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