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24
25 #ifndef SHARE_GC_SHENANDOAH_SHENANDOAHSCANREMEMBERED_HPP
26 #define SHARE_GC_SHENANDOAH_SHENANDOAHSCANREMEMBERED_HPP
27
28 // Terminology used within this source file:
29 //
30 // Card Entry: This is the information that identifies whether a
31 // particular card-table entry is Clean or Dirty. A clean
32 // card entry denotes that the associated memory does not
33 // hold references to young-gen memory.
34 //
35 // Card Region, aka
36 // Card Memory: This is the region of memory that is assocated with a
37 // particular card entry.
38 //
39 // Card Cluster: A card cluster represents 64 card entries. A card
40 // cluster is the minimal amount of work performed at a
41 // time by a parallel thread. Note that the work required
42 // to scan a card cluster is somewhat variable in that the
43 // required effort depends on how many cards are dirty, how
44 // many references are held within the objects that span a
45 // DIRTY card's memory, and on the size of the object
46 // that spans the end of a DIRTY card's memory (because
47 // that object, if it's not an array, may need to be scanned in
48 // its entirety, when the object is imprecisely dirtied. Imprecise
49 // dirtying is when the card corresponding to the object header
50 // is dirtied, rather than the card on which the updated field lives).
51 // To better balance work amongst them, parallel worker threads dynamically
52 // claim clusters and are flexible in the number of clusters they
53 // process.
54 //
55 // A cluster represents a "natural" quantum of work to be performed by
56 // a parallel GC thread's background remembered set scanning efforts.
57 // The notion of cluster is similar to the notion of stripe in the
58 // implementation of parallel GC card scanning. However, a cluster is
59 // typically smaller than a stripe, enabling finer grain division of
60 // labor between multiple threads, and potentially better load balancing
61 // when dirty cards are not uniformly distributed in the heap, as is often
62 // the case with generational workloads where more recently promoted objects
63 // may be dirtied more frequently that older objects.
64 //
65 // For illustration, consider the following possible JVM configurations:
66 //
67 // Scenario 1:
68 // RegionSize is 128 MB
69 // Span of a card entry is 512 B
70 // Each card table entry consumes 1 B
71 // Assume one long word (8 B)of the card table represents a cluster.
72 // This long word holds 8 card table entries, spanning a
73 // total of 8*512 B = 4 KB of the heap
74 // The number of clusters per region is 128 MB / 4 KB = 32 K
75 //
76 // Scenario 2:
77 // RegionSize is 128 MB
78 // Span of each card entry is 128 B
79 // Each card table entry consumes 1 bit
80 // Assume one int word (4 B) of the card table represents a cluster.
81 // This int word holds 32 b/1 b = 32 card table entries, spanning a
82 // total of 32 * 128 B = 4 KB of the heap
83 // The number of clusters per region is 128 MB / 4 KB = 32 K
84 //
85 // Scenario 3:
86 // RegionSize is 128 MB
87 // Span of each card entry is 512 B
88 // Each card table entry consumes 1 bit
89 // Assume one long word (8 B) of card table represents a cluster.
90 // This long word holds 64 b/ 1 b = 64 card table entries, spanning a
91 // total of 64 * 512 B = 32 KB of the heap
92 // The number of clusters per region is 128 MB / 32 KB = 4 K
93 //
94 // At the start of a new young-gen concurrent mark pass, the gang of
95 // Shenandoah worker threads collaborate in performing the following
96 // actions:
97 //
98 // Let old_regions = number of ShenandoahHeapRegion comprising
99 // old-gen memory
100 // Let region_size = ShenandoahHeapRegion::region_size_bytes()
101 // represent the number of bytes in each region
102 // Let clusters_per_region = region_size / 512
103 // Let rs represent the relevant RememberedSet implementation
104 // (an instance of ShenandoahDirectCardMarkRememberedSet or an instance
105 // of a to-be-implemented ShenandoahBufferWithSATBRememberedSet)
106 //
107 // for each ShenandoahHeapRegion old_region in the whole heap
108 // determine the cluster number of the first cluster belonging
109 // to that region
110 // for each cluster contained within that region
111 // Assure that exactly one worker thread processes each
112 // cluster, each thread making a series of invocations of the
113 // following:
114 //
115 // rs->process_clusters(worker_id, ReferenceProcessor *,
116 // ShenandoahConcurrentMark *, cluster_no, cluster_count,
117 // HeapWord *end_of_range, OopClosure *oops);
118 //
119 // For efficiency, divide up the clusters so that different threads
120 // are responsible for processing different clusters. Processing costs
121 // may vary greatly between clusters for the following reasons:
122 //
123 // a) some clusters contain mostly dirty cards and other
124 // clusters contain mostly clean cards
125 // b) some clusters contain mostly primitive data and other
126 // clusters contain mostly reference data
127 // c) some clusters are spanned by very large non-array objects that
128 // begin in some other cluster. When a large non-array object
129 // beginning in a preceding cluster spans large portions of
130 // this cluster, then because of imprecise dirtying, the
131 // portion of the object in this cluster may be clean, but
132 // will need to be processed by the worker responsible for
133 // this cluster, potentially increasing its work.
134 // d) in the case that the end of this cluster is spanned by a
135 // very large non-array object, the worker for this cluster will
136 // be responsible for processing the portion of the object
137 // in this cluster.
138 //
139 // Though an initial division of labor between marking threads may
140 // assign equal numbers of clusters to be scanned by each thread, it
141 // should be expected that some threads will finish their assigned
142 // work before others. Therefore, some amount of the full remembered
143 // set scanning effort should be held back and assigned incrementally
144 // to the threads that end up with excess capacity. Consider the
145 // following strategy for dividing labor:
146 //
147 // 1. Assume there are 8 marking threads and 1024 remembered
148 // set clusters to be scanned.
149 // 2. Assign each thread to scan 64 clusters. This leaves
150 // 512 (1024 - (8*64)) clusters to still be scanned.
151 // 3. As the 8 server threads complete previous cluster
152 // scanning assignments, issue each of the next 8 scanning
153 // assignments as units of 32 additional cluster each.
154 // In the case that there is high variance in effort
155 // associated with previous cluster scanning assignments,
156 // multiples of these next assignments may be serviced by
157 // the server threads that were previously assigned lighter
158 // workloads.
159 // 4. Make subsequent scanning assignments as follows:
160 // a) 8 assignments of size 16 clusters
161 // b) 8 assignments of size 8 clusters
162 // c) 16 assignments of size 4 clusters
163 //
164 // When there is no more remembered set processing work to be
165 // assigned to a newly idled worker thread, that thread can move
166 // on to work on other tasks associated with root scanning until such
167 // time as all clusters have been examined.
168 //
169 // Remembered set scanning is designed to run concurrently with
170 // mutator threads, with multiple concurrent workers. Furthermore, the
171 // current implementation of remembered set scanning never clears a
172 // card once it has been marked.
173 //
174 // These limitations will be addressed in future enhancements to the
175 // existing implementation.
176
177 #include "gc/shared/workerThread.hpp"
178 #include "gc/shenandoah/shenandoahCardStats.hpp"
179 #include "gc/shenandoah/shenandoahCardTable.hpp"
180 #include "gc/shenandoah/shenandoahNumberSeq.hpp"
181 #include "gc/shenandoah/shenandoahTaskqueue.hpp"
182 #include "memory/iterator.hpp"
183 #include "utilities/globalDefinitions.hpp"
184
185 class ShenandoahReferenceProcessor;
186 class ShenandoahConcurrentMark;
187 class ShenandoahHeap;
188 class ShenandoahHeapRegion;
189 class ShenandoahRegionIterator;
190 class ShenandoahMarkingContext;
191
192 class CardTable;
193 typedef CardTable::CardValue CardValue;
194
195 class ShenandoahDirectCardMarkRememberedSet: public CHeapObj<mtGC> {
196
197 private:
198
199 // Use symbolic constants defined in cardTable.hpp
200 // CardTable::card_shift = 9;
201 // CardTable::card_size = 512;
202 // CardTable::card_size_in_words = 64;
203 // CardTable::clean_card_val()
204 // CardTable::dirty_card_val()
205
206 const size_t LogCardValsPerIntPtr; // the number of card values (entries) in an intptr_t
207 const size_t LogCardSizeInWords; // the size of a card in heap word units
208
209 ShenandoahHeap *_heap;
210 ShenandoahCardTable *_card_table;
211 size_t _card_shift;
212 size_t _total_card_count;
213 size_t _cluster_count;
214 HeapWord *_whole_heap_base; // Points to first HeapWord of data contained within heap memory
215 CardValue* _byte_map; // Points to first entry within the card table
216 CardValue* _byte_map_base; // Points to byte_map minus the bias computed from address of heap memory
217
218 public:
219
220 // count is the number of cards represented by the card table.
221 ShenandoahDirectCardMarkRememberedSet(ShenandoahCardTable *card_table, size_t total_card_count);
222
223 // Card index is zero-based relative to _byte_map.
224 size_t last_valid_index() const;
225 size_t total_cards() const;
226 size_t card_index_for_addr(HeapWord *p) const;
227 HeapWord *addr_for_card_index(size_t card_index) const;
228 inline const CardValue* get_card_table_byte_map(bool write_table) const;
229 inline bool is_card_dirty(size_t card_index) const;
230 inline bool is_write_card_dirty(size_t card_index) const;
231 inline void mark_card_as_dirty(size_t card_index);
232 inline void mark_range_as_dirty(size_t card_index, size_t num_cards);
233 inline void mark_card_as_clean(size_t card_index);
234 inline void mark_range_as_clean(size_t card_index, size_t num_cards);
235 inline bool is_card_dirty(HeapWord *p) const;
236 inline void mark_card_as_dirty(HeapWord *p);
237 inline void mark_range_as_dirty(HeapWord *p, size_t num_heap_words);
238 inline void mark_card_as_clean(HeapWord *p);
239 inline void mark_range_as_clean(HeapWord *p, size_t num_heap_words);
240 inline size_t cluster_count() const;
241
242 // Called by GC thread at start of concurrent mark to exchange roles of read and write remembered sets.
243 // Not currently used because mutator write barrier does not honor changes to the location of card table.
244 // Instead of swap_remset, the current implementation of concurrent remembered set scanning does reset_remset
245 // in parallel threads, each invocation processing one entire HeapRegion at a time.
246 void swap_remset() { _card_table->swap_card_tables(); }
247
248 // Merge any dirty values from write table into the read table, while leaving
249 // the write table unchanged.
250 void merge_write_table(HeapWord* start, size_t word_count);
251
252 // Destructively copy the write table to the read table, and clean the write table.
253 void reset_remset(HeapWord* start, size_t word_count);
254
255 // Called by GC thread after scanning old remembered set in order to prepare for next GC pass
256 void clear_old_remset() { _card_table->clear_read_table(); }
257 };
258
259 // A ShenandoahCardCluster represents the minimal unit of work
260 // performed by independent parallel GC threads during scanning of
261 // remembered sets.
262 //
263 // The GC threads that perform card-table remembered set scanning may
264 // overwrite card-table entries to mark them as clean in the case that
265 // the associated memory no longer holds references to young-gen
266 // memory. Rather than access the card-table entries directly, all GC
267 // thread access to card-table information is made by way of the
268 // ShenandoahCardCluster data abstraction. This abstraction
269 // effectively manages access to multiple possible underlying
270 // remembered set implementations, including a traditional card-table
271 // approach and a SATB-based approach.
272 //
273 // The API services represent a compromise between efficiency and
274 // convenience.
275 //
276 // Multiple GC threads that scan the remembered set
277 // in parallel. The desire is to divide the complete scanning effort
278 // into multiple clusters of work that can be independently processed
279 // by individual threads without need for synchronizing efforts
280 // between the work performed by each task. The term "cluster" of
281 // work is similar to the term "stripe" as used in the implementation
282 // of Parallel GC.
283 //
284 // Complexity arises when an object to be scanned crosses the boundary
285 // between adjacent cluster regions. Here is the protocol that we currently
286 // follow:
287 //
288 // 1. The thread responsible for scanning the cards in a cluster modifies
289 // the associated card-table entries. Only cards that are dirty are
290 // processed, except as described below for the case of objects that
291 // straddle more than one card.
292 // 2. Object Arrays are precisely dirtied, so only the portion of the obj-array
293 // that overlaps the range of dirty cards in its cluster are scanned
294 // by each worker thread. This holds for portions of obj-arrays that extend
295 // over clusters processed by different workers, with each worked responsible
296 // for scanning the portion of the obj-array overlapping the dirty cards in
297 // its cluster.
298 // 3. Non-array objects are precisely dirtied by the interpreter and the compilers
299 // For such objects that extend over multiple cards, or even multiple clusters,
300 // the entire object is scanned by the worker that processes the (dirty) card on
301 // which the object's header lies. (However, GC workers should precisely dirty the
302 // cards with inter-regional/inter-generational pointers in the body of this object,
303 // thus making subsequent scans potentially less expensive.) Such larger non-array
304 // objects are relatively rare.
305 //
306 // A possible criticism:
307 // C. The representation of pointer location descriptive information
308 // within Klass representations is not designed for efficient
309 // "random access". An alternative approach to this design would
310 // be to scan very large objects multiple times, once for each
311 // cluster that is spanned by the object's range. This reduces
312 // unnecessary overscan, but it introduces different sorts of
313 // overhead effort:
314 // i) For each spanned cluster, we have to look up the start of
315 // the crossing object.
316 // ii) Each time we scan the very large object, we have to
317 // sequentially walk through its pointer location
318 // descriptors, skipping over all of the pointers that
319 // precede the start of the range of addresses that we
320 // consider relevant.
321
322
323 // Because old-gen heap memory is not necessarily contiguous, and
324 // because cards are not necessarily maintained for young-gen memory,
325 // consecutive card numbers do not necessarily correspond to consecutive
326 // address ranges. For the traditional direct-card-marking
327 // implementation of this interface, consecutive card numbers are
328 // likely to correspond to contiguous regions of memory, but this
329 // should not be assumed. Instead, rely only upon the following:
330 //
331 // 1. All card numbers for cards pertaining to the same
332 // ShenandoahHeapRegion are consecutively numbered.
333 // 2. In the case that neighboring ShenandoahHeapRegions both
334 // represent old-gen memory, the card regions that span the
335 // boundary between these neighboring heap regions will be
336 // consecutively numbered.
337 // 3. (A corollary) In the case that an old-gen object straddles the
338 // boundary between two heap regions, the card regions that
339 // correspond to the span of this object will be consecutively
340 // numbered.
341 //
342 // ShenandoahCardCluster abstracts access to the remembered set
343 // and also keeps track of crossing map information to allow efficient
344 // resolution of object start addresses.
345 //
346 // ShenandoahCardCluster supports all of the services of
347 // RememberedSet, plus it supports register_object() and lookup_object().
348 // Note that we only need to register the start addresses of the object that
349 // overlays the first address of a card; we need to do this for every card.
350 // In other words, register_object() checks if the object crosses a card boundary,
351 // and updates the offset value for each card that the object crosses into.
352 // For objects that don't straddle cards, nothing needs to be done.
353 //
354 // The RememberedSet template parameter is intended to represent either
355 // ShenandoahDirectCardMarkRememberedSet, or a to-be-implemented
356 // ShenandoahBufferWithSATBRememberedSet.
357 template<typename RememberedSet>
358 class ShenandoahCardCluster: public CHeapObj<mtGC> {
359
360 private:
361 RememberedSet *_rs;
362
363 public:
364 static const size_t CardsPerCluster = 64;
365
366 private:
367 typedef struct cross_map { uint8_t first; uint8_t last; } xmap;
368 typedef union crossing_info { uint16_t short_word; xmap offsets; } crossing_info;
369
370 // ObjectStartsInCardRegion bit is set within a crossing_info.offsets.start iff at least one object starts within
371 // a particular card region. We pack this bit into start byte under assumption that start byte is accessed less
372 // frequently than last byte. This is true when number of clean cards is greater than number of dirty cards.
373 static const uint16_t ObjectStartsInCardRegion = 0x80;
374 static const uint16_t FirstStartBits = 0x7f;
375
376 // Check that we have enough bits to store the largest possible offset into a card for an object start.
377 // The value for maximum card size is based on the constraints for GCCardSizeInBytes in gc_globals.hpp.
378 static const int MaxCardSize = NOT_LP64(512) LP64_ONLY(1024);
379 STATIC_ASSERT((MaxCardSize / HeapWordSize) - 1 <= FirstStartBits);
380
381 crossing_info *object_starts;
382
383 public:
384 // If we're setting first_start, assume the card has an object.
385 inline void set_first_start(size_t card_index, uint8_t value) {
386 object_starts[card_index].offsets.first = ObjectStartsInCardRegion | value;
387 }
388
389 inline void set_last_start(size_t card_index, uint8_t value) {
390 object_starts[card_index].offsets.last = value;
391 }
392
393 inline void set_starts_object_bit(size_t card_index) {
394 object_starts[card_index].offsets.first |= ObjectStartsInCardRegion;
395 }
396
397 inline void clear_starts_object_bit(size_t card_index) {
398 object_starts[card_index].offsets.first &= ~ObjectStartsInCardRegion;
399 }
400
401 // Returns true iff an object is known to start within the card memory associated with card card_index.
402 inline bool starts_object(size_t card_index) const {
403 return (object_starts[card_index].offsets.first & ObjectStartsInCardRegion) != 0;
404 }
405
406 inline void clear_objects_in_range(HeapWord *addr, size_t num_words) {
407 size_t card_index = _rs->card_index_for_addr(addr);
408 size_t last_card_index = _rs->card_index_for_addr(addr + num_words - 1);
409 while (card_index <= last_card_index)
410 object_starts[card_index++].short_word = 0;
411 }
412
413 ShenandoahCardCluster(RememberedSet *rs) {
414 _rs = rs;
415 // TODO: We don't really need object_starts entries for every card entry. We only need these for
416 // the card entries that correspond to old-gen memory. But for now, let's be quick and dirty.
417 object_starts = NEW_C_HEAP_ARRAY(crossing_info, rs->total_cards(), mtGC);
418 for (size_t i = 0; i < rs->total_cards(); i++) {
419 object_starts[i].short_word = 0;
420 }
421 }
422
423 ~ShenandoahCardCluster() {
424 FREE_C_HEAP_ARRAY(crossing_info, object_starts);
425 object_starts = nullptr;
426 }
427
428 // There is one entry within the object_starts array for each card entry.
429 //
430 // Suppose multiple garbage objects are coalesced during GC sweep
431 // into a single larger "free segment". As each two objects are
432 // coalesced together, the start information pertaining to the second
433 // object must be removed from the objects_starts array. If the
434 // second object had been been the first object within card memory,
435 // the new first object is the object that follows that object if
436 // that starts within the same card memory, or NoObject if the
437 // following object starts within the following cluster. If the
438 // second object had been the last object in the card memory,
439 // replace this entry with the newly coalesced object if it starts
440 // within the same card memory, or with NoObject if it starts in a
441 // preceding card's memory.
442 //
443 // Suppose a large free segment is divided into a smaller free
444 // segment and a new object. The second part of the newly divided
445 // memory must be registered as a new object, overwriting at most
446 // one first_start and one last_start entry. Note that one of the
447 // newly divided two objects might be a new GCLAB.
448 //
449 // Suppose postprocessing of a GCLAB finds that the original GCLAB
450 // has been divided into N objects. Each of the N newly allocated
451 // objects will be registered, overwriting at most one first_start
452 // and one last_start entries.
453 //
454 // No object registration operations are linear in the length of
455 // the registered objects.
456 //
457 // Consider further the following observations regarding object
458 // registration costs:
459 //
460 // 1. The cost is paid once for each old-gen object (Except when
461 // an object is demoted and repromoted, in which case we would
462 // pay the cost again).
463 // 2. The cost can be deferred so that there is no urgency during
464 // mutator copy-on-first-access promotion. Background GC
465 // threads will update the object_starts array by post-
466 // processing the contents of retired PLAB buffers.
467 // 3. The bet is that these costs are paid relatively rarely
468 // because:
469 // a) Most objects die young and objects that die in young-gen
470 // memory never need to be registered with the object_starts
471 // array.
472 // b) Most objects that are promoted into old-gen memory live
473 // there without further relocation for a relatively long
474 // time, so we get a lot of benefit from each investment
475 // in registering an object.
476
477 public:
478
479 // The starting locations of objects contained within old-gen memory
480 // are registered as part of the remembered set implementation. This
481 // information is required when scanning dirty card regions that are
482 // spanned by objects beginning within preceding card regions. It
483 // is necessary to find the first and last objects that begin within
484 // this card region. Starting addresses of objects are required to
485 // find the object headers, and object headers provide information
486 // about which fields within the object hold addresses.
487 //
488 // The old-gen memory allocator invokes register_object() for any
489 // object that is allocated within old-gen memory. This identifies
490 // the starting addresses of objects that span boundaries between
491 // card regions.
492 //
493 // It is not necessary to invoke register_object at the very instant
494 // an object is allocated. It is only necessary to invoke it
495 // prior to the next start of a garbage collection concurrent mark
496 // or concurrent update-references phase. An "ideal" time to register
497 // objects is during post-processing of a GCLAB after the GCLAB is
498 // retired due to depletion of its memory.
499 //
500 // register_object() does not perform synchronization. In the case
501 // that multiple threads are registering objects whose starting
502 // addresses are within the same cluster, races between these
503 // threads may result in corruption of the object-start data
504 // structures. Parallel GC threads should avoid registering objects
505 // residing within the same cluster by adhering to the following
506 // coordination protocols:
507 //
508 // 1. Align thread-local GCLAB buffers with some TBD multiple of
509 // card clusters. The card cluster size is 32 KB. If the
510 // desired GCLAB size is 128 KB, align the buffer on a multiple
511 // of 4 card clusters.
512 // 2. Post-process the contents of GCLAB buffers to register the
513 // objects allocated therein. Allow one GC thread at a
514 // time to do the post-processing of each GCLAB.
515 // 3. Since only one GC thread at a time is registering objects
516 // belonging to a particular allocation buffer, no locking
517 // is performed when registering these objects.
518 // 4. Any remnant of unallocated memory within an expended GC
519 // allocation buffer is not returned to the old-gen allocation
520 // pool until after the GC allocation buffer has been post
521 // processed. Before any remnant memory is returned to the
522 // old-gen allocation pool, the GC thread that scanned this GC
523 // allocation buffer performs a write-commit memory barrier.
524 // 5. Background GC threads that perform tenuring of young-gen
525 // objects without a GCLAB use a CAS lock before registering
526 // each tenured object. The CAS lock assures both mutual
527 // exclusion and memory coherency/visibility. Note that an
528 // object tenured by a background GC thread will not overlap
529 // with any of the clusters that are receiving tenured objects
530 // by way of GCLAB buffers. Multiple independent GC threads may
531 // attempt to tenure objects into a shared cluster. This is why
532 // sychronization may be necessary. Consider the following
533 // scenarios:
534 //
535 // a) If two objects are tenured into the same card region, each
536 // registration may attempt to modify the first-start or
537 // last-start information associated with that card region.
538 // Furthermore, because the representations of first-start
539 // and last-start information within the object_starts array
540 // entry uses different bits of a shared uint_16 to represent
541 // each, it is necessary to lock the entire card entry
542 // before modifying either the first-start or last-start
543 // information within the entry.
544 // b) Suppose GC thread X promotes a tenured object into
545 // card region A and this tenured object spans into
546 // neighboring card region B. Suppose GC thread Y (not equal
547 // to X) promotes a tenured object into cluster B. GC thread X
548 // will update the object_starts information for card A. No
549 // synchronization is required.
550 // c) In summary, when background GC threads register objects
551 // newly tenured into old-gen memory, they must acquire a
552 // mutual exclusion lock on the card that holds the starting
553 // address of the newly tenured object. This can be achieved
554 // by using a CAS instruction to assure that the previous
555 // values of first-offset and last-offset have not been
556 // changed since the same thread inquired as to their most
557 // current values.
558 //
559 // One way to minimize the need for synchronization between
560 // background tenuring GC threads is for each tenuring GC thread
561 // to promote young-gen objects into distinct dedicated cluster
562 // ranges.
563 // 6. The object_starts information is only required during the
564 // starting of concurrent marking and concurrent evacuation
565 // phases of GC. Before we start either of these GC phases, the
566 // JVM enters a safe point and all GC threads perform
567 // commit-write barriers to assure that access to the
568 // object_starts information is coherent.
569
570
571 // Notes on synchronization of register_object():
572 //
573 // 1. For efficiency, there is no locking in the implementation of register_object()
574 // 2. Thus, it is required that users of this service assure that concurrent/parallel invocations of
575 // register_object() do pertain to the same card's memory range. See discussion below to understand
576 // the risks.
577 // 3. When allocating from a TLAB or GCLAB, the mutual exclusion can be guaranteed by assuring that each
578 // LAB's start and end are aligned on card memory boundaries.
579 // 4. Use the same lock that guarantees exclusivity when performing free-list allocation within heap regions.
580 //
581 // Register the newly allocated object while we're holding the global lock since there's no synchronization
582 // built in to the implementation of register_object(). There are potential races when multiple independent
583 // threads are allocating objects, some of which might span the same card region. For example, consider
584 // a card table's memory region within which three objects are being allocated by three different threads:
585 //
586 // objects being "concurrently" allocated:
587 // [-----a------][-----b-----][--------------c------------------]
588 // [---- card table memory range --------------]
589 //
590 // Before any objects are allocated, this card's memory range holds no objects. Note that:
591 // allocation of object a wants to set the has-object, first-start, and last-start attributes of the preceding card region.
592 // allocation of object b wants to set the has-object, first-start, and last-start attributes of this card region.
593 // allocation of object c also wants to set the has-object, first-start, and last-start attributes of this card region.
594 //
595 // The thread allocating b and the thread allocating c can "race" in various ways, resulting in confusion, such as last-start
596 // representing object b while first-start represents object c. This is why we need to require all register_object()
597 // invocations associated with objects that are allocated from "free lists" to provide their own mutual exclusion locking
598 // mechanism.
599
600 // Reset the starts_object() information to false for all cards in the range between from and to.
601 void reset_object_range(HeapWord *from, HeapWord *to);
602
603 // register_object() requires that the caller hold the heap lock
604 // before calling it.
605 void register_object(HeapWord* address);
606
607 // register_object_without_lock() does not require that the caller hold
608 // the heap lock before calling it, under the assumption that the
609 // caller has assure no other thread will endeavor to concurrently
610 // register objects that start within the same card's memory region
611 // as address.
612 void register_object_without_lock(HeapWord* address);
613
614 // During the reference updates phase of GC, we walk through each old-gen memory region that was
615 // not part of the collection set and we invalidate all unmarked objects. As part of this effort,
616 // we coalesce neighboring dead objects in order to make future remembered set scanning more
617 // efficient (since future remembered set scanning of any card region containing consecutive
618 // dead objects can skip over all of them at once by reading only a single dead object header
619 // instead of having to read the header of each of the coalesced dead objects.
620 //
621 // At some future time, we may implement a further optimization: satisfy future allocation requests
622 // by carving new objects out of the range of memory that represents the coalesced dead objects.
623 //
624 // Suppose we want to combine several dead objects into a single coalesced object. How does this
625 // impact our representation of crossing map information?
626 // 1. If the newly coalesced range is contained entirely within a card range, that card's last
627 // start entry either remains the same or it is changed to the start of the coalesced region.
628 // 2. For the card that holds the start of the coalesced object, it will not impact the first start
629 // but it may impact the last start.
630 // 3. For following cards spanned entirely by the newly coalesced object, it will change starts_object
631 // to false (and make first-start and last-start "undefined").
632 // 4. For a following card that is spanned patially by the newly coalesced object, it may change
633 // first-start value, but it will not change the last-start value.
634 //
635 // The range of addresses represented by the arguments to coalesce_objects() must represent a range
636 // of memory that was previously occupied exactly by one or more previously registered objects. For
637 // convenience, it is legal to invoke coalesce_objects() with arguments that span a single previously
638 // registered object.
639 //
640 // The role of coalesce_objects is to change the crossing map information associated with all of the coalesced
641 // objects.
642 void coalesce_objects(HeapWord* address, size_t length_in_words);
643
644 // The typical use case is going to look something like this:
645 // for each heapregion that comprises old-gen memory
646 // for each card number that corresponds to this heap region
647 // scan the objects contained therein if the card is dirty
648 // To avoid excessive lookups in a sparse array, the API queries
649 // the card number pertaining to a particular address and then uses the
650 // card number for subsequent information lookups and stores.
651
652 // If starts_object(card_index), this returns the word offset within this card
653 // memory at which the first object begins. If !starts_object(card_index), the
654 // result is a don't care value -- asserts in a debug build.
655 size_t get_first_start(size_t card_index) const;
656
657 // If starts_object(card_index), this returns the word offset within this card
658 // memory at which the last object begins. If !starts_object(card_index), the
659 // result is a don't care value.
660 size_t get_last_start(size_t card_index) const;
661
662
663 // Given a card_index, return the starting address of the first block in the heap
664 // that straddles into the card. If the card is co-initial with an object, then
665 // this would return the starting address of the heap that this card covers.
666 // Expects to be called for a card affiliated with the old generation in
667 // generational mode.
668 HeapWord* block_start(size_t card_index) const;
669 };
670
671 // ShenandoahScanRemembered is a concrete class representing the
672 // ability to scan the old-gen remembered set for references to
673 // objects residing in young-gen memory.
674 //
675 // Scanning normally begins with an invocation of numRegions and ends
676 // after all clusters of all regions have been scanned.
677 //
678 // Throughout the scanning effort, the number of regions does not
679 // change.
680 //
681 // Even though the regions that comprise old-gen memory are not
682 // necessarily contiguous, the abstraction represented by this class
683 // identifies each of the old-gen regions with an integer value
684 // in the range from 0 to (numRegions() - 1) inclusive.
685 //
686
687 template<typename RememberedSet>
688 class ShenandoahScanRemembered: public CHeapObj<mtGC> {
689
690 private:
691 RememberedSet* _rs;
692 ShenandoahCardCluster<RememberedSet>* _scc;
693
694 // Global card stats (cumulative)
695 HdrSeq _card_stats_scan_rs[MAX_CARD_STAT_TYPE];
696 HdrSeq _card_stats_update_refs[MAX_CARD_STAT_TYPE];
697 // Per worker card stats (multiplexed by phase)
698 HdrSeq** _card_stats;
699
700 // The types of card metrics that we gather
701 const char* _card_stats_name[MAX_CARD_STAT_TYPE] = {
702 "dirty_run", "clean_run",
703 "dirty_cards", "clean_cards",
704 "max_dirty_run", "max_clean_run",
705 "dirty_scan_objs",
706 "alternations"
707 };
708
709 // The statistics are collected and logged separately for
710 // card-scans for initial marking, and for updating refs.
711 const char* _card_stat_log_type[MAX_CARD_STAT_LOG_TYPE] = {
712 "Scan Remembered Set", "Update Refs"
713 };
714
715 int _card_stats_log_counter[2] = {0, 0};
716
717 public:
718 // How to instantiate this object?
719 // ShenandoahDirectCardMarkRememberedSet *rs =
720 // new ShenandoahDirectCardMarkRememberedSet();
721 // scr = new
722 // ShenandoahScanRememberd<ShenandoahDirectCardMarkRememberedSet>(rs);
723 //
724 // or, after the planned implementation of
725 // ShenandoahBufferWithSATBRememberedSet has been completed:
726 //
727 // ShenandoahBufferWithSATBRememberedSet *rs =
728 // new ShenandoahBufferWithSATBRememberedSet();
729 // scr = new
730 // ShenandoahScanRememberd<ShenandoahBufferWithSATBRememberedSet>(rs);
731
732
733 ShenandoahScanRemembered(RememberedSet *rs) {
734 _rs = rs;
735 _scc = new ShenandoahCardCluster<RememberedSet>(rs);
736
737 // We allocate ParallelGCThreads worth even though we usually only
738 // use up to ConcGCThreads, because degenerate collections may employ
739 // ParallelGCThreads for remembered set scanning.
740 if (ShenandoahEnableCardStats) {
741 _card_stats = NEW_C_HEAP_ARRAY(HdrSeq*, ParallelGCThreads, mtGC);
742 for (uint i = 0; i < ParallelGCThreads; i++) {
743 _card_stats[i] = new HdrSeq[MAX_CARD_STAT_TYPE];
744 }
745 } else {
746 _card_stats = nullptr;
747 }
748 }
749
750 ~ShenandoahScanRemembered() {
751 delete _scc;
752 if (ShenandoahEnableCardStats) {
753 for (uint i = 0; i < ParallelGCThreads; i++) {
754 delete _card_stats[i];
755 }
756 FREE_C_HEAP_ARRAY(HdrSeq*, _card_stats);
757 _card_stats = nullptr;
758 }
759 assert(_card_stats == nullptr, "Error");
760 }
761
762 HdrSeq* card_stats(uint worker_id) {
763 assert(worker_id < ParallelGCThreads, "Error");
764 assert(ShenandoahEnableCardStats == (_card_stats != nullptr), "Error");
765 return ShenandoahEnableCardStats ? _card_stats[worker_id] : nullptr;
766 }
767
768 HdrSeq* card_stats_for_phase(CardStatLogType t) {
769 switch (t) {
770 case CARD_STAT_SCAN_RS:
771 return _card_stats_scan_rs;
772 case CARD_STAT_UPDATE_REFS:
773 return _card_stats_update_refs;
774 default:
775 guarantee(false, "No such CardStatLogType");
776 }
777 return nullptr; // Quiet compiler
778 }
779
780 // TODO: We really don't want to share all of these APIs with arbitrary consumers of the ShenandoahScanRemembered abstraction.
781 // But in the spirit of quick and dirty for the time being, I'm going to go ahead and publish everything for right now. Some
782 // of existing code already depends on having access to these services (because existing code has not been written to honor
783 // full abstraction of remembered set scanning. In the not too distant future, we want to try to make most, if not all, of
784 // these services private. Two problems with publicizing:
785 // 1. Allowing arbitrary users to reach beneath the hood allows the users to make assumptions about underlying implementation.
786 // This will make it more difficult to change underlying implementation at a future time, such as when we eventually experiment
787 // with SATB-based implementation of remembered set representation.
788 // 2. If we carefully control sharing of certain of these services, we can reduce the overhead of synchronization by assuring
789 // that all users follow protocols that avoid contention that might require synchronization. When we publish these APIs, we
790 // lose control over who and how the data is accessed. As a result, we are required to insert more defensive measures into
791 // the implementation, including synchronization locks.
792
793
794 // Card index is zero-based relative to first spanned card region.
795 size_t last_valid_index();
796 size_t total_cards();
797 size_t card_index_for_addr(HeapWord *p);
798 HeapWord *addr_for_card_index(size_t card_index);
799 bool is_card_dirty(size_t card_index);
800 bool is_write_card_dirty(size_t card_index) { return _rs->is_write_card_dirty(card_index); }
801 void mark_card_as_dirty(size_t card_index);
802 void mark_range_as_dirty(size_t card_index, size_t num_cards);
803 void mark_card_as_clean(size_t card_index);
804 void mark_range_as_clean(size_t card_index, size_t num_cards);
805 bool is_card_dirty(HeapWord *p);
806 void mark_card_as_dirty(HeapWord *p);
807 void mark_range_as_dirty(HeapWord *p, size_t num_heap_words);
808 void mark_card_as_clean(HeapWord *p);
809 void mark_range_as_clean(HeapWord *p, size_t num_heap_words);
810 size_t cluster_count();
811
812 // Called by GC thread at start of concurrent mark to exchange roles of read and write remembered sets.
813 void swap_remset() { _rs->swap_remset(); }
814
815 void reset_remset(HeapWord* start, size_t word_count) { _rs->reset_remset(start, word_count); }
816
817 void merge_write_table(HeapWord* start, size_t word_count) { _rs->merge_write_table(start, word_count); }
818
819 // Called by GC thread after scanning old remembered set in order to prepare for next GC pass
820 void clear_old_remset() { _rs->clear_old_remset(); }
821
822 size_t cluster_for_addr(HeapWord *addr);
823 HeapWord* addr_for_cluster(size_t cluster_no);
824
825 void reset_object_range(HeapWord *from, HeapWord *to);
826 void register_object(HeapWord *addr);
827 void register_object_without_lock(HeapWord *addr);
828 void coalesce_objects(HeapWord *addr, size_t length_in_words);
829
830 HeapWord* first_object_in_card(size_t card_index) {
831 if (_scc->starts_object(card_index)) {
832 return addr_for_card_index(card_index) + _scc->get_first_start(card_index);
833 } else {
834 return nullptr;
835 }
836 }
837
838 // Return true iff this object is "properly" registered.
839 bool verify_registration(HeapWord* address, ShenandoahMarkingContext* ctx);
840
841 // clear the cards to clean, and clear the object_starts info to no objects
842 void mark_range_as_empty(HeapWord *addr, size_t length_in_words);
843
844 // process_clusters() scans a portion of the remembered set
845 // for references from old gen into young. Several worker threads
846 // scan different portions of the remembered set by making parallel invocations
847 // of process_clusters() with each invocation scanning different
848 // "clusters" of the remembered set.
849 //
850 // An invocation of process_clusters() examines all of the
851 // intergenerational references spanned by `count` clusters starting
852 // with `first_cluster`. The `oops` argument is a worker-thread-local
853 // OopClosure that is applied to all "valid" references in the remembered set.
854 //
855 // A side-effect of executing process_clusters() is to update the remembered
856 // set entries (e.g. marking dirty cards clean if they no longer
857 // hold references to young-gen memory).
858 //
859 // An implementation of process_clusters() may choose to efficiently
860 // address more typical scenarios in the structure of remembered sets. E.g.
861 // in the generational setting, one might expect remembered sets to be very sparse
862 // (low mutation rates in the old generation leading to sparse dirty cards,
863 // each with very few intergenerational pointers). Specific implementations
864 // may choose to degrade gracefully as the sparsity assumption fails to hold,
865 // such as when there are sudden spikes in (premature) promotion or in the
866 // case of an underprovisioned, poorly-tuned, or poorly-shaped heap.
867 //
868 // At the start of a concurrent young generation marking cycle, we invoke process_clusters
869 // with ClosureType ShenandoahInitMarkRootsClosure.
870 //
871 // At the start of a concurrent evacuation phase, we invoke process_clusters with
872 // ClosureType ShenandoahEvacuateUpdateRootsClosure.
873
874 // All template expansions require methods to be defined in the inline.hpp file, but larger
875 // such methods need not be declared as inline.
876 template <typename ClosureType>
877 void process_clusters(size_t first_cluster, size_t count, HeapWord *end_of_range, ClosureType *oops,
878 bool use_write_table, uint worker_id);
879
880 template <typename ClosureType>
881 inline void process_humongous_clusters(ShenandoahHeapRegion* r, size_t first_cluster, size_t count,
882 HeapWord *end_of_range, ClosureType *oops, bool use_write_table);
883
884 template <typename ClosureType>
885 inline void process_region_slice(ShenandoahHeapRegion* region, size_t offset, size_t clusters, HeapWord* end_of_range,
886 ClosureType *cl, bool use_write_table, uint worker_id);
887
888 // To Do:
889 // Create subclasses of ShenandoahInitMarkRootsClosure and
890 // ShenandoahEvacuateUpdateRootsClosure and any other closures
891 // that need to participate in remembered set scanning. Within the
892 // subclasses, add a (probably templated) instance variable that
893 // refers to the associated ShenandoahCardCluster object. Use this
894 // ShenandoahCardCluster instance to "enhance" the do_oops
895 // processing so that we can:
896 //
897 // 1. Avoid processing references that correspond to clean card
898 // regions, and
899 // 2. Set card status to CLEAN when the associated card region no
900 // longer holds inter-generatioanal references.
901 //
902 // To enable efficient implementation of these behaviors, we
903 // probably also want to add a few fields into the
904 // ShenandoahCardCluster object that allow us to precompute and
905 // remember the addresses at which card status is going to change
906 // from dirty to clean and clean to dirty. The do_oops
907 // implementations will want to update this value each time they
908 // cross one of these boundaries.
909 void roots_do(OopIterateClosure* cl);
910
911 // Log stats related to card/RS stats for given phase t
912 void log_card_stats(uint nworkers, CardStatLogType t) PRODUCT_RETURN;
913 private:
914 // Log stats for given worker id related into given summary card/RS stats
915 void log_worker_card_stats(uint worker_id, HdrSeq* sum_stats) PRODUCT_RETURN;
916
917 // Log given stats
918 inline void log_card_stats(HdrSeq* stats) PRODUCT_RETURN;
919
920 // Merge the stats from worked_id into the given summary stats, and clear the worker_id's stats.
921 void merge_worker_card_stats_cumulative(HdrSeq* worker_stats, HdrSeq* sum_stats) PRODUCT_RETURN;
922 };
923
924
925 // A ShenandoahRegionChunk represents a contiguous interval of a ShenandoahHeapRegion, typically representing
926 // work to be done by a worker thread.
927 struct ShenandoahRegionChunk {
928 ShenandoahHeapRegion *_r; // The region of which this represents a chunk
929 size_t _chunk_offset; // HeapWordSize offset
930 size_t _chunk_size; // HeapWordSize qty
931 };
932
933 // ShenandoahRegionChunkIterator divides the total remembered set scanning effort into ShenandoahRegionChunks
934 // that are assigned one at a time to worker threads. (Here, we use the terms `assignments` and `chunks`
935 // interchangeably.) Note that the effort required to scan a range of memory is not necessarily a linear
936 // function of the size of the range. Some memory ranges hold only a small number of live objects.
937 // Some ranges hold primarily primitive (non-pointer) data. We start with larger chunk sizes because larger chunks
938 // reduce coordination overhead. We expect that the GC worker threads that receive more difficult assignments
939 // will work longer on those chunks. Meanwhile, other worker will threads repeatedly accept and complete multiple
940 // easier chunks. As the total amount of work remaining to be completed decreases, we decrease the size of chunks
941 // given to individual threads. This reduces the likelihood of significant imbalance between worker thread assignments
942 // when there is less meaningful work to be performed by the remaining worker threads while they wait for
943 // worker threads with difficult assignments to finish, reducing the overall duration of the phase.
944
945 class ShenandoahRegionChunkIterator : public StackObj {
946 private:
947 // The largest chunk size is 4 MiB, measured in words. Otherwise, remembered set scanning may become too unbalanced.
948 // If the largest chunk size is too small, there is too much overhead sifting out assignments to individual worker threads.
949 static const size_t _maximum_chunk_size_words = (4 * 1024 * 1024) / HeapWordSize;
950
951 static const size_t _clusters_in_smallest_chunk = 4;
952
953 // smallest_chunk_size is 4 clusters. Each cluster spans 128 KiB.
954 // This is computed from CardTable::card_size_in_words() *
955 // ShenandoahCardCluster<ShenandoahDirectCardMarkRememberedSet>::CardsPerCluster;
956 static size_t smallest_chunk_size_words() {
957 return _clusters_in_smallest_chunk * CardTable::card_size_in_words() *
958 ShenandoahCardCluster<ShenandoahDirectCardMarkRememberedSet>::CardsPerCluster;
959 }
960
961 // The total remembered set scanning effort is divided into chunks of work that are assigned to individual worker tasks.
962 // The chunks of assigned work are divided into groups, where the size of the typical group (_regular_group_size) is half the
963 // total number of regions. The first group may be larger than
964 // _regular_group_size in the case that the first group's chunk
965 // size is less than the region size. The last group may be larger
966 // than _regular_group_size because no group is allowed to
967 // have smaller assignments than _smallest_chunk_size, which is 128 KB.
968
969 // Under normal circumstances, no configuration needs more than _maximum_groups (default value of 16).
970 // The first group "effectively" processes chunks of size 1 MiB (or smaller for smaller region sizes).
971 // The last group processes chunks of size 128 KiB. There are four groups total.
972
973 // group[0] is 4 MiB chunk size (_maximum_chunk_size_words)
974 // group[1] is 2 MiB chunk size
975 // group[2] is 1 MiB chunk size
976 // group[3] is 512 KiB chunk size
977 // group[4] is 256 KiB chunk size
978 // group[5] is 128 Kib shunk size (_smallest_chunk_size_words = 4 * 64 * 64
979 static const size_t _maximum_groups = 6;
980
981 const ShenandoahHeap* _heap;
982
983 const size_t _regular_group_size; // Number of chunks in each group
984 const size_t _first_group_chunk_size_b4_rebalance;
985 const size_t _num_groups; // Number of groups in this configuration
986 const size_t _total_chunks;
987
988 shenandoah_padding(0);
989 volatile size_t _index;
990 shenandoah_padding(1);
991
992 size_t _region_index[_maximum_groups]; // The region index for the first region spanned by this group
993 size_t _group_offset[_maximum_groups]; // The offset at which group begins within first region spanned by this group
994 size_t _group_chunk_size[_maximum_groups]; // The size of each chunk within this group
995 size_t _group_entries[_maximum_groups]; // Total chunks spanned by this group and the ones before it.
996
997 // No implicit copying: iterators should be passed by reference to capture the state
998 NONCOPYABLE(ShenandoahRegionChunkIterator);
999
1000 // Makes use of _heap.
1001 size_t calc_regular_group_size();
1002
1003 // Makes use of _regular_group_size, which must be initialized before call.
1004 size_t calc_first_group_chunk_size_b4_rebalance();
1005
1006 // Makes use of _regular_group_size and _first_group_chunk_size_b4_rebalance, both of which must be initialized before call.
1007 size_t calc_num_groups();
1008
1009 // Makes use of _regular_group_size, _first_group_chunk_size_b4_rebalance, which must be initialized before call.
1010 size_t calc_total_chunks();
1011
1012 public:
1013 ShenandoahRegionChunkIterator(size_t worker_count);
1014 ShenandoahRegionChunkIterator(ShenandoahHeap* heap, size_t worker_count);
1015
1016 // Reset iterator to default state
1017 void reset();
1018
1019 // Fills in assignment with next chunk of work and returns true iff there is more work.
1020 // Otherwise, returns false. This is multi-thread-safe.
1021 inline bool next(struct ShenandoahRegionChunk *assignment);
1022
1023 // This is *not* MT safe. However, in the absence of multithreaded access, it
1024 // can be used to determine if there is more work to do.
1025 inline bool has_next() const;
1026 };
1027
1028 typedef ShenandoahScanRemembered<ShenandoahDirectCardMarkRememberedSet> RememberedScanner;
1029
1030 class ShenandoahScanRememberedTask : public WorkerTask {
1031 private:
1032 ShenandoahObjToScanQueueSet* _queue_set;
1033 ShenandoahObjToScanQueueSet* _old_queue_set;
1034 ShenandoahReferenceProcessor* _rp;
1035 ShenandoahRegionChunkIterator* _work_list;
1036 bool _is_concurrent;
1037
1038 public:
1039 ShenandoahScanRememberedTask(ShenandoahObjToScanQueueSet* queue_set,
1040 ShenandoahObjToScanQueueSet* old_queue_set,
1041 ShenandoahReferenceProcessor* rp,
1042 ShenandoahRegionChunkIterator* work_list,
1043 bool is_concurrent);
1044
1045 void work(uint worker_id);
1046 void do_work(uint worker_id);
1047 };
1048
1049 // After Full GC is done, reconstruct the remembered set by iterating over OLD regions,
1050 // registering all objects between bottom() and top(), and dirtying the cards containing
1051 // cross-generational pointers.
1052 class ShenandoahReconstructRememberedSetTask : public WorkerTask {
1053 private:
1054 ShenandoahRegionIterator* _regions;
1055
1056 public:
1057 explicit ShenandoahReconstructRememberedSetTask(ShenandoahRegionIterator* regions);
1058
1059 void work(uint worker_id) override;
1060 };
1061
1062 #endif // SHARE_GC_SHENANDOAH_SHENANDOAHSCANREMEMBERED_HPP