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25
26 #ifndef SHARE_GC_SHENANDOAH_SHENANDOAHSCANREMEMBERED_HPP
27 #define SHARE_GC_SHENANDOAH_SHENANDOAHSCANREMEMBERED_HPP
28
29 // Terminology used within this source file:
30 //
31 // Card Entry: This is the information that identifies whether a
32 // particular card-table entry is Clean or Dirty. A clean
33 // card entry denotes that the associated memory does not
34 // hold references to young-gen memory.
35 //
36 // Card Region, aka
37 // Card Memory: This is the region of memory that is assocated with a
38 // particular card entry.
39 //
40 // Card Cluster: A card cluster represents 64 card entries. A card
41 // cluster is the minimal amount of work performed at a
42 // time by a parallel thread. Note that the work required
43 // to scan a card cluster is somewhat variable in that the
44 // required effort depends on how many cards are dirty, how
45 // many references are held within the objects that span a
46 // DIRTY card's memory, and on the size of the object
47 // that spans the end of a DIRTY card's memory (because
48 // that object will be scanned in its entirety). For these
49 // reasons, it is advisable for the multiple worker threads
50 // to be flexible in the number of clusters to be
51 // processed by each thread.
52 //
53 // A cluster represents a "natural" quantum of work to be performed by
54 // a parallel GC thread's background remembered set scanning efforts.
55 // The notion of cluster is similar to the notion of stripe in the
56 // implementation of parallel GC card scanning. However, a cluster is
57 // typically smaller than a stripe, enabling finer grain division of
58 // labor between multiple threads.
59 //
60 // For illustration, consider the following possible JVM configurations:
61 //
62 // Scenario 1:
63 // RegionSize is 128 MB
64 // Span of a card entry is 512 B
65 // Each card table entry consumes 1 B
66 // Assume one long word of card table entries represents a cluster.
67 // This long word holds 8 card table entries, spanning a
68 // total of 4KB
69 // The number of clusters per region is 128 MB / 4 KB = 32K
70 //
71 // Scenario 2:
72 // RegionSize is 128 MB
73 // Span of each card entry is 128 B
74 // Each card table entry consumes 1 bit
75 // Assume one int word of card tables represents a cluster.
76 // This int word holds 32 card table entries, spanning a
77 // total of 4KB
78 // The number of clusters per region is 128 MB / 4 KB = 32K
79 //
80 // Scenario 3:
81 // RegionSize is 128 MB
82 // Span of each card entry is 512 B
83 // Each card table entry consumes 1 bit
84 // Assume one long word of card tables represents a cluster.
85 // This long word holds 64 card table entries, spanning a
86 // total of 32 KB
87 // The number of clusters per region is 128 MB / 32 KB = 4K
88 //
89 // At the start of a new young-gen concurrent mark pass, the gang of
90 // Shenandoah worker threads collaborate in performing the following
91 // actions:
92 //
93 // Let old_regions = number of ShenandoahHeapRegion comprising
94 // old-gen memory
95 // Let region_size = ShenandoahHeapRegion::region_size_bytes()
96 // represent the number of bytes in each region
97 // Let clusters_per_region = region_size / 512
98 // Let rs represent the relevant RememberedSet implementation
99 // (an instance of ShenandoahDirectCardMarkRememberedSet or an instance
100 // of a to-be-implemented ShenandoahBufferWithSATBRememberedSet)
101 //
102 // for each ShenandoahHeapRegion old_region in the whole heap
103 // determine the cluster number of the first cluster belonging
104 // to that region
105 // for each cluster contained within that region
106 // Assure that exactly one worker thread initializes each
107 // cluster of overreach memory by invoking:
108 //
109 // rs->initialize_overreach(cluster_no, cluster_count)
110 //
111 // in separate threads. (Divide up the clusters so that
112 // different threads are responsible for initializing different
113 // clusters. Initialization cost is essentially identical for
114 // each cluster.)
115 //
116 // Next, we repeat the process for invocations of process_clusters.
117 // for each ShenandoahHeapRegion old_region in the whole heap
118 // determine the cluster number of the first cluster belonging
119 // to that region
120 // for each cluster contained within that region
121 // Assure that exactly one worker thread processes each
122 // cluster, each thread making a series of invocations of the
123 // following:
124 //
125 // rs->process_clusters(worker_id, ReferenceProcessor *,
126 // ShenandoahConcurrentMark *, cluster_no, cluster_count,
127 // HeapWord *end_of_range, OopClosure *oops);
128 //
129 // For efficiency, divide up the clusters so that different threads
130 // are responsible for processing different clusters. Processing costs
131 // may vary greatly between clusters for the following reasons:
132 //
133 // a) some clusters contain mostly dirty cards and other
134 // clusters contain mostly clean cards
135 // b) some clusters contain mostly primitive data and other
136 // clusters contain mostly reference data
137 // c) some clusters are spanned by very large objects that
138 // begin in some other cluster. When a large object
139 // beginning in a preceding cluster spans large portions of
140 // this cluster, the processing of this cluster gets a
141 // "free ride" because the thread responsible for processing
142 // the cluster that holds the object's header does the
143 // processing.
144 // d) in the case that the end of this cluster is spanned by a
145 // very large object, the processing of this cluster will
146 // be responsible for examining the entire object,
147 // potentially requiring this thread to process large amounts
148 // of memory pertaining to other clusters.
149 //
150 // Though an initial division of labor between marking threads may
151 // assign equal numbers of clusters to be scanned by each thread, it
152 // should be expected that some threads will finish their assigned
153 // work before others. Therefore, some amount of the full remembered
154 // set scanning effort should be held back and assigned incrementally
155 // to the threads that end up with excess capacity. Consider the
156 // following strategy for dividing labor:
157 //
158 // 1. Assume there are 8 marking threads and 1024 remembered
159 // set clusters to be scanned.
160 // 2. Assign each thread to scan 64 clusters. This leaves
161 // 512 (1024 - (8*64)) clusters to still be scanned.
162 // 3. As the 8 server threads complete previous cluster
163 // scanning assignments, issue each of the next 8 scanning
164 // assignments as units of 32 additional cluster each.
165 // In the case that there is high variance in effort
166 // associated with previous cluster scanning assignments,
167 // multiples of these next assignments may be serviced by
168 // the server threads that were previously assigned lighter
169 // workloads.
170 // 4. Make subsequent scanning assignments as follows:
171 // a) 8 assignments of size 16 clusters
172 // b) 8 assignments of size 8 clusters
173 // c) 16 assignments of size 4 clusters
174 //
175 // When there is no more remembered set processing work to be
176 // assigned to a newly idled worker thread, that thread can move
177 // on to work on other tasks associated with root scanning until such
178 // time as all clusters have been examined.
179 //
180 // Once all clusters have been processed, the gang of GC worker
181 // threads collaborate to merge the overreach data.
182 //
183 // for each ShenandoahHeapRegion old_region in the whole heap
184 // determine the cluster number of the first cluster belonging
185 // to that region
186 // for each cluster contained within that region
187 // Assure that exactly one worker thread initializes each
188 // cluster of overreach memory by invoking:
189 //
190 // rs->merge_overreach(cluster_no, cluster_count)
191 //
192 // in separate threads. (Divide up the clusters so that
193 // different threads are responsible for merging different
194 // clusters. Merging cost is essentially identical for
195 // each cluster.)
196 //
197 // Though remembered set scanning is designed to run concurrently with
198 // mutator threads, the current implementation of remembered set
199 // scanning runs in parallel during a GC safepoint. Furthermore, the
200 // current implementation of remembered set scanning never clears a
201 // card once it has been marked. Since the current implementation
202 // never clears marked pages, the current implementation does not
203 // invoke initialize_overreach() or merge_overreach().
204 //
205 // These limitations will be addressed in future enhancements to the
206 // existing implementation.
207
208 #include <stdint.h>
209 #include "memory/iterator.hpp"
210 #include "gc/shared/workgroup.hpp"
211 #include "gc/shenandoah/shenandoahCardTable.hpp"
212 #include "gc/shenandoah/shenandoahHeapRegion.hpp"
213 #include "gc/shenandoah/shenandoahTaskqueue.hpp"
214
215 class ShenandoahReferenceProcessor;
216 class ShenandoahConcurrentMark;
217 class ShenandoahHeap;
218 class ShenandoahRegionIterator;
219 class ShenandoahMarkingContext;
220
221 class CardTable;
222
223 class ShenandoahDirectCardMarkRememberedSet: public CHeapObj<mtGC> {
224
225 private:
226
227 // Use symbolic constants defined in cardTable.hpp
228 // CardTable::card_shift = 9;
229 // CardTable::card_size = 512;
230 // CardTable::card_size_in_words = 64;
231
232 // CardTable::clean_card_val()
233 // CardTable::dirty_card_val()
234
235 ShenandoahHeap *_heap;
236 ShenandoahCardTable *_card_table;
237 size_t _card_shift;
238 size_t _total_card_count;
239 size_t _cluster_count;
240 HeapWord *_whole_heap_base; // Points to first HeapWord of data contained within heap memory
241 HeapWord *_whole_heap_end;
242 uint8_t *_byte_map; // Points to first entry within the card table
243 uint8_t *_byte_map_base; // Points to byte_map minus the bias computed from address of heap memory
244 uint8_t *_overreach_map; // Points to first entry within the overreach card table
245 uint8_t *_overreach_map_base; // Points to overreach_map minus the bias computed from address of heap memory
246
247 uint64_t _wide_clean_value;
248
249 public:
250 // count is the number of cards represented by the card table.
251 ShenandoahDirectCardMarkRememberedSet(ShenandoahCardTable *card_table, size_t total_card_count);
252 ~ShenandoahDirectCardMarkRememberedSet();
253
254 // Card index is zero-based relative to _byte_map.
255 size_t total_cards();
256 size_t card_index_for_addr(HeapWord *p);
257 HeapWord *addr_for_card_index(size_t card_index);
258 bool is_card_dirty(size_t card_index);
259 bool is_write_card_dirty(size_t card_index);
260 void mark_card_as_dirty(size_t card_index);
261 void mark_range_as_dirty(size_t card_index, size_t num_cards);
262 void mark_card_as_clean(size_t card_index);
263 void mark_read_card_as_clean(size_t card_index);
264 void mark_range_as_clean(size_t card_index, size_t num_cards);
265 void mark_overreach_card_as_dirty(size_t card_index);
266 bool is_card_dirty(HeapWord *p);
267 void mark_card_as_dirty(HeapWord *p);
268 void mark_range_as_dirty(HeapWord *p, size_t num_heap_words);
269 void mark_card_as_clean(HeapWord *p);
270 void mark_range_as_clean(HeapWord *p, size_t num_heap_words);
271 void mark_overreach_card_as_dirty(void *p);
272 size_t cluster_count();
273
274 // Called by multiple GC threads at start of concurrent mark and evacuation phases. Each parallel GC thread typically
275 // initializes a different subranges of all overreach entries.
276 void initialize_overreach(size_t first_cluster, size_t count);
277
278 // Called by GC thread at end of concurrent mark or evacuation phase. Each parallel GC thread typically merges different
279 // subranges of all overreach entries.
280 void merge_overreach(size_t first_cluster, size_t count);
281
282 // Called by GC thread at start of concurrent mark to exchange roles of read and write remembered sets.
283 // Not currently used because mutator write barrier does not honor changes to the location of card table.
284 void swap_remset() { _card_table->swap_card_tables(); }
285
286 HeapWord* whole_heap_base() { return _whole_heap_base; }
287 HeapWord* whole_heap_end() { return _whole_heap_end; }
288
289 // Instead of swap_remset, the current implementation of concurrent remembered set scanning does reset_remset
290 // in parallel threads, each invocation processing one entire HeapRegion at a time. Processing of a region
291 // consists of copying the write table to the read table and cleaning the write table.
292 void reset_remset(HeapWord* start, size_t word_count) {
293 size_t card_index = card_index_for_addr(start);
294 size_t num_cards = word_count / CardTable::card_size_in_words;
295 size_t iterations = num_cards / (sizeof (intptr_t) / sizeof (CardTable::CardValue));
296 intptr_t* read_table_ptr = (intptr_t*) &(_card_table->read_byte_map())[card_index];
297 intptr_t* write_table_ptr = (intptr_t*) &(_card_table->write_byte_map())[card_index];
298 for (size_t i = 0; i < iterations; i++) {
299 *read_table_ptr++ = *write_table_ptr;
300 *write_table_ptr++ = CardTable::clean_card_row_val();
301 }
302 }
303
304 // Called by GC thread after scanning old remembered set in order to prepare for next GC pass
305 void clear_old_remset() { _card_table->clear_read_table(); }
306
307 };
308
309 // A ShenandoahCardCluster represents the minimal unit of work
310 // performed by independent parallel GC threads during scanning of
311 // remembered sets.
312 //
313 // The GC threads that perform card-table remembered set scanning may
314 // overwrite card-table entries to mark them as clean in the case that
315 // the associated memory no longer holds references to young-gen
316 // memory. Rather than access the card-table entries directly, all GC
317 // thread access to card-table information is made by way of the
318 // ShenandoahCardCluster data abstraction. This abstraction
319 // effectively manages access to multiple possible underlying
320 // remembered set implementations, including a traditional card-table
321 // approach and a SATB-based approach.
322 //
323 // The API services represent a compromise between efficiency and
324 // convenience.
325 //
326 // In the initial implementation, we assume that scanning of card
327 // table entries occurs only while the JVM is at a safe point. Thus,
328 // there is no synchronization required between GC threads that are
329 // scanning card-table entries and marking certain entries that were
330 // previously dirty as clean, and mutator threads which would possibly
331 // be marking certain card-table entries as dirty.
332 //
333 // There is however a need to implement concurrency control and memory
334 // coherency between multiple GC threads that scan the remembered set
335 // in parallel. The desire is to divide the complete scanning effort
336 // into multiple clusters of work that can be independently processed
337 // by individual threads without need for synchronizing efforts
338 // between the work performed by each task. The term "cluster" of
339 // work is similar to the term "stripe" as used in the implementation
340 // of Parallel GC.
341 //
342 // Complexity arises when an object to be scanned crosses the boundary
343 // between adjacent cluster regions. Here is the protocol that is
344 // followed:
345 //
346 // 1. We implement a supplemental data structure known as the overreach
347 // card table. The thread that is responsible for scanning each
348 // cluster of card-table entries is granted exclusive access to
349 // modify the associated card-table entries. In the case that a
350 // thread scans a very large object that reaches into one or more
351 // following clusters, that thread has exclusive access to the
352 // overreach card table for all of the entries belonging to the
353 // following clusters that are spanned by this large object.
354 // After all clusters have been scanned, the scanning threads
355 // briefly synchronize to merge the contents of the overreach
356 // entries with the traditional card table entries using logical-
357 // and operations.
358 // 2. Every object is scanned in its "entirety" by the thread that is
359 // responsible for the cluster that holds its starting address.
360 // Entirety is in quotes because there are various situations in
361 // which some portions of the object will not be scanned by this
362 // thread:
363 // a) If an object spans multiple card regions, all of which are
364 // contained within the same cluster, the scanning thread
365 // consults the existing card-table entries and does not scan
366 // portions of the object that are not currently dirty.
367 // b) For any cluster that is spanned in its entirety by a very
368 // large object, the GC thread that scans this object assumes
369 // full responsibility for maintenance of the associated
370 // card-table entries.
371 // c) If a cluster is partially spanned by an object originating
372 // in a preceding cluster, the portion of the object that
373 // partially spans the following cluster is scanned in its
374 // entirety (because the thread that is responsible for
375 // scanning the object cannot rely upon the card-table entries
376 // associated with the following cluster). Whenever references
377 // to young-gen memory are found within the scanned data, the
378 // associated overreach card table entries are marked as dirty
379 // by the scanning thread.
380 // 3. If a cluster is spanned in its entirety by an object that
381 // originates within a preceding cluster's memory, the thread
382 // assigned to examine this cluster does absolutely nothing. The
383 // thread assigned to scan the cluster that holds the object's
384 // starting address takes full responsibility for scanning the
385 // entire object and updating the associated card-table entries.
386 // 4. If a cluster is spanned partially by an object that originates
387 // within a preceding cluster's memory, the thread assigned to
388 // examine this cluster marks the card-table entry as clean for
389 // each card table that is fully spanned by this overreaching
390 // object. If a card-table entry's memory is partially spanned
391 // by the overreaching object, the thread sets the card-table
392 // entry to clean if it was previously dirty and if the portion
393 // of the card-table entry's memory that is not spanned by the
394 // overreaching object does not hold pointers to young-gen
395 // memory.
396 // 5. While examining a particular card belonging to a particular
397 // cluster, if an object reaches beyond the end of its card
398 // memory, the thread "scans" all portions of the object that
399 // correspond to DIRTY card entries within the current cluster and
400 // all portions of the object that reach into following clustesr.
401 // After this object is scanned, continue scanning with the memory
402 // that follows this object if this memory pertains to the same
403 // cluster. Otherwise, consider this cluster's memory to have
404 // been fully examined.
405 //
406 // Discussion:
407 // Though this design results from careful consideration of multiple
408 // design objectives, it is subject to various criticisms. Some
409 // discussion of the design choices is provided here:
410 //
411 // 1. Note that remembered sets are a heuristic technique to avoid
412 // the need to scan all of old-gen memory with each young-gen
413 // collection. If we sometimes scan a bit more memory than is
414 // absolutely necessary, that should be considered a reasonable
415 // compromise. This compromise is already present in the sizing
416 // of card table memory areas. Note that a single dirty pointer
417 // within a 512-byte card region forces the "unnecessary" scanning
418 // of 63 = ((512 - 8 = 504) / 8) pointers.
419 // 2. One undesirable aspect of this design is that we sometimes have
420 // to scan large amounts of memory belonging to very large
421 // objects, even for parts of the very large object that do not
422 // correspond to dirty card table entries. Note that this design
423 // limits the amount of non-dirty scanning that might have to
424 // be performed for these very large objects. In particular, only
425 // the last part of the very large object that extends into but
426 // does not completely span a particular cluster is unnecessarily
427 // scanned. Thus, for each very large object, the maximum
428 // over-scan is the size of memory spanned by a single cluster.
429 // 3. The representation of pointer location descriptive information
430 // within Klass representations is not designed for efficient
431 // "random access". An alternative approach to this design would
432 // be to scan very large objects multiple times, once for each
433 // cluster that is spanned by the object's range. This reduces
434 // unnecessary overscan, but it introduces different sorts of
435 // overhead effort:
436 // i) For each spanned cluster, we have to look up the start of
437 // the crossing object.
438 // ii) Each time we scan the very large object, we have to
439 // sequentially walk through its pointer location
440 // descriptors, skipping over all of the pointers that
441 // precede the start of the range of addresses that we
442 // consider relevant.
443
444
445 // Because old-gen heap memory is not necessarily contiguous, and
446 // because cards are not necessarily maintained for young-gen memory,
447 // consecutive card numbers do not necessarily correspond to consecutive
448 // address ranges. For the traditional direct-card-marking
449 // implementation of this interface, consecutive card numbers are
450 // likely to correspond to contiguous regions of memory, but this
451 // should not be assumed. Instead, rely only upon the following:
452 //
453 // 1. All card numbers for cards pertaining to the same
454 // ShenandoahHeapRegion are consecutively numbered.
455 // 2. In the case that neighboring ShenandoahHeapRegions both
456 // represent old-gen memory, the card regions that span the
457 // boundary between these neighboring heap regions will be
458 // consecutively numbered.
459 // 3. (A corollary) In the case that an old-gen object spans the
460 // boundary between two heap regions, the card regions that
461 // correspond to the span of this object will be consecutively
462 // numbered.
463
464
465 // ShenandoahCardCluster abstracts access to the remembered set
466 // and also keeps track of crossing map information to allow efficient
467 // resolution of object start addresses.
468 //
469 // ShenandoahCardCluster supports all of the services of
470 // RememberedSet, plus it supports register_object() and lookup_object().
471 //
472 // There are two situations under which we need to know the location
473 // at which the object spanning the start of a particular card-table
474 // memory region begins:
475 //
476 // 1. When we begin to scan dirty card memory that is not the
477 // first card region within a cluster, and the object that
478 // crosses into this card memory was not previously scanned,
479 // we need to find where that object starts so we can scan it.
480 // (Asides: if the objects starts within a previous cluster, it
481 // has already been scanned. If the object starts within this
482 // cluster and it spans at least one card region that is dirty
483 // and precedes this card region within the cluster, then it has
484 // already been scanned.)
485 // 2. When we are otherwise done scanning a complete cluster, if the
486 // last object within the cluster reaches into the following
487 // cluster, we need to scan this object. Thus, we need to find
488 // its starting location.
489 //
490 // The RememberedSet template parameter is intended to represent either
491 // ShenandoahDirectCardMarkRememberedSet, or a to-be-implemented
492 // ShenandoahBufferWithSATBRememberedSet.
493 template<typename RememberedSet>
494 class ShenandoahCardCluster: public CHeapObj<mtGC> {
495
496 private:
497 RememberedSet *_rs;
498
499 public:
500 static const size_t CardsPerCluster = 64;
501
502 private:
503 // This bit is set iff at least one object starts within a
504 // particular card region.
505 static const uint16_t ObjectStartsInCardRegion = 0x8000;
506 static const uint16_t FirstStartBits = 0x003f;
507 static const uint16_t LastStartBits = 0x0fc0;
508 static const uint16_t FirstStartShift = 0;
509 static const uint16_t LastStartShift = 6;
510
511 uint16_t *object_starts;
512
513 public:
514 inline void set_first_start(size_t card_index, uint8_t value) {
515 object_starts[card_index] &= ~FirstStartBits;
516 object_starts[card_index] |= (FirstStartBits & (value << FirstStartShift));
517 }
518
519 inline void set_last_start(size_t card_index, uint8_t value) {
520 object_starts[card_index] &= ~LastStartBits;
521 object_starts[card_index] |= (LastStartBits & (value << LastStartShift));
522 }
523
524 inline void set_has_object_bit(size_t card_index) {
525 object_starts[card_index] |= ObjectStartsInCardRegion;
526 }
527
528 inline void clear_has_object_bit(size_t card_index) {
529 object_starts[card_index] &= ~ObjectStartsInCardRegion;
530 }
531
532 inline void clear_objects_in_range(HeapWord *addr, size_t num_words) {
533 size_t card_index = _rs->card_index_for_addr(addr);
534 size_t last_card_index = _rs->card_index_for_addr(addr + num_words - 1);
535 while (card_index <= last_card_index)
536 object_starts[card_index++] = 0;
537 }
538
539 ShenandoahCardCluster(RememberedSet *rs) {
540 _rs = rs;
541 // TODO: We don't really need object_starts entries for every card entry. We only need these for
542 // the card entries that correspond to old-gen memory. But for now, let's be quick and dirty.
543 object_starts = (uint16_t *) malloc(rs->total_cards() * sizeof(uint16_t));
544 if (object_starts == NULL)
545 fatal("Insufficient memory for initializing heap");
546 for (size_t i = 0; i < rs->total_cards(); i++)
547 object_starts[i] = 0;
548 }
549
550 ~ShenandoahCardCluster() {
551 if (object_starts != NULL)
552 free(object_starts);
553 }
554
555 // There is one entry within the object_starts array for each
556 // card entry. The interpretation of the data contained within each
557 // object_starts entry is described below:
558 //
559 // Bits 0x003f: Value ranges from 0-63, which is multiplied by 8
560 // to obtain the offset at which the first object
561 // beginning within this card region begins.
562 // Bits 0x0fc0: Value ranges from 0-63, which is multiplied by 8 to
563 // obtain the offset at which the last object beginning
564 // within this card region begins.
565 // Bits 0x8000: This bit is on if an object starts within this card
566 // region.
567 // Bits 0x7000: Reserved for future uses
568 //
569 // In the most recent implementation of ShenandoahScanRemembered::process_clusters(),
570 // there is no need for the get_crossing_object_start() method function, so there is no
571 // need to maintain the following information. The comment is left in place for now in
572 // case we find it necessary to add support for this service at a later time.
573 //
574 // Bits 0x7fff: If no object starts within this card region, the
575 // remaining bits of the object_starts array represent
576 // the absolute word offset within the enclosing
577 // cluster's memory of the starting address for the
578 // object that spans the start of this card region's
579 // memory. If the spanning object begins in memory
580 // that precedes this card region's cluster, the value
581 // stored in these bits is the special value 0x7fff.
582 // (Note that the maximum value required to represent a
583 // spanning object from within the current cluster is
584 // ((63 * 64) - 8), which equals 0x0fbf.
585 //
586 // In the absence of the need to support get_crossing_object_start(),
587 // here is discussion of performance:
588 //
589 // Suppose multiple garbage objects are coalesced during GC sweep
590 // into a single larger "free segment". As each two objects are
591 // coalesced together, the start information pertaining to the second
592 // object must be removed from the objects_starts array. If the
593 // second object had been been the first object within card memory,
594 // the new first object is the object that follows that object if
595 // that starts within the same card memory, or NoObject if the
596 // following object starts within the following cluster. If the
597 // second object had been the last object in the card memory,
598 // replace this entry with the newly coalesced object if it starts
599 // within the same card memory, or with NoObject if it starts in a
600 // preceding card's memory.
601 //
602 // Suppose a large free segment is divided into a smaller free
603 // segment and a new object. The second part of the newly divided
604 // memory must be registered as a new object, overwriting at most
605 // one first_start and one last_start entry. Note that one of the
606 // newly divided two objects might be a new GCLAB.
607 //
608 // Suppose postprocessing of a GCLAB finds that the original GCLAB
609 // has been divided into N objects. Each of the N newly allocated
610 // objects will be registered, overwriting at most one first_start
611 // and one last_start entries.
612 //
613 // No object registration operations are linear in the length of
614 // the registered objects.
615 //
616 // Consider further the following observations regarding object
617 // registration costs:
618 //
619 // 1. The cost is paid once for each old-gen object (Except when
620 // an object is demoted and repromoted, in which case we would
621 // pay the cost again).
622 // 2. The cost can be deferred so that there is no urgency during
623 // mutator copy-on-first-access promotion. Background GC
624 // threads will update the object_starts array by post-
625 // processing the contents of retired GCLAB buffers.
626 // 3. The bet is that these costs are paid relatively rarely
627 // because:
628 // a) Most objects die young and objects that die in young-gen
629 // memory never need to be registered with the object_starts
630 // array.
631 // b) Most objects that are promoted into old-gen memory live
632 // there without further relocation for a relatively long
633 // time, so we get a lot of benefit from each investment
634 // in registering an object.
635
636 public:
637
638 // The starting locations of objects contained within old-gen memory
639 // are registered as part of the remembered set implementation. This
640 // information is required when scanning dirty card regions that are
641 // spanned by objects beginning within preceding card regions. It
642 // is necessary to find the first and last objects that begin within
643 // this card region. Starting addresses of objects are required to
644 // find the object headers, and object headers provide information
645 // about which fields within the object hold addresses.
646 //
647 // The old-gen memory allocator invokes register_object() for any
648 // object that is allocated within old-gen memory. This identifies
649 // the starting addresses of objects that span boundaries between
650 // card regions.
651 //
652 // It is not necessary to invoke register_object at the very instant
653 // an object is allocated. It is only necessary to invoke it
654 // prior to the next start of a garbage collection concurrent mark
655 // or concurrent update-references phase. An "ideal" time to register
656 // objects is during post-processing of a GCLAB after the GCLAB is
657 // retired due to depletion of its memory.
658 //
659 // register_object() does not perform synchronization. In the case
660 // that multiple threads are registering objects whose starting
661 // addresses are within the same cluster, races between these
662 // threads may result in corruption of the object-start data
663 // structures. Parallel GC threads should avoid registering objects
664 // residing within the same cluster by adhering to the following
665 // coordination protocols:
666 //
667 // 1. Align thread-local GCLAB buffers with some TBD multiple of
668 // card clusters. The card cluster size is 32 KB. If the
669 // desired GCLAB size is 128 KB, align the buffer on a multiple
670 // of 4 card clusters.
671 // 2. Post-process the contents of GCLAB buffers to register the
672 // objects allocated therein. Allow one GC thread at a
673 // time to do the post-processing of each GCLAB.
674 // 3. Since only one GC thread at a time is registering objects
675 // belonging to a particular allocation buffer, no locking
676 // is performed when registering these objects.
677 // 4. Any remnant of unallocated memory within an expended GC
678 // allocation buffer is not returned to the old-gen allocation
679 // pool until after the GC allocation buffer has been post
680 // processed. Before any remnant memory is returned to the
681 // old-gen allocation pool, the GC thread that scanned this GC
682 // allocation buffer performs a write-commit memory barrier.
683 // 5. Background GC threads that perform tenuring of young-gen
684 // objects without a GCLAB use a CAS lock before registering
685 // each tenured object. The CAS lock assures both mutual
686 // exclusion and memory coherency/visibility. Note that an
687 // object tenured by a background GC thread will not overlap
688 // with any of the clusters that are receiving tenured objects
689 // by way of GCLAB buffers. Multiple independent GC threads may
690 // attempt to tenure objects into a shared cluster. This is why
691 // sychronization may be necessary. Consider the following
692 // scenarios:
693 //
694 // a) If two objects are tenured into the same card region, each
695 // registration may attempt to modify the first-start or
696 // last-start information associated with that card region.
697 // Furthermore, because the representations of first-start
698 // and last-start information within the object_starts array
699 // entry uses different bits of a shared uint_16 to represent
700 // each, it is necessary to lock the entire card entry
701 // before modifying either the first-start or last-start
702 // information within the entry.
703 // b) Suppose GC thread X promotes a tenured object into
704 // card region A and this tenured object spans into
705 // neighboring card region B. Suppose GC thread Y (not equal
706 // to X) promotes a tenured object into cluster B. GC thread X
707 // will update the object_starts information for card A. No
708 // synchronization is required.
709 // c) In summary, when background GC threads register objects
710 // newly tenured into old-gen memory, they must acquire a
711 // mutual exclusion lock on the card that holds the starting
712 // address of the newly tenured object. This can be achieved
713 // by using a CAS instruction to assure that the previous
714 // values of first-offset and last-offset have not been
715 // changed since the same thread inquired as to their most
716 // current values.
717 //
718 // One way to minimize the need for synchronization between
719 // background tenuring GC threads is for each tenuring GC thread
720 // to promote young-gen objects into distinct dedicated cluster
721 // ranges.
722 // 6. The object_starts information is only required during the
723 // starting of concurrent marking and concurrent evacuation
724 // phases of GC. Before we start either of these GC phases, the
725 // JVM enters a safe point and all GC threads perform
726 // commit-write barriers to assure that access to the
727 // object_starts information is coherent.
728
729
730 // Notes on synchronization of register_object():
731 //
732 // 1. For efficiency, there is no locking in the implementation of register_object()
733 // 2. Thus, it is required that users of this service assure that concurrent/parallel invocations of
734 // register_object() do pertain to the same card's memory range. See discussion below to undestand
735 // the risks.
736 // 3. When allocating from a TLAB or GCLAB, the mutual exclusion can be guaranteed by assuring that each
737 // LAB's start and end are aligned on card memory boundaries.
738 // 4. Use the same lock that guarantees exclusivity when performing free-list allocation within heap regions.
739 //
740 // Register the newly allocated object while we're holding the global lock since there's no synchronization
741 // built in to the implementation of register_object(). There are potential races when multiple independent
742 // threads are allocating objects, some of which might span the same card region. For example, consider
743 // a card table's memory region within which three objects are being allocated by three different threads:
744 //
745 // objects being "concurrently" allocated:
746 // [-----a------][-----b-----][--------------c------------------]
747 // [---- card table memory range --------------]
748 //
749 // Before any objects are allocated, this card's memory range holds no objects. Note that:
750 // allocation of object a wants to set the has-object, first-start, and last-start attributes of the preceding card region.
751 // allocation of object b wants to set the has-object, first-start, and last-start attributes of this card region.
752 // allocation of object c also wants to set the has-object, first-start, and last-start attributes of this card region.
753 //
754 // The thread allocating b and the thread allocating c can "race" in various ways, resulting in confusion, such as last-start
755 // representing object b while first-start represents object c. This is why we need to require all register_object()
756 // invocations associated with objects that are allocated from "free lists" to provide their own mutual exclusion locking
757 // mechanism.
758
759 // Reset the has_object() information to false for all cards in the range between from and to.
760 void reset_object_range(HeapWord *from, HeapWord *to);
761
762 // register_object() requires that the caller hold the heap lock
763 // before calling it.
764 void register_object(HeapWord* address);
765
766 // register_object_wo_lock() does not require that the caller hold
767 // the heap lock before calling it, under the assumption that the
768 // caller has assure no other thread will endeavor to concurrently
769 // register objects that start within the same card's memory region
770 // as address.
771 void register_object_wo_lock(HeapWord* address);
772
773 // During the reference updates phase of GC, we walk through each old-gen memory region that was
774 // not part of the collection set and we invalidate all unmarked objects. As part of this effort,
775 // we coalesce neighboring dead objects in order to make future remembered set scanning more
776 // efficient (since future remembered set scanning of any card region containing consecutive
777 // dead objects can skip over all of them at once by reading only a single dead object header
778 // instead of having to read the header of each of the coalesced dead objects.
779 //
780 // At some future time, we may implement a further optimization: satisfy future allocation requests
781 // by carving new objects out of the range of memory that represents the coalesced dead objects.
782 //
783 // Suppose we want to combine several dead objects into a single coalesced object. How does this
784 // impact our representation of crossing map information?
785 // 1. If the newly coalesced range is contained entirely within a card range, that card's last
786 // start entry either remains the same or it is changed to the start of the coalesced region.
787 // 2. For the card that holds the start of the coalesced object, it will not impact the first start
788 // but it may impact the last start.
789 // 3. For following cards spanned entirely by the newly coalesced object, it will change has_object
790 // to false (and make first-start and last-start "undefined").
791 // 4. For a following card that is spanned patially by the newly coalesced object, it may change
792 // first-start value, but it will not change the last-start value.
793 //
794 // The range of addresses represented by the arguments to coalesce_objects() must represent a range
795 // of memory that was previously occupied exactly by one or more previously registered objects. For
796 // convenience, it is legal to invoke coalesce_objects() with arguments that span a single previously
797 // registered object.
798 //
799 // The role of coalesce_objects is to change the crossing map information associated with all of the coalesced
800 // objects.
801 void coalesce_objects(HeapWord* address, size_t length_in_words);
802
803 // The typical use case is going to look something like this:
804 // for each heapregion that comprises old-gen memory
805 // for each card number that corresponds to this heap region
806 // scan the objects contained therein if the card is dirty
807 // To avoid excessive lookups in a sparse array, the API queries
808 // the card number pertaining to a particular address and then uses the
809 // card noumber for subsequent information lookups and stores.
810
811 // Returns true iff an object is known to start within the card memory
812 // associated with addr p.
813 // Returns true iff an object is known to start within the card memory
814 // associated with addr p.
815 bool has_object(size_t card_index);
816
817 // If has_object(card_index), this returns the word offset within this card
818 // memory at which the first object begins. If !has_object(card_index), the
819 // result is a don't care value.
820 size_t get_first_start(size_t card_index);
821
822 // If has_object(card_index), this returns the word offset within this card
823 // memory at which the last object begins. If !has_object(card_index), the
824 // result is a don't care value.
825 size_t get_last_start(size_t card_index);
826
827 };
828
829 // ShenandoahScanRemembered is a concrete class representing the
830 // ability to scan the old-gen remembered set for references to
831 // objects residing in young-gen memory.
832 //
833 // Scanning normally begins with an invocation of numRegions and ends
834 // after all clusters of all regions have been scanned.
835 //
836 // Throughout the scanning effort, the number of regions does not
837 // change.
838 //
839 // Even though the regions that comprise old-gen memory are not
840 // necessarily contiguous, the abstraction represented by this class
841 // identifies each of the old-gen regions with an integer value
842 // in the range from 0 to (numRegions() - 1) inclusive.
843 //
844
845 template<typename RememberedSet>
846 class ShenandoahScanRemembered: public CHeapObj<mtGC> {
847
848 private:
849
850 RememberedSet* _rs;
851 ShenandoahCardCluster<RememberedSet>* _scc;
852
853 public:
854 // How to instantiate this object?
855 // ShenandoahDirectCardMarkRememberedSet *rs =
856 // new ShenandoahDirectCardMarkRememberedSet();
857 // scr = new
858 // ShenandoahScanRememberd<ShenandoahDirectCardMarkRememberedSet>(rs);
859 //
860 // or, after the planned implementation of
861 // ShenandoahBufferWithSATBRememberedSet has been completed:
862 //
863 // ShenandoahBufferWithSATBRememberedSet *rs =
864 // new ShenandoahBufferWithSATBRememberedSet();
865 // scr = new
866 // ShenandoahScanRememberd<ShenandoahBufferWithSATBRememberedSet>(rs);
867
868
869 ShenandoahScanRemembered(RememberedSet *rs) {
870 _rs = rs;
871 _scc = new ShenandoahCardCluster<RememberedSet>(rs);
872 }
873
874 ~ShenandoahScanRemembered() {
875 delete _scc;
876 }
877
878 // TODO: We really don't want to share all of these APIs with arbitrary consumers of the ShenandoahScanRemembered abstraction.
879 // 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
880 // of existing code already depends on having access to these services (because existing code has not been written to honor
881 // full abstraction of remembered set scanning. In the not too distant future, we want to try to make most, if not all, of
882 // these services private. Two problems with publicizing:
883 // 1. Allowing arbitrary users to reach beneath the hood allows the users to make assumptions about underlying implementation.
884 // This will make it more difficult to change underlying implementation at a future time, such as when we eventually experiment
885 // with SATB-based implementation of remembered set representation.
886 // 2. If we carefully control sharing of certain of these services, we can reduce the overhead of synchronization by assuring
887 // that all users follow protocols that avoid contention that might require synchronization. When we publish these APIs, we
888 // lose control over who and how the data is accessed. As a result, we are required to insert more defensive measures into
889 // the implementation, including synchronization locks.
890
891
892 // Card index is zero-based relative to first spanned card region.
893 size_t total_cards();
894 size_t card_index_for_addr(HeapWord *p);
895 HeapWord *addr_for_card_index(size_t card_index);
896 bool is_card_dirty(size_t card_index);
897 bool is_write_card_dirty(size_t card_index) { return _rs->is_write_card_dirty(card_index); }
898 void mark_card_as_dirty(size_t card_index);
899 void mark_range_as_dirty(size_t card_index, size_t num_cards);
900 void mark_card_as_clean(size_t card_index);
901 void mark_read_card_as_clean(size_t card_index) { _rs->mark_read_card_clean(card_index); }
902 void mark_range_as_clean(size_t card_index, size_t num_cards);
903 void mark_overreach_card_as_dirty(size_t card_index);
904 bool is_card_dirty(HeapWord *p);
905 void mark_card_as_dirty(HeapWord *p);
906 void mark_range_as_dirty(HeapWord *p, size_t num_heap_words);
907 void mark_card_as_clean(HeapWord *p);
908 void mark_range_as_clean(HeapWord *p, size_t num_heap_words);
909 void mark_overreach_card_as_dirty(void *p);
910 size_t cluster_count();
911 void initialize_overreach(size_t first_cluster, size_t count);
912 void merge_overreach(size_t first_cluster, size_t count);
913
914 // Called by GC thread at start of concurrent mark to exchange roles of read and write remembered sets.
915 void swap_remset() { _rs->swap_remset(); }
916
917 void reset_remset(HeapWord* start, size_t word_count) { _rs->reset_remset(start, word_count); }
918
919 // Called by GC thread after scanning old remembered set in order to prepare for next GC pass
920 void clear_old_remset() { _rs->clear_old_remset(); }
921
922 size_t cluster_for_addr(HeapWord *addr);
923
924 void reset_object_range(HeapWord *from, HeapWord *to);
925 void register_object(HeapWord *addr);
926 void register_object_wo_lock(HeapWord *addr);
927 void coalesce_objects(HeapWord *addr, size_t length_in_words);
928
929 // Return true iff this object is "properly" registered.
930 bool verify_registration(HeapWord* address, ShenandoahMarkingContext* ctx);
931
932 // clear the cards to clean, and clear the object_starts info to no objects
933 void mark_range_as_empty(HeapWord *addr, size_t length_in_words);
934
935 // process_clusters() scans a portion of the remembered set during a JVM
936 // safepoint as part of the root scanning activities that serve to
937 // initiate concurrent scanning and concurrent evacuation. Multiple
938 // threads may scan different portions of the remembered set by
939 // making parallel invocations of process_clusters() with each
940 // invocation scanning different clusters of the remembered set.
941 //
942 // An invocation of process_clusters() examines all of the
943 // intergenerational references spanned by count clusters starting
944 // with first_cluster. The oops argument is assumed to represent a
945 // thread-local OopClosure into which addresses of intergenerational
946 // pointer values will be accumulated for the purposes of root scanning.
947 //
948 // A side effect of executing process_clusters() is to update the card
949 // table entries, marking dirty cards as clean if they no longer
950 // hold references to young-gen memory. (THIS IS NOT YET IMPLEMENTED.)
951 //
952 // The implementation of process_clusters() is designed to efficiently
953 // minimize work in the large majority of cases for which the
954 // associated cluster has very few dirty card-table entries.
955 //
956 // At initialization of concurrent marking, invoke process_clusters with
957 // ClosureType equal to ShenandoahInitMarkRootsClosure.
958 //
959 // At initialization of concurrent evacuation, invoke process_clusters with
960 // ClosureType equal to ShenandoahEvacuateUpdateRootsClosure.
961
962 // This is big enough it probably shouldn't be in-lined. On the other hand, there are only a few places this
963 // code is called from, so it might as well be in-lined. The "real" reason I'm inlining at the moment is because
964 // the template expansions were making it difficult for the link/loader to resolve references to the template-
965 // parameterized implementations of this service.
966 template <typename ClosureType>
967 inline void process_clusters(size_t first_cluster, size_t count, HeapWord *end_of_range, ClosureType *oops);
968
969 template <typename ClosureType>
970 inline void process_clusters(size_t first_cluster, size_t count, HeapWord *end_of_range, ClosureType *oops, bool use_write_table);
971
972 template <typename ClosureType>
973 inline void process_region(ShenandoahHeapRegion* region, ClosureType *cl);
974
975 template <typename ClosureType>
976 inline void process_region(ShenandoahHeapRegion* region, ClosureType *cl, bool use_write_table);
977
978 // To Do:
979 // Create subclasses of ShenandoahInitMarkRootsClosure and
980 // ShenandoahEvacuateUpdateRootsClosure and any other closures
981 // that need to participate in remembered set scanning. Within the
982 // subclasses, add a (probably templated) instance variable that
983 // refers to the associated ShenandoahCardCluster object. Use this
984 // ShenandoahCardCluster instance to "enhance" the do_oops
985 // processing so that we can:
986 //
987 // 1. Avoid processing references that correspond to clean card
988 // regions, and
989 // 2. Set card status to CLEAN when the associated card region no
990 // longer holds inter-generatioanal references.
991 //
992 // To enable efficient implementation of these behaviors, we
993 // probably also want to add a few fields into the
994 // ShenandoahCardCluster object that allow us to precompute and
995 // remember the addresses at which card status is going to change
996 // from dirty to clean and clean to dirty. The do_oops
997 // implementations will want to update this value each time they
998 // cross one of these boundaries.
999 void roots_do(OopIterateClosure* cl);
1000 };
1001
1002 typedef ShenandoahScanRemembered<ShenandoahDirectCardMarkRememberedSet> RememberedScanner;
1003
1004 class ShenandoahScanRememberedTask : public AbstractGangTask {
1005 private:
1006 ShenandoahObjToScanQueueSet* _queue_set;
1007 ShenandoahObjToScanQueueSet* _old_queue_set;
1008 ShenandoahReferenceProcessor* _rp;
1009 ShenandoahRegionIterator* _regions;
1010 public:
1011 ShenandoahScanRememberedTask(ShenandoahObjToScanQueueSet* queue_set,
1012 ShenandoahObjToScanQueueSet* old_queue_set,
1013 ShenandoahReferenceProcessor* rp,
1014 ShenandoahRegionIterator* regions);
1015
1016 void work(uint worker_id);
1017 };
1018 #endif // SHARE_GC_SHENANDOAH_SHENANDOAHSCANREMEMBERED_HPP