1 /*
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3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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5 * This code is free software; you can redistribute it and/or modify it
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23 */
24
25 // ReferenceProcessor class encapsulates the per-"collector" processing
26 // of java.lang.Reference objects for GC. The interface is useful for supporting
27 // a generational abstraction, in particular when there are multiple
28 // generations that are being independently collected -- possibly
29 // concurrently and/or incrementally. Note, however, that the
30 // ReferenceProcessor class abstracts away from a generational setting
31 // by using only a heap interval (called "span" below), thus allowing
32 // its use in a straightforward manner in a general, non-generational
33 // setting.
34 //
35 // The basic idea is that each ReferenceProcessor object concerns
36 // itself with ("weak") reference processing in a specific "span"
37 // of the heap of interest to a specific collector. Currently,
38 // the span is a convex interval of the heap, but, efficiency
39 // apart, there seems to be no reason it couldn't be extended
40 // (with appropriate modifications) to any "non-convex interval".
41
42 // forward references
43 class ReferencePolicy;
44 class AbstractRefProcTaskExecutor;
45 class DiscoveredList;
46
47 class ReferenceProcessor : public CHeapObj {
48 protected:
49 // End of list marker
50 static oop _sentinelRef;
51 MemRegion _span; // (right-open) interval of heap
52 // subject to wkref discovery
53 bool _discovering_refs; // true when discovery enabled
54 bool _discovery_is_atomic; // if discovery is atomic wrt
55 // other collectors in configuration
56 bool _discovery_is_mt; // true if reference discovery is MT.
57 // If true, setting "next" field of a discovered refs list requires
58 // write barrier(s). (Must be true if used in a collector in which
59 // elements of a discovered list may be moved during discovery: for
60 // example, a collector like Garbage-First that moves objects during a
61 // long-term concurrent marking phase that does weak reference
62 // discovery.)
63 bool _discovered_list_needs_barrier;
64 BarrierSet* _bs; // Cached copy of BarrierSet.
65 bool _enqueuing_is_done; // true if all weak references enqueued
66 bool _processing_is_mt; // true during phases when
67 // reference processing is MT.
68 int _next_id; // round-robin counter in
69 // support of work distribution
70
71 // For collectors that do not keep GC marking information
72 // in the object header, this field holds a closure that
73 // helps the reference processor determine the reachability
74 // of an oop (the field is currently initialized to NULL for
75 // all collectors but the CMS collector).
76 BoolObjectClosure* _is_alive_non_header;
77
78 // Soft ref clearing policies
79 // . the default policy
80 static ReferencePolicy* _default_soft_ref_policy;
81 // . the "clear all" policy
82 static ReferencePolicy* _always_clear_soft_ref_policy;
83 // . the current policy below is either one of the above
84 ReferencePolicy* _current_soft_ref_policy;
85
86 // The discovered ref lists themselves
87
88 // The active MT'ness degree of the queues below
89 int _num_q;
90 // The maximum MT'ness degree of the queues below
91 int _max_num_q;
92 // Arrays of lists of oops, one per thread
93 DiscoveredList* _discoveredSoftRefs;
94 DiscoveredList* _discoveredWeakRefs;
95 DiscoveredList* _discoveredFinalRefs;
96 DiscoveredList* _discoveredPhantomRefs;
97
98 public:
99 int num_q() { return _num_q; }
100 void set_mt_degree(int v) { _num_q = v; }
101 DiscoveredList* discovered_soft_refs() { return _discoveredSoftRefs; }
102 static oop sentinel_ref() { return _sentinelRef; }
103 static oop* adr_sentinel_ref() { return &_sentinelRef; }
104 ReferencePolicy* setup_policy(bool always_clear) {
105 _current_soft_ref_policy = always_clear ?
106 _always_clear_soft_ref_policy : _default_soft_ref_policy;
107 _current_soft_ref_policy->setup(); // snapshot the policy threshold
108 return _current_soft_ref_policy;
109 }
110
111 public:
112 // Process references with a certain reachability level.
113 void process_discovered_reflist(DiscoveredList refs_lists[],
114 ReferencePolicy* policy,
115 bool clear_referent,
116 BoolObjectClosure* is_alive,
117 OopClosure* keep_alive,
118 VoidClosure* complete_gc,
119 AbstractRefProcTaskExecutor* task_executor);
120
121 void process_phaseJNI(BoolObjectClosure* is_alive,
122 OopClosure* keep_alive,
123 VoidClosure* complete_gc);
124
125 // Work methods used by the method process_discovered_reflist
126 // Phase1: keep alive all those referents that are otherwise
127 // dead but which must be kept alive by policy (and their closure).
128 void process_phase1(DiscoveredList& refs_list,
129 ReferencePolicy* policy,
130 BoolObjectClosure* is_alive,
131 OopClosure* keep_alive,
132 VoidClosure* complete_gc);
133 // Phase2: remove all those references whose referents are
134 // reachable.
135 inline void process_phase2(DiscoveredList& refs_list,
136 BoolObjectClosure* is_alive,
137 OopClosure* keep_alive,
138 VoidClosure* complete_gc) {
139 if (discovery_is_atomic()) {
140 // complete_gc is ignored in this case for this phase
141 pp2_work(refs_list, is_alive, keep_alive);
142 } else {
143 assert(complete_gc != NULL, "Error");
144 pp2_work_concurrent_discovery(refs_list, is_alive,
145 keep_alive, complete_gc);
146 }
147 }
148 // Work methods in support of process_phase2
149 void pp2_work(DiscoveredList& refs_list,
150 BoolObjectClosure* is_alive,
151 OopClosure* keep_alive);
152 void pp2_work_concurrent_discovery(
153 DiscoveredList& refs_list,
154 BoolObjectClosure* is_alive,
155 OopClosure* keep_alive,
156 VoidClosure* complete_gc);
157 // Phase3: process the referents by either clearing them
158 // or keeping them alive (and their closure)
159 void process_phase3(DiscoveredList& refs_list,
160 bool clear_referent,
161 BoolObjectClosure* is_alive,
162 OopClosure* keep_alive,
163 VoidClosure* complete_gc);
164
165 // Enqueue references with a certain reachability level
166 void enqueue_discovered_reflist(DiscoveredList& refs_list, HeapWord* pending_list_addr);
167
168 // "Preclean" all the discovered reference lists
169 // by removing references with strongly reachable referents.
170 // The first argument is a predicate on an oop that indicates
171 // its (strong) reachability and the second is a closure that
172 // may be used to incrementalize or abort the precleaning process.
173 // The caller is responsible for taking care of potential
174 // interference with concurrent operations on these lists
175 // (or predicates involved) by other threads. Currently
176 // only used by the CMS collector. should_unload_classes is
177 // used to aid assertion checking when classes are collected.
178 void preclean_discovered_references(BoolObjectClosure* is_alive,
179 OopClosure* keep_alive,
180 VoidClosure* complete_gc,
181 YieldClosure* yield,
182 bool should_unload_classes);
183
184 // Delete entries in the discovered lists that have
185 // either a null referent or are not active. Such
186 // Reference objects can result from the clearing
187 // or enqueueing of Reference objects concurrent
188 // with their discovery by a (concurrent) collector.
189 // For a definition of "active" see java.lang.ref.Reference;
190 // Refs are born active, become inactive when enqueued,
191 // and never become active again. The state of being
192 // active is encoded as follows: A Ref is active
193 // if and only if its "next" field is NULL.
194 void clean_up_discovered_references();
195 void clean_up_discovered_reflist(DiscoveredList& refs_list);
196
197 // Returns the name of the discovered reference list
198 // occupying the i / _num_q slot.
199 const char* list_name(int i);
200
201 void enqueue_discovered_reflists(HeapWord* pending_list_addr, AbstractRefProcTaskExecutor* task_executor);
202
203 protected:
204 // "Preclean" the given discovered reference list
205 // by removing references with strongly reachable referents.
206 // Currently used in support of CMS only.
207 void preclean_discovered_reflist(DiscoveredList& refs_list,
208 BoolObjectClosure* is_alive,
209 OopClosure* keep_alive,
210 VoidClosure* complete_gc,
211 YieldClosure* yield);
212
213 int next_id() {
214 int id = _next_id;
215 if (++_next_id == _num_q) {
216 _next_id = 0;
217 }
218 return id;
219 }
220 DiscoveredList* get_discovered_list(ReferenceType rt);
221 inline void add_to_discovered_list_mt(DiscoveredList& refs_list, oop obj,
222 HeapWord* discovered_addr);
223 void verify_ok_to_handle_reflists() PRODUCT_RETURN;
224
225 void abandon_partial_discovered_list(DiscoveredList& refs_list);
226
227 // Calculate the number of jni handles.
228 unsigned int count_jni_refs();
229
230 // Balances reference queues.
231 void balance_queues(DiscoveredList ref_lists[]);
232
233 // Update (advance) the soft ref master clock field.
234 void update_soft_ref_master_clock();
235
236 public:
237 // constructor
238 ReferenceProcessor():
239 _span((HeapWord*)NULL, (HeapWord*)NULL),
240 _discoveredSoftRefs(NULL), _discoveredWeakRefs(NULL),
241 _discoveredFinalRefs(NULL), _discoveredPhantomRefs(NULL),
242 _discovering_refs(false),
243 _discovery_is_atomic(true),
244 _enqueuing_is_done(false),
245 _discovery_is_mt(false),
246 _discovered_list_needs_barrier(false),
247 _bs(NULL),
248 _is_alive_non_header(NULL),
249 _num_q(0),
250 _max_num_q(0),
251 _processing_is_mt(false),
252 _next_id(0)
253 {}
254
255 ReferenceProcessor(MemRegion span, bool atomic_discovery,
256 bool mt_discovery,
257 int mt_degree = 1,
258 bool mt_processing = false,
259 bool discovered_list_needs_barrier = false);
260
261 // Allocates and initializes a reference processor.
262 static ReferenceProcessor* create_ref_processor(
263 MemRegion span,
264 bool atomic_discovery,
265 bool mt_discovery,
266 BoolObjectClosure* is_alive_non_header = NULL,
267 int parallel_gc_threads = 1,
268 bool mt_processing = false,
269 bool discovered_list_needs_barrier = false);
270
271 // RefDiscoveryPolicy values
272 enum DiscoveryPolicy {
273 ReferenceBasedDiscovery = 0,
274 ReferentBasedDiscovery = 1,
275 DiscoveryPolicyMin = ReferenceBasedDiscovery,
276 DiscoveryPolicyMax = ReferentBasedDiscovery
277 };
278
279 static void init_statics();
280
281 public:
282 // get and set "is_alive_non_header" field
283 BoolObjectClosure* is_alive_non_header() {
284 return _is_alive_non_header;
285 }
286 void set_is_alive_non_header(BoolObjectClosure* is_alive_non_header) {
287 _is_alive_non_header = is_alive_non_header;
288 }
289
290 // get and set span
291 MemRegion span() { return _span; }
292 void set_span(MemRegion span) { _span = span; }
293
294 // start and stop weak ref discovery
295 void enable_discovery() { _discovering_refs = true; }
296 void disable_discovery() { _discovering_refs = false; }
297 bool discovery_enabled() { return _discovering_refs; }
298
299 // whether discovery is atomic wrt other collectors
300 bool discovery_is_atomic() const { return _discovery_is_atomic; }
301 void set_atomic_discovery(bool atomic) { _discovery_is_atomic = atomic; }
302
303 // whether discovery is done by multiple threads same-old-timeously
304 bool discovery_is_mt() const { return _discovery_is_mt; }
305 void set_mt_discovery(bool mt) { _discovery_is_mt = mt; }
306
307 // Whether we are in a phase when _processing_ is MT.
308 bool processing_is_mt() const { return _processing_is_mt; }
309 void set_mt_processing(bool mt) { _processing_is_mt = mt; }
310
311 // whether all enqueuing of weak references is complete
312 bool enqueuing_is_done() { return _enqueuing_is_done; }
313 void set_enqueuing_is_done(bool v) { _enqueuing_is_done = v; }
314
315 // iterate over oops
316 void weak_oops_do(OopClosure* f); // weak roots
317 static void oops_do(OopClosure* f); // strong root(s)
318
319 // Balance each of the discovered lists.
320 void balance_all_queues();
321
322 // Discover a Reference object, using appropriate discovery criteria
323 bool discover_reference(oop obj, ReferenceType rt);
324
325 // Process references found during GC (called by the garbage collector)
326 void process_discovered_references(BoolObjectClosure* is_alive,
327 OopClosure* keep_alive,
328 VoidClosure* complete_gc,
329 AbstractRefProcTaskExecutor* task_executor);
330
331 public:
332 // Enqueue references at end of GC (called by the garbage collector)
333 bool enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor = NULL);
334
335 // If a discovery is in process that is being superceded, abandon it: all
336 // the discovered lists will be empty, and all the objects on them will
337 // have NULL discovered fields. Must be called only at a safepoint.
338 void abandon_partial_discovery();
339
340 // debugging
341 void verify_no_references_recorded() PRODUCT_RETURN;
342 static void verify();
343
344 // clear the discovered lists (unlinking each entry).
345 void clear_discovered_references() PRODUCT_RETURN;
346 };
347
348 // A utility class to disable reference discovery in
349 // the scope which contains it, for given ReferenceProcessor.
350 class NoRefDiscovery: StackObj {
351 private:
352 ReferenceProcessor* _rp;
353 bool _was_discovering_refs;
354 public:
355 NoRefDiscovery(ReferenceProcessor* rp) : _rp(rp) {
356 _was_discovering_refs = _rp->discovery_enabled();
357 if (_was_discovering_refs) {
358 _rp->disable_discovery();
359 }
360 }
361
362 ~NoRefDiscovery() {
363 if (_was_discovering_refs) {
364 _rp->enable_discovery();
365 }
366 }
367 };
368
369
370 // A utility class to temporarily mutate the span of the
371 // given ReferenceProcessor in the scope that contains it.
372 class ReferenceProcessorSpanMutator: StackObj {
373 private:
374 ReferenceProcessor* _rp;
375 MemRegion _saved_span;
376
377 public:
378 ReferenceProcessorSpanMutator(ReferenceProcessor* rp,
379 MemRegion span):
380 _rp(rp) {
381 _saved_span = _rp->span();
382 _rp->set_span(span);
383 }
384
385 ~ReferenceProcessorSpanMutator() {
386 _rp->set_span(_saved_span);
387 }
388 };
389
390 // A utility class to temporarily change the MT'ness of
391 // reference discovery for the given ReferenceProcessor
392 // in the scope that contains it.
393 class ReferenceProcessorMTMutator: StackObj {
394 private:
395 ReferenceProcessor* _rp;
396 bool _saved_mt;
397
398 public:
399 ReferenceProcessorMTMutator(ReferenceProcessor* rp,
400 bool mt):
401 _rp(rp) {
402 _saved_mt = _rp->discovery_is_mt();
403 _rp->set_mt_discovery(mt);
404 }
405
406 ~ReferenceProcessorMTMutator() {
407 _rp->set_mt_discovery(_saved_mt);
408 }
409 };
410
411
412 // A utility class to temporarily change the disposition
413 // of the "is_alive_non_header" closure field of the
414 // given ReferenceProcessor in the scope that contains it.
415 class ReferenceProcessorIsAliveMutator: StackObj {
416 private:
417 ReferenceProcessor* _rp;
418 BoolObjectClosure* _saved_cl;
419
420 public:
421 ReferenceProcessorIsAliveMutator(ReferenceProcessor* rp,
422 BoolObjectClosure* cl):
423 _rp(rp) {
424 _saved_cl = _rp->is_alive_non_header();
425 _rp->set_is_alive_non_header(cl);
426 }
427
428 ~ReferenceProcessorIsAliveMutator() {
429 _rp->set_is_alive_non_header(_saved_cl);
430 }
431 };
432
433 // A utility class to temporarily change the disposition
434 // of the "discovery_is_atomic" field of the
435 // given ReferenceProcessor in the scope that contains it.
436 class ReferenceProcessorAtomicMutator: StackObj {
437 private:
438 ReferenceProcessor* _rp;
439 bool _saved_atomic_discovery;
440
441 public:
442 ReferenceProcessorAtomicMutator(ReferenceProcessor* rp,
443 bool atomic):
444 _rp(rp) {
445 _saved_atomic_discovery = _rp->discovery_is_atomic();
446 _rp->set_atomic_discovery(atomic);
447 }
448
449 ~ReferenceProcessorAtomicMutator() {
450 _rp->set_atomic_discovery(_saved_atomic_discovery);
451 }
452 };
453
454
455 // A utility class to temporarily change the MT processing
456 // disposition of the given ReferenceProcessor instance
457 // in the scope that contains it.
458 class ReferenceProcessorMTProcMutator: StackObj {
459 private:
460 ReferenceProcessor* _rp;
461 bool _saved_mt;
462
463 public:
464 ReferenceProcessorMTProcMutator(ReferenceProcessor* rp,
465 bool mt):
466 _rp(rp) {
467 _saved_mt = _rp->processing_is_mt();
468 _rp->set_mt_processing(mt);
469 }
470
471 ~ReferenceProcessorMTProcMutator() {
472 _rp->set_mt_processing(_saved_mt);
473 }
474 };
475
476
477 // This class is an interface used to implement task execution for the
478 // reference processing.
479 class AbstractRefProcTaskExecutor {
480 public:
481
482 // Abstract tasks to execute.
483 class ProcessTask;
484 class EnqueueTask;
485
486 // Executes a task using worker threads.
487 virtual void execute(ProcessTask& task) = 0;
488 virtual void execute(EnqueueTask& task) = 0;
489
490 // Switch to single threaded mode.
491 virtual void set_single_threaded_mode() { };
492 };
493
494 // Abstract reference processing task to execute.
495 class AbstractRefProcTaskExecutor::ProcessTask {
496 protected:
497 ProcessTask(ReferenceProcessor& ref_processor,
498 DiscoveredList refs_lists[],
499 bool marks_oops_alive)
500 : _ref_processor(ref_processor),
501 _refs_lists(refs_lists),
502 _marks_oops_alive(marks_oops_alive)
503 { }
504
505 public:
506 virtual void work(unsigned int work_id, BoolObjectClosure& is_alive,
507 OopClosure& keep_alive,
508 VoidClosure& complete_gc) = 0;
509
510 // Returns true if a task marks some oops as alive.
511 bool marks_oops_alive() const
512 { return _marks_oops_alive; }
513
514 protected:
515 ReferenceProcessor& _ref_processor;
516 DiscoveredList* _refs_lists;
517 const bool _marks_oops_alive;
518 };
519
520 // Abstract reference processing task to execute.
521 class AbstractRefProcTaskExecutor::EnqueueTask {
522 protected:
523 EnqueueTask(ReferenceProcessor& ref_processor,
524 DiscoveredList refs_lists[],
525 HeapWord* pending_list_addr,
526 oop sentinel_ref,
527 int n_queues)
528 : _ref_processor(ref_processor),
529 _refs_lists(refs_lists),
530 _pending_list_addr(pending_list_addr),
531 _sentinel_ref(sentinel_ref),
532 _n_queues(n_queues)
533 { }
534
535 public:
536 virtual void work(unsigned int work_id) = 0;
537
538 protected:
539 ReferenceProcessor& _ref_processor;
540 DiscoveredList* _refs_lists;
541 HeapWord* _pending_list_addr;
542 oop _sentinel_ref;
543 int _n_queues;
544 };