1 /*
2 * Copyright (c) 2001, 2015, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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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.inline.hpp"
33 #include "runtime/java.hpp"
34 #include "runtime/os.hpp"
35 #include "utilities/macros.hpp"
36
37 CardTableRS::CardTableRS(MemRegion whole_heap) :
38 GenRemSet(),
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) return v;
84 }
85 ShouldNotReachHere();
86 return 0;
87 }
88
89 void CardTableRS::prepare_for_younger_refs_iterate(bool parallel) {
90 // Parallel or sequential, we must always set the prev to equal the
91 // last one written.
92 if (parallel) {
93 // Find a parallel value to be used next.
94 jbyte next_val = find_unused_youngergenP_card_value();
95 set_cur_youngergen_card_val(next_val);
96
97 } else {
98 // In an sequential traversal we will always write youngergen, so that
99 // the inline barrier is correct.
100 set_cur_youngergen_card_val(youngergen_card);
101 }
102 }
103
104 void CardTableRS::younger_refs_iterate(Generation* g,
105 OopsInGenClosure* blk,
106 uint n_threads) {
107 _last_cur_val_in_gen[g->level()+1] = cur_youngergen_card_val();
108 g->younger_refs_iterate(blk, n_threads);
109 }
110
111 inline bool ClearNoncleanCardWrapper::clear_card(jbyte* entry) {
112 if (_is_par) {
113 return clear_card_parallel(entry);
114 } else {
115 return clear_card_serial(entry);
116 }
117 }
118
119 inline bool ClearNoncleanCardWrapper::clear_card_parallel(jbyte* entry) {
120 while (true) {
121 // In the parallel case, we may have to do this several times.
122 jbyte entry_val = *entry;
123 assert(entry_val != CardTableRS::clean_card_val(),
124 "We shouldn't be looking at clean cards, and this should "
125 "be the only place they get cleaned.");
126 if (CardTableRS::card_is_dirty_wrt_gen_iter(entry_val)
127 || _ct->is_prev_youngergen_card_val(entry_val)) {
128 jbyte res =
129 Atomic::cmpxchg(CardTableRS::clean_card_val(), entry, entry_val);
130 if (res == entry_val) {
131 break;
132 } else {
133 assert(res == CardTableRS::cur_youngergen_and_prev_nonclean_card,
134 "The CAS above should only fail if another thread did "
135 "a GC write barrier.");
136 }
137 } else if (entry_val ==
138 CardTableRS::cur_youngergen_and_prev_nonclean_card) {
139 // Parallelism shouldn't matter in this case. Only the thread
140 // assigned to scan the card should change this value.
141 *entry = _ct->cur_youngergen_card_val();
142 break;
143 } else {
144 assert(entry_val == _ct->cur_youngergen_card_val(),
145 "Should be the only possibility.");
146 // In this case, the card was clean before, and become
147 // cur_youngergen only because of processing of a promoted object.
148 // We don't have to look at the card.
149 return false;
150 }
151 }
152 return true;
153 }
154
155
156 inline bool ClearNoncleanCardWrapper::clear_card_serial(jbyte* entry) {
157 jbyte entry_val = *entry;
158 assert(entry_val != CardTableRS::clean_card_val(),
159 "We shouldn't be looking at clean cards, and this should "
160 "be the only place they get cleaned.");
161 assert(entry_val != CardTableRS::cur_youngergen_and_prev_nonclean_card,
162 "This should be possible in the sequential case.");
163 *entry = CardTableRS::clean_card_val();
164 return true;
165 }
166
167 ClearNoncleanCardWrapper::ClearNoncleanCardWrapper(
168 DirtyCardToOopClosure* dirty_card_closure, CardTableRS* ct, bool is_par) :
169 _dirty_card_closure(dirty_card_closure), _ct(ct), _is_par(is_par) {
170 }
171
172 bool ClearNoncleanCardWrapper::is_word_aligned(jbyte* entry) {
173 return (((intptr_t)entry) & (BytesPerWord-1)) == 0;
174 }
175
176 // The regions are visited in *decreasing* address order.
177 // This order aids with imprecise card marking, where a dirty
178 // card may cause scanning, and summarization marking, of objects
179 // that extend onto subsequent cards.
180 void ClearNoncleanCardWrapper::do_MemRegion(MemRegion mr) {
181 assert(mr.word_size() > 0, "Error");
182 assert(_ct->is_aligned(mr.start()), "mr.start() should be card aligned");
183 // mr.end() may not necessarily be card aligned.
184 jbyte* cur_entry = _ct->byte_for(mr.last());
185 const jbyte* limit = _ct->byte_for(mr.start());
186 HeapWord* end_of_non_clean = mr.end();
187 HeapWord* start_of_non_clean = end_of_non_clean;
188 while (cur_entry >= limit) {
189 HeapWord* cur_hw = _ct->addr_for(cur_entry);
190 if ((*cur_entry != CardTableRS::clean_card_val()) && clear_card(cur_entry)) {
191 // Continue the dirty range by opening the
192 // dirty window one card to the left.
193 start_of_non_clean = cur_hw;
194 } else {
195 // We hit a "clean" card; process any non-empty
196 // "dirty" range accumulated so far.
197 if (start_of_non_clean < end_of_non_clean) {
198 const MemRegion mrd(start_of_non_clean, end_of_non_clean);
199 _dirty_card_closure->do_MemRegion(mrd);
200 }
201
202 // fast forward through potential continuous whole-word range of clean cards beginning at a word-boundary
203 if (is_word_aligned(cur_entry)) {
204 jbyte* cur_row = cur_entry - BytesPerWord;
205 while (cur_row >= limit && *((intptr_t*)cur_row) == CardTableRS::clean_card_row()) {
206 cur_row -= BytesPerWord;
207 }
208 cur_entry = cur_row + BytesPerWord;
209 cur_hw = _ct->addr_for(cur_entry);
210 }
211
212 // Reset the dirty window, while continuing to look
213 // for the next dirty card that will start a
214 // new dirty window.
215 end_of_non_clean = cur_hw;
216 start_of_non_clean = cur_hw;
217 }
218 // Note that "cur_entry" leads "start_of_non_clean" in
219 // its leftward excursion after this point
220 // in the loop and, when we hit the left end of "mr",
221 // will point off of the left end of the card-table
222 // for "mr".
223 cur_entry--;
224 }
225 // If the first card of "mr" was dirty, we will have
226 // been left with a dirty window, co-initial with "mr",
227 // which we now process.
228 if (start_of_non_clean < end_of_non_clean) {
229 const MemRegion mrd(start_of_non_clean, end_of_non_clean);
230 _dirty_card_closure->do_MemRegion(mrd);
231 }
232 }
233
234 // clean (by dirty->clean before) ==> cur_younger_gen
235 // dirty ==> cur_youngergen_and_prev_nonclean_card
236 // precleaned ==> cur_youngergen_and_prev_nonclean_card
237 // prev-younger-gen ==> cur_youngergen_and_prev_nonclean_card
238 // cur-younger-gen ==> cur_younger_gen
239 // cur_youngergen_and_prev_nonclean_card ==> no change.
240 void CardTableRS::write_ref_field_gc_par(void* field, oop new_val) {
241 jbyte* entry = ct_bs()->byte_for(field);
242 do {
243 jbyte entry_val = *entry;
244 // We put this first because it's probably the most common case.
245 if (entry_val == clean_card_val()) {
246 // No threat of contention with cleaning threads.
247 *entry = cur_youngergen_card_val();
248 return;
249 } else if (card_is_dirty_wrt_gen_iter(entry_val)
250 || is_prev_youngergen_card_val(entry_val)) {
251 // Mark it as both cur and prev youngergen; card cleaning thread will
252 // eventually remove the previous stuff.
253 jbyte new_val = cur_youngergen_and_prev_nonclean_card;
254 jbyte res = Atomic::cmpxchg(new_val, entry, entry_val);
255 // Did the CAS succeed?
256 if (res == entry_val) return;
257 // Otherwise, retry, to see the new value.
258 continue;
259 } else {
260 assert(entry_val == cur_youngergen_and_prev_nonclean_card
261 || entry_val == cur_youngergen_card_val(),
262 "should be only possibilities.");
263 return;
264 }
265 } while (true);
266 }
267
268 void CardTableRS::younger_refs_in_space_iterate(Space* sp,
269 OopsInGenClosure* cl,
270 uint n_threads) {
271 const MemRegion urasm = sp->used_region_at_save_marks();
272 #ifdef ASSERT
273 // Convert the assertion check to a warning if we are running
274 // CMS+ParNew until related bug is fixed.
275 MemRegion ur = sp->used_region();
276 assert(ur.contains(urasm) || (UseConcMarkSweepGC),
277 err_msg("Did you forget to call save_marks()? "
278 "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in "
279 "[" PTR_FORMAT ", " PTR_FORMAT ")",
280 p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end())));
281 // In the case of CMS+ParNew, issue a warning
282 if (!ur.contains(urasm)) {
283 assert(UseConcMarkSweepGC, "Tautology: see assert above");
284 warning("CMS+ParNew: Did you forget to call save_marks()? "
285 "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in "
286 "[" PTR_FORMAT ", " PTR_FORMAT ")",
287 p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end()));
288 MemRegion ur2 = sp->used_region();
289 MemRegion urasm2 = sp->used_region_at_save_marks();
290 if (!ur.equals(ur2)) {
291 warning("CMS+ParNew: Flickering used_region()!!");
292 }
293 if (!urasm.equals(urasm2)) {
294 warning("CMS+ParNew: Flickering used_region_at_save_marks()!!");
295 }
296 ShouldNotReachHere();
297 }
298 #endif
299 _ct_bs->non_clean_card_iterate_possibly_parallel(sp, urasm, cl, this, n_threads);
300 }
301
302 void CardTableRS::clear_into_younger(Generation* old_gen) {
303 assert(old_gen->level() == 1, "Should only be called for the old generation");
304 // The card tables for the youngest gen need never be cleared.
305 // There's a bit of subtlety in the clear() and invalidate()
306 // methods that we exploit here and in invalidate_or_clear()
307 // below to avoid missing cards at the fringes. If clear() or
308 // invalidate() are changed in the future, this code should
309 // be revisited. 20040107.ysr
310 clear(old_gen->prev_used_region());
311 }
312
313 void CardTableRS::invalidate_or_clear(Generation* old_gen) {
314 assert(old_gen->level() == 1, "Should only be called for the old generation");
315 // Invalidate the cards for the currently occupied part of
316 // the old generation and clear the cards for the
317 // unoccupied part of the generation (if any, making use
318 // of that generation's prev_used_region to determine that
319 // region). No need to do anything for the youngest
320 // generation. Also see note#20040107.ysr above.
321 MemRegion used_mr = old_gen->used_region();
322 MemRegion to_be_cleared_mr = old_gen->prev_used_region().minus(used_mr);
323 if (!to_be_cleared_mr.is_empty()) {
324 clear(to_be_cleared_mr);
325 }
326 invalidate(used_mr);
327 }
328
329
330 class VerifyCleanCardClosure: public OopClosure {
331 private:
332 HeapWord* _boundary;
333 HeapWord* _begin;
334 HeapWord* _end;
335 protected:
336 template <class T> void do_oop_work(T* p) {
337 HeapWord* jp = (HeapWord*)p;
338 assert(jp >= _begin && jp < _end,
339 err_msg("Error: jp " PTR_FORMAT " should be within "
340 "[_begin, _end) = [" PTR_FORMAT "," PTR_FORMAT ")",
341 p2i(jp), p2i(_begin), p2i(_end)));
342 oop obj = oopDesc::load_decode_heap_oop(p);
343 guarantee(obj == NULL || (HeapWord*)obj >= _boundary,
344 err_msg("pointer " PTR_FORMAT " at " PTR_FORMAT " on "
345 "clean card crosses boundary" PTR_FORMAT,
346 p2i((HeapWord*)obj), p2i(jp), p2i(_boundary)));
347 }
348
349 public:
350 VerifyCleanCardClosure(HeapWord* b, HeapWord* begin, HeapWord* end) :
351 _boundary(b), _begin(begin), _end(end) {
352 assert(b <= begin,
353 err_msg("Error: boundary " PTR_FORMAT " should be at or below begin " PTR_FORMAT,
354 p2i(b), p2i(begin)));
355 assert(begin <= end,
356 err_msg("Error: begin " PTR_FORMAT " should be strictly below end " PTR_FORMAT,
357 p2i(begin), p2i(end)));
358 }
359
360 virtual void do_oop(oop* p) { VerifyCleanCardClosure::do_oop_work(p); }
361 virtual void do_oop(narrowOop* p) { VerifyCleanCardClosure::do_oop_work(p); }
362 };
363
364 class VerifyCTSpaceClosure: public SpaceClosure {
365 private:
366 CardTableRS* _ct;
367 HeapWord* _boundary;
368 public:
369 VerifyCTSpaceClosure(CardTableRS* ct, HeapWord* boundary) :
370 _ct(ct), _boundary(boundary) {}
371 virtual void do_space(Space* s) { _ct->verify_space(s, _boundary); }
372 };
373
374 class VerifyCTGenClosure: public GenCollectedHeap::GenClosure {
375 CardTableRS* _ct;
376 public:
377 VerifyCTGenClosure(CardTableRS* ct) : _ct(ct) {}
378 void do_generation(Generation* gen) {
379 // Skip the youngest generation.
380 if (gen->level() == 0) return;
381 // Normally, we're interested in pointers to younger generations.
382 VerifyCTSpaceClosure blk(_ct, gen->reserved().start());
383 gen->space_iterate(&blk, true);
384 }
385 };
386
387 void CardTableRS::verify_space(Space* s, HeapWord* gen_boundary) {
388 // We don't need to do young-gen spaces.
389 if (s->end() <= gen_boundary) return;
390 MemRegion used = s->used_region();
391
392 jbyte* cur_entry = byte_for(used.start());
393 jbyte* limit = byte_after(used.last());
394 while (cur_entry < limit) {
395 if (*cur_entry == CardTableModRefBS::clean_card) {
396 jbyte* first_dirty = cur_entry+1;
397 while (first_dirty < limit &&
398 *first_dirty == CardTableModRefBS::clean_card) {
399 first_dirty++;
400 }
401 // If the first object is a regular object, and it has a
402 // young-to-old field, that would mark the previous card.
403 HeapWord* boundary = addr_for(cur_entry);
404 HeapWord* end = (first_dirty >= limit) ? used.end() : addr_for(first_dirty);
405 HeapWord* boundary_block = s->block_start(boundary);
406 HeapWord* begin = boundary; // Until proven otherwise.
407 HeapWord* start_block = boundary_block; // Until proven otherwise.
408 if (boundary_block < boundary) {
409 if (s->block_is_obj(boundary_block) && s->obj_is_alive(boundary_block)) {
410 oop boundary_obj = oop(boundary_block);
411 if (!boundary_obj->is_objArray() &&
412 !boundary_obj->is_typeArray()) {
413 guarantee(cur_entry > byte_for(used.start()),
414 "else boundary would be boundary_block");
415 if (*byte_for(boundary_block) != CardTableModRefBS::clean_card) {
416 begin = boundary_block + s->block_size(boundary_block);
417 start_block = begin;
418 }
419 }
420 }
421 }
422 // Now traverse objects until end.
423 if (begin < end) {
424 MemRegion mr(begin, end);
425 VerifyCleanCardClosure verify_blk(gen_boundary, begin, end);
426 for (HeapWord* cur = start_block; cur < end; cur += s->block_size(cur)) {
427 if (s->block_is_obj(cur) && s->obj_is_alive(cur)) {
428 oop(cur)->oop_iterate_no_header(&verify_blk, mr);
429 }
430 }
431 }
432 cur_entry = first_dirty;
433 } else {
434 // We'd normally expect that cur_youngergen_and_prev_nonclean_card
435 // is a transient value, that cannot be in the card table
436 // except during GC, and thus assert that:
437 // guarantee(*cur_entry != cur_youngergen_and_prev_nonclean_card,
438 // "Illegal CT value");
439 // That however, need not hold, as will become clear in the
440 // following...
441
442 // We'd normally expect that if we are in the parallel case,
443 // we can't have left a prev value (which would be different
444 // from the current value) in the card table, and so we'd like to
445 // assert that:
446 // guarantee(cur_youngergen_card_val() == youngergen_card
447 // || !is_prev_youngergen_card_val(*cur_entry),
448 // "Illegal CT value");
449 // That, however, may not hold occasionally, because of
450 // CMS or MSC in the old gen. To wit, consider the
451 // following two simple illustrative scenarios:
452 // (a) CMS: Consider the case where a large object L
453 // spanning several cards is allocated in the old
454 // gen, and has a young gen reference stored in it, dirtying
455 // some interior cards. A young collection scans the card,
456 // finds a young ref and installs a youngergenP_n value.
457 // L then goes dead. Now a CMS collection starts,
458 // finds L dead and sweeps it up. Assume that L is
459 // abutting _unallocated_blk, so _unallocated_blk is
460 // adjusted down to (below) L. Assume further that
461 // no young collection intervenes during this CMS cycle.
462 // The next young gen cycle will not get to look at this
463 // youngergenP_n card since it lies in the unoccupied
464 // part of the space.
465 // Some young collections later the blocks on this
466 // card can be re-allocated either due to direct allocation
467 // or due to absorbing promotions. At this time, the
468 // before-gc verification will fail the above assert.
469 // (b) MSC: In this case, an object L with a young reference
470 // is on a card that (therefore) holds a youngergen_n value.
471 // Suppose also that L lies towards the end of the used
472 // the used space before GC. An MSC collection
473 // occurs that compacts to such an extent that this
474 // card is no longer in the occupied part of the space.
475 // Since current code in MSC does not always clear cards
476 // in the unused part of old gen, this stale youngergen_n
477 // value is left behind and can later be covered by
478 // an object when promotion or direct allocation
479 // re-allocates that part of the heap.
480 //
481 // Fortunately, the presence of such stale card values is
482 // "only" a minor annoyance in that subsequent young collections
483 // might needlessly scan such cards, but would still never corrupt
484 // the heap as a result. However, it's likely not to be a significant
485 // performance inhibitor in practice. For instance,
486 // some recent measurements with unoccupied cards eagerly cleared
487 // out to maintain this invariant, showed next to no
488 // change in young collection times; of course one can construct
489 // degenerate examples where the cost can be significant.)
490 // Note, in particular, that if the "stale" card is modified
491 // after re-allocation, it would be dirty, not "stale". Thus,
492 // we can never have a younger ref in such a card and it is
493 // safe not to scan that card in any collection. [As we see
494 // below, we do some unnecessary scanning
495 // in some cases in the current parallel scanning algorithm.]
496 //
497 // The main point below is that the parallel card scanning code
498 // deals correctly with these stale card values. There are two main
499 // cases to consider where we have a stale "younger gen" value and a
500 // "derivative" case to consider, where we have a stale
501 // "cur_younger_gen_and_prev_non_clean" value, as will become
502 // apparent in the case analysis below.
503 // o Case 1. If the stale value corresponds to a younger_gen_n
504 // value other than the cur_younger_gen value then the code
505 // treats this as being tantamount to a prev_younger_gen
506 // card. This means that the card may be unnecessarily scanned.
507 // There are two sub-cases to consider:
508 // o Case 1a. Let us say that the card is in the occupied part
509 // of the generation at the time the collection begins. In
510 // that case the card will be either cleared when it is scanned
511 // for young pointers, or will be set to cur_younger_gen as a
512 // result of promotion. (We have elided the normal case where
513 // the scanning thread and the promoting thread interleave
514 // possibly resulting in a transient
515 // cur_younger_gen_and_prev_non_clean value before settling
516 // to cur_younger_gen. [End Case 1a.]
517 // o Case 1b. Consider now the case when the card is in the unoccupied
518 // part of the space which becomes occupied because of promotions
519 // into it during the current young GC. In this case the card
520 // will never be scanned for young references. The current
521 // code will set the card value to either
522 // cur_younger_gen_and_prev_non_clean or leave
523 // it with its stale value -- because the promotions didn't
524 // result in any younger refs on that card. Of these two
525 // cases, the latter will be covered in Case 1a during
526 // a subsequent scan. To deal with the former case, we need
527 // to further consider how we deal with a stale value of
528 // cur_younger_gen_and_prev_non_clean in our case analysis
529 // below. This we do in Case 3 below. [End Case 1b]
530 // [End Case 1]
531 // o Case 2. If the stale value corresponds to cur_younger_gen being
532 // a value not necessarily written by a current promotion, the
533 // card will not be scanned by the younger refs scanning code.
534 // (This is OK since as we argued above such cards cannot contain
535 // any younger refs.) The result is that this value will be
536 // treated as a prev_younger_gen value in a subsequent collection,
537 // which is addressed in Case 1 above. [End Case 2]
538 // o Case 3. We here consider the "derivative" case from Case 1b. above
539 // because of which we may find a stale
540 // cur_younger_gen_and_prev_non_clean card value in the table.
541 // Once again, as in Case 1, we consider two subcases, depending
542 // on whether the card lies in the occupied or unoccupied part
543 // of the space at the start of the young collection.
544 // o Case 3a. Let us say the card is in the occupied part of
545 // the old gen at the start of the young collection. In that
546 // case, the card will be scanned by the younger refs scanning
547 // code which will set it to cur_younger_gen. In a subsequent
548 // scan, the card will be considered again and get its final
549 // correct value. [End Case 3a]
550 // o Case 3b. Now consider the case where the card is in the
551 // unoccupied part of the old gen, and is occupied as a result
552 // of promotions during thus young gc. In that case,
553 // the card will not be scanned for younger refs. The presence
554 // of newly promoted objects on the card will then result in
555 // its keeping the value cur_younger_gen_and_prev_non_clean
556 // value, which we have dealt with in Case 3 here. [End Case 3b]
557 // [End Case 3]
558 //
559 // (Please refer to the code in the helper class
560 // ClearNonCleanCardWrapper and in CardTableModRefBS for details.)
561 //
562 // The informal arguments above can be tightened into a formal
563 // correctness proof and it behooves us to write up such a proof,
564 // or to use model checking to prove that there are no lingering
565 // concerns.
566 //
567 // Clearly because of Case 3b one cannot bound the time for
568 // which a card will retain what we have called a "stale" value.
569 // However, one can obtain a Loose upper bound on the redundant
570 // work as a result of such stale values. Note first that any
571 // time a stale card lies in the occupied part of the space at
572 // the start of the collection, it is scanned by younger refs
573 // code and we can define a rank function on card values that
574 // declines when this is so. Note also that when a card does not
575 // lie in the occupied part of the space at the beginning of a
576 // young collection, its rank can either decline or stay unchanged.
577 // In this case, no extra work is done in terms of redundant
578 // younger refs scanning of that card.
579 // Then, the case analysis above reveals that, in the worst case,
580 // any such stale card will be scanned unnecessarily at most twice.
581 //
582 // It is nonetheless advisable to try and get rid of some of this
583 // redundant work in a subsequent (low priority) re-design of
584 // the card-scanning code, if only to simplify the underlying
585 // state machine analysis/proof. ysr 1/28/2002. XXX
586 cur_entry++;
587 }
588 }
589 }
590
591 void CardTableRS::verify() {
592 // At present, we only know how to verify the card table RS for
593 // generational heaps.
594 VerifyCTGenClosure blk(this);
595 GenCollectedHeap::heap()->generation_iterate(&blk, false);
596 _ct_bs->verify();
597 }