1 /* 2 * Copyright (c) 2001, 2013, 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_implementation/shared/gcTimer.hpp" 29 #include "gc_implementation/shared/gcTraceTime.hpp" 30 #include "gc_interface/collectedHeap.hpp" 31 #include "gc_interface/collectedHeap.inline.hpp" 32 #include "memory/referencePolicy.hpp" 33 #include "memory/referenceProcessor.hpp" 34 #include "oops/oop.inline.hpp" 35 #include "runtime/java.hpp" 36 #include "runtime/jniHandles.hpp" 37 38 ReferencePolicy* ReferenceProcessor::_always_clear_soft_ref_policy = NULL; 39 ReferencePolicy* ReferenceProcessor::_default_soft_ref_policy = NULL; 40 bool ReferenceProcessor::_pending_list_uses_discovered_field = false; 41 jlong ReferenceProcessor::_soft_ref_timestamp_clock = 0; 42 43 void referenceProcessor_init() { 44 ReferenceProcessor::init_statics(); 45 } 46 47 void ReferenceProcessor::init_statics() { 48 // We need a monotonically non-deccreasing time in ms but 49 // os::javaTimeMillis() does not guarantee monotonicity. 50 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC; 51 52 // Initialize the soft ref timestamp clock. 53 _soft_ref_timestamp_clock = now; 54 // Also update the soft ref clock in j.l.r.SoftReference 55 java_lang_ref_SoftReference::set_clock(_soft_ref_timestamp_clock); 56 57 _always_clear_soft_ref_policy = new AlwaysClearPolicy(); 58 _default_soft_ref_policy = new COMPILER2_PRESENT(LRUMaxHeapPolicy()) 59 NOT_COMPILER2(LRUCurrentHeapPolicy()); 60 if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) { 61 vm_exit_during_initialization("Could not allocate reference policy object"); 62 } 63 guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery || 64 RefDiscoveryPolicy == ReferentBasedDiscovery, 65 "Unrecongnized RefDiscoveryPolicy"); 66 _pending_list_uses_discovered_field = JDK_Version::current().pending_list_uses_discovered_field(); 67 } 68 69 void ReferenceProcessor::enable_discovery(bool verify_disabled, bool check_no_refs) { 70 #ifdef ASSERT 71 // Verify that we're not currently discovering refs 72 assert(!verify_disabled || !_discovering_refs, "nested call?"); 73 74 if (check_no_refs) { 75 // Verify that the discovered lists are empty 76 verify_no_references_recorded(); 77 } 78 #endif // ASSERT 79 80 // Someone could have modified the value of the static 81 // field in the j.l.r.SoftReference class that holds the 82 // soft reference timestamp clock using reflection or 83 // Unsafe between GCs. Unconditionally update the static 84 // field in ReferenceProcessor here so that we use the new 85 // value during reference discovery. 86 87 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock(); 88 _discovering_refs = true; 89 } 90 91 ReferenceProcessor::ReferenceProcessor(MemRegion span, 92 bool mt_processing, 93 uint mt_processing_degree, 94 bool mt_discovery, 95 uint mt_discovery_degree, 96 bool atomic_discovery, 97 BoolObjectClosure* is_alive_non_header, 98 bool discovered_list_needs_barrier) : 99 _discovering_refs(false), 100 _enqueuing_is_done(false), 101 _is_alive_non_header(is_alive_non_header), 102 _discovered_list_needs_barrier(discovered_list_needs_barrier), 103 _processing_is_mt(mt_processing), 104 _next_id(0) 105 { 106 _span = span; 107 _discovery_is_atomic = atomic_discovery; 108 _discovery_is_mt = mt_discovery; 109 _num_q = MAX2(1U, mt_processing_degree); 110 _max_num_q = MAX2(_num_q, mt_discovery_degree); 111 _discovered_refs = NEW_C_HEAP_ARRAY(DiscoveredList, 112 _max_num_q * number_of_subclasses_of_ref(), mtGC); 113 114 if (_discovered_refs == NULL) { 115 vm_exit_during_initialization("Could not allocated RefProc Array"); 116 } 117 _discoveredSoftRefs = &_discovered_refs[0]; 118 _discoveredWeakRefs = &_discoveredSoftRefs[_max_num_q]; 119 _discoveredFinalRefs = &_discoveredWeakRefs[_max_num_q]; 120 _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_q]; 121 122 // Initialize all entries to NULL 123 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) { 124 _discovered_refs[i].set_head(NULL); 125 _discovered_refs[i].set_length(0); 126 } 127 128 setup_policy(false /* default soft ref policy */); 129 } 130 131 #ifndef PRODUCT 132 void ReferenceProcessor::verify_no_references_recorded() { 133 guarantee(!_discovering_refs, "Discovering refs?"); 134 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) { 135 guarantee(_discovered_refs[i].is_empty(), 136 "Found non-empty discovered list"); 137 } 138 } 139 #endif 140 141 void ReferenceProcessor::weak_oops_do(OopClosure* f) { 142 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) { 143 if (UseCompressedOops) { 144 f->do_oop((narrowOop*)_discovered_refs[i].adr_head()); 145 } else { 146 f->do_oop((oop*)_discovered_refs[i].adr_head()); 147 } 148 } 149 } 150 151 void ReferenceProcessor::update_soft_ref_master_clock() { 152 // Update (advance) the soft ref master clock field. This must be done 153 // after processing the soft ref list. 154 155 // We need a monotonically non-deccreasing time in ms but 156 // os::javaTimeMillis() does not guarantee monotonicity. 157 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC; 158 jlong soft_ref_clock = java_lang_ref_SoftReference::clock(); 159 assert(soft_ref_clock == _soft_ref_timestamp_clock, "soft ref clocks out of sync"); 160 161 NOT_PRODUCT( 162 if (now < _soft_ref_timestamp_clock) { 163 warning("time warp: "INT64_FORMAT" to "INT64_FORMAT, 164 _soft_ref_timestamp_clock, now); 165 } 166 ) 167 // The values of now and _soft_ref_timestamp_clock are set using 168 // javaTimeNanos(), which is guaranteed to be monotonically 169 // non-decreasing provided the underlying platform provides such 170 // a time source (and it is bug free). 171 // In product mode, however, protect ourselves from non-monotonicty. 172 if (now > _soft_ref_timestamp_clock) { 173 _soft_ref_timestamp_clock = now; 174 java_lang_ref_SoftReference::set_clock(now); 175 } 176 // Else leave clock stalled at its old value until time progresses 177 // past clock value. 178 } 179 180 size_t ReferenceProcessor::total_count(DiscoveredList lists[]) { 181 size_t total = 0; 182 for (uint i = 0; i < _max_num_q; ++i) { 183 total += lists[i].length(); 184 } 185 return total; 186 } 187 188 ReferenceProcessorStats ReferenceProcessor::process_discovered_references( 189 BoolObjectClosure* is_alive, 190 OopClosure* keep_alive, 191 VoidClosure* complete_gc, 192 AbstractRefProcTaskExecutor* task_executor, 193 GCTimer* gc_timer) { 194 NOT_PRODUCT(verify_ok_to_handle_reflists()); 195 196 assert(!enqueuing_is_done(), "If here enqueuing should not be complete"); 197 // Stop treating discovered references specially. 198 disable_discovery(); 199 200 // If discovery was concurrent, someone could have modified 201 // the value of the static field in the j.l.r.SoftReference 202 // class that holds the soft reference timestamp clock using 203 // reflection or Unsafe between when discovery was enabled and 204 // now. Unconditionally update the static field in ReferenceProcessor 205 // here so that we use the new value during processing of the 206 // discovered soft refs. 207 208 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock(); 209 210 bool trace_time = PrintGCDetails && PrintReferenceGC; 211 212 // Soft references 213 size_t soft_count = 0; 214 { 215 GCTraceTime tt("SoftReference", trace_time, false, gc_timer); 216 soft_count = 217 process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true, 218 is_alive, keep_alive, complete_gc, task_executor); 219 } 220 221 update_soft_ref_master_clock(); 222 223 // Weak references 224 size_t weak_count = 0; 225 { 226 GCTraceTime tt("WeakReference", trace_time, false, gc_timer); 227 weak_count = 228 process_discovered_reflist(_discoveredWeakRefs, NULL, true, 229 is_alive, keep_alive, complete_gc, task_executor); 230 } 231 232 // Final references 233 size_t final_count = 0; 234 { 235 GCTraceTime tt("FinalReference", trace_time, false, gc_timer); 236 final_count = 237 process_discovered_reflist(_discoveredFinalRefs, NULL, false, 238 is_alive, keep_alive, complete_gc, task_executor); 239 } 240 241 // Phantom references 242 size_t phantom_count = 0; 243 { 244 GCTraceTime tt("PhantomReference", trace_time, false, gc_timer); 245 phantom_count = 246 process_discovered_reflist(_discoveredPhantomRefs, NULL, false, 247 is_alive, keep_alive, complete_gc, task_executor); 248 } 249 250 // Weak global JNI references. It would make more sense (semantically) to 251 // traverse these simultaneously with the regular weak references above, but 252 // that is not how the JDK1.2 specification is. See #4126360. Native code can 253 // thus use JNI weak references to circumvent the phantom references and 254 // resurrect a "post-mortem" object. 255 { 256 GCTraceTime tt("JNI Weak Reference", trace_time, false, gc_timer); 257 if (task_executor != NULL) { 258 task_executor->set_single_threaded_mode(); 259 } 260 process_phaseJNI(is_alive, keep_alive, complete_gc); 261 } 262 263 return ReferenceProcessorStats(soft_count, weak_count, final_count, phantom_count); 264 } 265 266 #ifndef PRODUCT 267 // Calculate the number of jni handles. 268 uint ReferenceProcessor::count_jni_refs() { 269 class AlwaysAliveClosure: public BoolObjectClosure { 270 public: 271 virtual bool do_object_b(oop obj) { return true; } 272 }; 273 274 class CountHandleClosure: public OopClosure { 275 private: 276 int _count; 277 public: 278 CountHandleClosure(): _count(0) {} 279 void do_oop(oop* unused) { _count++; } 280 void do_oop(narrowOop* unused) { ShouldNotReachHere(); } 281 int count() { return _count; } 282 }; 283 CountHandleClosure global_handle_count; 284 AlwaysAliveClosure always_alive; 285 JNIHandles::weak_oops_do(&always_alive, &global_handle_count); 286 return global_handle_count.count(); 287 } 288 #endif 289 290 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive, 291 OopClosure* keep_alive, 292 VoidClosure* complete_gc) { 293 #ifndef PRODUCT 294 if (PrintGCDetails && PrintReferenceGC) { 295 unsigned int count = count_jni_refs(); 296 gclog_or_tty->print(", %u refs", count); 297 } 298 #endif 299 JNIHandles::weak_oops_do(is_alive, keep_alive); 300 complete_gc->do_void(); 301 } 302 303 304 template <class T> 305 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref, 306 AbstractRefProcTaskExecutor* task_executor) { 307 308 // Remember old value of pending references list 309 T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr(); 310 T old_pending_list_value = *pending_list_addr; 311 312 // Enqueue references that are not made active again, and 313 // clear the decks for the next collection (cycle). 314 ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor); 315 // Do the post-barrier on pending_list_addr missed in 316 // enqueue_discovered_reflist. 317 oopDesc::bs()->write_ref_field(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr)); 318 319 // Stop treating discovered references specially. 320 ref->disable_discovery(); 321 322 // Return true if new pending references were added 323 return old_pending_list_value != *pending_list_addr; 324 } 325 326 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) { 327 NOT_PRODUCT(verify_ok_to_handle_reflists()); 328 if (UseCompressedOops) { 329 return enqueue_discovered_ref_helper<narrowOop>(this, task_executor); 330 } else { 331 return enqueue_discovered_ref_helper<oop>(this, task_executor); 332 } 333 } 334 335 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list, 336 HeapWord* pending_list_addr) { 337 // Given a list of refs linked through the "discovered" field 338 // (java.lang.ref.Reference.discovered), self-loop their "next" field 339 // thus distinguishing them from active References, then 340 // prepend them to the pending list. 341 // BKWRD COMPATIBILITY NOTE: For older JDKs (prior to the fix for 4956777), 342 // the "next" field is used to chain the pending list, not the discovered 343 // field. 344 345 if (TraceReferenceGC && PrintGCDetails) { 346 gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list " 347 INTPTR_FORMAT, (address)refs_list.head()); 348 } 349 350 oop obj = NULL; 351 oop next_d = refs_list.head(); 352 if (pending_list_uses_discovered_field()) { // New behaviour 353 // Walk down the list, self-looping the next field 354 // so that the References are not considered active. 355 while (obj != next_d) { 356 obj = next_d; 357 assert(obj->is_instanceRef(), "should be reference object"); 358 next_d = java_lang_ref_Reference::discovered(obj); 359 if (TraceReferenceGC && PrintGCDetails) { 360 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT, 361 (void *)obj, (void *)next_d); 362 } 363 assert(java_lang_ref_Reference::next(obj) == NULL, 364 "Reference not active; should not be discovered"); 365 // Self-loop next, so as to make Ref not active. 366 java_lang_ref_Reference::set_next_raw(obj, obj); 367 if (next_d == obj) { // obj is last 368 // Swap refs_list into pendling_list_addr and 369 // set obj's discovered to what we read from pending_list_addr. 370 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr); 371 // Need post-barrier on pending_list_addr above; 372 // see special post-barrier code at the end of 373 // enqueue_discovered_reflists() further below. 374 java_lang_ref_Reference::set_discovered_raw(obj, old); // old may be NULL 375 oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), old); 376 } 377 } 378 } else { // Old behaviour 379 // Walk down the list, copying the discovered field into 380 // the next field and clearing the discovered field. 381 while (obj != next_d) { 382 obj = next_d; 383 assert(obj->is_instanceRef(), "should be reference object"); 384 next_d = java_lang_ref_Reference::discovered(obj); 385 if (TraceReferenceGC && PrintGCDetails) { 386 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT, 387 (void *)obj, (void *)next_d); 388 } 389 assert(java_lang_ref_Reference::next(obj) == NULL, 390 "The reference should not be enqueued"); 391 if (next_d == obj) { // obj is last 392 // Swap refs_list into pendling_list_addr and 393 // set obj's next to what we read from pending_list_addr. 394 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr); 395 // Need oop_check on pending_list_addr above; 396 // see special oop-check code at the end of 397 // enqueue_discovered_reflists() further below. 398 if (old == NULL) { 399 // obj should be made to point to itself, since 400 // pending list was empty. 401 java_lang_ref_Reference::set_next(obj, obj); 402 } else { 403 java_lang_ref_Reference::set_next(obj, old); 404 } 405 } else { 406 java_lang_ref_Reference::set_next(obj, next_d); 407 } 408 java_lang_ref_Reference::set_discovered(obj, (oop) NULL); 409 } 410 } 411 } 412 413 // Parallel enqueue task 414 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask { 415 public: 416 RefProcEnqueueTask(ReferenceProcessor& ref_processor, 417 DiscoveredList discovered_refs[], 418 HeapWord* pending_list_addr, 419 int n_queues) 420 : EnqueueTask(ref_processor, discovered_refs, 421 pending_list_addr, n_queues) 422 { } 423 424 virtual void work(unsigned int work_id) { 425 assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds"); 426 // Simplest first cut: static partitioning. 427 int index = work_id; 428 // The increment on "index" must correspond to the maximum number of queues 429 // (n_queues) with which that ReferenceProcessor was created. That 430 // is because of the "clever" way the discovered references lists were 431 // allocated and are indexed into. 432 assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected"); 433 for (int j = 0; 434 j < ReferenceProcessor::number_of_subclasses_of_ref(); 435 j++, index += _n_queues) { 436 _ref_processor.enqueue_discovered_reflist( 437 _refs_lists[index], _pending_list_addr); 438 _refs_lists[index].set_head(NULL); 439 _refs_lists[index].set_length(0); 440 } 441 } 442 }; 443 444 // Enqueue references that are not made active again 445 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr, 446 AbstractRefProcTaskExecutor* task_executor) { 447 if (_processing_is_mt && task_executor != NULL) { 448 // Parallel code 449 RefProcEnqueueTask tsk(*this, _discovered_refs, 450 pending_list_addr, _max_num_q); 451 task_executor->execute(tsk); 452 } else { 453 // Serial code: call the parent class's implementation 454 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) { 455 enqueue_discovered_reflist(_discovered_refs[i], pending_list_addr); 456 _discovered_refs[i].set_head(NULL); 457 _discovered_refs[i].set_length(0); 458 } 459 } 460 } 461 462 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) { 463 _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref); 464 oop discovered = java_lang_ref_Reference::discovered(_ref); 465 assert(_discovered_addr && discovered->is_oop_or_null(), 466 "discovered field is bad"); 467 _next = discovered; 468 _referent_addr = java_lang_ref_Reference::referent_addr(_ref); 469 _referent = java_lang_ref_Reference::referent(_ref); 470 assert(Universe::heap()->is_in_reserved_or_null(_referent), 471 "Wrong oop found in java.lang.Reference object"); 472 assert(allow_null_referent ? 473 _referent->is_oop_or_null() 474 : _referent->is_oop(), 475 "bad referent"); 476 } 477 478 void DiscoveredListIterator::remove() { 479 assert(_ref->is_oop(), "Dropping a bad reference"); 480 oop_store_raw(_discovered_addr, NULL); 481 482 // First _prev_next ref actually points into DiscoveredList (gross). 483 oop new_next; 484 if (_next == _ref) { 485 // At the end of the list, we should make _prev point to itself. 486 // If _ref is the first ref, then _prev_next will be in the DiscoveredList, 487 // and _prev will be NULL. 488 new_next = _prev; 489 } else { 490 new_next = _next; 491 } 492 493 if (UseCompressedOops) { 494 // Remove Reference object from list. 495 oopDesc::encode_store_heap_oop((narrowOop*)_prev_next, new_next); 496 } else { 497 // Remove Reference object from list. 498 oopDesc::store_heap_oop((oop*)_prev_next, new_next); 499 } 500 NOT_PRODUCT(_removed++); 501 _refs_list.dec_length(1); 502 } 503 504 // Make the Reference object active again. 505 void DiscoveredListIterator::make_active() { 506 // For G1 we don't want to use set_next - it 507 // will dirty the card for the next field of 508 // the reference object and will fail 509 // CT verification. 510 if (UseG1GC) { 511 HeapWord* next_addr = java_lang_ref_Reference::next_addr(_ref); 512 if (UseCompressedOops) { 513 oopDesc::bs()->write_ref_field_pre((narrowOop*)next_addr, NULL); 514 } else { 515 oopDesc::bs()->write_ref_field_pre((oop*)next_addr, NULL); 516 } 517 java_lang_ref_Reference::set_next_raw(_ref, NULL); 518 } else { 519 java_lang_ref_Reference::set_next(_ref, NULL); 520 } 521 } 522 523 void DiscoveredListIterator::clear_referent() { 524 oop_store_raw(_referent_addr, NULL); 525 } 526 527 // NOTE: process_phase*() are largely similar, and at a high level 528 // merely iterate over the extant list applying a predicate to 529 // each of its elements and possibly removing that element from the 530 // list and applying some further closures to that element. 531 // We should consider the possibility of replacing these 532 // process_phase*() methods by abstracting them into 533 // a single general iterator invocation that receives appropriate 534 // closures that accomplish this work. 535 536 // (SoftReferences only) Traverse the list and remove any SoftReferences whose 537 // referents are not alive, but that should be kept alive for policy reasons. 538 // Keep alive the transitive closure of all such referents. 539 void 540 ReferenceProcessor::process_phase1(DiscoveredList& refs_list, 541 ReferencePolicy* policy, 542 BoolObjectClosure* is_alive, 543 OopClosure* keep_alive, 544 VoidClosure* complete_gc) { 545 assert(policy != NULL, "Must have a non-NULL policy"); 546 DiscoveredListIterator iter(refs_list, keep_alive, is_alive); 547 // Decide which softly reachable refs should be kept alive. 548 while (iter.has_next()) { 549 iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */)); 550 bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive(); 551 if (referent_is_dead && 552 !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) { 553 if (TraceReferenceGC) { 554 gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s" ") by policy", 555 (void *)iter.obj(), iter.obj()->klass()->internal_name()); 556 } 557 // Remove Reference object from list 558 iter.remove(); 559 // Make the Reference object active again 560 iter.make_active(); 561 // keep the referent around 562 iter.make_referent_alive(); 563 iter.move_to_next(); 564 } else { 565 iter.next(); 566 } 567 } 568 // Close the reachable set 569 complete_gc->do_void(); 570 NOT_PRODUCT( 571 if (PrintGCDetails && TraceReferenceGC) { 572 gclog_or_tty->print_cr(" Dropped %d dead Refs out of %d " 573 "discovered Refs by policy, from list " INTPTR_FORMAT, 574 iter.removed(), iter.processed(), (address)refs_list.head()); 575 } 576 ) 577 } 578 579 // Traverse the list and remove any Refs that are not active, or 580 // whose referents are either alive or NULL. 581 void 582 ReferenceProcessor::pp2_work(DiscoveredList& refs_list, 583 BoolObjectClosure* is_alive, 584 OopClosure* keep_alive) { 585 assert(discovery_is_atomic(), "Error"); 586 DiscoveredListIterator iter(refs_list, keep_alive, is_alive); 587 while (iter.has_next()) { 588 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */)); 589 DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());) 590 assert(next == NULL, "Should not discover inactive Reference"); 591 if (iter.is_referent_alive()) { 592 if (TraceReferenceGC) { 593 gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)", 594 (void *)iter.obj(), iter.obj()->klass()->internal_name()); 595 } 596 // The referent is reachable after all. 597 // Remove Reference object from list. 598 iter.remove(); 599 // Update the referent pointer as necessary: Note that this 600 // should not entail any recursive marking because the 601 // referent must already have been traversed. 602 iter.make_referent_alive(); 603 iter.move_to_next(); 604 } else { 605 iter.next(); 606 } 607 } 608 NOT_PRODUCT( 609 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) { 610 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d " 611 "Refs in discovered list " INTPTR_FORMAT, 612 iter.removed(), iter.processed(), (address)refs_list.head()); 613 } 614 ) 615 } 616 617 void 618 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList& refs_list, 619 BoolObjectClosure* is_alive, 620 OopClosure* keep_alive, 621 VoidClosure* complete_gc) { 622 assert(!discovery_is_atomic(), "Error"); 623 DiscoveredListIterator iter(refs_list, keep_alive, is_alive); 624 while (iter.has_next()) { 625 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */)); 626 HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj()); 627 oop next = java_lang_ref_Reference::next(iter.obj()); 628 if ((iter.referent() == NULL || iter.is_referent_alive() || 629 next != NULL)) { 630 assert(next->is_oop_or_null(), "bad next field"); 631 // Remove Reference object from list 632 iter.remove(); 633 // Trace the cohorts 634 iter.make_referent_alive(); 635 if (UseCompressedOops) { 636 keep_alive->do_oop((narrowOop*)next_addr); 637 } else { 638 keep_alive->do_oop((oop*)next_addr); 639 } 640 iter.move_to_next(); 641 } else { 642 iter.next(); 643 } 644 } 645 // Now close the newly reachable set 646 complete_gc->do_void(); 647 NOT_PRODUCT( 648 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) { 649 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d " 650 "Refs in discovered list " INTPTR_FORMAT, 651 iter.removed(), iter.processed(), (address)refs_list.head()); 652 } 653 ) 654 } 655 656 // Traverse the list and process the referents, by either 657 // clearing them or keeping them (and their reachable 658 // closure) alive. 659 void 660 ReferenceProcessor::process_phase3(DiscoveredList& refs_list, 661 bool clear_referent, 662 BoolObjectClosure* is_alive, 663 OopClosure* keep_alive, 664 VoidClosure* complete_gc) { 665 ResourceMark rm; 666 DiscoveredListIterator iter(refs_list, keep_alive, is_alive); 667 while (iter.has_next()) { 668 iter.update_discovered(); 669 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */)); 670 if (clear_referent) { 671 // NULL out referent pointer 672 iter.clear_referent(); 673 } else { 674 // keep the referent around 675 iter.make_referent_alive(); 676 } 677 if (TraceReferenceGC) { 678 gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending", 679 clear_referent ? "cleared " : "", 680 (void *)iter.obj(), iter.obj()->klass()->internal_name()); 681 } 682 assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference"); 683 iter.next(); 684 } 685 // Remember to update the next pointer of the last ref. 686 iter.update_discovered(); 687 // Close the reachable set 688 complete_gc->do_void(); 689 } 690 691 void 692 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) { 693 oop obj = NULL; 694 oop next = refs_list.head(); 695 while (next != obj) { 696 obj = next; 697 next = java_lang_ref_Reference::discovered(obj); 698 java_lang_ref_Reference::set_discovered_raw(obj, NULL); 699 } 700 refs_list.set_head(NULL); 701 refs_list.set_length(0); 702 } 703 704 void 705 ReferenceProcessor::abandon_partial_discovered_list(DiscoveredList& refs_list) { 706 clear_discovered_references(refs_list); 707 } 708 709 void ReferenceProcessor::abandon_partial_discovery() { 710 // loop over the lists 711 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) { 712 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) { 713 gclog_or_tty->print_cr("\nAbandoning %s discovered list", list_name(i)); 714 } 715 abandon_partial_discovered_list(_discovered_refs[i]); 716 } 717 } 718 719 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask { 720 public: 721 RefProcPhase1Task(ReferenceProcessor& ref_processor, 722 DiscoveredList refs_lists[], 723 ReferencePolicy* policy, 724 bool marks_oops_alive) 725 : ProcessTask(ref_processor, refs_lists, marks_oops_alive), 726 _policy(policy) 727 { } 728 virtual void work(unsigned int i, BoolObjectClosure& is_alive, 729 OopClosure& keep_alive, 730 VoidClosure& complete_gc) 731 { 732 Thread* thr = Thread::current(); 733 int refs_list_index = ((WorkerThread*)thr)->id(); 734 _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy, 735 &is_alive, &keep_alive, &complete_gc); 736 } 737 private: 738 ReferencePolicy* _policy; 739 }; 740 741 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask { 742 public: 743 RefProcPhase2Task(ReferenceProcessor& ref_processor, 744 DiscoveredList refs_lists[], 745 bool marks_oops_alive) 746 : ProcessTask(ref_processor, refs_lists, marks_oops_alive) 747 { } 748 virtual void work(unsigned int i, BoolObjectClosure& is_alive, 749 OopClosure& keep_alive, 750 VoidClosure& complete_gc) 751 { 752 _ref_processor.process_phase2(_refs_lists[i], 753 &is_alive, &keep_alive, &complete_gc); 754 } 755 }; 756 757 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask { 758 public: 759 RefProcPhase3Task(ReferenceProcessor& ref_processor, 760 DiscoveredList refs_lists[], 761 bool clear_referent, 762 bool marks_oops_alive) 763 : ProcessTask(ref_processor, refs_lists, marks_oops_alive), 764 _clear_referent(clear_referent) 765 { } 766 virtual void work(unsigned int i, BoolObjectClosure& is_alive, 767 OopClosure& keep_alive, 768 VoidClosure& complete_gc) 769 { 770 // Don't use "refs_list_index" calculated in this way because 771 // balance_queues() has moved the Ref's into the first n queues. 772 // Thread* thr = Thread::current(); 773 // int refs_list_index = ((WorkerThread*)thr)->id(); 774 // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent, 775 _ref_processor.process_phase3(_refs_lists[i], _clear_referent, 776 &is_alive, &keep_alive, &complete_gc); 777 } 778 private: 779 bool _clear_referent; 780 }; 781 782 void ReferenceProcessor::set_discovered(oop ref, oop value) { 783 java_lang_ref_Reference::set_discovered_raw(ref, value); 784 if (_discovered_list_needs_barrier) { 785 oopDesc::bs()->write_ref_field(ref, value); 786 } 787 } 788 789 // Balances reference queues. 790 // Move entries from all queues[0, 1, ..., _max_num_q-1] to 791 // queues[0, 1, ..., _num_q-1] because only the first _num_q 792 // corresponding to the active workers will be processed. 793 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[]) 794 { 795 // calculate total length 796 size_t total_refs = 0; 797 if (TraceReferenceGC && PrintGCDetails) { 798 gclog_or_tty->print_cr("\nBalance ref_lists "); 799 } 800 801 for (uint i = 0; i < _max_num_q; ++i) { 802 total_refs += ref_lists[i].length(); 803 if (TraceReferenceGC && PrintGCDetails) { 804 gclog_or_tty->print("%d ", ref_lists[i].length()); 805 } 806 } 807 if (TraceReferenceGC && PrintGCDetails) { 808 gclog_or_tty->print_cr(" = %d", total_refs); 809 } 810 size_t avg_refs = total_refs / _num_q + 1; 811 uint to_idx = 0; 812 for (uint from_idx = 0; from_idx < _max_num_q; from_idx++) { 813 bool move_all = false; 814 if (from_idx >= _num_q) { 815 move_all = ref_lists[from_idx].length() > 0; 816 } 817 while ((ref_lists[from_idx].length() > avg_refs) || 818 move_all) { 819 assert(to_idx < _num_q, "Sanity Check!"); 820 if (ref_lists[to_idx].length() < avg_refs) { 821 // move superfluous refs 822 size_t refs_to_move; 823 // Move all the Ref's if the from queue will not be processed. 824 if (move_all) { 825 refs_to_move = MIN2(ref_lists[from_idx].length(), 826 avg_refs - ref_lists[to_idx].length()); 827 } else { 828 refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs, 829 avg_refs - ref_lists[to_idx].length()); 830 } 831 832 assert(refs_to_move > 0, "otherwise the code below will fail"); 833 834 oop move_head = ref_lists[from_idx].head(); 835 oop move_tail = move_head; 836 oop new_head = move_head; 837 // find an element to split the list on 838 for (size_t j = 0; j < refs_to_move; ++j) { 839 move_tail = new_head; 840 new_head = java_lang_ref_Reference::discovered(new_head); 841 } 842 843 // Add the chain to the to list. 844 if (ref_lists[to_idx].head() == NULL) { 845 // to list is empty. Make a loop at the end. 846 set_discovered(move_tail, move_tail); 847 } else { 848 set_discovered(move_tail, ref_lists[to_idx].head()); 849 } 850 ref_lists[to_idx].set_head(move_head); 851 ref_lists[to_idx].inc_length(refs_to_move); 852 853 // Remove the chain from the from list. 854 if (move_tail == new_head) { 855 // We found the end of the from list. 856 ref_lists[from_idx].set_head(NULL); 857 } else { 858 ref_lists[from_idx].set_head(new_head); 859 } 860 ref_lists[from_idx].dec_length(refs_to_move); 861 if (ref_lists[from_idx].length() == 0) { 862 break; 863 } 864 } else { 865 to_idx = (to_idx + 1) % _num_q; 866 } 867 } 868 } 869 #ifdef ASSERT 870 size_t balanced_total_refs = 0; 871 for (uint i = 0; i < _max_num_q; ++i) { 872 balanced_total_refs += ref_lists[i].length(); 873 if (TraceReferenceGC && PrintGCDetails) { 874 gclog_or_tty->print("%d ", ref_lists[i].length()); 875 } 876 } 877 if (TraceReferenceGC && PrintGCDetails) { 878 gclog_or_tty->print_cr(" = %d", balanced_total_refs); 879 gclog_or_tty->flush(); 880 } 881 assert(total_refs == balanced_total_refs, "Balancing was incomplete"); 882 #endif 883 } 884 885 void ReferenceProcessor::balance_all_queues() { 886 balance_queues(_discoveredSoftRefs); 887 balance_queues(_discoveredWeakRefs); 888 balance_queues(_discoveredFinalRefs); 889 balance_queues(_discoveredPhantomRefs); 890 } 891 892 size_t 893 ReferenceProcessor::process_discovered_reflist( 894 DiscoveredList refs_lists[], 895 ReferencePolicy* policy, 896 bool clear_referent, 897 BoolObjectClosure* is_alive, 898 OopClosure* keep_alive, 899 VoidClosure* complete_gc, 900 AbstractRefProcTaskExecutor* task_executor) 901 { 902 bool mt_processing = task_executor != NULL && _processing_is_mt; 903 // If discovery used MT and a dynamic number of GC threads, then 904 // the queues must be balanced for correctness if fewer than the 905 // maximum number of queues were used. The number of queue used 906 // during discovery may be different than the number to be used 907 // for processing so don't depend of _num_q < _max_num_q as part 908 // of the test. 909 bool must_balance = _discovery_is_mt; 910 911 if ((mt_processing && ParallelRefProcBalancingEnabled) || 912 must_balance) { 913 balance_queues(refs_lists); 914 } 915 916 size_t total_list_count = total_count(refs_lists); 917 918 if (PrintReferenceGC && PrintGCDetails) { 919 gclog_or_tty->print(", %u refs", total_list_count); 920 } 921 922 // Phase 1 (soft refs only): 923 // . Traverse the list and remove any SoftReferences whose 924 // referents are not alive, but that should be kept alive for 925 // policy reasons. Keep alive the transitive closure of all 926 // such referents. 927 if (policy != NULL) { 928 if (mt_processing) { 929 RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/); 930 task_executor->execute(phase1); 931 } else { 932 for (uint i = 0; i < _max_num_q; i++) { 933 process_phase1(refs_lists[i], policy, 934 is_alive, keep_alive, complete_gc); 935 } 936 } 937 } else { // policy == NULL 938 assert(refs_lists != _discoveredSoftRefs, 939 "Policy must be specified for soft references."); 940 } 941 942 // Phase 2: 943 // . Traverse the list and remove any refs whose referents are alive. 944 if (mt_processing) { 945 RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/); 946 task_executor->execute(phase2); 947 } else { 948 for (uint i = 0; i < _max_num_q; i++) { 949 process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc); 950 } 951 } 952 953 // Phase 3: 954 // . Traverse the list and process referents as appropriate. 955 if (mt_processing) { 956 RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/); 957 task_executor->execute(phase3); 958 } else { 959 for (uint i = 0; i < _max_num_q; i++) { 960 process_phase3(refs_lists[i], clear_referent, 961 is_alive, keep_alive, complete_gc); 962 } 963 } 964 965 return total_list_count; 966 } 967 968 void ReferenceProcessor::clean_up_discovered_references() { 969 // loop over the lists 970 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) { 971 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) { 972 gclog_or_tty->print_cr( 973 "\nScrubbing %s discovered list of Null referents", 974 list_name(i)); 975 } 976 clean_up_discovered_reflist(_discovered_refs[i]); 977 } 978 } 979 980 void ReferenceProcessor::clean_up_discovered_reflist(DiscoveredList& refs_list) { 981 assert(!discovery_is_atomic(), "Else why call this method?"); 982 DiscoveredListIterator iter(refs_list, NULL, NULL); 983 while (iter.has_next()) { 984 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */)); 985 oop next = java_lang_ref_Reference::next(iter.obj()); 986 assert(next->is_oop_or_null(), "bad next field"); 987 // If referent has been cleared or Reference is not active, 988 // drop it. 989 if (iter.referent() == NULL || next != NULL) { 990 debug_only( 991 if (PrintGCDetails && TraceReferenceGC) { 992 gclog_or_tty->print_cr("clean_up_discovered_list: Dropping Reference: " 993 INTPTR_FORMAT " with next field: " INTPTR_FORMAT 994 " and referent: " INTPTR_FORMAT, 995 (void *)iter.obj(), (void *)next, (void *)iter.referent()); 996 } 997 ) 998 // Remove Reference object from list 999 iter.remove(); 1000 iter.move_to_next(); 1001 } else { 1002 iter.next(); 1003 } 1004 } 1005 NOT_PRODUCT( 1006 if (PrintGCDetails && TraceReferenceGC) { 1007 gclog_or_tty->print( 1008 " Removed %d Refs with NULL referents out of %d discovered Refs", 1009 iter.removed(), iter.processed()); 1010 } 1011 ) 1012 } 1013 1014 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) { 1015 uint id = 0; 1016 // Determine the queue index to use for this object. 1017 if (_discovery_is_mt) { 1018 // During a multi-threaded discovery phase, 1019 // each thread saves to its "own" list. 1020 Thread* thr = Thread::current(); 1021 id = thr->as_Worker_thread()->id(); 1022 } else { 1023 // single-threaded discovery, we save in round-robin 1024 // fashion to each of the lists. 1025 if (_processing_is_mt) { 1026 id = next_id(); 1027 } 1028 } 1029 assert(0 <= id && id < _max_num_q, "Id is out-of-bounds (call Freud?)"); 1030 1031 // Get the discovered queue to which we will add 1032 DiscoveredList* list = NULL; 1033 switch (rt) { 1034 case REF_OTHER: 1035 // Unknown reference type, no special treatment 1036 break; 1037 case REF_SOFT: 1038 list = &_discoveredSoftRefs[id]; 1039 break; 1040 case REF_WEAK: 1041 list = &_discoveredWeakRefs[id]; 1042 break; 1043 case REF_FINAL: 1044 list = &_discoveredFinalRefs[id]; 1045 break; 1046 case REF_PHANTOM: 1047 list = &_discoveredPhantomRefs[id]; 1048 break; 1049 case REF_NONE: 1050 // we should not reach here if we are an InstanceRefKlass 1051 default: 1052 ShouldNotReachHere(); 1053 } 1054 if (TraceReferenceGC && PrintGCDetails) { 1055 gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, list); 1056 } 1057 return list; 1058 } 1059 1060 inline void 1061 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list, 1062 oop obj, 1063 HeapWord* discovered_addr) { 1064 assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller"); 1065 // First we must make sure this object is only enqueued once. CAS in a non null 1066 // discovered_addr. 1067 oop current_head = refs_list.head(); 1068 // The last ref must have its discovered field pointing to itself. 1069 oop next_discovered = (current_head != NULL) ? current_head : obj; 1070 1071 // Note: In the case of G1, this specific pre-barrier is strictly 1072 // not necessary because the only case we are interested in 1073 // here is when *discovered_addr is NULL (see the CAS further below), 1074 // so this will expand to nothing. As a result, we have manually 1075 // elided this out for G1, but left in the test for some future 1076 // collector that might have need for a pre-barrier here, e.g.:- 1077 // oopDesc::bs()->write_ref_field_pre((oop* or narrowOop*)discovered_addr, next_discovered); 1078 assert(!_discovered_list_needs_barrier || UseG1GC, 1079 "Need to check non-G1 collector: " 1080 "may need a pre-write-barrier for CAS from NULL below"); 1081 oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr, 1082 NULL); 1083 if (retest == NULL) { 1084 // This thread just won the right to enqueue the object. 1085 // We have separate lists for enqueueing, so no synchronization 1086 // is necessary. 1087 refs_list.set_head(obj); 1088 refs_list.inc_length(1); 1089 if (_discovered_list_needs_barrier) { 1090 oopDesc::bs()->write_ref_field((void*)discovered_addr, next_discovered); 1091 } 1092 1093 if (TraceReferenceGC) { 1094 gclog_or_tty->print_cr("Discovered reference (mt) (" INTPTR_FORMAT ": %s)", 1095 (void *)obj, obj->klass()->internal_name()); 1096 } 1097 } else { 1098 // If retest was non NULL, another thread beat us to it: 1099 // The reference has already been discovered... 1100 if (TraceReferenceGC) { 1101 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)", 1102 (void *)obj, obj->klass()->internal_name()); 1103 } 1104 } 1105 } 1106 1107 #ifndef PRODUCT 1108 // Non-atomic (i.e. concurrent) discovery might allow us 1109 // to observe j.l.References with NULL referents, being those 1110 // cleared concurrently by mutators during (or after) discovery. 1111 void ReferenceProcessor::verify_referent(oop obj) { 1112 bool da = discovery_is_atomic(); 1113 oop referent = java_lang_ref_Reference::referent(obj); 1114 assert(da ? referent->is_oop() : referent->is_oop_or_null(), 1115 err_msg("Bad referent " INTPTR_FORMAT " found in Reference " 1116 INTPTR_FORMAT " during %satomic discovery ", 1117 (void *)referent, (void *)obj, da ? "" : "non-")); 1118 } 1119 #endif 1120 1121 // We mention two of several possible choices here: 1122 // #0: if the reference object is not in the "originating generation" 1123 // (or part of the heap being collected, indicated by our "span" 1124 // we don't treat it specially (i.e. we scan it as we would 1125 // a normal oop, treating its references as strong references). 1126 // This means that references can't be discovered unless their 1127 // referent is also in the same span. This is the simplest, 1128 // most "local" and most conservative approach, albeit one 1129 // that may cause weak references to be enqueued least promptly. 1130 // We call this choice the "ReferenceBasedDiscovery" policy. 1131 // #1: the reference object may be in any generation (span), but if 1132 // the referent is in the generation (span) being currently collected 1133 // then we can discover the reference object, provided 1134 // the object has not already been discovered by 1135 // a different concurrently running collector (as may be the 1136 // case, for instance, if the reference object is in CMS and 1137 // the referent in DefNewGeneration), and provided the processing 1138 // of this reference object by the current collector will 1139 // appear atomic to every other collector in the system. 1140 // (Thus, for instance, a concurrent collector may not 1141 // discover references in other generations even if the 1142 // referent is in its own generation). This policy may, 1143 // in certain cases, enqueue references somewhat sooner than 1144 // might Policy #0 above, but at marginally increased cost 1145 // and complexity in processing these references. 1146 // We call this choice the "RefeferentBasedDiscovery" policy. 1147 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) { 1148 // Make sure we are discovering refs (rather than processing discovered refs). 1149 if (!_discovering_refs || !RegisterReferences) { 1150 return false; 1151 } 1152 // We only discover active references. 1153 oop next = java_lang_ref_Reference::next(obj); 1154 if (next != NULL) { // Ref is no longer active 1155 return false; 1156 } 1157 1158 HeapWord* obj_addr = (HeapWord*)obj; 1159 if (RefDiscoveryPolicy == ReferenceBasedDiscovery && 1160 !_span.contains(obj_addr)) { 1161 // Reference is not in the originating generation; 1162 // don't treat it specially (i.e. we want to scan it as a normal 1163 // object with strong references). 1164 return false; 1165 } 1166 1167 // We only discover references whose referents are not (yet) 1168 // known to be strongly reachable. 1169 if (is_alive_non_header() != NULL) { 1170 verify_referent(obj); 1171 if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) { 1172 return false; // referent is reachable 1173 } 1174 } 1175 if (rt == REF_SOFT) { 1176 // For soft refs we can decide now if these are not 1177 // current candidates for clearing, in which case we 1178 // can mark through them now, rather than delaying that 1179 // to the reference-processing phase. Since all current 1180 // time-stamp policies advance the soft-ref clock only 1181 // at a major collection cycle, this is always currently 1182 // accurate. 1183 if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) { 1184 return false; 1185 } 1186 } 1187 1188 ResourceMark rm; // Needed for tracing. 1189 1190 HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj); 1191 const oop discovered = java_lang_ref_Reference::discovered(obj); 1192 assert(discovered->is_oop_or_null(), "bad discovered field"); 1193 if (discovered != NULL) { 1194 // The reference has already been discovered... 1195 if (TraceReferenceGC) { 1196 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)", 1197 (void *)obj, obj->klass()->internal_name()); 1198 } 1199 if (RefDiscoveryPolicy == ReferentBasedDiscovery) { 1200 // assumes that an object is not processed twice; 1201 // if it's been already discovered it must be on another 1202 // generation's discovered list; so we won't discover it. 1203 return false; 1204 } else { 1205 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery, 1206 "Unrecognized policy"); 1207 // Check assumption that an object is not potentially 1208 // discovered twice except by concurrent collectors that potentially 1209 // trace the same Reference object twice. 1210 assert(UseConcMarkSweepGC || UseG1GC, 1211 "Only possible with a concurrent marking collector"); 1212 return true; 1213 } 1214 } 1215 1216 if (RefDiscoveryPolicy == ReferentBasedDiscovery) { 1217 verify_referent(obj); 1218 // Discover if and only if EITHER: 1219 // .. reference is in our span, OR 1220 // .. we are an atomic collector and referent is in our span 1221 if (_span.contains(obj_addr) || 1222 (discovery_is_atomic() && 1223 _span.contains(java_lang_ref_Reference::referent(obj)))) { 1224 // should_enqueue = true; 1225 } else { 1226 return false; 1227 } 1228 } else { 1229 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery && 1230 _span.contains(obj_addr), "code inconsistency"); 1231 } 1232 1233 // Get the right type of discovered queue head. 1234 DiscoveredList* list = get_discovered_list(rt); 1235 if (list == NULL) { 1236 return false; // nothing special needs to be done 1237 } 1238 1239 if (_discovery_is_mt) { 1240 add_to_discovered_list_mt(*list, obj, discovered_addr); 1241 } else { 1242 // If "_discovered_list_needs_barrier", we do write barriers when 1243 // updating the discovered reference list. Otherwise, we do a raw store 1244 // here: the field will be visited later when processing the discovered 1245 // references. 1246 oop current_head = list->head(); 1247 // The last ref must have its discovered field pointing to itself. 1248 oop next_discovered = (current_head != NULL) ? current_head : obj; 1249 1250 // As in the case further above, since we are over-writing a NULL 1251 // pre-value, we can safely elide the pre-barrier here for the case of G1. 1252 // e.g.:- oopDesc::bs()->write_ref_field_pre((oop* or narrowOop*)discovered_addr, next_discovered); 1253 assert(discovered == NULL, "control point invariant"); 1254 assert(!_discovered_list_needs_barrier || UseG1GC, 1255 "For non-G1 collector, may need a pre-write-barrier for CAS from NULL below"); 1256 oop_store_raw(discovered_addr, next_discovered); 1257 if (_discovered_list_needs_barrier) { 1258 oopDesc::bs()->write_ref_field((void*)discovered_addr, next_discovered); 1259 } 1260 list->set_head(obj); 1261 list->inc_length(1); 1262 1263 if (TraceReferenceGC) { 1264 gclog_or_tty->print_cr("Discovered reference (" INTPTR_FORMAT ": %s)", 1265 (void *)obj, obj->klass()->internal_name()); 1266 } 1267 } 1268 assert(obj->is_oop(), "Discovered a bad reference"); 1269 verify_referent(obj); 1270 return true; 1271 } 1272 1273 // Preclean the discovered references by removing those 1274 // whose referents are alive, and by marking from those that 1275 // are not active. These lists can be handled here 1276 // in any order and, indeed, concurrently. 1277 void ReferenceProcessor::preclean_discovered_references( 1278 BoolObjectClosure* is_alive, 1279 OopClosure* keep_alive, 1280 VoidClosure* complete_gc, 1281 YieldClosure* yield, 1282 GCTimer* gc_timer) { 1283 1284 NOT_PRODUCT(verify_ok_to_handle_reflists()); 1285 1286 // Soft references 1287 { 1288 GCTraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC, 1289 false, gc_timer); 1290 for (uint i = 0; i < _max_num_q; i++) { 1291 if (yield->should_return()) { 1292 return; 1293 } 1294 preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive, 1295 keep_alive, complete_gc, yield); 1296 } 1297 } 1298 1299 // Weak references 1300 { 1301 GCTraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC, 1302 false, gc_timer); 1303 for (uint i = 0; i < _max_num_q; i++) { 1304 if (yield->should_return()) { 1305 return; 1306 } 1307 preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive, 1308 keep_alive, complete_gc, yield); 1309 } 1310 } 1311 1312 // Final references 1313 { 1314 GCTraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC, 1315 false, gc_timer); 1316 for (uint i = 0; i < _max_num_q; i++) { 1317 if (yield->should_return()) { 1318 return; 1319 } 1320 preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive, 1321 keep_alive, complete_gc, yield); 1322 } 1323 } 1324 1325 // Phantom references 1326 { 1327 GCTraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC, 1328 false, gc_timer); 1329 for (uint i = 0; i < _max_num_q; i++) { 1330 if (yield->should_return()) { 1331 return; 1332 } 1333 preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive, 1334 keep_alive, complete_gc, yield); 1335 } 1336 } 1337 } 1338 1339 // Walk the given discovered ref list, and remove all reference objects 1340 // whose referents are still alive, whose referents are NULL or which 1341 // are not active (have a non-NULL next field). NOTE: When we are 1342 // thus precleaning the ref lists (which happens single-threaded today), 1343 // we do not disable refs discovery to honour the correct semantics of 1344 // java.lang.Reference. As a result, we need to be careful below 1345 // that ref removal steps interleave safely with ref discovery steps 1346 // (in this thread). 1347 void 1348 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList& refs_list, 1349 BoolObjectClosure* is_alive, 1350 OopClosure* keep_alive, 1351 VoidClosure* complete_gc, 1352 YieldClosure* yield) { 1353 DiscoveredListIterator iter(refs_list, keep_alive, is_alive); 1354 while (iter.has_next()) { 1355 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */)); 1356 oop obj = iter.obj(); 1357 oop next = java_lang_ref_Reference::next(obj); 1358 if (iter.referent() == NULL || iter.is_referent_alive() || 1359 next != NULL) { 1360 // The referent has been cleared, or is alive, or the Reference is not 1361 // active; we need to trace and mark its cohort. 1362 if (TraceReferenceGC) { 1363 gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)", 1364 (void *)iter.obj(), iter.obj()->klass()->internal_name()); 1365 } 1366 // Remove Reference object from list 1367 iter.remove(); 1368 // Keep alive its cohort. 1369 iter.make_referent_alive(); 1370 if (UseCompressedOops) { 1371 narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj); 1372 keep_alive->do_oop(next_addr); 1373 } else { 1374 oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj); 1375 keep_alive->do_oop(next_addr); 1376 } 1377 iter.move_to_next(); 1378 } else { 1379 iter.next(); 1380 } 1381 } 1382 // Close the reachable set 1383 complete_gc->do_void(); 1384 1385 NOT_PRODUCT( 1386 if (PrintGCDetails && PrintReferenceGC && (iter.processed() > 0)) { 1387 gclog_or_tty->print_cr(" Dropped %d Refs out of %d " 1388 "Refs in discovered list " INTPTR_FORMAT, 1389 iter.removed(), iter.processed(), (address)refs_list.head()); 1390 } 1391 ) 1392 } 1393 1394 const char* ReferenceProcessor::list_name(uint i) { 1395 assert(i >= 0 && i <= _max_num_q * number_of_subclasses_of_ref(), 1396 "Out of bounds index"); 1397 1398 int j = i / _max_num_q; 1399 switch (j) { 1400 case 0: return "SoftRef"; 1401 case 1: return "WeakRef"; 1402 case 2: return "FinalRef"; 1403 case 3: return "PhantomRef"; 1404 } 1405 ShouldNotReachHere(); 1406 return NULL; 1407 } 1408 1409 #ifndef PRODUCT 1410 void ReferenceProcessor::verify_ok_to_handle_reflists() { 1411 // empty for now 1412 } 1413 #endif 1414 1415 #ifndef PRODUCT 1416 void ReferenceProcessor::clear_discovered_references() { 1417 guarantee(!_discovering_refs, "Discovering refs?"); 1418 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) { 1419 clear_discovered_references(_discovered_refs[i]); 1420 } 1421 } 1422 1423 #endif // PRODUCT