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