referenceProcessor.cpp revision 9149:a8a8604f890f
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 "memory/allocation.hpp" 35#include "oops/oop.inline.hpp" 36#include "runtime/java.hpp" 37#include "runtime/jniHandles.hpp" 38 39ReferencePolicy* ReferenceProcessor::_always_clear_soft_ref_policy = NULL; 40ReferencePolicy* ReferenceProcessor::_default_soft_ref_policy = NULL; 41jlong ReferenceProcessor::_soft_ref_timestamp_clock = 0; 42 43void referenceProcessor_init() { 44 ReferenceProcessor::init_statics(); 45} 46 47void ReferenceProcessor::init_statics() { 48 // We need a monotonically non-decreasing 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#if defined(COMPILER2) || INCLUDE_JVMCI 59 _default_soft_ref_policy = new LRUMaxHeapPolicy(); 60#else 61 _default_soft_ref_policy = new LRUCurrentHeapPolicy(); 62#endif 63 if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) { 64 vm_exit_during_initialization("Could not allocate reference policy object"); 65 } 66 guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery || 67 RefDiscoveryPolicy == ReferentBasedDiscovery, 68 "Unrecognized RefDiscoveryPolicy"); 69} 70 71void ReferenceProcessor::enable_discovery(bool check_no_refs) { 72#ifdef ASSERT 73 // Verify that we're not currently discovering refs 74 assert(!_discovering_refs, "nested call?"); 75 76 if (check_no_refs) { 77 // Verify that the discovered lists are empty 78 verify_no_references_recorded(); 79 } 80#endif // ASSERT 81 82 // Someone could have modified the value of the static 83 // field in the j.l.r.SoftReference class that holds the 84 // soft reference timestamp clock using reflection or 85 // Unsafe between GCs. Unconditionally update the static 86 // field in ReferenceProcessor here so that we use the new 87 // value during reference discovery. 88 89 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock(); 90 _discovering_refs = true; 91} 92 93ReferenceProcessor::ReferenceProcessor(MemRegion span, 94 bool mt_processing, 95 uint mt_processing_degree, 96 bool mt_discovery, 97 uint mt_discovery_degree, 98 bool atomic_discovery, 99 BoolObjectClosure* is_alive_non_header) : 100 _discovering_refs(false), 101 _enqueuing_is_done(false), 102 _is_alive_non_header(is_alive_non_header), 103 _processing_is_mt(mt_processing), 104 _next_id(0) 105{ 106 _span = span; 107 _discovery_is_atomic = atomic_discovery; 108 _discovery_is_mt = mt_discovery; 109 _num_q = MAX2(1U, mt_processing_degree); 110 _max_num_q = MAX2(_num_q, mt_discovery_degree); 111 _discovered_refs = NEW_C_HEAP_ARRAY(DiscoveredList, 112 _max_num_q * number_of_subclasses_of_ref(), mtGC); 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 _discoveredCleanerRefs = &_discoveredPhantomRefs[_max_num_q]; 122 123 // Initialize all entries to NULL 124 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) { 125 _discovered_refs[i].set_head(NULL); 126 _discovered_refs[i].set_length(0); 127 } 128 129 setup_policy(false /* default soft ref policy */); 130} 131 132#ifndef PRODUCT 133void ReferenceProcessor::verify_no_references_recorded() { 134 guarantee(!_discovering_refs, "Discovering refs?"); 135 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) { 136 guarantee(_discovered_refs[i].is_empty(), 137 "Found non-empty discovered list"); 138 } 139} 140#endif 141 142void ReferenceProcessor::weak_oops_do(OopClosure* f) { 143 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) { 144 if (UseCompressedOops) { 145 f->do_oop((narrowOop*)_discovered_refs[i].adr_head()); 146 } else { 147 f->do_oop((oop*)_discovered_refs[i].adr_head()); 148 } 149 } 150} 151 152void ReferenceProcessor::update_soft_ref_master_clock() { 153 // Update (advance) the soft ref master clock field. This must be done 154 // after processing the soft ref list. 155 156 // We need a monotonically non-decreasing time in ms but 157 // os::javaTimeMillis() does not guarantee monotonicity. 158 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC; 159 jlong soft_ref_clock = java_lang_ref_SoftReference::clock(); 160 assert(soft_ref_clock == _soft_ref_timestamp_clock, "soft ref clocks out of sync"); 161 162 NOT_PRODUCT( 163 if (now < _soft_ref_timestamp_clock) { 164 warning("time warp: " JLONG_FORMAT " to " JLONG_FORMAT, 165 _soft_ref_timestamp_clock, now); 166 } 167 ) 168 // The values of now and _soft_ref_timestamp_clock are set using 169 // javaTimeNanos(), which is guaranteed to be monotonically 170 // non-decreasing provided the underlying platform provides such 171 // a time source (and it is bug free). 172 // In product mode, however, protect ourselves from non-monotonicity. 173 if (now > _soft_ref_timestamp_clock) { 174 _soft_ref_timestamp_clock = now; 175 java_lang_ref_SoftReference::set_clock(now); 176 } 177 // Else leave clock stalled at its old value until time progresses 178 // past clock value. 179} 180 181size_t ReferenceProcessor::total_count(DiscoveredList lists[]) { 182 size_t total = 0; 183 for (uint i = 0; i < _max_num_q; ++i) { 184 total += lists[i].length(); 185 } 186 return total; 187} 188 189static void log_ref_count(size_t count, bool doit) { 190 if (doit) { 191 gclog_or_tty->print(", " SIZE_FORMAT " refs", count); 192 } 193} 194 195class GCRefTraceTime : public StackObj { 196 GCTraceTimeImpl _gc_trace_time; 197 public: 198 GCRefTraceTime(const char* title, bool doit, GCTimer* timer, size_t count) : 199 _gc_trace_time(title, doit, false, timer) { 200 log_ref_count(count, doit); 201 } 202}; 203 204ReferenceProcessorStats ReferenceProcessor::process_discovered_references( 205 BoolObjectClosure* is_alive, 206 OopClosure* keep_alive, 207 VoidClosure* complete_gc, 208 AbstractRefProcTaskExecutor* task_executor, 209 GCTimer* gc_timer) { 210 211 assert(!enqueuing_is_done(), "If here enqueuing should not be complete"); 212 // Stop treating discovered references specially. 213 disable_discovery(); 214 215 // If discovery was concurrent, someone could have modified 216 // the value of the static field in the j.l.r.SoftReference 217 // class that holds the soft reference timestamp clock using 218 // reflection or Unsafe between when discovery was enabled and 219 // now. Unconditionally update the static field in ReferenceProcessor 220 // here so that we use the new value during processing of the 221 // discovered soft refs. 222 223 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock(); 224 225 bool trace_time = PrintGCDetails && PrintReferenceGC; 226 227 // Include cleaners in phantom statistics. We expect Cleaner 228 // references to be temporary, and don't want to deal with 229 // possible incompatibilities arising from making it more visible. 230 ReferenceProcessorStats stats( 231 total_count(_discoveredSoftRefs), 232 total_count(_discoveredWeakRefs), 233 total_count(_discoveredFinalRefs), 234 total_count(_discoveredPhantomRefs) + total_count(_discoveredCleanerRefs)); 235 236 // Soft references 237 { 238 GCRefTraceTime tt("SoftReference", trace_time, gc_timer, stats.soft_count()); 239 process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true, 240 is_alive, keep_alive, complete_gc, task_executor); 241 } 242 243 update_soft_ref_master_clock(); 244 245 // Weak references 246 { 247 GCRefTraceTime tt("WeakReference", trace_time, gc_timer, stats.weak_count()); 248 process_discovered_reflist(_discoveredWeakRefs, NULL, true, 249 is_alive, keep_alive, complete_gc, task_executor); 250 } 251 252 // Final references 253 { 254 GCRefTraceTime tt("FinalReference", trace_time, gc_timer, stats.final_count()); 255 process_discovered_reflist(_discoveredFinalRefs, NULL, false, 256 is_alive, keep_alive, complete_gc, task_executor); 257 } 258 259 // Phantom references 260 { 261 GCRefTraceTime tt("PhantomReference", trace_time, gc_timer, stats.phantom_count()); 262 process_discovered_reflist(_discoveredPhantomRefs, NULL, false, 263 is_alive, keep_alive, complete_gc, task_executor); 264 265 // Process cleaners, but include them in phantom timing. We expect 266 // Cleaner references to be temporary, and don't want to deal with 267 // possible incompatibilities arising from making it more visible. 268 process_discovered_reflist(_discoveredCleanerRefs, NULL, true, 269 is_alive, keep_alive, complete_gc, task_executor); 270 } 271 272 // Weak global JNI references. It would make more sense (semantically) to 273 // traverse these simultaneously with the regular weak references above, but 274 // that is not how the JDK1.2 specification is. See #4126360. Native code can 275 // thus use JNI weak references to circumvent the phantom references and 276 // resurrect a "post-mortem" object. 277 { 278 GCTraceTime tt("JNI Weak Reference", trace_time, false, gc_timer); 279 NOT_PRODUCT(log_ref_count(count_jni_refs(), trace_time);) 280 if (task_executor != NULL) { 281 task_executor->set_single_threaded_mode(); 282 } 283 process_phaseJNI(is_alive, keep_alive, complete_gc); 284 } 285 286 return stats; 287} 288 289#ifndef PRODUCT 290// Calculate the number of jni handles. 291uint ReferenceProcessor::count_jni_refs() { 292 class AlwaysAliveClosure: public BoolObjectClosure { 293 public: 294 virtual bool do_object_b(oop obj) { return true; } 295 }; 296 297 class CountHandleClosure: public OopClosure { 298 private: 299 int _count; 300 public: 301 CountHandleClosure(): _count(0) {} 302 void do_oop(oop* unused) { _count++; } 303 void do_oop(narrowOop* unused) { ShouldNotReachHere(); } 304 int count() { return _count; } 305 }; 306 CountHandleClosure global_handle_count; 307 AlwaysAliveClosure always_alive; 308 JNIHandles::weak_oops_do(&always_alive, &global_handle_count); 309 return global_handle_count.count(); 310} 311#endif 312 313void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive, 314 OopClosure* keep_alive, 315 VoidClosure* complete_gc) { 316 JNIHandles::weak_oops_do(is_alive, keep_alive); 317 complete_gc->do_void(); 318} 319 320 321template <class T> 322bool enqueue_discovered_ref_helper(ReferenceProcessor* ref, 323 AbstractRefProcTaskExecutor* task_executor) { 324 325 // Remember old value of pending references list 326 T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr(); 327 T old_pending_list_value = *pending_list_addr; 328 329 // Enqueue references that are not made active again, and 330 // clear the decks for the next collection (cycle). 331 ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor); 332 // Do the post-barrier on pending_list_addr missed in 333 // enqueue_discovered_reflist. 334 oopDesc::bs()->write_ref_field(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr)); 335 336 // Stop treating discovered references specially. 337 ref->disable_discovery(); 338 339 // Return true if new pending references were added 340 return old_pending_list_value != *pending_list_addr; 341} 342 343bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) { 344 if (UseCompressedOops) { 345 return enqueue_discovered_ref_helper<narrowOop>(this, task_executor); 346 } else { 347 return enqueue_discovered_ref_helper<oop>(this, task_executor); 348 } 349} 350 351void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list, 352 HeapWord* pending_list_addr) { 353 // Given a list of refs linked through the "discovered" field 354 // (java.lang.ref.Reference.discovered), self-loop their "next" field 355 // thus distinguishing them from active References, then 356 // prepend them to the pending list. 357 // 358 // The Java threads will see the Reference objects linked together through 359 // the discovered field. Instead of trying to do the write barrier updates 360 // in all places in the reference processor where we manipulate the discovered 361 // field we make sure to do the barrier here where we anyway iterate through 362 // all linked Reference objects. Note that it is important to not dirty any 363 // cards during reference processing since this will cause card table 364 // verification to fail for G1. 365 if (TraceReferenceGC && PrintGCDetails) { 366 gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list " 367 INTPTR_FORMAT, p2i(refs_list.head())); 368 } 369 370 oop obj = NULL; 371 oop next_d = refs_list.head(); 372 // Walk down the list, self-looping the next field 373 // so that the References are not considered active. 374 while (obj != next_d) { 375 obj = next_d; 376 assert(obj->is_instanceRef(), "should be reference object"); 377 next_d = java_lang_ref_Reference::discovered(obj); 378 if (TraceReferenceGC && PrintGCDetails) { 379 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT, 380 p2i(obj), p2i(next_d)); 381 } 382 assert(java_lang_ref_Reference::next(obj) == NULL, 383 "Reference not active; should not be discovered"); 384 // Self-loop next, so as to make Ref not active. 385 java_lang_ref_Reference::set_next_raw(obj, obj); 386 if (next_d != obj) { 387 oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), next_d); 388 } else { 389 // This is the last object. 390 // Swap refs_list into pending_list_addr and 391 // set obj's discovered to what we read from pending_list_addr. 392 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr); 393 // Need post-barrier on pending_list_addr. See enqueue_discovered_ref_helper() above. 394 java_lang_ref_Reference::set_discovered_raw(obj, old); // old may be NULL 395 oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), old); 396 } 397 } 398} 399 400// Parallel enqueue task 401class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask { 402public: 403 RefProcEnqueueTask(ReferenceProcessor& ref_processor, 404 DiscoveredList discovered_refs[], 405 HeapWord* pending_list_addr, 406 int n_queues) 407 : EnqueueTask(ref_processor, discovered_refs, 408 pending_list_addr, n_queues) 409 { } 410 411 virtual void work(unsigned int work_id) { 412 assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds"); 413 // Simplest first cut: static partitioning. 414 int index = work_id; 415 // The increment on "index" must correspond to the maximum number of queues 416 // (n_queues) with which that ReferenceProcessor was created. That 417 // is because of the "clever" way the discovered references lists were 418 // allocated and are indexed into. 419 assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected"); 420 for (int j = 0; 421 j < ReferenceProcessor::number_of_subclasses_of_ref(); 422 j++, index += _n_queues) { 423 _ref_processor.enqueue_discovered_reflist( 424 _refs_lists[index], _pending_list_addr); 425 _refs_lists[index].set_head(NULL); 426 _refs_lists[index].set_length(0); 427 } 428 } 429}; 430 431// Enqueue references that are not made active again 432void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr, 433 AbstractRefProcTaskExecutor* task_executor) { 434 if (_processing_is_mt && task_executor != NULL) { 435 // Parallel code 436 RefProcEnqueueTask tsk(*this, _discovered_refs, 437 pending_list_addr, _max_num_q); 438 task_executor->execute(tsk); 439 } else { 440 // Serial code: call the parent class's implementation 441 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) { 442 enqueue_discovered_reflist(_discovered_refs[i], pending_list_addr); 443 _discovered_refs[i].set_head(NULL); 444 _discovered_refs[i].set_length(0); 445 } 446 } 447} 448 449void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) { 450 _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref); 451 oop discovered = java_lang_ref_Reference::discovered(_ref); 452 assert(_discovered_addr && discovered->is_oop_or_null(), 453 "Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered)); 454 _next = discovered; 455 _referent_addr = java_lang_ref_Reference::referent_addr(_ref); 456 _referent = java_lang_ref_Reference::referent(_ref); 457 assert(Universe::heap()->is_in_reserved_or_null(_referent), 458 "Wrong oop found in java.lang.Reference object"); 459 assert(allow_null_referent ? 460 _referent->is_oop_or_null() 461 : _referent->is_oop(), 462 "Expected an oop%s for referent field at " PTR_FORMAT, 463 (allow_null_referent ? " or NULL" : ""), 464 p2i(_referent)); 465} 466 467void DiscoveredListIterator::remove() { 468 assert(_ref->is_oop(), "Dropping a bad reference"); 469 oop_store_raw(_discovered_addr, NULL); 470 471 // First _prev_next ref actually points into DiscoveredList (gross). 472 oop new_next; 473 if (_next == _ref) { 474 // At the end of the list, we should make _prev point to itself. 475 // If _ref is the first ref, then _prev_next will be in the DiscoveredList, 476 // and _prev will be NULL. 477 new_next = _prev; 478 } else { 479 new_next = _next; 480 } 481 // Remove Reference object from discovered list. Note that G1 does not need a 482 // pre-barrier here because we know the Reference has already been found/marked, 483 // that's how it ended up in the discovered list in the first place. 484 oop_store_raw(_prev_next, new_next); 485 NOT_PRODUCT(_removed++); 486 _refs_list.dec_length(1); 487} 488 489void DiscoveredListIterator::clear_referent() { 490 oop_store_raw(_referent_addr, NULL); 491} 492 493// NOTE: process_phase*() are largely similar, and at a high level 494// merely iterate over the extant list applying a predicate to 495// each of its elements and possibly removing that element from the 496// list and applying some further closures to that element. 497// We should consider the possibility of replacing these 498// process_phase*() methods by abstracting them into 499// a single general iterator invocation that receives appropriate 500// closures that accomplish this work. 501 502// (SoftReferences only) Traverse the list and remove any SoftReferences whose 503// referents are not alive, but that should be kept alive for policy reasons. 504// Keep alive the transitive closure of all such referents. 505void 506ReferenceProcessor::process_phase1(DiscoveredList& refs_list, 507 ReferencePolicy* policy, 508 BoolObjectClosure* is_alive, 509 OopClosure* keep_alive, 510 VoidClosure* complete_gc) { 511 assert(policy != NULL, "Must have a non-NULL policy"); 512 DiscoveredListIterator iter(refs_list, keep_alive, is_alive); 513 // Decide which softly reachable refs should be kept alive. 514 while (iter.has_next()) { 515 iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */)); 516 bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive(); 517 if (referent_is_dead && 518 !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) { 519 if (TraceReferenceGC) { 520 gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s" ") by policy", 521 p2i(iter.obj()), iter.obj()->klass()->internal_name()); 522 } 523 // Remove Reference object from list 524 iter.remove(); 525 // keep the referent around 526 iter.make_referent_alive(); 527 iter.move_to_next(); 528 } else { 529 iter.next(); 530 } 531 } 532 // Close the reachable set 533 complete_gc->do_void(); 534 NOT_PRODUCT( 535 if (PrintGCDetails && TraceReferenceGC) { 536 gclog_or_tty->print_cr(" Dropped " SIZE_FORMAT " dead Refs out of " SIZE_FORMAT 537 " discovered Refs by policy, from list " INTPTR_FORMAT, 538 iter.removed(), iter.processed(), p2i(refs_list.head())); 539 } 540 ) 541} 542 543// Traverse the list and remove any Refs that are not active, or 544// whose referents are either alive or NULL. 545void 546ReferenceProcessor::pp2_work(DiscoveredList& refs_list, 547 BoolObjectClosure* is_alive, 548 OopClosure* keep_alive) { 549 assert(discovery_is_atomic(), "Error"); 550 DiscoveredListIterator iter(refs_list, keep_alive, is_alive); 551 while (iter.has_next()) { 552 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */)); 553 DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());) 554 assert(next == NULL, "Should not discover inactive Reference"); 555 if (iter.is_referent_alive()) { 556 if (TraceReferenceGC) { 557 gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)", 558 p2i(iter.obj()), iter.obj()->klass()->internal_name()); 559 } 560 // The referent is reachable after all. 561 // Remove Reference object from list. 562 iter.remove(); 563 // Update the referent pointer as necessary: Note that this 564 // should not entail any recursive marking because the 565 // referent must already have been traversed. 566 iter.make_referent_alive(); 567 iter.move_to_next(); 568 } else { 569 iter.next(); 570 } 571 } 572 NOT_PRODUCT( 573 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) { 574 gclog_or_tty->print_cr(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT 575 " Refs in discovered list " INTPTR_FORMAT, 576 iter.removed(), iter.processed(), p2i(refs_list.head())); 577 } 578 ) 579} 580 581void 582ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList& refs_list, 583 BoolObjectClosure* is_alive, 584 OopClosure* keep_alive, 585 VoidClosure* complete_gc) { 586 assert(!discovery_is_atomic(), "Error"); 587 DiscoveredListIterator iter(refs_list, keep_alive, is_alive); 588 while (iter.has_next()) { 589 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */)); 590 HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj()); 591 oop next = java_lang_ref_Reference::next(iter.obj()); 592 if ((iter.referent() == NULL || iter.is_referent_alive() || 593 next != NULL)) { 594 assert(next->is_oop_or_null(), "Expected an oop or NULL for next field at " PTR_FORMAT, p2i(next)); 595 // Remove Reference object from list 596 iter.remove(); 597 // Trace the cohorts 598 iter.make_referent_alive(); 599 if (UseCompressedOops) { 600 keep_alive->do_oop((narrowOop*)next_addr); 601 } else { 602 keep_alive->do_oop((oop*)next_addr); 603 } 604 iter.move_to_next(); 605 } else { 606 iter.next(); 607 } 608 } 609 // Now close the newly reachable set 610 complete_gc->do_void(); 611 NOT_PRODUCT( 612 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) { 613 gclog_or_tty->print_cr(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT 614 " Refs in discovered list " INTPTR_FORMAT, 615 iter.removed(), iter.processed(), p2i(refs_list.head())); 616 } 617 ) 618} 619 620// Traverse the list and process the referents, by either 621// clearing them or keeping them (and their reachable 622// closure) alive. 623void 624ReferenceProcessor::process_phase3(DiscoveredList& refs_list, 625 bool clear_referent, 626 BoolObjectClosure* is_alive, 627 OopClosure* keep_alive, 628 VoidClosure* complete_gc) { 629 ResourceMark rm; 630 DiscoveredListIterator iter(refs_list, keep_alive, is_alive); 631 while (iter.has_next()) { 632 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */)); 633 if (clear_referent) { 634 // NULL out referent pointer 635 iter.clear_referent(); 636 } else { 637 // keep the referent around 638 iter.make_referent_alive(); 639 } 640 if (TraceReferenceGC) { 641 gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending", 642 clear_referent ? "cleared " : "", 643 p2i(iter.obj()), iter.obj()->klass()->internal_name()); 644 } 645 assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference"); 646 iter.next(); 647 } 648 // Close the reachable set 649 complete_gc->do_void(); 650} 651 652void 653ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) { 654 oop obj = NULL; 655 oop next = refs_list.head(); 656 while (next != obj) { 657 obj = next; 658 next = java_lang_ref_Reference::discovered(obj); 659 java_lang_ref_Reference::set_discovered_raw(obj, NULL); 660 } 661 refs_list.set_head(NULL); 662 refs_list.set_length(0); 663} 664 665void ReferenceProcessor::abandon_partial_discovery() { 666 // loop over the lists 667 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) { 668 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) { 669 gclog_or_tty->print_cr("\nAbandoning %s discovered list", list_name(i)); 670 } 671 clear_discovered_references(_discovered_refs[i]); 672 } 673} 674 675class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask { 676public: 677 RefProcPhase1Task(ReferenceProcessor& ref_processor, 678 DiscoveredList refs_lists[], 679 ReferencePolicy* policy, 680 bool marks_oops_alive) 681 : ProcessTask(ref_processor, refs_lists, marks_oops_alive), 682 _policy(policy) 683 { } 684 virtual void work(unsigned int i, BoolObjectClosure& is_alive, 685 OopClosure& keep_alive, 686 VoidClosure& complete_gc) 687 { 688 Thread* thr = Thread::current(); 689 int refs_list_index = ((WorkerThread*)thr)->id(); 690 _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy, 691 &is_alive, &keep_alive, &complete_gc); 692 } 693private: 694 ReferencePolicy* _policy; 695}; 696 697class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask { 698public: 699 RefProcPhase2Task(ReferenceProcessor& ref_processor, 700 DiscoveredList refs_lists[], 701 bool marks_oops_alive) 702 : ProcessTask(ref_processor, refs_lists, marks_oops_alive) 703 { } 704 virtual void work(unsigned int i, BoolObjectClosure& is_alive, 705 OopClosure& keep_alive, 706 VoidClosure& complete_gc) 707 { 708 _ref_processor.process_phase2(_refs_lists[i], 709 &is_alive, &keep_alive, &complete_gc); 710 } 711}; 712 713class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask { 714public: 715 RefProcPhase3Task(ReferenceProcessor& ref_processor, 716 DiscoveredList refs_lists[], 717 bool clear_referent, 718 bool marks_oops_alive) 719 : ProcessTask(ref_processor, refs_lists, marks_oops_alive), 720 _clear_referent(clear_referent) 721 { } 722 virtual void work(unsigned int i, BoolObjectClosure& is_alive, 723 OopClosure& keep_alive, 724 VoidClosure& complete_gc) 725 { 726 // Don't use "refs_list_index" calculated in this way because 727 // balance_queues() has moved the Ref's into the first n queues. 728 // Thread* thr = Thread::current(); 729 // int refs_list_index = ((WorkerThread*)thr)->id(); 730 // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent, 731 _ref_processor.process_phase3(_refs_lists[i], _clear_referent, 732 &is_alive, &keep_alive, &complete_gc); 733 } 734private: 735 bool _clear_referent; 736}; 737 738// Balances reference queues. 739// Move entries from all queues[0, 1, ..., _max_num_q-1] to 740// queues[0, 1, ..., _num_q-1] because only the first _num_q 741// corresponding to the active workers will be processed. 742void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[]) 743{ 744 // calculate total length 745 size_t total_refs = 0; 746 if (TraceReferenceGC && PrintGCDetails) { 747 gclog_or_tty->print_cr("\nBalance ref_lists "); 748 } 749 750 for (uint i = 0; i < _max_num_q; ++i) { 751 total_refs += ref_lists[i].length(); 752 if (TraceReferenceGC && PrintGCDetails) { 753 gclog_or_tty->print(SIZE_FORMAT " ", ref_lists[i].length()); 754 } 755 } 756 if (TraceReferenceGC && PrintGCDetails) { 757 gclog_or_tty->print_cr(" = " SIZE_FORMAT, total_refs); 758 } 759 size_t avg_refs = total_refs / _num_q + 1; 760 uint to_idx = 0; 761 for (uint from_idx = 0; from_idx < _max_num_q; from_idx++) { 762 bool move_all = false; 763 if (from_idx >= _num_q) { 764 move_all = ref_lists[from_idx].length() > 0; 765 } 766 while ((ref_lists[from_idx].length() > avg_refs) || 767 move_all) { 768 assert(to_idx < _num_q, "Sanity Check!"); 769 if (ref_lists[to_idx].length() < avg_refs) { 770 // move superfluous refs 771 size_t refs_to_move; 772 // Move all the Ref's if the from queue will not be processed. 773 if (move_all) { 774 refs_to_move = MIN2(ref_lists[from_idx].length(), 775 avg_refs - ref_lists[to_idx].length()); 776 } else { 777 refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs, 778 avg_refs - ref_lists[to_idx].length()); 779 } 780 781 assert(refs_to_move > 0, "otherwise the code below will fail"); 782 783 oop move_head = ref_lists[from_idx].head(); 784 oop move_tail = move_head; 785 oop new_head = move_head; 786 // find an element to split the list on 787 for (size_t j = 0; j < refs_to_move; ++j) { 788 move_tail = new_head; 789 new_head = java_lang_ref_Reference::discovered(new_head); 790 } 791 792 // Add the chain to the to list. 793 if (ref_lists[to_idx].head() == NULL) { 794 // to list is empty. Make a loop at the end. 795 java_lang_ref_Reference::set_discovered_raw(move_tail, move_tail); 796 } else { 797 java_lang_ref_Reference::set_discovered_raw(move_tail, ref_lists[to_idx].head()); 798 } 799 ref_lists[to_idx].set_head(move_head); 800 ref_lists[to_idx].inc_length(refs_to_move); 801 802 // Remove the chain from the from list. 803 if (move_tail == new_head) { 804 // We found the end of the from list. 805 ref_lists[from_idx].set_head(NULL); 806 } else { 807 ref_lists[from_idx].set_head(new_head); 808 } 809 ref_lists[from_idx].dec_length(refs_to_move); 810 if (ref_lists[from_idx].length() == 0) { 811 break; 812 } 813 } else { 814 to_idx = (to_idx + 1) % _num_q; 815 } 816 } 817 } 818#ifdef ASSERT 819 size_t balanced_total_refs = 0; 820 for (uint i = 0; i < _max_num_q; ++i) { 821 balanced_total_refs += ref_lists[i].length(); 822 if (TraceReferenceGC && PrintGCDetails) { 823 gclog_or_tty->print(SIZE_FORMAT " ", ref_lists[i].length()); 824 } 825 } 826 if (TraceReferenceGC && PrintGCDetails) { 827 gclog_or_tty->print_cr(" = " SIZE_FORMAT, balanced_total_refs); 828 gclog_or_tty->flush(); 829 } 830 assert(total_refs == balanced_total_refs, "Balancing was incomplete"); 831#endif 832} 833 834void ReferenceProcessor::balance_all_queues() { 835 balance_queues(_discoveredSoftRefs); 836 balance_queues(_discoveredWeakRefs); 837 balance_queues(_discoveredFinalRefs); 838 balance_queues(_discoveredPhantomRefs); 839 balance_queues(_discoveredCleanerRefs); 840} 841 842void ReferenceProcessor::process_discovered_reflist( 843 DiscoveredList refs_lists[], 844 ReferencePolicy* policy, 845 bool clear_referent, 846 BoolObjectClosure* is_alive, 847 OopClosure* keep_alive, 848 VoidClosure* complete_gc, 849 AbstractRefProcTaskExecutor* task_executor) 850{ 851 bool mt_processing = task_executor != NULL && _processing_is_mt; 852 // If discovery used MT and a dynamic number of GC threads, then 853 // the queues must be balanced for correctness if fewer than the 854 // maximum number of queues were used. The number of queue used 855 // during discovery may be different than the number to be used 856 // for processing so don't depend of _num_q < _max_num_q as part 857 // of the test. 858 bool must_balance = _discovery_is_mt; 859 860 if ((mt_processing && ParallelRefProcBalancingEnabled) || 861 must_balance) { 862 balance_queues(refs_lists); 863 } 864 865 // Phase 1 (soft refs only): 866 // . Traverse the list and remove any SoftReferences whose 867 // referents are not alive, but that should be kept alive for 868 // policy reasons. Keep alive the transitive closure of all 869 // such referents. 870 if (policy != NULL) { 871 if (mt_processing) { 872 RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/); 873 task_executor->execute(phase1); 874 } else { 875 for (uint i = 0; i < _max_num_q; i++) { 876 process_phase1(refs_lists[i], policy, 877 is_alive, keep_alive, complete_gc); 878 } 879 } 880 } else { // policy == NULL 881 assert(refs_lists != _discoveredSoftRefs, 882 "Policy must be specified for soft references."); 883 } 884 885 // Phase 2: 886 // . Traverse the list and remove any refs whose referents are alive. 887 if (mt_processing) { 888 RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/); 889 task_executor->execute(phase2); 890 } else { 891 for (uint i = 0; i < _max_num_q; i++) { 892 process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc); 893 } 894 } 895 896 // Phase 3: 897 // . Traverse the list and process referents as appropriate. 898 if (mt_processing) { 899 RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/); 900 task_executor->execute(phase3); 901 } else { 902 for (uint i = 0; i < _max_num_q; i++) { 903 process_phase3(refs_lists[i], clear_referent, 904 is_alive, keep_alive, complete_gc); 905 } 906 } 907} 908 909inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) { 910 uint id = 0; 911 // Determine the queue index to use for this object. 912 if (_discovery_is_mt) { 913 // During a multi-threaded discovery phase, 914 // each thread saves to its "own" list. 915 Thread* thr = Thread::current(); 916 id = thr->as_Worker_thread()->id(); 917 } else { 918 // single-threaded discovery, we save in round-robin 919 // fashion to each of the lists. 920 if (_processing_is_mt) { 921 id = next_id(); 922 } 923 } 924 assert(id < _max_num_q, "Id is out-of-bounds (call Freud?)"); 925 926 // Get the discovered queue to which we will add 927 DiscoveredList* list = NULL; 928 switch (rt) { 929 case REF_OTHER: 930 // Unknown reference type, no special treatment 931 break; 932 case REF_SOFT: 933 list = &_discoveredSoftRefs[id]; 934 break; 935 case REF_WEAK: 936 list = &_discoveredWeakRefs[id]; 937 break; 938 case REF_FINAL: 939 list = &_discoveredFinalRefs[id]; 940 break; 941 case REF_PHANTOM: 942 list = &_discoveredPhantomRefs[id]; 943 break; 944 case REF_CLEANER: 945 list = &_discoveredCleanerRefs[id]; 946 break; 947 case REF_NONE: 948 // we should not reach here if we are an InstanceRefKlass 949 default: 950 ShouldNotReachHere(); 951 } 952 if (TraceReferenceGC && PrintGCDetails) { 953 gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, p2i(list)); 954 } 955 return list; 956} 957 958inline void 959ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list, 960 oop obj, 961 HeapWord* discovered_addr) { 962 assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller"); 963 // First we must make sure this object is only enqueued once. CAS in a non null 964 // discovered_addr. 965 oop current_head = refs_list.head(); 966 // The last ref must have its discovered field pointing to itself. 967 oop next_discovered = (current_head != NULL) ? current_head : obj; 968 969 oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr, 970 NULL); 971 if (retest == NULL) { 972 // This thread just won the right to enqueue the object. 973 // We have separate lists for enqueueing, so no synchronization 974 // is necessary. 975 refs_list.set_head(obj); 976 refs_list.inc_length(1); 977 978 if (TraceReferenceGC) { 979 gclog_or_tty->print_cr("Discovered reference (mt) (" INTPTR_FORMAT ": %s)", 980 p2i(obj), obj->klass()->internal_name()); 981 } 982 } else { 983 // If retest was non NULL, another thread beat us to it: 984 // The reference has already been discovered... 985 if (TraceReferenceGC) { 986 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)", 987 p2i(obj), obj->klass()->internal_name()); 988 } 989 } 990} 991 992#ifndef PRODUCT 993// Non-atomic (i.e. concurrent) discovery might allow us 994// to observe j.l.References with NULL referents, being those 995// cleared concurrently by mutators during (or after) discovery. 996void ReferenceProcessor::verify_referent(oop obj) { 997 bool da = discovery_is_atomic(); 998 oop referent = java_lang_ref_Reference::referent(obj); 999 assert(da ? referent->is_oop() : referent->is_oop_or_null(), 1000 "Bad referent " INTPTR_FORMAT " found in Reference " 1001 INTPTR_FORMAT " during %satomic discovery ", 1002 p2i(referent), p2i(obj), da ? "" : "non-"); 1003} 1004#endif 1005 1006// We mention two of several possible choices here: 1007// #0: if the reference object is not in the "originating generation" 1008// (or part of the heap being collected, indicated by our "span" 1009// we don't treat it specially (i.e. we scan it as we would 1010// a normal oop, treating its references as strong references). 1011// This means that references can't be discovered unless their 1012// referent is also in the same span. This is the simplest, 1013// most "local" and most conservative approach, albeit one 1014// that may cause weak references to be enqueued least promptly. 1015// We call this choice the "ReferenceBasedDiscovery" policy. 1016// #1: the reference object may be in any generation (span), but if 1017// the referent is in the generation (span) being currently collected 1018// then we can discover the reference object, provided 1019// the object has not already been discovered by 1020// a different concurrently running collector (as may be the 1021// case, for instance, if the reference object is in CMS and 1022// the referent in DefNewGeneration), and provided the processing 1023// of this reference object by the current collector will 1024// appear atomic to every other collector in the system. 1025// (Thus, for instance, a concurrent collector may not 1026// discover references in other generations even if the 1027// referent is in its own generation). This policy may, 1028// in certain cases, enqueue references somewhat sooner than 1029// might Policy #0 above, but at marginally increased cost 1030// and complexity in processing these references. 1031// We call this choice the "RefeferentBasedDiscovery" policy. 1032bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) { 1033 // Make sure we are discovering refs (rather than processing discovered refs). 1034 if (!_discovering_refs || !RegisterReferences) { 1035 return false; 1036 } 1037 // We only discover active references. 1038 oop next = java_lang_ref_Reference::next(obj); 1039 if (next != NULL) { // Ref is no longer active 1040 return false; 1041 } 1042 1043 HeapWord* obj_addr = (HeapWord*)obj; 1044 if (RefDiscoveryPolicy == ReferenceBasedDiscovery && 1045 !_span.contains(obj_addr)) { 1046 // Reference is not in the originating generation; 1047 // don't treat it specially (i.e. we want to scan it as a normal 1048 // object with strong references). 1049 return false; 1050 } 1051 1052 // We only discover references whose referents are not (yet) 1053 // known to be strongly reachable. 1054 if (is_alive_non_header() != NULL) { 1055 verify_referent(obj); 1056 if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) { 1057 return false; // referent is reachable 1058 } 1059 } 1060 if (rt == REF_SOFT) { 1061 // For soft refs we can decide now if these are not 1062 // current candidates for clearing, in which case we 1063 // can mark through them now, rather than delaying that 1064 // to the reference-processing phase. Since all current 1065 // time-stamp policies advance the soft-ref clock only 1066 // at a full collection cycle, this is always currently 1067 // accurate. 1068 if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) { 1069 return false; 1070 } 1071 } 1072 1073 ResourceMark rm; // Needed for tracing. 1074 1075 HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj); 1076 const oop discovered = java_lang_ref_Reference::discovered(obj); 1077 assert(discovered->is_oop_or_null(), "Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered)); 1078 if (discovered != NULL) { 1079 // The reference has already been discovered... 1080 if (TraceReferenceGC) { 1081 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)", 1082 p2i(obj), obj->klass()->internal_name()); 1083 } 1084 if (RefDiscoveryPolicy == ReferentBasedDiscovery) { 1085 // assumes that an object is not processed twice; 1086 // if it's been already discovered it must be on another 1087 // generation's discovered list; so we won't discover it. 1088 return false; 1089 } else { 1090 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery, 1091 "Unrecognized policy"); 1092 // Check assumption that an object is not potentially 1093 // discovered twice except by concurrent collectors that potentially 1094 // trace the same Reference object twice. 1095 assert(UseConcMarkSweepGC || UseG1GC, 1096 "Only possible with a concurrent marking collector"); 1097 return true; 1098 } 1099 } 1100 1101 if (RefDiscoveryPolicy == ReferentBasedDiscovery) { 1102 verify_referent(obj); 1103 // Discover if and only if EITHER: 1104 // .. reference is in our span, OR 1105 // .. we are an atomic collector and referent is in our span 1106 if (_span.contains(obj_addr) || 1107 (discovery_is_atomic() && 1108 _span.contains(java_lang_ref_Reference::referent(obj)))) { 1109 // should_enqueue = true; 1110 } else { 1111 return false; 1112 } 1113 } else { 1114 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery && 1115 _span.contains(obj_addr), "code inconsistency"); 1116 } 1117 1118 // Get the right type of discovered queue head. 1119 DiscoveredList* list = get_discovered_list(rt); 1120 if (list == NULL) { 1121 return false; // nothing special needs to be done 1122 } 1123 1124 if (_discovery_is_mt) { 1125 add_to_discovered_list_mt(*list, obj, discovered_addr); 1126 } else { 1127 // We do a raw store here: the field will be visited later when processing 1128 // the discovered references. 1129 oop current_head = list->head(); 1130 // The last ref must have its discovered field pointing to itself. 1131 oop next_discovered = (current_head != NULL) ? current_head : obj; 1132 1133 assert(discovered == NULL, "control point invariant"); 1134 oop_store_raw(discovered_addr, next_discovered); 1135 list->set_head(obj); 1136 list->inc_length(1); 1137 1138 if (TraceReferenceGC) { 1139 gclog_or_tty->print_cr("Discovered reference (" INTPTR_FORMAT ": %s)", 1140 p2i(obj), obj->klass()->internal_name()); 1141 } 1142 } 1143 assert(obj->is_oop(), "Discovered a bad reference"); 1144 verify_referent(obj); 1145 return true; 1146} 1147 1148// Preclean the discovered references by removing those 1149// whose referents are alive, and by marking from those that 1150// are not active. These lists can be handled here 1151// in any order and, indeed, concurrently. 1152void ReferenceProcessor::preclean_discovered_references( 1153 BoolObjectClosure* is_alive, 1154 OopClosure* keep_alive, 1155 VoidClosure* complete_gc, 1156 YieldClosure* yield, 1157 GCTimer* gc_timer) { 1158 1159 // Soft references 1160 { 1161 GCTraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC, 1162 false, gc_timer); 1163 for (uint i = 0; i < _max_num_q; i++) { 1164 if (yield->should_return()) { 1165 return; 1166 } 1167 preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive, 1168 keep_alive, complete_gc, yield); 1169 } 1170 } 1171 1172 // Weak references 1173 { 1174 GCTraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC, 1175 false, gc_timer); 1176 for (uint i = 0; i < _max_num_q; i++) { 1177 if (yield->should_return()) { 1178 return; 1179 } 1180 preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive, 1181 keep_alive, complete_gc, yield); 1182 } 1183 } 1184 1185 // Final references 1186 { 1187 GCTraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC, 1188 false, gc_timer); 1189 for (uint i = 0; i < _max_num_q; i++) { 1190 if (yield->should_return()) { 1191 return; 1192 } 1193 preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive, 1194 keep_alive, complete_gc, yield); 1195 } 1196 } 1197 1198 // Phantom references 1199 { 1200 GCTraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC, 1201 false, gc_timer); 1202 for (uint i = 0; i < _max_num_q; i++) { 1203 if (yield->should_return()) { 1204 return; 1205 } 1206 preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive, 1207 keep_alive, complete_gc, yield); 1208 } 1209 1210 // Cleaner references. Included in timing for phantom references. We 1211 // expect Cleaner references to be temporary, and don't want to deal with 1212 // possible incompatibilities arising from making it more visible. 1213 for (uint i = 0; i < _max_num_q; i++) { 1214 if (yield->should_return()) { 1215 return; 1216 } 1217 preclean_discovered_reflist(_discoveredCleanerRefs[i], is_alive, 1218 keep_alive, complete_gc, yield); 1219 } 1220 } 1221} 1222 1223// Walk the given discovered ref list, and remove all reference objects 1224// whose referents are still alive, whose referents are NULL or which 1225// are not active (have a non-NULL next field). NOTE: When we are 1226// thus precleaning the ref lists (which happens single-threaded today), 1227// we do not disable refs discovery to honor the correct semantics of 1228// java.lang.Reference. As a result, we need to be careful below 1229// that ref removal steps interleave safely with ref discovery steps 1230// (in this thread). 1231void 1232ReferenceProcessor::preclean_discovered_reflist(DiscoveredList& refs_list, 1233 BoolObjectClosure* is_alive, 1234 OopClosure* keep_alive, 1235 VoidClosure* complete_gc, 1236 YieldClosure* yield) { 1237 DiscoveredListIterator iter(refs_list, keep_alive, is_alive); 1238 while (iter.has_next()) { 1239 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */)); 1240 oop obj = iter.obj(); 1241 oop next = java_lang_ref_Reference::next(obj); 1242 if (iter.referent() == NULL || iter.is_referent_alive() || 1243 next != NULL) { 1244 // The referent has been cleared, or is alive, or the Reference is not 1245 // active; we need to trace and mark its cohort. 1246 if (TraceReferenceGC) { 1247 gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)", 1248 p2i(iter.obj()), iter.obj()->klass()->internal_name()); 1249 } 1250 // Remove Reference object from list 1251 iter.remove(); 1252 // Keep alive its cohort. 1253 iter.make_referent_alive(); 1254 if (UseCompressedOops) { 1255 narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj); 1256 keep_alive->do_oop(next_addr); 1257 } else { 1258 oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj); 1259 keep_alive->do_oop(next_addr); 1260 } 1261 iter.move_to_next(); 1262 } else { 1263 iter.next(); 1264 } 1265 } 1266 // Close the reachable set 1267 complete_gc->do_void(); 1268 1269 NOT_PRODUCT( 1270 if (PrintGCDetails && PrintReferenceGC && (iter.processed() > 0)) { 1271 gclog_or_tty->print_cr(" Dropped " SIZE_FORMAT " Refs out of " SIZE_FORMAT 1272 " Refs in discovered list " INTPTR_FORMAT, 1273 iter.removed(), iter.processed(), p2i(refs_list.head())); 1274 } 1275 ) 1276} 1277 1278const char* ReferenceProcessor::list_name(uint i) { 1279 assert(i <= _max_num_q * number_of_subclasses_of_ref(), 1280 "Out of bounds index"); 1281 1282 int j = i / _max_num_q; 1283 switch (j) { 1284 case 0: return "SoftRef"; 1285 case 1: return "WeakRef"; 1286 case 2: return "FinalRef"; 1287 case 3: return "PhantomRef"; 1288 case 4: return "CleanerRef"; 1289 } 1290 ShouldNotReachHere(); 1291 return NULL; 1292} 1293 1294