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 "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 // The indexing in this array is slightly odd. We want to access
107 // the old generation record here, which is at index 2.
108 _last_cur_val_in_gen[2] = cur_youngergen_card_val();
109 g->younger_refs_iterate(blk);
110 }
111
112 inline bool ClearNoncleanCardWrapper::clear_card(jbyte* entry) {
113 if (_is_par) {
114 return clear_card_parallel(entry);
115 } else {
116 return clear_card_serial(entry);
117 }
118 }
119
120 inline bool ClearNoncleanCardWrapper::clear_card_parallel(jbyte* entry) {
121 while (true) {
122 // In the parallel case, we may have to do this several times.
123 jbyte entry_val = *entry;
124 assert(entry_val != CardTableRS::clean_card_val(),
125 "We shouldn't be looking at clean cards, and this should "
126 "be the only place they get cleaned.");
127 if (CardTableRS::card_is_dirty_wrt_gen_iter(entry_val)
128 || _ct->is_prev_youngergen_card_val(entry_val)) {
129 jbyte res =
130 Atomic::cmpxchg(CardTableRS::clean_card_val(), entry, entry_val);
131 if (res == entry_val) {
132 break;
133 } else {
134 assert(res == CardTableRS::cur_youngergen_and_prev_nonclean_card,
135 "The CAS above should only fail if another thread did "
136 "a GC write barrier.");
137 }
138 } else if (entry_val ==
139 CardTableRS::cur_youngergen_and_prev_nonclean_card) {
140 // Parallelism shouldn't matter in this case. Only the thread
141 // assigned to scan the card should change this value.
142 *entry = _ct->cur_youngergen_card_val();
143 break;
144 } else {
145 assert(entry_val == _ct->cur_youngergen_card_val(),
146 "Should be the only possibility.");
147 // In this case, the card was clean before, and become
148 // cur_youngergen only because of processing of a promoted object.
149 // We don't have to look at the card.
150 return false;
151 }
152 }
153 return true;
154 }
155
156
157 inline bool ClearNoncleanCardWrapper::clear_card_serial(jbyte* entry) {
158 jbyte entry_val = *entry;
159 assert(entry_val != CardTableRS::clean_card_val(),
160 "We shouldn't be looking at clean cards, and this should "
161 "be the only place they get cleaned.");
162 assert(entry_val != CardTableRS::cur_youngergen_and_prev_nonclean_card,
163 "This should be possible in the sequential case.");
164 *entry = CardTableRS::clean_card_val();
165 return true;
166 }
167
168 ClearNoncleanCardWrapper::ClearNoncleanCardWrapper(
169 DirtyCardToOopClosure* dirty_card_closure, CardTableRS* ct) :
170 _dirty_card_closure(dirty_card_closure), _ct(ct) {
171 // Cannot yet substitute active_workers for n_par_threads
172 // in the case where parallelism is being turned off by
173 // setting n_par_threads to 0.
174 _is_par = (GenCollectedHeap::heap()->n_par_threads() > 0);
175 assert(!_is_par ||
176 (GenCollectedHeap::heap()->n_par_threads() ==
177 GenCollectedHeap::heap()->workers()->active_workers()), "Mismatch");
178 }
179
180 bool ClearNoncleanCardWrapper::is_word_aligned(jbyte* entry) {
181 return (((intptr_t)entry) & (BytesPerWord-1)) == 0;
182 }
183
184 // The regions are visited in *decreasing* address order.
185 // This order aids with imprecise card marking, where a dirty
186 // card may cause scanning, and summarization marking, of objects
187 // that extend onto subsequent cards.
188 void ClearNoncleanCardWrapper::do_MemRegion(MemRegion mr) {
189 assert(mr.word_size() > 0, "Error");
190 assert(_ct->is_aligned(mr.start()), "mr.start() should be card aligned");
191 // mr.end() may not necessarily be card aligned.
192 jbyte* cur_entry = _ct->byte_for(mr.last());
193 const jbyte* limit = _ct->byte_for(mr.start());
194 HeapWord* end_of_non_clean = mr.end();
195 HeapWord* start_of_non_clean = end_of_non_clean;
196 while (cur_entry >= limit) {
197 HeapWord* cur_hw = _ct->addr_for(cur_entry);
198 if ((*cur_entry != CardTableRS::clean_card_val()) && clear_card(cur_entry)) {
199 // Continue the dirty range by opening the
200 // dirty window one card to the left.
201 start_of_non_clean = cur_hw;
202 } else {
203 // We hit a "clean" card; process any non-empty
204 // "dirty" range accumulated so far.
205 if (start_of_non_clean < end_of_non_clean) {
206 const MemRegion mrd(start_of_non_clean, end_of_non_clean);
207 _dirty_card_closure->do_MemRegion(mrd);
208 }
209
210 // fast forward through potential continuous whole-word range of clean cards beginning at a word-boundary
211 if (is_word_aligned(cur_entry)) {
212 jbyte* cur_row = cur_entry - BytesPerWord;
213 while (cur_row >= limit && *((intptr_t*)cur_row) == CardTableRS::clean_card_row()) {
214 cur_row -= BytesPerWord;
215 }
216 cur_entry = cur_row + BytesPerWord;
217 cur_hw = _ct->addr_for(cur_entry);
218 }
219
220 // Reset the dirty window, while continuing to look
221 // for the next dirty card that will start a
222 // new dirty window.
223 end_of_non_clean = cur_hw;
224 start_of_non_clean = cur_hw;
225 }
226 // Note that "cur_entry" leads "start_of_non_clean" in
227 // its leftward excursion after this point
228 // in the loop and, when we hit the left end of "mr",
229 // will point off of the left end of the card-table
230 // for "mr".
231 cur_entry--;
232 }
233 // If the first card of "mr" was dirty, we will have
234 // been left with a dirty window, co-initial with "mr",
235 // which we now process.
236 if (start_of_non_clean < end_of_non_clean) {
237 const MemRegion mrd(start_of_non_clean, end_of_non_clean);
238 _dirty_card_closure->do_MemRegion(mrd);
239 }
240 }
241
242 // clean (by dirty->clean before) ==> cur_younger_gen
243 // dirty ==> cur_youngergen_and_prev_nonclean_card
244 // precleaned ==> cur_youngergen_and_prev_nonclean_card
245 // prev-younger-gen ==> cur_youngergen_and_prev_nonclean_card
246 // cur-younger-gen ==> cur_younger_gen
247 // cur_youngergen_and_prev_nonclean_card ==> no change.
248 void CardTableRS::write_ref_field_gc_par(void* field, oop new_val) {
249 jbyte* entry = ct_bs()->byte_for(field);
250 do {
251 jbyte entry_val = *entry;
252 // We put this first because it's probably the most common case.
253 if (entry_val == clean_card_val()) {
254 // No threat of contention with cleaning threads.
255 *entry = cur_youngergen_card_val();
256 return;
257 } else if (card_is_dirty_wrt_gen_iter(entry_val)
258 || is_prev_youngergen_card_val(entry_val)) {
259 // Mark it as both cur and prev youngergen; card cleaning thread will
260 // eventually remove the previous stuff.
261 jbyte new_val = cur_youngergen_and_prev_nonclean_card;
262 jbyte res = Atomic::cmpxchg(new_val, entry, entry_val);
263 // Did the CAS succeed?
264 if (res == entry_val) return;
265 // Otherwise, retry, to see the new value.
266 continue;
267 } else {
268 assert(entry_val == cur_youngergen_and_prev_nonclean_card
269 || entry_val == cur_youngergen_card_val(),
270 "should be only possibilities.");
271 return;
272 }
273 } while (true);
274 }
275
276 void CardTableRS::younger_refs_in_space_iterate(Space* sp,
277 OopsInGenClosure* cl) {
278 const MemRegion urasm = sp->used_region_at_save_marks();
279 #ifdef ASSERT
280 // Convert the assertion check to a warning if we are running
281 // CMS+ParNew until related bug is fixed.
282 MemRegion ur = sp->used_region();
283 assert(ur.contains(urasm) || (UseConcMarkSweepGC),
284 err_msg("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 // In the case of CMS+ParNew, issue a warning
289 if (!ur.contains(urasm)) {
290 assert(UseConcMarkSweepGC, "Tautology: see assert above");
291 warning("CMS+ParNew: Did you forget to call save_marks()? "
292 "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in "
293 "[" PTR_FORMAT ", " PTR_FORMAT ")",
294 p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end()));
295 MemRegion ur2 = sp->used_region();
296 MemRegion urasm2 = sp->used_region_at_save_marks();
297 if (!ur.equals(ur2)) {
298 warning("CMS+ParNew: Flickering used_region()!!");
299 }
300 if (!urasm.equals(urasm2)) {
301 warning("CMS+ParNew: Flickering used_region_at_save_marks()!!");
302 }
303 ShouldNotReachHere();
304 }
305 #endif
306 _ct_bs->non_clean_card_iterate_possibly_parallel(sp, urasm, cl, this);
307 }
308
309 void CardTableRS::clear_into_younger(Generation* old_gen) {
310 assert(old_gen == GenCollectedHeap::heap()->old_gen(),
311 "Should only be called for the old generation");
312 // The card tables for the youngest gen need never be cleared.
313 // There's a bit of subtlety in the clear() and invalidate()
314 // methods that we exploit here and in invalidate_or_clear()
315 // below to avoid missing cards at the fringes. If clear() or
316 // invalidate() are changed in the future, this code should
317 // be revisited. 20040107.ysr
318 clear(old_gen->prev_used_region());
319 }
320
321 void CardTableRS::invalidate_or_clear(Generation* old_gen) {
322 assert(old_gen == GenCollectedHeap::heap()->old_gen(),
323 "Should only be called for the old generation");
324 // Invalidate the cards for the currently occupied part of
325 // the old generation and clear the cards for the
326 // unoccupied part of the generation (if any, making use
327 // of that generation's prev_used_region to determine that
328 // region). No need to do anything for the youngest
329 // generation. Also see note#20040107.ysr above.
330 MemRegion used_mr = old_gen->used_region();
331 MemRegion to_be_cleared_mr = old_gen->prev_used_region().minus(used_mr);
332 if (!to_be_cleared_mr.is_empty()) {
333 clear(to_be_cleared_mr);
334 }
335 invalidate(used_mr);
336 }
337
338
339 class VerifyCleanCardClosure: public OopClosure {
340 private:
341 HeapWord* _boundary;
342 HeapWord* _begin;
343 HeapWord* _end;
344 protected:
345 template <class T> void do_oop_work(T* p) {
346 HeapWord* jp = (HeapWord*)p;
347 assert(jp >= _begin && jp < _end,
348 err_msg("Error: jp " PTR_FORMAT " should be within "
349 "[_begin, _end) = [" PTR_FORMAT "," PTR_FORMAT ")",
350 p2i(jp), p2i(_begin), p2i(_end)));
351 oop obj = oopDesc::load_decode_heap_oop(p);
352 guarantee(obj == NULL || (HeapWord*)obj >= _boundary,
353 err_msg("pointer " PTR_FORMAT " at " PTR_FORMAT " on "
354 "clean card crosses boundary" PTR_FORMAT,
355 p2i((HeapWord*)obj), p2i(jp), p2i(_boundary)));
356 }
357
358 public:
359 VerifyCleanCardClosure(HeapWord* b, HeapWord* begin, HeapWord* end) :
360 _boundary(b), _begin(begin), _end(end) {
361 assert(b <= begin,
362 err_msg("Error: boundary " PTR_FORMAT " should be at or below begin " PTR_FORMAT,
363 p2i(b), p2i(begin)));
364 assert(begin <= end,
365 err_msg("Error: begin " PTR_FORMAT " should be strictly below end " PTR_FORMAT,
366 p2i(begin), p2i(end)));
367 }
368
369 virtual void do_oop(oop* p) { VerifyCleanCardClosure::do_oop_work(p); }
370 virtual void do_oop(narrowOop* p) { VerifyCleanCardClosure::do_oop_work(p); }
371 };
372
373 class VerifyCTSpaceClosure: public SpaceClosure {
374 private:
375 CardTableRS* _ct;
376 HeapWord* _boundary;
377 public:
378 VerifyCTSpaceClosure(CardTableRS* ct, HeapWord* boundary) :
379 _ct(ct), _boundary(boundary) {}
380 virtual void do_space(Space* s) { _ct->verify_space(s, _boundary); }
381 };
382
383 class VerifyCTGenClosure: public GenCollectedHeap::GenClosure {
384 CardTableRS* _ct;
385 public:
386 VerifyCTGenClosure(CardTableRS* ct) : _ct(ct) {}
387 void do_generation(Generation* gen) {
388 // Skip the youngest generation.
389 if (gen == GenCollectedHeap::heap()->young_gen()) {
390 return;
391 }
392 // Normally, we're interested in pointers to younger generations.
393 VerifyCTSpaceClosure blk(_ct, gen->reserved().start());
394 gen->space_iterate(&blk, true);
395 }
396 };
397
398 void CardTableRS::verify_space(Space* s, HeapWord* gen_boundary) {
399 // We don't need to do young-gen spaces.
400 if (s->end() <= gen_boundary) return;
401 MemRegion used = s->used_region();
402
403 jbyte* cur_entry = byte_for(used.start());
404 jbyte* limit = byte_after(used.last());
405 while (cur_entry < limit) {
406 if (*cur_entry == CardTableModRefBS::clean_card) {
407 jbyte* first_dirty = cur_entry+1;
408 while (first_dirty < limit &&
409 *first_dirty == CardTableModRefBS::clean_card) {
410 first_dirty++;
411 }
412 // If the first object is a regular object, and it has a
413 // young-to-old field, that would mark the previous card.
414 HeapWord* boundary = addr_for(cur_entry);
415 HeapWord* end = (first_dirty >= limit) ? used.end() : addr_for(first_dirty);
416 HeapWord* boundary_block = s->block_start(boundary);
417 HeapWord* begin = boundary; // Until proven otherwise.
418 HeapWord* start_block = boundary_block; // Until proven otherwise.
419 if (boundary_block < boundary) {
420 if (s->block_is_obj(boundary_block) && s->obj_is_alive(boundary_block)) {
421 oop boundary_obj = oop(boundary_block);
422 if (!boundary_obj->is_objArray() &&
423 !boundary_obj->is_typeArray()) {
424 guarantee(cur_entry > byte_for(used.start()),
425 "else boundary would be boundary_block");
426 if (*byte_for(boundary_block) != CardTableModRefBS::clean_card) {
427 begin = boundary_block + s->block_size(boundary_block);
428 start_block = begin;
429 }
430 }
431 }
432 }
433 // Now traverse objects until end.
434 if (begin < end) {
435 MemRegion mr(begin, end);
436 VerifyCleanCardClosure verify_blk(gen_boundary, begin, end);
437 for (HeapWord* cur = start_block; cur < end; cur += s->block_size(cur)) {
438 if (s->block_is_obj(cur) && s->obj_is_alive(cur)) {
439 oop(cur)->oop_iterate_no_header(&verify_blk, mr);
440 }
441 }
442 }
443 cur_entry = first_dirty;
444 } else {
445 // We'd normally expect that cur_youngergen_and_prev_nonclean_card
446 // is a transient value, that cannot be in the card table
447 // except during GC, and thus assert that:
448 // guarantee(*cur_entry != cur_youngergen_and_prev_nonclean_card,
449 // "Illegal CT value");
450 // That however, need not hold, as will become clear in the
451 // following...
452
453 // We'd normally expect that if we are in the parallel case,
454 // we can't have left a prev value (which would be different
455 // from the current value) in the card table, and so we'd like to
456 // assert that:
457 // guarantee(cur_youngergen_card_val() == youngergen_card
458 // || !is_prev_youngergen_card_val(*cur_entry),
459 // "Illegal CT value");
460 // That, however, may not hold occasionally, because of
461 // CMS or MSC in the old gen. To wit, consider the
462 // following two simple illustrative scenarios:
463 // (a) CMS: Consider the case where a large object L
464 // spanning several cards is allocated in the old
465 // gen, and has a young gen reference stored in it, dirtying
466 // some interior cards. A young collection scans the card,
467 // finds a young ref and installs a youngergenP_n value.
468 // L then goes dead. Now a CMS collection starts,
469 // finds L dead and sweeps it up. Assume that L is
470 // abutting _unallocated_blk, so _unallocated_blk is
471 // adjusted down to (below) L. Assume further that
472 // no young collection intervenes during this CMS cycle.
473 // The next young gen cycle will not get to look at this
474 // youngergenP_n card since it lies in the unoccupied
475 // part of the space.
476 // Some young collections later the blocks on this
477 // card can be re-allocated either due to direct allocation
478 // or due to absorbing promotions. At this time, the
479 // before-gc verification will fail the above assert.
480 // (b) MSC: In this case, an object L with a young reference
481 // is on a card that (therefore) holds a youngergen_n value.
482 // Suppose also that L lies towards the end of the used
483 // the used space before GC. An MSC collection
484 // occurs that compacts to such an extent that this
485 // card is no longer in the occupied part of the space.
486 // Since current code in MSC does not always clear cards
487 // in the unused part of old gen, this stale youngergen_n
488 // value is left behind and can later be covered by
489 // an object when promotion or direct allocation
490 // re-allocates that part of the heap.
491 //
492 // Fortunately, the presence of such stale card values is
493 // "only" a minor annoyance in that subsequent young collections
494 // might needlessly scan such cards, but would still never corrupt
495 // the heap as a result. However, it's likely not to be a significant
496 // performance inhibitor in practice. For instance,
497 // some recent measurements with unoccupied cards eagerly cleared
498 // out to maintain this invariant, showed next to no
499 // change in young collection times; of course one can construct
500 // degenerate examples where the cost can be significant.)
501 // Note, in particular, that if the "stale" card is modified
502 // after re-allocation, it would be dirty, not "stale". Thus,
503 // we can never have a younger ref in such a card and it is
504 // safe not to scan that card in any collection. [As we see
505 // below, we do some unnecessary scanning
506 // in some cases in the current parallel scanning algorithm.]
507 //
508 // The main point below is that the parallel card scanning code
509 // deals correctly with these stale card values. There are two main
510 // cases to consider where we have a stale "younger gen" value and a
511 // "derivative" case to consider, where we have a stale
512 // "cur_younger_gen_and_prev_non_clean" value, as will become
513 // apparent in the case analysis below.
514 // o Case 1. If the stale value corresponds to a younger_gen_n
515 // value other than the cur_younger_gen value then the code
516 // treats this as being tantamount to a prev_younger_gen
517 // card. This means that the card may be unnecessarily scanned.
518 // There are two sub-cases to consider:
519 // o Case 1a. Let us say that the card is in the occupied part
520 // of the generation at the time the collection begins. In
521 // that case the card will be either cleared when it is scanned
522 // for young pointers, or will be set to cur_younger_gen as a
523 // result of promotion. (We have elided the normal case where
524 // the scanning thread and the promoting thread interleave
525 // possibly resulting in a transient
526 // cur_younger_gen_and_prev_non_clean value before settling
527 // to cur_younger_gen. [End Case 1a.]
528 // o Case 1b. Consider now the case when the card is in the unoccupied
529 // part of the space which becomes occupied because of promotions
530 // into it during the current young GC. In this case the card
531 // will never be scanned for young references. The current
532 // code will set the card value to either
533 // cur_younger_gen_and_prev_non_clean or leave
534 // it with its stale value -- because the promotions didn't
535 // result in any younger refs on that card. Of these two
536 // cases, the latter will be covered in Case 1a during
537 // a subsequent scan. To deal with the former case, we need
538 // to further consider how we deal with a stale value of
539 // cur_younger_gen_and_prev_non_clean in our case analysis
540 // below. This we do in Case 3 below. [End Case 1b]
541 // [End Case 1]
542 // o Case 2. If the stale value corresponds to cur_younger_gen being
543 // a value not necessarily written by a current promotion, the
544 // card will not be scanned by the younger refs scanning code.
545 // (This is OK since as we argued above such cards cannot contain
546 // any younger refs.) The result is that this value will be
547 // treated as a prev_younger_gen value in a subsequent collection,
548 // which is addressed in Case 1 above. [End Case 2]
549 // o Case 3. We here consider the "derivative" case from Case 1b. above
550 // because of which we may find a stale
551 // cur_younger_gen_and_prev_non_clean card value in the table.
552 // Once again, as in Case 1, we consider two subcases, depending
553 // on whether the card lies in the occupied or unoccupied part
554 // of the space at the start of the young collection.
555 // o Case 3a. Let us say the card is in the occupied part of
556 // the old gen at the start of the young collection. In that
557 // case, the card will be scanned by the younger refs scanning
558 // code which will set it to cur_younger_gen. In a subsequent
559 // scan, the card will be considered again and get its final
560 // correct value. [End Case 3a]
561 // o Case 3b. Now consider the case where the card is in the
562 // unoccupied part of the old gen, and is occupied as a result
563 // of promotions during thus young gc. In that case,
564 // the card will not be scanned for younger refs. The presence
565 // of newly promoted objects on the card will then result in
566 // its keeping the value cur_younger_gen_and_prev_non_clean
567 // value, which we have dealt with in Case 3 here. [End Case 3b]
568 // [End Case 3]
569 //
570 // (Please refer to the code in the helper class
571 // ClearNonCleanCardWrapper and in CardTableModRefBS for details.)
572 //
573 // The informal arguments above can be tightened into a formal
574 // correctness proof and it behooves us to write up such a proof,
575 // or to use model checking to prove that there are no lingering
576 // concerns.
577 //
578 // Clearly because of Case 3b one cannot bound the time for
579 // which a card will retain what we have called a "stale" value.
580 // However, one can obtain a Loose upper bound on the redundant
581 // work as a result of such stale values. Note first that any
582 // time a stale card lies in the occupied part of the space at
583 // the start of the collection, it is scanned by younger refs
584 // code and we can define a rank function on card values that
585 // declines when this is so. Note also that when a card does not
586 // lie in the occupied part of the space at the beginning of a
587 // young collection, its rank can either decline or stay unchanged.
588 // In this case, no extra work is done in terms of redundant
589 // younger refs scanning of that card.
590 // Then, the case analysis above reveals that, in the worst case,
591 // any such stale card will be scanned unnecessarily at most twice.
592 //
593 // It is nonetheless advisable to try and get rid of some of this
594 // redundant work in a subsequent (low priority) re-design of
595 // the card-scanning code, if only to simplify the underlying
596 // state machine analysis/proof. ysr 1/28/2002. XXX
597 cur_entry++;
598 }
599 }
600 }
601
602 void CardTableRS::verify() {
603 // At present, we only know how to verify the card table RS for
604 // generational heaps.
605 VerifyCTGenClosure blk(this);
606 GenCollectedHeap::heap()->generation_iterate(&blk, false);
607 _ct_bs->verify();
608 }