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