly removing that element from the 483 // list and applying some further closures to that element. 484 // We should consider the possibility of replacing these 485 // process_phase*() methods by abstracting them into 486 // a single general iterator invocation that receives appropriate 487 // closures that accomplish this work. 488 489 // (SoftReferences only) Traverse the list and remove any SoftReferences whose 490 // referents are not alive, but that should be kept alive for policy reasons. 491 // Keep alive the transitive closure of all such referents. 492 void 493 ReferenceProcessor::process_phase1(DiscoveredList& refs_list, 494 ReferencePolicy* policy, 495 BoolObjectClosure* is_alive, 496 OopClosure* keep_alive, 497 VoidClosure* complete_gc) { 498 assert(policy != NULL, "Must have a non-NULL policy"); 499 DiscoveredListIterator iter(refs_list, keep_alive, is_alive); 500 // Decide which softly reachable refs should be kept alive. 501 while (iter.has_next()) { 502 iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */)); 503 bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive(); 504 if (referent_is_dead && 505 !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) { 506 if (TraceReferenceGC) { 507 gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s" ") by policy", 508 p2i(iter.obj()), iter.obj()->klass()->internal_name()); 509 } 510 // Remove Reference object from list 511 iter.remove(); 512 // keep the referent around 513 iter.make_referent_alive(); 514 iter.move_to_next(); 515 } else { 516 iter.next(); 517 } 518 } 519 // Close the reachable set 520 complete_gc->do_void(); 521 NOT_PRODUCT( 522 if (PrintGCDetails && TraceReferenceGC) { 523 gclog_or_tty->print_cr(" Dropped " SIZE_FORMAT " dead Refs out of " SIZE_FORMAT 524 " discovered Refs by policy, from list " INTPTR_FORMAT, 525 iter.removed(), iter.processed(), p2i(refs_list.head())); 526 } 527 ) 528 } 529 530 // Traverse the list and remove any Refs that are not active, or 531 // whose referents are either alive or NULL. 532 void 533 ReferenceProcessor::pp2_work(DiscoveredList& refs_list, 534 BoolObjectClosure* is_alive, 535 OopClosure* keep_alive) { 536 assert(discovery_is_atomic(), "Error"); 537 DiscoveredListIterator iter(refs_list, keep_alive, is_alive); 538 while (iter.has_next()) { 539 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */)); 540 DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());) 541 assert(next == NULL, "Should not discover inactive Reference"); 542 if (iter.is_referent_alive()) { 543 if (TraceReferenceGC) { 544 gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)", 545 p2i(iter.obj()), iter.obj()->klass()->internal_name()); 546 } 547 // The referent is reachable after all. 548 // Remove Reference object from list. 549 iter.remove(); 550 // Update the referent pointer as necessary: Note that this 551 // should not entail any recursive marking because the 552 // referent must already have been traversed. 553 iter.make_referent_alive(); 554 iter.move_to_next(); 555 } else { 556 iter.next(); 557 } 558 } 559 NOT_PRODUCT( 560 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) { 561 gclog_or_tty->print_cr(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT 562 " Refs in discovered list " INTPTR_FORMAT, 563 iter.removed(), iter.processed(), p2i(refs_list.head())); 564 } 565 ) 566 } 567 568 void 569 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList& refs_list, 570 BoolObjectClosure* is_alive, 571 OopClosure* keep_alive, 572 VoidClosure* complete_gc) { 573 assert(!discovery_is_atomic(), "Error"); 574 DiscoveredListIterator iter(refs_list, keep_alive, is_alive); 575 while (iter.has_next()) { 576 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */)); 577 HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj()); 578 oop next = java_lang_ref_Reference::next(iter.obj()); 579 if ((iter.referent() == NULL || iter.is_referent_alive() || 580 next != NULL)) { 581 assert(next->is_oop_or_null(), err_msg("Expected an oop or NULL for next field at " PTR_FORMAT, p2i(next))); 582 // Remove Reference object from list 583 iter.remove(); 584 // Trace the cohorts 585 iter.make_referent_alive(); 586 if (UseCompressedOops) { 587 keep_alive->do_oop((narrowOop*)next_addr); 588 } else { 589 keep_alive->do_oop((oop*)next_addr); 590 } 591 iter.move_to_next(); 592 } else { 593 iter.next(); 594 } 595 } 596 // Now close the newly reachable set 597 complete_gc->do_void(); 598 NOT_PRODUCT( 599 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) { 600 gclog_or_tty->print_cr(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT 601 " Refs in discovered list " INTPTR_FORMAT, 602 iter.removed(), iter.processed(), p2i(refs_list.head())); 603 } 604 ) 605 } 606 607 // Traverse the list and process the referents, by either 608 // clearing them or keeping them (and their reachable 609 // closure) alive. 610 void 611 ReferenceProcessor::process_phase3(DiscoveredList& refs_list, 612 bool clear_referent, 613 BoolObjectClosure* is_alive, 614 OopClosure* keep_alive, 615 VoidClosure* complete_gc) { 616 ResourceMark rm; 617 DiscoveredListIterator iter(refs_list, keep_alive, is_alive); 618 while (iter.has_next()) { 619 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */)); 620 if (clear_referent) { 621 // NULL out referent pointer 622 iter.clear_referent(); 623 } else { 624 // keep the referent around 625 iter.make_referent_alive(); 626 } 627 if (TraceReferenceGC) { 628 gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending", 629 clear_referent ? "cleared " : "", 630 p2i(iter.obj()), iter.obj()->klass()->internal_name()); 631 } 632 assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference"); 633 iter.next(); 634 } 635 // Close the reachable set 636 complete_gc->do_void(); 637 } 638 639 void 640 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) { 641 oop obj = NULL; 642 oop next = refs_list.head(); 643 while (next != obj) { 644 obj = next; 645 next = java_lang_ref_Reference::discovered(obj); 646 java_lang_ref_Reference::set_discovered_raw(obj, NULL); 647 } 648 refs_list.set_head(NULL); 649 refs_list.set_length(0); 650 } 651 652 void ReferenceProcessor::abandon_partial_discovery() { 653 // loop over the lists 654 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) { 655 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) { 656 gclog_or_tty->print_cr("\nAbandoning %s discovered list", list_name(i)); 657 } 658 clear_discovered_references(_discovered_refs[i]); 659 } 660 } 661 662 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask { 663 public: 664 RefProcPhase1Task(ReferenceProcessor& ref_processor, 665 DiscoveredList refs_lists[], 666 ReferencePolicy* policy, 667 bool marks_oops_alive) 668 : ProcessTask(ref_processor, refs_lists, marks_oops_alive), 669 _policy(policy) 670 { } 671 virtual void work(unsigned int i, BoolObjectClosure& is_alive, 672 OopClosure& keep_alive, 673 VoidClosure& complete_gc) 674 { 675 Thread* thr = Thread::current(); 676 int refs_list_index = ((WorkerThread*)thr)->id(); 677 _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy, 678 &is_alive, &keep_alive, &complete_gc); 679 } 680 private: 681 ReferencePolicy* _policy; 682 }; 683 684 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask { 685 public: 686 RefProcPhase2Task(ReferenceProcessor& ref_processor, 687 DiscoveredList refs_lists[], 688 bool marks_oops_alive) 689 : ProcessTask(ref_processor, refs_lists, marks_oops_alive) 690 { } 691 virtual void work(unsigned int i, BoolObjectClosure& is_alive, 692 OopClosure& keep_alive, 693 VoidClosure& complete_gc) 694 { 695 _ref_processor.process_phase2(_refs_lists[i], 696 &is_alive, &keep_alive, &complete_gc); 697 } 698 }; 699 700 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask { 701 public: 702 RefProcPhase3Task(ReferenceProcessor& ref_processor, 703 DiscoveredList refs_lists[], 704 bool clear_referent, 705 bool marks_oops_alive) 706 : ProcessTask(ref_processor, refs_lists, marks_oops_alive), 707 _clear_referent(clear_referent) 708 { } 709 virtual void work(unsigned int i, BoolObjectClosure& is_alive, 710 OopClosure& keep_alive, 711 VoidClosure& complete_gc) 712 { 713 // Don't use "refs_list_index" calculated in this way because 714 // balance_queues() has moved the Ref's into the first n queues. 715 // Thread* thr = Thread::current(); 716 // int refs_list_index = ((WorkerThread*)thr)->id(); 717 // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent, 718 _ref_processor.process_phase3(_refs_lists[i], _clear_referent, 719 &is_alive, &keep_alive, &complete_gc); 720 } 721 private: 722 bool _clear_referent; 723 }; 724 725 // Balances reference queues. 726 // Move entries from all queues[0, 1, ..., _max_num_q-1] to 727 // queues[0, 1, ..., _num_q-1] because only the first _num_q 728 // corresponding to the active workers will be processed. 729 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[]) 730 { 731 // calculate total length 732 size_t total_refs = 0; 733 if (TraceReferenceGC && PrintGCDetails) { 734 gclog_or_tty->print_cr("\nBalance ref_lists "); 735 } 736 737 for (uint i = 0; i < _max_num_q; ++i) { 738 total_refs += ref_lists[i].length(); 739 if (TraceReferenceGC && PrintGCDetails) { 740 gclog_or_tty->print(SIZE_FORMAT " ", ref_lists[i].length()); 741 } 742 } 743 if (TraceReferenceGC && PrintGCDetails) { 744 gclog_or_tty->print_cr(" = " SIZE_FORMAT, total_refs); 745 } 746 size_t avg_refs = total_refs / _num_q + 1; 747 uint to_idx = 0; 748 for (uint from_idx = 0; from_idx < _max_num_q; from_idx++) { 749 bool move_all = false; 750 if (from_idx >= _num_q) { 751 move_all = ref_lists[from_idx].length() > 0; 752 } 753 while ((ref_lists[from_idx].length() > avg_refs) || 754 move_all) { 755 assert(to_idx < _num_q, "Sanity Check!"); 756 if (ref_lists[to_idx].length() < avg_refs) { 757 // move superfluous refs 758 size_t refs_to_move; 759 // Move all the Ref's if the from queue will not be processed. 760 if (move_all) { 761 refs_to_move = MIN2(ref_lists[from_idx].length(), 762 avg_refs - ref_lists[to_idx].length()); 763 } else { 764 refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs, 765 avg_refs - ref_lists[to_idx].length()); 766 } 767 768 assert(refs_to_move > 0, "otherwise the code below will fail"); 769 770 oop move_head = ref_lists[from_idx].head(); 771 oop move_tail = move_head; 772 oop new_head = move_head; 773 // find an element to split the list on 774 for (size_t j = 0; j < refs_to_move; ++j) { 775 move_tail = new_head; 776 new_head = java_lang_ref_Reference::discovered(new_head); 777 } 778 779 // Add the chain to the to list. 780 if (ref_lists[to_idx].head() == NULL) { 781 // to list is empty. Make a loop at the end. 782 java_lang_ref_Reference::set_discovered_raw(move_tail, move_tail); 783 } else { 784 java_lang_ref_Reference::set_discovered_raw(move_tail, ref_lists[to_idx].head()); 785 } 786 ref_lists[to_idx].set_head(move_head); 787 ref_lists[to_idx].inc_length(refs_to_move); 788 789 // Remove the chain from the from list. 790 if (move_tail == new_head) { 791 // We found the end of the from list. 792 ref_lists[from_idx].set_head(NULL); 793 } else { 794 ref_lists[from_idx].set_head(new_head); 795 } 796 ref_lists[from_idx].dec_length(refs_to_move); 797 if (ref_lists[from_idx].length() == 0) { 798 break; 799 } 800 } else { 801 to_idx = (to_idx + 1) % _num_q; 802 } 803 } 804 } 805 #ifdef ASSERT 806 size_t balanced_total_refs = 0; 807 for (uint i = 0; i < _max_num_q; ++i) { 808 balanced_total_refs += ref_lists[i].length(); 809 if (TraceReferenceGC && PrintGCDetails) { 810 gclog_or_tty->print(SIZE_FORMAT " ", ref_lists[i].length()); 811 } 812 } 813 if (TraceReferenceGC && PrintGCDetails) { 814 gclog_or_tty->print_cr(" = " SIZE_FORMAT, balanced_total_refs); 815 gclog_or_tty->flush(); 816 } 817 assert(total_refs == balanced_total_refs, "Balancing was incomplete"); 818 #endif 819 } 820 821 void ReferenceProcessor::balance_all_queues() { 822 balance_queues(_discoveredSoftRefs); 823 balance_queues(_discoveredWeakRefs); 824 balance_queues(_discoveredFinalRefs); 825 balance_queues(_discoveredPhantomRefs); 826 balance_queues(_discoveredCleanerRefs); 827 } 828 829 size_t 830 ReferenceProcessor::process_discovered_reflist( 831 DiscoveredList refs_lists[], 832 ReferencePolicy* policy, 833 bool clear_referent, 834 BoolObjectClosure* is_alive, 835 OopClosure* keep_alive, 836 VoidClosure* complete_gc, 837 AbstractRefProcTaskExecutor* task_executor) 838 { 839 bool mt_processing = task_executor != NULL && _processing_is_mt; 840 // If discovery used MT and a dynamic number of GC threads, then 841 // the queues must be balanced for correctness if fewer than the 842 // maximum number of queues were used. The number of queue used 843 // during discovery may be different than the number to be used 844 // for processing so don't depend of _num_q < _max_num_q as part 845 // of the test. 846 bool must_balance = _discovery_is_mt; 847 848 if ((mt_processing && ParallelRefProcBalancingEnabled) || 849 must_balance) { 850 balance_queues(refs_lists); 851 } 852 853 size_t total_list_count = total_count(refs_lists); 854 855 if (PrintReferenceGC && PrintGCDetails) { 856 gclog_or_tty->print(", " SIZE_FORMAT " refs", total_list_count); 857 } 858 859 // Phase 1 (soft refs only): 860 // . Traverse the list and remove any SoftReferences whose 861 // referents are not alive, but that should be kept alive for 862 // policy reasons. Keep alive the transitive closure of all 863 // such referents. 864 if (policy != NULL) { 865 if (mt_processing) { 866 RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/); 867 task_executor->execute(phase1); 868 } else { 869 for (uint i = 0; i < _max_num_q; i++) { 870 process_phase1(refs_lists[i], policy, 871 is_alive, keep_alive, complete_gc); 872 } 873 } 874 } else { // policy == NULL 875 assert(refs_lists != _discoveredSoftRefs, 876 "Policy must be specified for soft references."); 877 } 878 879 // Phase 2: 880 // . Traverse the list and remove any refs whose referents are alive. 881 if (mt_processing) { 882 RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/); 883 task_executor->execute(phase2); 884 } else { 885 for (uint i = 0; i < _max_num_q; i++) { 886 process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc); 887 } 888 } 889 890 // Phase 3: 891 // . Traverse the list and process referents as appropriate. 892 if (mt_processing) { 893 RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/); 894 task_executor->execute(phase3); 895 } else { 896 for (uint i = 0; i < _max_num_q; i++) { 897 process_phase3(refs_lists[i], clear_referent, 898 is_alive, keep_alive, complete_gc); 899 } 900 } 901 902 return total_list_count; 903 } 904 905 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) { 906 uint id = 0; 907 // Determine the queue index to use for this object. 908 if (_discovery_is_mt) { 909 // During a multi-threaded discovery phase, 910 // each thread saves to its "own" list. 911 Thread* thr = Thread::current(); 912 id = thr->as_Worker_thread()->id(); 913 } else { 914 // single-threaded discovery, we save in round-robin 915 // fashion to each of the lists. 916 if (_processing_is_mt) { 917 id = next_id(); 918 } 919 } 920 assert(id < _max_num_q, "Id is out-of-bounds (call Freud?)"); 921 922 // Get the discovered queue to which we will add 923 DiscoveredList* list = NULL; 924 switch (rt) { 925 case REF_OTHER: 926 // Unknown reference type, no special treatment 927 break; 928 case REF_SOFT: 929 list = &_discoveredSoftRefs[id]; 930 break; 931 case REF_WEAK: 932 list = &_discoveredWeakRefs[id]; 933 break; 934 case REF_FINAL: 935 list = &_discoveredFinalRefs[id]; 936 break; 937 case REF_PHANTOM: 938 list = &_discoveredPhantomRefs[id]; 939 break; 940 case REF_CLEANER: 941 list = &_discoveredCleanerRefs[id]; 942 break; 943 case REF_NONE: 944 // we should not reach here if we are an InstanceRefKlass 945 default: 946 ShouldNotReachHere(); 947 } 948 if (TraceReferenceGC && PrintGCDetails) { 949 gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, p2i(list)); 950 } 951 return list; 952 } 953 954 inline void 955 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list, 956 oop obj, 957 HeapWord* discovered_addr) { 958 assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller"); 959 // First we must make sure this object is only enqueued once. CAS in a non null 960 // discovered_addr. 961 oop current_head = refs_list.head(); 962 // The last ref must have its discovered field pointing to itself. 963 oop next_discovered = (current_head != NULL) ? current_head : obj; 964 965 oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr, 966 NULL); 967 if (retest == NULL) { 968 // This thread just won the right to enqueue the object. 969 // We have separate lists for enqueueing, so no synchronization 970 // is necessary. 971 refs_list.set_head(obj); 972 refs_list.inc_length(1); 973 974 if (TraceReferenceGC) { 975 gclog_or_tty->print_cr("Discovered reference (mt) (" INTPTR_FORMAT ": %s)", 976 p2i(obj), obj->klass()->internal_name()); 977 } 978 } else { 979 // If retest was non NULL, another thread beat us to it: 980 // The reference has already been discovered... 981 if (TraceReferenceGC) { 982 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)", 983 p2i(obj), obj->klass()->internal_name()); 984 } 985 } 986 } 987 988 #ifndef PRODUCT 989 // Non-atomic (i.e. concurrent) discovery might allow us 990 // to observe j.l.References with NULL referents, being those 991 // cleared concurrently by mutators during (or after) discovery. 992 void ReferenceProcessor::verify_referent(oop obj) { 993 bool da = discovery_is_atomic(); 994 oop referent = java_lang_ref_Reference::referent(obj); 995 assert(da ? referent->is_oop() : referent->is_oop_or_null(), 996 err_msg("Bad referent " INTPTR_FORMAT " found in Reference " 997 INTPTR_FORMAT " during %satomic discovery ", 998 p2i(referent), p2i(obj), da ? "" : "non-")); 999 } 1000 #endif 1001 1002 // We mention two of several possible choices here: 1003 // #0: if the reference object is not in the "originating generation" 1004 // (or part of the heap being collected, indicated by our "span" 1005 // we don't treat it specially (i.e. we scan it as we would 1006 // a normal oop, treating its references as strong references). 1007 // This means that references can't be discovered unless their 1008 // referent is also in the same span. This is the simplest, 1009 // most "local" and most conservative approach, albeit one 1010 // that may cause weak references to be enqueued least promptly. 1011 // We call this choice the "ReferenceBasedDiscovery" policy. 1012 // #1: the reference object may be in any generation (span), but if 1013 // the referent is in the generation (span) being currently collected 1014 // then we can discover the reference object, provided 1015 // the object has not already been discovered by 1016 // a different concurrently running collector (as may be the 1017 // case, for instance, if the reference object is in CMS and 1018 // the referent in DefNewGeneration), and provided the processing 1019 // of this reference object by the current collector will 1020 // appear atomic to every other collector in the system. 1021 // (Thus, for instance, a concurrent collector may not 1022 // discover references in other generations even if the 1023 // referent is in its own generation). This policy may, 1024 // in certain cases, enqueue references somewhat sooner than 1025 // might Policy #0 above, but at marginally increased cost 1026 // and complexity in processing these references. 1027 // We call this choice the "RefeferentBasedDiscovery" policy. 1028 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) { 1029 // Make sure we are discovering refs (rather than processing discovered refs). 1030 if (!_discovering_refs || !RegisterReferences) { 1031 return false; 1032 } 1033 // We only discover active references. 1034 oop next = java_lang_ref_Reference::next(obj); 1035 if (next != NULL) { // Ref is no longer active 1036 return false; 1037 } 1038 1039 HeapWord* obj_addr = (HeapWord*)obj; 1040 if (RefDiscoveryPolicy == ReferenceBasedDiscovery && 1041 !_span.contains(obj_addr)) { 1042 // Reference is not in the originating generation; 1043 // don't treat it specially (i.e. we want to scan it as a normal 1044 // object with strong references). 1045 return false; 1046 } 1047 1048 // We only discover references whose referents are not (yet) 1049 // known to be strongly reachable. 1050 if (is_alive_non_header() != NULL) { 1051 verify_referent(obj); 1052 if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) { 1053 return false; // referent is reachable 1054 } 1055 } 1056 if (rt == REF_SOFT) { 1057 // For soft refs we can decide now if these are not 1058 // current candidates for clearing, in which case we 1059 // can mark through them now, rather than delaying that 1060 // to the reference-processing phase. Since all current 1061 // time-stamp policies advance the soft-ref clock only 1062 // at a full collection cycle, this is always currently 1063 // accurate. 1064 if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) { 1065 return false; 1066 } 1067 } 1068 1069 ResourceMark rm; // Needed for tracing. 1070 1071 HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj); 1072 const oop discovered = java_lang_ref_Reference::discovered(obj); 1073 assert(discovered->is_oop_or_null(), err_msg("Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered))); 1074 if (discovered != NULL) { 1075 // The reference has already been discovered... 1076 if (TraceReferenceGC) { 1077 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)", 1078 p2i(obj), obj->klass()->internal_name()); 1079 } 1080 if (RefDiscoveryPolicy == ReferentBasedDiscovery) { 1081 // assumes that an object is not processed twice; 1082 // if it's been already discovered it must be on another 1083 // generation's discovered list; so we won't discover it. 1084 return false; 1085 } else { 1086 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery, 1087 "Unrecognized policy"); 1088 // Check assumption that an object is not potentially 1089 // discovered twice except by concurrent collectors that potentially 1090 // trace the same Reference object twice. 1091 assert(UseConcMarkSweepGC || UseG1GC, 1092 "Only possible with a concurrent marking collector"); 1093 return true; 1094 } 1095 } 1096 1097 if (RefDiscoveryPolicy == ReferentBasedDiscovery) { 1098 verify_referent(obj); 1099 // Discover if and only if EITHER: 1100 // .. reference is in our span, OR 1101 // .. we are an atomic collector and referent is in our span 1102 if (_span.contains(obj_addr) || 1103 (discovery_is_atomic() && 1104 _span.contains(java_lang_ref_Reference::referent(obj)))) { 1105 // should_enqueue = true; 1106 } else { 1107 return false; 1108 } 1109 } else { 1110 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery && 1111 _span.contains(obj_addr), "code inconsistency"); 1112 } 1113 1114 // Get the right type of discovered queue head. 1115 DiscoveredList* list = get_discovered_list(rt); 1116 if (list == NULL) { 1117 return false; // nothing special needs to be done 1118 } 1119 1120 if (_discovery_is_mt) { 1121 add_to_discovered_list_mt(*list, obj, discovered_addr); 1122 } else { 1123 // We do a raw store here: the field will be visited later when processing 1124 // the discovered references. 1125 oop current_head = list->head(); 1126 // The last ref must have its discovered field pointing to itself. 1127 oop next_discovered = (current_head != NULL) ? current_head : obj; 1128 1129 assert(discovered == NULL, "control point invariant"); 1130 oop_store_raw(discovered_addr, next_discovered); 1131 list->set_head(obj); 1132 list->inc_length(1); 1133 1134 if (TraceReferenceGC) { 1135 gclog_or_tty->print_cr("Discovered reference (" INTPTR_FORMAT ": %s)", 1136 p2i(obj), obj->klass()->internal_name()); 1137 } 1138 } 1139 assert(obj->is_oop(), "Discovered a bad reference"); 1140 verify_referent(obj); 1141 return true; 1142 } 1143 1144 // Preclean the discovered references by removing those 1145 // whose referents are alive, and by marking from those that 1146 // are not active. These lists can be handled here 1147 // in any order and, indeed, concurrently. 1148 void ReferenceProcessor::preclean_discovered_references( 1149 BoolObjectClosure* is_alive, 1150 OopClosure* keep_alive, 1151 VoidClosure* complete_gc, 1152 YieldClosure* yield, 1153 GCTimer* gc_timer, 1154 GCId gc_id) { 1155 1156 // Soft references 1157 { 1158 GCTraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC, 1159 false, gc_timer, gc_id); 1160 for (uint i = 0; i < _max_num_q; i++) { 1161 if (yield->should_return()) { 1162 return; 1163 } 1164 preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive, 1165 keep_alive, complete_gc, yield); 1166 } 1167 } 1168 1169 // Weak references 1170 { 1171 GCTraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC, 1172 false, gc_timer, gc_id); 1173 for (uint i = 0; i < _max_num_q; i++) { 1174 if (yield->should_return()) { 1175 return; 1176 } 1177 preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive, 1178 keep_alive, complete_gc, yield); 1179 } 1180 } 1181 1182 // Final references 1183 { 1184 GCTraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC, 1185 false, gc_timer, gc_id); 1186 for (uint i = 0; i < _max_num_q; i++) { 1187 if (yield->should_return()) { 1188 return; 1189 } 1190 preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive, 1191 keep_alive, complete_gc, yield); 1192 } 1193 } 1194 1195 // Phantom references 1196 { 1197 GCTraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC, 1198 false, gc_timer, gc_id); 1199 for (uint i = 0; i < _max_num_q; i++) { 1200 if (yield->should_return()) { 1201 return; 1202 } 1203 preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive, 1204 keep_alive, complete_gc, yield); 1205 } 1206 1207 // Cleaner references. Included in timing for phantom references. We 1208 // expect Cleaner references to be temporary, and don't want to deal with 1209 // possible incompatibilities arising from making it more visible. 1210 for (uint i = 0; i < _max_num_q; i++) { 1211 if (yield->should_return()) { 1212 return; 1213 } 1214 preclean_discovered_reflist(_discoveredCleanerRefs[i], is_alive, 1215 keep_alive, complete_gc, yield); 1216 } 1217 } 1218 } 1219 1220 // Walk the given discovered ref list, and remove all reference objects 1221 // whose referents are still alive, whose referents are NULL or which 1222 // are not active (have a non-NULL next field). NOTE: When we are 1223 // thus precleaning the ref lists (which happens single-threaded today), 1224 // we do not disable refs discovery to honor the correct semantics of 1225 // java.lang.Reference. As a result, we need to be careful below 1226 // that ref removal steps interleave safely with ref discovery steps 1227 // (in this thread). 1228 void 1229 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList& refs_list, 1230 BoolObjectClosure* is_alive, 1231 OopClosure* keep_alive, 1232 VoidClosure* complete_gc, 1233 YieldClosure* yield) { 1234 DiscoveredListIterator iter(refs_list, keep_alive, is_alive); 1235 while (iter.has_next()) { 1236 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */)); 1237 oop obj = iter.obj(); 1238 oop next = java_lang_ref_Reference::next(obj); 1239 if (iter.referent() == NULL || iter.is_referent_alive() || 1240 next != NULL) { 1241 // The referent has been cleared, or is alive, or the Reference is not 1242 // active; we need to trace and mark its cohort. 1243 if (TraceReferenceGC) { 1244 gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)", 1245 p2i(iter.obj()), iter.obj()->klass()->internal_name()); 1246 } 1247 // Remove Reference object from list 1248 iter.remove(); 1249 // Keep alive its cohort. 1250 iter.make_referent_alive(); 1251 if (UseCompressedOops) { 1252 narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj); 1253 keep_alive->do_oop(next_addr); 1254 } else { 1255 oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj); 1256 keep_alive->do_oop(next_addr); 1257 } 1258 iter.move_to_next(); 1259 } else { 1260 iter.next(); 1261 } 1262 } 1263 // Close the reachable set 1264 complete_gc->do_void(); 1265 1266 NOT_PRODUCT( 1267 if (PrintGCDetails && PrintReferenceGC && (iter.processed() > 0)) { 1268 gclog_or_tty->print_cr(" Dropped " SIZE_FORMAT " Refs out of " SIZE_FORMAT 1269 " Refs in discovered list " INTPTR_FORMAT, 1270 iter.removed(), iter.processed(), p2i(refs_list.head())); 1271 } 1272 ) 1273 } 1274 1275 const char* ReferenceProcessor::list_name(uint i) { 1276 assert(i <= _max_num_q * number_of_subclasses_of_ref(), 1277 "Out of bounds index"); 1278 1279 int j = i / _max_num_q; 1280 switch (j) { 1281 case 0: return "SoftRef"; 1282 case 1: return "WeakRef"; 1283 case 2: return "FinalRef"; 1284 case 3: return "PhantomRef"; 1285 case 4: return "CleanerRef"; 1286 } 1287 ShouldNotReachHere(); 1288 return NULL; 1289 } 1290