1 /*
2 * Copyright (c) 2001, 2017, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
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23 */
24
25 #include "precompiled.hpp"
26 #include "gc/shared/cardTableRS.hpp"
27 #include "gc/shared/genCollectedHeap.hpp"
28 #include "gc/shared/generation.hpp"
29 #include "gc/shared/space.inline.hpp"
30 #include "memory/allocation.inline.hpp"
31 #include "oops/oop.inline.hpp"
32 #include "runtime/atomic.hpp"
33 #include "runtime/java.hpp"
34 #include "runtime/os.hpp"
35 #include "utilities/macros.hpp"
36
37 CardTableRS::CardTableRS(MemRegion whole_heap) :
38 _bs(NULL),
39 _cur_youngergen_card_val(youngergenP1_card)
40 {
41 _ct_bs = new CardTableModRefBSForCTRS(whole_heap);
42 _ct_bs->initialize();
43 set_bs(_ct_bs);
44 // max_gens is really GenCollectedHeap::heap()->gen_policy()->number_of_generations()
45 // (which is always 2, young & old), but GenCollectedHeap has not been initialized yet.
46 uint max_gens = 2;
47 _last_cur_val_in_gen = NEW_C_HEAP_ARRAY3(jbyte, max_gens + 1,
48 mtGC, CURRENT_PC, AllocFailStrategy::RETURN_NULL);
49 if (_last_cur_val_in_gen == NULL) {
50 vm_exit_during_initialization("Could not create last_cur_val_in_gen array.");
51 }
52 for (uint i = 0; i < max_gens + 1; i++) {
53 _last_cur_val_in_gen[i] = clean_card_val();
54 }
55 _ct_bs->set_CTRS(this);
56 }
57
58 CardTableRS::~CardTableRS() {
59 if (_ct_bs) {
60 delete _ct_bs;
61 _ct_bs = NULL;
62 }
63 if (_last_cur_val_in_gen) {
64 FREE_C_HEAP_ARRAY(jbyte, _last_cur_val_in_gen);
65 }
66 }
67
68 void CardTableRS::resize_covered_region(MemRegion new_region) {
69 _ct_bs->resize_covered_region(new_region);
70 }
71
72 jbyte CardTableRS::find_unused_youngergenP_card_value() {
73 for (jbyte v = youngergenP1_card;
74 v < cur_youngergen_and_prev_nonclean_card;
75 v++) {
76 bool seen = false;
77 for (int g = 0; g < _regions_to_iterate; g++) {
78 if (_last_cur_val_in_gen[g] == v) {
79 seen = true;
80 break;
81 }
82 }
83 if (!seen) {
84 return v;
85 }
86 }
87 ShouldNotReachHere();
88 return 0;
89 }
90
91 void CardTableRS::prepare_for_younger_refs_iterate(bool parallel) {
92 // Parallel or sequential, we must always set the prev to equal the
93 // last one written.
94 if (parallel) {
95 // Find a parallel value to be used next.
96 jbyte next_val = find_unused_youngergenP_card_value();
97 set_cur_youngergen_card_val(next_val);
98
99 } else {
100 // In an sequential traversal we will always write youngergen, so that
101 // the inline barrier is correct.
102 set_cur_youngergen_card_val(youngergen_card);
103 }
104 }
105
106 void CardTableRS::younger_refs_iterate(Generation* g,
107 OopsInGenClosure* blk,
108 uint n_threads) {
109 // The indexing in this array is slightly odd. We want to access
110 // the old generation record here, which is at index 2.
111 _last_cur_val_in_gen[2] = cur_youngergen_card_val();
112 g->younger_refs_iterate(blk, n_threads);
113 }
114
115 inline bool ClearNoncleanCardWrapper::clear_card(jbyte* entry) {
116 if (_is_par) {
117 return clear_card_parallel(entry);
118 } else {
119 return clear_card_serial(entry);
120 }
121 }
122
123 inline bool ClearNoncleanCardWrapper::clear_card_parallel(jbyte* entry) {
124 while (true) {
125 // In the parallel case, we may have to do this several times.
126 jbyte entry_val = *entry;
127 assert(entry_val != CardTableRS::clean_card_val(),
128 "We shouldn't be looking at clean cards, and this should "
129 "be the only place they get cleaned.");
130 if (CardTableRS::card_is_dirty_wrt_gen_iter(entry_val)
131 || _ct->is_prev_youngergen_card_val(entry_val)) {
132 jbyte res =
133 Atomic::cmpxchg(CardTableRS::clean_card_val(), entry, entry_val);
134 if (res == entry_val) {
135 break;
136 } else {
137 assert(res == CardTableRS::cur_youngergen_and_prev_nonclean_card,
138 "The CAS above should only fail if another thread did "
139 "a GC write barrier.");
140 }
141 } else if (entry_val ==
142 CardTableRS::cur_youngergen_and_prev_nonclean_card) {
143 // Parallelism shouldn't matter in this case. Only the thread
144 // assigned to scan the card should change this value.
145 *entry = _ct->cur_youngergen_card_val();
146 break;
147 } else {
148 assert(entry_val == _ct->cur_youngergen_card_val(),
149 "Should be the only possibility.");
150 // In this case, the card was clean before, and become
151 // cur_youngergen only because of processing of a promoted object.
152 // We don't have to look at the card.
153 return false;
154 }
155 }
156 return true;
157 }
158
159
160 inline bool ClearNoncleanCardWrapper::clear_card_serial(jbyte* entry) {
161 jbyte entry_val = *entry;
162 assert(entry_val != CardTableRS::clean_card_val(),
163 "We shouldn't be looking at clean cards, and this should "
164 "be the only place they get cleaned.");
165 assert(entry_val != CardTableRS::cur_youngergen_and_prev_nonclean_card,
166 "This should be possible in the sequential case.");
167 *entry = CardTableRS::clean_card_val();
168 return true;
169 }
170
171 ClearNoncleanCardWrapper::ClearNoncleanCardWrapper(
172 DirtyCardToOopClosure* dirty_card_closure, CardTableRS* ct, bool is_par) :
173 _dirty_card_closure(dirty_card_closure), _ct(ct), _is_par(is_par) {
174 }
175
176 bool ClearNoncleanCardWrapper::is_word_aligned(jbyte* entry) {
177 return (((intptr_t)entry) & (BytesPerWord-1)) == 0;
178 }
179
180 // The regions are visited in *decreasing* address order.
181 // This order aids with imprecise card marking, where a dirty
182 // card may cause scanning, and summarization marking, of objects
183 // that extend onto subsequent cards.
184 void ClearNoncleanCardWrapper::do_MemRegion(MemRegion mr) {
185 assert(mr.word_size() > 0, "Error");
186 assert(_ct->is_aligned(mr.start()), "mr.start() should be card aligned");
187 // mr.end() may not necessarily be card aligned.
188 jbyte* cur_entry = _ct->byte_for(mr.last());
189 const jbyte* limit = _ct->byte_for(mr.start());
190 HeapWord* end_of_non_clean = mr.end();
191 HeapWord* start_of_non_clean = end_of_non_clean;
192 while (cur_entry >= limit) {
193 HeapWord* cur_hw = _ct->addr_for(cur_entry);
194 if ((*cur_entry != CardTableRS::clean_card_val()) && clear_card(cur_entry)) {
195 // Continue the dirty range by opening the
196 // dirty window one card to the left.
197 start_of_non_clean = cur_hw;
198 } else {
199 // We hit a "clean" card; process any non-empty
200 // "dirty" range accumulated so far.
201 if (start_of_non_clean < end_of_non_clean) {
202 const MemRegion mrd(start_of_non_clean, end_of_non_clean);
203 _dirty_card_closure->do_MemRegion(mrd);
204 }
205
206 // fast forward through potential continuous whole-word range of clean cards beginning at a word-boundary
207 if (is_word_aligned(cur_entry)) {
208 jbyte* cur_row = cur_entry - BytesPerWord;
209 while (cur_row >= limit && *((intptr_t*)cur_row) == CardTableRS::clean_card_row()) {
210 cur_row -= BytesPerWord;
211 }
212 cur_entry = cur_row + BytesPerWord;
213 cur_hw = _ct->addr_for(cur_entry);
214 }
215
216 // Reset the dirty window, while continuing to look
217 // for the next dirty card that will start a
218 // new dirty window.
219 end_of_non_clean = cur_hw;
220 start_of_non_clean = cur_hw;
221 }
222 // Note that "cur_entry" leads "start_of_non_clean" in
223 // its leftward excursion after this point
224 // in the loop and, when we hit the left end of "mr",
225 // will point off of the left end of the card-table
226 // for "mr".
227 cur_entry--;
228 }
229 // If the first card of "mr" was dirty, we will have
230 // been left with a dirty window, co-initial with "mr",
231 // which we now process.
232 if (start_of_non_clean < end_of_non_clean) {
233 const MemRegion mrd(start_of_non_clean, end_of_non_clean);
234 _dirty_card_closure->do_MemRegion(mrd);
235 }
236 }
237
238 // clean (by dirty->clean before) ==> cur_younger_gen
239 // dirty ==> cur_youngergen_and_prev_nonclean_card
240 // precleaned ==> cur_youngergen_and_prev_nonclean_card
241 // prev-younger-gen ==> cur_youngergen_and_prev_nonclean_card
242 // cur-younger-gen ==> cur_younger_gen
243 // cur_youngergen_and_prev_nonclean_card ==> no change.
244 void CardTableRS::write_ref_field_gc_par(void* field, oop new_val) {
245 volatile jbyte* entry = _ct_bs->byte_for(field);
246 do {
247 jbyte entry_val = *entry;
248 // We put this first because it's probably the most common case.
249 if (entry_val == clean_card_val()) {
250 // No threat of contention with cleaning threads.
251 *entry = cur_youngergen_card_val();
252 return;
253 } else if (card_is_dirty_wrt_gen_iter(entry_val)
254 || is_prev_youngergen_card_val(entry_val)) {
255 // Mark it as both cur and prev youngergen; card cleaning thread will
256 // eventually remove the previous stuff.
257 jbyte new_val = cur_youngergen_and_prev_nonclean_card;
258 jbyte res = Atomic::cmpxchg(new_val, entry, entry_val);
259 // Did the CAS succeed?
260 if (res == entry_val) return;
261 // Otherwise, retry, to see the new value.
262 continue;
263 } else {
264 assert(entry_val == cur_youngergen_and_prev_nonclean_card
265 || entry_val == cur_youngergen_card_val(),
266 "should be only possibilities.");
267 return;
268 }
269 } while (true);
270 }
271
272 void CardTableRS::younger_refs_in_space_iterate(Space* sp,
273 OopsInGenClosure* cl,
274 uint n_threads) {
275 const MemRegion urasm = sp->used_region_at_save_marks();
276 #ifdef ASSERT
277 // Convert the assertion check to a warning if we are running
278 // CMS+ParNew until related bug is fixed.
279 MemRegion ur = sp->used_region();
280 assert(ur.contains(urasm) || (UseConcMarkSweepGC),
281 "Did you forget to call save_marks()? "
282 "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in "
283 "[" PTR_FORMAT ", " PTR_FORMAT ")",
284 p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end()));
285 // In the case of CMS+ParNew, issue a warning
286 if (!ur.contains(urasm)) {
287 assert(UseConcMarkSweepGC, "Tautology: see assert above");
288 log_warning(gc)("CMS+ParNew: Did you forget to call save_marks()? "
289 "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in "
290 "[" PTR_FORMAT ", " PTR_FORMAT ")",
291 p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end()));
292 MemRegion ur2 = sp->used_region();
293 MemRegion urasm2 = sp->used_region_at_save_marks();
294 if (!ur.equals(ur2)) {
295 log_warning(gc)("CMS+ParNew: Flickering used_region()!!");
296 }
297 if (!urasm.equals(urasm2)) {
298 log_warning(gc)("CMS+ParNew: Flickering used_region_at_save_marks()!!");
299 }
300 ShouldNotReachHere();
301 }
302 #endif
303 _ct_bs->non_clean_card_iterate_possibly_parallel(sp, urasm, cl, this, n_threads);
304 }
305
306 void CardTableRS::clear_into_younger(Generation* old_gen) {
307 assert(GenCollectedHeap::heap()->is_old_gen(old_gen),
308 "Should only be called for the old generation");
309 // The card tables for the youngest gen need never be cleared.
310 // There's a bit of subtlety in the clear() and invalidate()
311 // methods that we exploit here and in invalidate_or_clear()
312 // below to avoid missing cards at the fringes. If clear() or
313 // invalidate() are changed in the future, this code should
314 // be revisited. 20040107.ysr
315 clear(old_gen->prev_used_region());
316 }
317
318 void CardTableRS::invalidate_or_clear(Generation* old_gen) {
319 assert(GenCollectedHeap::heap()->is_old_gen(old_gen),
320 "Should only be called for the old generation");
321 // Invalidate the cards for the currently occupied part of
322 // the old generation and clear the cards for the
323 // unoccupied part of the generation (if any, making use
324 // of that generation's prev_used_region to determine that
325 // region). No need to do anything for the youngest
326 // generation. Also see note#20040107.ysr above.
327 MemRegion used_mr = old_gen->used_region();
328 MemRegion to_be_cleared_mr = old_gen->prev_used_region().minus(used_mr);
329 if (!to_be_cleared_mr.is_empty()) {
330 clear(to_be_cleared_mr);
331 }
332 invalidate(used_mr);
333 }
334
335
336 class VerifyCleanCardClosure: public OopClosure {
337 private:
338 HeapWord* _boundary;
339 HeapWord* _begin;
340 HeapWord* _end;
341 protected:
342 template <class T> void do_oop_work(T* p) {
343 HeapWord* jp = (HeapWord*)p;
344 assert(jp >= _begin && jp < _end,
345 "Error: jp " PTR_FORMAT " should be within "
346 "[_begin, _end) = [" PTR_FORMAT "," PTR_FORMAT ")",
347 p2i(jp), p2i(_begin), p2i(_end));
348 oop obj = oopDesc::load_decode_heap_oop(p);
349 guarantee(obj == NULL || (HeapWord*)obj >= _boundary,
350 "pointer " PTR_FORMAT " at " PTR_FORMAT " on "
351 "clean card crosses boundary" PTR_FORMAT,
352 p2i(obj), p2i(jp), p2i(_boundary));
353 }
354
355 public:
356 VerifyCleanCardClosure(HeapWord* b, HeapWord* begin, HeapWord* end) :
357 _boundary(b), _begin(begin), _end(end) {
358 assert(b <= begin,
359 "Error: boundary " PTR_FORMAT " should be at or below begin " PTR_FORMAT,
360 p2i(b), p2i(begin));
361 assert(begin <= end,
362 "Error: begin " PTR_FORMAT " should be strictly below end " PTR_FORMAT,
363 p2i(begin), p2i(end));
364 }
365
366 virtual void do_oop(oop* p) { VerifyCleanCardClosure::do_oop_work(p); }
367 virtual void do_oop(narrowOop* p) { VerifyCleanCardClosure::do_oop_work(p); }
368 };
369
370 class VerifyCTSpaceClosure: public SpaceClosure {
371 private:
372 CardTableRS* _ct;
373 HeapWord* _boundary;
374 public:
375 VerifyCTSpaceClosure(CardTableRS* ct, HeapWord* boundary) :
376 _ct(ct), _boundary(boundary) {}
377 virtual void do_space(Space* s) { _ct->verify_space(s, _boundary); }
378 };
379
380 class VerifyCTGenClosure: public GenCollectedHeap::GenClosure {
381 CardTableRS* _ct;
382 public:
383 VerifyCTGenClosure(CardTableRS* ct) : _ct(ct) {}
384 void do_generation(Generation* gen) {
385 // Skip the youngest generation.
386 if (GenCollectedHeap::heap()->is_young_gen(gen)) {
387 return;
388 }
389 // Normally, we're interested in pointers to younger generations.
390 VerifyCTSpaceClosure blk(_ct, gen->reserved().start());
391 gen->space_iterate(&blk, true);
392 }
393 };
394
395 void CardTableRS::verify_space(Space* s, HeapWord* gen_boundary) {
396 // We don't need to do young-gen spaces.
397 if (s->end() <= gen_boundary) return;
398 MemRegion used = s->used_region();
399
400 jbyte* cur_entry = byte_for(used.start());
401 jbyte* limit = byte_after(used.last());
402 while (cur_entry < limit) {
403 if (*cur_entry == clean_card_val()) {
404 jbyte* first_dirty = cur_entry+1;
405 while (first_dirty < limit &&
406 *first_dirty == clean_card_val()) {
407 first_dirty++;
408 }
409 // If the first object is a regular object, and it has a
410 // young-to-old field, that would mark the previous card.
411 HeapWord* boundary = addr_for(cur_entry);
412 HeapWord* end = (first_dirty >= limit) ? used.end() : addr_for(first_dirty);
413 HeapWord* boundary_block = s->block_start(boundary);
414 HeapWord* begin = boundary; // Until proven otherwise.
415 HeapWord* start_block = boundary_block; // Until proven otherwise.
416 if (boundary_block < boundary) {
417 if (s->block_is_obj(boundary_block) && s->obj_is_alive(boundary_block)) {
418 oop boundary_obj = oop(boundary_block);
419 if (!boundary_obj->is_objArray() &&
420 !boundary_obj->is_typeArray()) {
421 guarantee(cur_entry > byte_for(used.start()),
422 "else boundary would be boundary_block");
423 if (*byte_for(boundary_block) != clean_card_val()) {
424 begin = boundary_block + s->block_size(boundary_block);
425 start_block = begin;
426 }
427 }
428 }
429 }
430 // Now traverse objects until end.
431 if (begin < end) {
432 MemRegion mr(begin, end);
433 VerifyCleanCardClosure verify_blk(gen_boundary, begin, end);
434 for (HeapWord* cur = start_block; cur < end; cur += s->block_size(cur)) {
435 if (s->block_is_obj(cur) && s->obj_is_alive(cur)) {
436 oop(cur)->oop_iterate_no_header(&verify_blk, mr);
437 }
438 }
439 }
440 cur_entry = first_dirty;
441 } else {
442 // We'd normally expect that cur_youngergen_and_prev_nonclean_card
443 // is a transient value, that cannot be in the card table
444 // except during GC, and thus assert that:
445 // guarantee(*cur_entry != cur_youngergen_and_prev_nonclean_card,
446 // "Illegal CT value");
447 // That however, need not hold, as will become clear in the
448 // following...
449
450 // We'd normally expect that if we are in the parallel case,
451 // we can't have left a prev value (which would be different
452 // from the current value) in the card table, and so we'd like to
453 // assert that:
454 // guarantee(cur_youngergen_card_val() == youngergen_card
455 // || !is_prev_youngergen_card_val(*cur_entry),
456 // "Illegal CT value");
457 // That, however, may not hold occasionally, because of
458 // CMS or MSC in the old gen. To wit, consider the
459 // following two simple illustrative scenarios:
460 // (a) CMS: Consider the case where a large object L
461 // spanning several cards is allocated in the old
462 // gen, and has a young gen reference stored in it, dirtying
463 // some interior cards. A young collection scans the card,
464 // finds a young ref and installs a youngergenP_n value.
465 // L then goes dead. Now a CMS collection starts,
466 // finds L dead and sweeps it up. Assume that L is
467 // abutting _unallocated_blk, so _unallocated_blk is
468 // adjusted down to (below) L. Assume further that
469 // no young collection intervenes during this CMS cycle.
470 // The next young gen cycle will not get to look at this
471 // youngergenP_n card since it lies in the unoccupied
472 // part of the space.
473 // Some young collections later the blocks on this
474 // card can be re-allocated either due to direct allocation
475 // or due to absorbing promotions. At this time, the
476 // before-gc verification will fail the above assert.
477 // (b) MSC: In this case, an object L with a young reference
478 // is on a card that (therefore) holds a youngergen_n value.
479 // Suppose also that L lies towards the end of the used
480 // the used space before GC. An MSC collection
481 // occurs that compacts to such an extent that this
482 // card is no longer in the occupied part of the space.
483 // Since current code in MSC does not always clear cards
484 // in the unused part of old gen, this stale youngergen_n
485 // value is left behind and can later be covered by
486 // an object when promotion or direct allocation
487 // re-allocates that part of the heap.
488 //
489 // Fortunately, the presence of such stale card values is
490 // "only" a minor annoyance in that subsequent young collections
491 // might needlessly scan such cards, but would still never corrupt
492 // the heap as a result. However, it's likely not to be a significant
493 // performance inhibitor in practice. For instance,
494 // some recent measurements with unoccupied cards eagerly cleared
495 // out to maintain this invariant, showed next to no
496 // change in young collection times; of course one can construct
497 // degenerate examples where the cost can be significant.)
498 // Note, in particular, that if the "stale" card is modified
499 // after re-allocation, it would be dirty, not "stale". Thus,
500 // we can never have a younger ref in such a card and it is
501 // safe not to scan that card in any collection. [As we see
502 // below, we do some unnecessary scanning
503 // in some cases in the current parallel scanning algorithm.]
504 //
505 // The main point below is that the parallel card scanning code
506 // deals correctly with these stale card values. There are two main
507 // cases to consider where we have a stale "young gen" value and a
508 // "derivative" case to consider, where we have a stale
509 // "cur_younger_gen_and_prev_non_clean" value, as will become
510 // apparent in the case analysis below.
511 // o Case 1. If the stale value corresponds to a younger_gen_n
512 // value other than the cur_younger_gen value then the code
513 // treats this as being tantamount to a prev_younger_gen
514 // card. This means that the card may be unnecessarily scanned.
515 // There are two sub-cases to consider:
516 // o Case 1a. Let us say that the card is in the occupied part
517 // of the generation at the time the collection begins. In
518 // that case the card will be either cleared when it is scanned
519 // for young pointers, or will be set to cur_younger_gen as a
520 // result of promotion. (We have elided the normal case where
521 // the scanning thread and the promoting thread interleave
522 // possibly resulting in a transient
523 // cur_younger_gen_and_prev_non_clean value before settling
524 // to cur_younger_gen. [End Case 1a.]
525 // o Case 1b. Consider now the case when the card is in the unoccupied
526 // part of the space which becomes occupied because of promotions
527 // into it during the current young GC. In this case the card
528 // will never be scanned for young references. The current
529 // code will set the card value to either
530 // cur_younger_gen_and_prev_non_clean or leave
531 // it with its stale value -- because the promotions didn't
532 // result in any younger refs on that card. Of these two
533 // cases, the latter will be covered in Case 1a during
534 // a subsequent scan. To deal with the former case, we need
535 // to further consider how we deal with a stale value of
536 // cur_younger_gen_and_prev_non_clean in our case analysis
537 // below. This we do in Case 3 below. [End Case 1b]
538 // [End Case 1]
539 // o Case 2. If the stale value corresponds to cur_younger_gen being
540 // a value not necessarily written by a current promotion, the
541 // card will not be scanned by the younger refs scanning code.
542 // (This is OK since as we argued above such cards cannot contain
543 // any younger refs.) The result is that this value will be
544 // treated as a prev_younger_gen value in a subsequent collection,
545 // which is addressed in Case 1 above. [End Case 2]
546 // o Case 3. We here consider the "derivative" case from Case 1b. above
547 // because of which we may find a stale
548 // cur_younger_gen_and_prev_non_clean card value in the table.
549 // Once again, as in Case 1, we consider two subcases, depending
550 // on whether the card lies in the occupied or unoccupied part
551 // of the space at the start of the young collection.
552 // o Case 3a. Let us say the card is in the occupied part of
553 // the old gen at the start of the young collection. In that
554 // case, the card will be scanned by the younger refs scanning
555 // code which will set it to cur_younger_gen. In a subsequent
556 // scan, the card will be considered again and get its final
557 // correct value. [End Case 3a]
558 // o Case 3b. Now consider the case where the card is in the
559 // unoccupied part of the old gen, and is occupied as a result
560 // of promotions during thus young gc. In that case,
561 // the card will not be scanned for younger refs. The presence
562 // of newly promoted objects on the card will then result in
563 // its keeping the value cur_younger_gen_and_prev_non_clean
564 // value, which we have dealt with in Case 3 here. [End Case 3b]
565 // [End Case 3]
566 //
567 // (Please refer to the code in the helper class
568 // ClearNonCleanCardWrapper and in CardTableModRefBS for details.)
569 //
570 // The informal arguments above can be tightened into a formal
571 // correctness proof and it behooves us to write up such a proof,
572 // or to use model checking to prove that there are no lingering
573 // concerns.
574 //
575 // Clearly because of Case 3b one cannot bound the time for
576 // which a card will retain what we have called a "stale" value.
577 // However, one can obtain a Loose upper bound on the redundant
578 // work as a result of such stale values. Note first that any
579 // time a stale card lies in the occupied part of the space at
580 // the start of the collection, it is scanned by younger refs
581 // code and we can define a rank function on card values that
582 // declines when this is so. Note also that when a card does not
583 // lie in the occupied part of the space at the beginning of a
584 // young collection, its rank can either decline or stay unchanged.
585 // In this case, no extra work is done in terms of redundant
586 // younger refs scanning of that card.
587 // Then, the case analysis above reveals that, in the worst case,
588 // any such stale card will be scanned unnecessarily at most twice.
589 //
590 // It is nonetheless advisable to try and get rid of some of this
591 // redundant work in a subsequent (low priority) re-design of
592 // the card-scanning code, if only to simplify the underlying
593 // state machine analysis/proof. ysr 1/28/2002. XXX
594 cur_entry++;
595 }
596 }
597 }
598
599 void CardTableRS::verify() {
600 // At present, we only know how to verify the card table RS for
601 // generational heaps.
602 VerifyCTGenClosure blk(this);
603 GenCollectedHeap::heap()->generation_iterate(&blk, false);
604 _ct_bs->verify();
605 }