1 /*
2 * Copyright (c) 2001, 2013, 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.
18 *
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.hpp"
31 #include "oops/oop.inline.hpp"
32 #include "runtime/java.hpp"
33 #include "runtime/os.hpp"
34 #include "utilities/macros.hpp"
35 #if INCLUDE_ALL_GCS
36 #include "gc_implementation/g1/concurrentMark.hpp"
37 #include "gc_implementation/g1/g1SATBCardTableModRefBS.hpp"
38 #endif // INCLUDE_ALL_GCS
39
40 CardTableRS::CardTableRS(MemRegion whole_heap,
41 int max_covered_regions) :
42 GenRemSet(),
43 _cur_youngergen_card_val(youngergenP1_card),
44 _regions_to_iterate(max_covered_regions - 1)
45 {
46 #if INCLUDE_ALL_GCS
47 if (UseG1GC) {
48 _ct_bs = new G1SATBCardTableLoggingModRefBS(whole_heap,
49 max_covered_regions);
50 } else {
51 _ct_bs = new CardTableModRefBSForCTRS(whole_heap, max_covered_regions);
52 }
53 #else
54 _ct_bs = new CardTableModRefBSForCTRS(whole_heap, max_covered_regions);
55 #endif
56 set_bs(_ct_bs);
57 _last_cur_val_in_gen = NEW_C_HEAP_ARRAY3(jbyte, GenCollectedHeap::max_gens + 1,
58 mtGC, 0, AllocFailStrategy::RETURN_NULL);
59 if (_last_cur_val_in_gen == NULL) {
60 vm_exit_during_initialization("Could not create last_cur_val_in_gen array.");
61 }
62 for (int i = 0; i < GenCollectedHeap::max_gens + 1; i++) {
63 _last_cur_val_in_gen[i] = clean_card_val();
64 }
65 _ct_bs->set_CTRS(this);
66 }
67
68 CardTableRS::~CardTableRS() {
69 if (_ct_bs) {
70 delete _ct_bs;
71 _ct_bs = NULL;
72 }
73 if (_last_cur_val_in_gen) {
74 FREE_C_HEAP_ARRAY(jbyte, _last_cur_val_in_gen, mtInternal);
75 }
76 }
77
78 void CardTableRS::resize_covered_region(MemRegion new_region) {
79 _ct_bs->resize_covered_region(new_region);
80 }
81
82 jbyte CardTableRS::find_unused_youngergenP_card_value() {
83 for (jbyte v = youngergenP1_card;
84 v < cur_youngergen_and_prev_nonclean_card;
85 v++) {
86 bool seen = false;
87 for (int g = 0; g < _regions_to_iterate; g++) {
88 if (_last_cur_val_in_gen[g] == v) {
89 seen = true;
90 break;
91 }
92 }
93 if (!seen) return v;
94 }
95 ShouldNotReachHere();
96 return 0;
97 }
98
99 void CardTableRS::prepare_for_younger_refs_iterate(bool parallel) {
100 // Parallel or sequential, we must always set the prev to equal the
101 // last one written.
102 if (parallel) {
103 // Find a parallel value to be used next.
104 jbyte next_val = find_unused_youngergenP_card_value();
105 set_cur_youngergen_card_val(next_val);
106
107 } else {
108 // In an sequential traversal we will always write youngergen, so that
109 // the inline barrier is correct.
110 set_cur_youngergen_card_val(youngergen_card);
111 }
112 }
113
114 void CardTableRS::younger_refs_iterate(Generation* g,
115 OopsInGenClosure* blk) {
116 _last_cur_val_in_gen[g->level()+1] = cur_youngergen_card_val();
117 g->younger_refs_iterate(blk);
118 }
119
120 inline bool ClearNoncleanCardWrapper::clear_card(jbyte* entry) {
121 if (_is_par) {
122 return clear_card_parallel(entry);
123 } else {
124 return clear_card_serial(entry);
125 }
126 }
127
128 inline bool ClearNoncleanCardWrapper::clear_card_parallel(jbyte* entry) {
129 while (true) {
130 // In the parallel case, we may have to do this several times.
131 jbyte entry_val = *entry;
132 assert(entry_val != CardTableRS::clean_card_val(),
133 "We shouldn't be looking at clean cards, and this should "
134 "be the only place they get cleaned.");
135 if (CardTableRS::card_is_dirty_wrt_gen_iter(entry_val)
136 || _ct->is_prev_youngergen_card_val(entry_val)) {
137 jbyte res =
138 Atomic::cmpxchg(CardTableRS::clean_card_val(), entry, entry_val);
139 if (res == entry_val) {
140 break;
141 } else {
142 assert(res == CardTableRS::cur_youngergen_and_prev_nonclean_card,
143 "The CAS above should only fail if another thread did "
144 "a GC write barrier.");
145 }
146 } else if (entry_val ==
147 CardTableRS::cur_youngergen_and_prev_nonclean_card) {
148 // Parallelism shouldn't matter in this case. Only the thread
149 // assigned to scan the card should change this value.
150 *entry = _ct->cur_youngergen_card_val();
151 break;
152 } else {
153 assert(entry_val == _ct->cur_youngergen_card_val(),
154 "Should be the only possibility.");
155 // In this case, the card was clean before, and become
156 // cur_youngergen only because of processing of a promoted object.
157 // We don't have to look at the card.
158 return false;
159 }
160 }
161 return true;
162 }
163
164
165 inline bool ClearNoncleanCardWrapper::clear_card_serial(jbyte* entry) {
166 jbyte entry_val = *entry;
167 assert(entry_val != CardTableRS::clean_card_val(),
168 "We shouldn't be looking at clean cards, and this should "
169 "be the only place they get cleaned.");
170 assert(entry_val != CardTableRS::cur_youngergen_and_prev_nonclean_card,
171 "This should be possible in the sequential case.");
172 *entry = CardTableRS::clean_card_val();
173 return true;
174 }
175
176 ClearNoncleanCardWrapper::ClearNoncleanCardWrapper(
177 DirtyCardToOopClosure* dirty_card_closure, CardTableRS* ct) :
178 _dirty_card_closure(dirty_card_closure), _ct(ct) {
179 // Cannot yet substitute active_workers for n_par_threads
180 // in the case where parallelism is being turned off by
181 // setting n_par_threads to 0.
182 _is_par = (SharedHeap::heap()->n_par_threads() > 0);
183 assert(!_is_par ||
184 (SharedHeap::heap()->n_par_threads() ==
185 SharedHeap::heap()->workers()->active_workers()), "Mismatch");
186 }
187
188 bool ClearNoncleanCardWrapper::is_word_aligned(jbyte* entry) {
189 return (((intptr_t)entry) & (BytesPerWord-1)) == 0;
190 }
191
192 void ClearNoncleanCardWrapper::do_MemRegion(MemRegion mr) {
193 assert(mr.word_size() > 0, "Error");
194 assert(_ct->is_aligned(mr.start()), "mr.start() should be card aligned");
195 // mr.end() may not necessarily be card aligned.
196 jbyte* cur_entry = _ct->byte_for(mr.last());
197 const jbyte* limit = _ct->byte_for(mr.start());
198 HeapWord* end_of_non_clean = mr.end();
199 HeapWord* start_of_non_clean = end_of_non_clean;
200 while (cur_entry >= limit) {
201 HeapWord* cur_hw = _ct->addr_for(cur_entry);
202 if ((*cur_entry != CardTableRS::clean_card_val()) && clear_card(cur_entry)) {
203 // Continue the dirty range by opening the
204 // dirty window one card to the left.
205 start_of_non_clean = cur_hw;
206 } else {
207 // We hit a "clean" card; process any non-empty
208 // "dirty" range accumulated so far.
209 if (start_of_non_clean < end_of_non_clean) {
210 const MemRegion mrd(start_of_non_clean, end_of_non_clean);
211 _dirty_card_closure->do_MemRegion(mrd);
212 }
213
214 // fast forward through potential continuous whole-word range of clean cards beginning at a word-boundary
215 if (is_word_aligned(cur_entry)) {
216 jbyte* cur_row = cur_entry - BytesPerWord;
217 while (cur_row >= limit && *((intptr_t*)cur_row) == CardTableRS::clean_card_row()) {
218 cur_row -= BytesPerWord;
219 }
220 cur_entry = cur_row + BytesPerWord;
221 cur_hw = _ct->addr_for(cur_entry);
222 }
223
224 // Reset the dirty window, while continuing to look
225 // for the next dirty card that will start a
226 // new dirty window.
227 end_of_non_clean = cur_hw;
228 start_of_non_clean = cur_hw;
229 }
230 // Note that "cur_entry" leads "start_of_non_clean" in
231 // its leftward excursion after this point
232 // in the loop and, when we hit the left end of "mr",
233 // will point off of the left end of the card-table
234 // for "mr".
235 cur_entry--;
236 }
237 // If the first card of "mr" was dirty, we will have
238 // been left with a dirty window, co-initial with "mr",
239 // which we now process.
240 if (start_of_non_clean < end_of_non_clean) {
241 const MemRegion mrd(start_of_non_clean, end_of_non_clean);
242 _dirty_card_closure->do_MemRegion(mrd);
243 }
244 }
245
246 // clean (by dirty->clean before) ==> cur_younger_gen
247 // dirty ==> cur_youngergen_and_prev_nonclean_card
248 // precleaned ==> cur_youngergen_and_prev_nonclean_card
249 // prev-younger-gen ==> cur_youngergen_and_prev_nonclean_card
250 // cur-younger-gen ==> cur_younger_gen
251 // cur_youngergen_and_prev_nonclean_card ==> no change.
252 void CardTableRS::write_ref_field_gc_par(void* field, oop new_val) {
253 jbyte* entry = ct_bs()->byte_for(field);
254 do {
255 jbyte entry_val = *entry;
256 // We put this first because it's probably the most common case.
257 if (entry_val == clean_card_val()) {
258 // No threat of contention with cleaning threads.
259 *entry = cur_youngergen_card_val();
260 return;
261 } else if (card_is_dirty_wrt_gen_iter(entry_val)
262 || is_prev_youngergen_card_val(entry_val)) {
263 // Mark it as both cur and prev youngergen; card cleaning thread will
264 // eventually remove the previous stuff.
265 jbyte new_val = cur_youngergen_and_prev_nonclean_card;
266 jbyte res = Atomic::cmpxchg(new_val, entry, entry_val);
267 // Did the CAS succeed?
268 if (res == entry_val) return;
269 // Otherwise, retry, to see the new value.
270 continue;
271 } else {
272 assert(entry_val == cur_youngergen_and_prev_nonclean_card
273 || entry_val == cur_youngergen_card_val(),
274 "should be only possibilities.");
275 return;
276 }
277 } while (true);
278 }
279
280 void CardTableRS::younger_refs_in_space_iterate(Space* sp,
281 OopsInGenClosure* cl) {
282 const MemRegion urasm = sp->used_region_at_save_marks();
283 #ifdef ASSERT
284 // Convert the assertion check to a warning if we are running
285 // CMS+ParNew until related bug is fixed.
286 MemRegion ur = sp->used_region();
287 assert(ur.contains(urasm) || (UseConcMarkSweepGC && UseParNewGC),
288 err_msg("Did you forget to call save_marks()? "
289 "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in "
290 "[" PTR_FORMAT ", " PTR_FORMAT ")",
291 urasm.start(), urasm.end(), ur.start(), ur.end()));
292 // In the case of CMS+ParNew, issue a warning
293 if (!ur.contains(urasm)) {
294 assert(UseConcMarkSweepGC && UseParNewGC, "Tautology: see assert above");
295 warning("CMS+ParNew: Did you forget to call save_marks()? "
296 "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in "
297 "[" PTR_FORMAT ", " PTR_FORMAT ")",
298 urasm.start(), urasm.end(), ur.start(), ur.end());
299 MemRegion ur2 = sp->used_region();
300 MemRegion urasm2 = sp->used_region_at_save_marks();
301 if (!ur.equals(ur2)) {
302 warning("CMS+ParNew: Flickering used_region()!!");
303 }
304 if (!urasm.equals(urasm2)) {
305 warning("CMS+ParNew: Flickering used_region_at_save_marks()!!");
306 }
307 ShouldNotReachHere();
308 }
309 #endif
310 _ct_bs->non_clean_card_iterate_possibly_parallel(sp, urasm, cl, this);
311 }
312
313 void CardTableRS::clear_into_younger(Generation* old_gen) {
314 assert(old_gen->level() == 1, "Should only be called for the old generation");
315 GenCollectedHeap* gch = GenCollectedHeap::heap();
316 // Generations younger than gen have been evacuated. We can clear
317 // card table entries for gen (we know that it has no pointers
318 // to younger gens) and for those below. The card tables for
319 // the youngest gen need never be cleared.
320 // There's a bit of subtlety in the clear() and invalidate()
321 // methods that we exploit here and in invalidate_or_clear()
322 // below to avoid missing cards at the fringes. If clear() or
323 // invalidate() are changed in the future, this code should
324 // be revisited. 20040107.ysr
325 clear(old_gen->prev_used_region());
326 }
327
328 void CardTableRS::invalidate_or_clear(Generation* old_gen) {
329 assert(old_gen->level() == 1, "Should only be called for the old generation");
330 // For generation gen invalidate the cards for the currently
331 // occupied part of that generation and clear the cards for the
332 // unoccupied part of the generation (if any, making use
333 // of that generation's prev_used_region to determine that
334 // region). No need to do anything for the youngest
335 // generation. Also see note#20040107.ysr above.
336 MemRegion used_mr = old_gen->used_region();
337 MemRegion to_be_cleared_mr = old_gen->prev_used_region().minus(used_mr);
338 if (!to_be_cleared_mr.is_empty()) {
339 clear(to_be_cleared_mr);
340 }
341 invalidate(used_mr);
342 }
343
344
345 class VerifyCleanCardClosure: public OopClosure {
346 private:
347 HeapWord* _boundary;
348 HeapWord* _begin;
349 HeapWord* _end;
350 protected:
351 template <class T> void do_oop_work(T* p) {
352 HeapWord* jp = (HeapWord*)p;
353 assert(jp >= _begin && jp < _end,
354 err_msg("Error: jp " PTR_FORMAT " should be within "
355 "[_begin, _end) = [" PTR_FORMAT "," PTR_FORMAT ")",
356 jp, _begin, _end));
357 oop obj = oopDesc::load_decode_heap_oop(p);
358 guarantee(obj == NULL || (HeapWord*)obj >= _boundary,
359 err_msg("pointer " PTR_FORMAT " at " PTR_FORMAT " on "
360 "clean card crosses boundary" PTR_FORMAT,
361 (HeapWord*)obj, jp, _boundary));
362 }
363
364 public:
365 VerifyCleanCardClosure(HeapWord* b, HeapWord* begin, HeapWord* end) :
366 _boundary(b), _begin(begin), _end(end) {
367 assert(b <= begin,
368 err_msg("Error: boundary " PTR_FORMAT " should be at or below begin " PTR_FORMAT,
369 b, begin));
370 assert(begin <= end,
371 err_msg("Error: begin " PTR_FORMAT " should be strictly below end " PTR_FORMAT,
372 begin, end));
373 }
374
375 virtual void do_oop(oop* p) { VerifyCleanCardClosure::do_oop_work(p); }
376 virtual void do_oop(narrowOop* p) { VerifyCleanCardClosure::do_oop_work(p); }
377 };
378
379 class VerifyCTSpaceClosure: public SpaceClosure {
380 private:
381 CardTableRS* _ct;
382 HeapWord* _boundary;
383 public:
384 VerifyCTSpaceClosure(CardTableRS* ct, HeapWord* boundary) :
385 _ct(ct), _boundary(boundary) {}
386 virtual void do_space(Space* s) { _ct->verify_space(s, _boundary); }
387 };
388
389 class VerifyCTGenClosure: public GenCollectedHeap::GenClosure {
390 CardTableRS* _ct;
391 public:
392 VerifyCTGenClosure(CardTableRS* ct) : _ct(ct) {}
393 void do_generation(Generation* gen) {
394 // Skip the youngest generation.
395 if (gen->level() == 0) return;
396 // Normally, we're interested in pointers to younger generations.
397 VerifyCTSpaceClosure blk(_ct, gen->reserved().start());
398 gen->space_iterate(&blk, true);
399 }
400 };
401
402 void CardTableRS::verify_space(Space* s, HeapWord* gen_boundary) {
403 // We don't need to do young-gen spaces.
404 if (s->end() <= gen_boundary) return;
405 MemRegion used = s->used_region();
406
407 jbyte* cur_entry = byte_for(used.start());
408 jbyte* limit = byte_after(used.last());
409 while (cur_entry < limit) {
410 if (*cur_entry == CardTableModRefBS::clean_card) {
411 jbyte* first_dirty = cur_entry+1;
412 while (first_dirty < limit &&
413 *first_dirty == CardTableModRefBS::clean_card) {
414 first_dirty++;
415 }
416 // If the first object is a regular object, and it has a
417 // young-to-old field, that would mark the previous card.
418 HeapWord* boundary = addr_for(cur_entry);
419 HeapWord* end = (first_dirty >= limit) ? used.end() : addr_for(first_dirty);
420 HeapWord* boundary_block = s->block_start(boundary);
421 HeapWord* begin = boundary; // Until proven otherwise.
422 HeapWord* start_block = boundary_block; // Until proven otherwise.
423 if (boundary_block < boundary) {
424 if (s->block_is_obj(boundary_block) && s->obj_is_alive(boundary_block)) {
425 oop boundary_obj = oop(boundary_block);
426 if (!boundary_obj->is_objArray() &&
427 !boundary_obj->is_typeArray()) {
428 guarantee(cur_entry > byte_for(used.start()),
429 "else boundary would be boundary_block");
430 if (*byte_for(boundary_block) != CardTableModRefBS::clean_card) {
431 begin = boundary_block + s->block_size(boundary_block);
432 start_block = begin;
433 }
434 }
435 }
436 }
437 // Now traverse objects until end.
438 if (begin < end) {
439 MemRegion mr(begin, end);
440 VerifyCleanCardClosure verify_blk(gen_boundary, begin, end);
441 for (HeapWord* cur = start_block; cur < end; cur += s->block_size(cur)) {
442 if (s->block_is_obj(cur) && s->obj_is_alive(cur)) {
443 oop(cur)->oop_iterate_no_header(&verify_blk, mr);
444 }
445 }
446 }
447 cur_entry = first_dirty;
448 } else {
449 // We'd normally expect that cur_youngergen_and_prev_nonclean_card
450 // is a transient value, that cannot be in the card table
451 // except during GC, and thus assert that:
452 // guarantee(*cur_entry != cur_youngergen_and_prev_nonclean_card,
453 // "Illegal CT value");
454 // That however, need not hold, as will become clear in the
455 // following...
456
457 // We'd normally expect that if we are in the parallel case,
458 // we can't have left a prev value (which would be different
459 // from the current value) in the card table, and so we'd like to
460 // assert that:
461 // guarantee(cur_youngergen_card_val() == youngergen_card
462 // || !is_prev_youngergen_card_val(*cur_entry),
463 // "Illegal CT value");
464 // That, however, may not hold occasionally, because of
465 // CMS or MSC in the old gen. To wit, consider the
466 // following two simple illustrative scenarios:
467 // (a) CMS: Consider the case where a large object L
468 // spanning several cards is allocated in the old
469 // gen, and has a young gen reference stored in it, dirtying
470 // some interior cards. A young collection scans the card,
471 // finds a young ref and installs a youngergenP_n value.
472 // L then goes dead. Now a CMS collection starts,
473 // finds L dead and sweeps it up. Assume that L is
474 // abutting _unallocated_blk, so _unallocated_blk is
475 // adjusted down to (below) L. Assume further that
476 // no young collection intervenes during this CMS cycle.
477 // The next young gen cycle will not get to look at this
478 // youngergenP_n card since it lies in the unoccupied
479 // part of the space.
480 // Some young collections later the blocks on this
481 // card can be re-allocated either due to direct allocation
482 // or due to absorbing promotions. At this time, the
483 // before-gc verification will fail the above assert.
484 // (b) MSC: In this case, an object L with a young reference
485 // is on a card that (therefore) holds a youngergen_n value.
486 // Suppose also that L lies towards the end of the used
487 // the used space before GC. An MSC collection
488 // occurs that compacts to such an extent that this
489 // card is no longer in the occupied part of the space.
490 // Since current code in MSC does not always clear cards
491 // in the unused part of old gen, this stale youngergen_n
492 // value is left behind and can later be covered by
493 // an object when promotion or direct allocation
494 // re-allocates that part of the heap.
495 //
496 // Fortunately, the presence of such stale card values is
497 // "only" a minor annoyance in that subsequent young collections
498 // might needlessly scan such cards, but would still never corrupt
499 // the heap as a result. However, it's likely not to be a significant
500 // performance inhibitor in practice. For instance,
501 // some recent measurements with unoccupied cards eagerly cleared
502 // out to maintain this invariant, showed next to no
503 // change in young collection times; of course one can construct
504 // degenerate examples where the cost can be significant.)
505 // Note, in particular, that if the "stale" card is modified
506 // after re-allocation, it would be dirty, not "stale". Thus,
507 // we can never have a younger ref in such a card and it is
508 // safe not to scan that card in any collection. [As we see
509 // below, we do some unnecessary scanning
510 // in some cases in the current parallel scanning algorithm.]
511 //
512 // The main point below is that the parallel card scanning code
513 // deals correctly with these stale card values. There are two main
514 // cases to consider where we have a stale "younger gen" value and a
515 // "derivative" case to consider, where we have a stale
516 // "cur_younger_gen_and_prev_non_clean" value, as will become
517 // apparent in the case analysis below.
518 // o Case 1. If the stale value corresponds to a younger_gen_n
519 // value other than the cur_younger_gen value then the code
520 // treats this as being tantamount to a prev_younger_gen
521 // card. This means that the card may be unnecessarily scanned.
522 // There are two sub-cases to consider:
523 // o Case 1a. Let us say that the card is in the occupied part
524 // of the generation at the time the collection begins. In
525 // that case the card will be either cleared when it is scanned
526 // for young pointers, or will be set to cur_younger_gen as a
527 // result of promotion. (We have elided the normal case where
528 // the scanning thread and the promoting thread interleave
529 // possibly resulting in a transient
530 // cur_younger_gen_and_prev_non_clean value before settling
531 // to cur_younger_gen. [End Case 1a.]
532 // o Case 1b. Consider now the case when the card is in the unoccupied
533 // part of the space which becomes occupied because of promotions
534 // into it during the current young GC. In this case the card
535 // will never be scanned for young references. The current
536 // code will set the card value to either
537 // cur_younger_gen_and_prev_non_clean or leave
538 // it with its stale value -- because the promotions didn't
539 // result in any younger refs on that card. Of these two
540 // cases, the latter will be covered in Case 1a during
541 // a subsequent scan. To deal with the former case, we need
542 // to further consider how we deal with a stale value of
543 // cur_younger_gen_and_prev_non_clean in our case analysis
544 // below. This we do in Case 3 below. [End Case 1b]
545 // [End Case 1]
546 // o Case 2. If the stale value corresponds to cur_younger_gen being
547 // a value not necessarily written by a current promotion, the
548 // card will not be scanned by the younger refs scanning code.
549 // (This is OK since as we argued above such cards cannot contain
550 // any younger refs.) The result is that this value will be
551 // treated as a prev_younger_gen value in a subsequent collection,
552 // which is addressed in Case 1 above. [End Case 2]
553 // o Case 3. We here consider the "derivative" case from Case 1b. above
554 // because of which we may find a stale
555 // cur_younger_gen_and_prev_non_clean card value in the table.
556 // Once again, as in Case 1, we consider two subcases, depending
557 // on whether the card lies in the occupied or unoccupied part
558 // of the space at the start of the young collection.
559 // o Case 3a. Let us say the card is in the occupied part of
560 // the old gen at the start of the young collection. In that
561 // case, the card will be scanned by the younger refs scanning
562 // code which will set it to cur_younger_gen. In a subsequent
563 // scan, the card will be considered again and get its final
564 // correct value. [End Case 3a]
565 // o Case 3b. Now consider the case where the card is in the
566 // unoccupied part of the old gen, and is occupied as a result
567 // of promotions during thus young gc. In that case,
568 // the card will not be scanned for younger refs. The presence
569 // of newly promoted objects on the card will then result in
570 // its keeping the value cur_younger_gen_and_prev_non_clean
571 // value, which we have dealt with in Case 3 here. [End Case 3b]
572 // [End Case 3]
573 //
574 // (Please refer to the code in the helper class
575 // ClearNonCleanCardWrapper and in CardTableModRefBS for details.)
576 //
577 // The informal arguments above can be tightened into a formal
578 // correctness proof and it behooves us to write up such a proof,
579 // or to use model checking to prove that there are no lingering
580 // concerns.
581 //
582 // Clearly because of Case 3b one cannot bound the time for
583 // which a card will retain what we have called a "stale" value.
584 // However, one can obtain a Loose upper bound on the redundant
585 // work as a result of such stale values. Note first that any
586 // time a stale card lies in the occupied part of the space at
587 // the start of the collection, it is scanned by younger refs
588 // code and we can define a rank function on card values that
589 // declines when this is so. Note also that when a card does not
590 // lie in the occupied part of the space at the beginning of a
591 // young collection, its rank can either decline or stay unchanged.
592 // In this case, no extra work is done in terms of redundant
593 // younger refs scanning of that card.
594 // Then, the case analysis above reveals that, in the worst case,
595 // any such stale card will be scanned unnecessarily at most twice.
596 //
597 // It is nonethelss advisable to try and get rid of some of this
598 // redundant work in a subsequent (low priority) re-design of
599 // the card-scanning code, if only to simplify the underlying
600 // state machine analysis/proof. ysr 1/28/2002. XXX
601 cur_entry++;
602 }
603 }
604 }
605
606 void CardTableRS::verify() {
607 // At present, we only know how to verify the card table RS for
608 // generational heaps.
609 VerifyCTGenClosure blk(this);
610 CollectedHeap* ch = Universe::heap();
611
612 if (ch->kind() == CollectedHeap::GenCollectedHeap) {
613 GenCollectedHeap::heap()->generation_iterate(&blk, false);
614 _ct_bs->verify();
615 }
616 }
617
618
619 void CardTableRS::verify_aligned_region_empty(MemRegion mr) {
620 if (!mr.is_empty()) {
621 jbyte* cur_entry = byte_for(mr.start());
622 jbyte* limit = byte_after(mr.last());
623 // The region mr may not start on a card boundary so
624 // the first card may reflect a write to the space
625 // just prior to mr.
626 if (!is_aligned(mr.start())) {
627 cur_entry++;
628 }
629 for (;cur_entry < limit; cur_entry++) {
630 guarantee(*cur_entry == CardTableModRefBS::clean_card,
631 "Unexpected dirty card found");
632 }
633 }
634 }