Thu, 07 Feb 2013 16:05:48 -0500
Merge
1 /*
2 * Copyright (c) 2001, 2012, 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 */
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
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 jbyte[GenCollectedHeap::max_gens + 1];
58 if (_last_cur_val_in_gen == NULL) {
59 vm_exit_during_initialization("Could not last_cur_val_in_gen array.");
60 }
61 for (int i = 0; i < GenCollectedHeap::max_gens + 1; i++) {
62 _last_cur_val_in_gen[i] = clean_card_val();
63 }
64 _ct_bs->set_CTRS(this);
65 }
67 void CardTableRS::resize_covered_region(MemRegion new_region) {
68 _ct_bs->resize_covered_region(new_region);
69 }
71 jbyte CardTableRS::find_unused_youngergenP_card_value() {
72 for (jbyte v = youngergenP1_card;
73 v < cur_youngergen_and_prev_nonclean_card;
74 v++) {
75 bool seen = false;
76 for (int g = 0; g < _regions_to_iterate; g++) {
77 if (_last_cur_val_in_gen[g] == v) {
78 seen = true;
79 break;
80 }
81 }
82 if (!seen) return v;
83 }
84 ShouldNotReachHere();
85 return 0;
86 }
88 void CardTableRS::prepare_for_younger_refs_iterate(bool parallel) {
89 // Parallel or sequential, we must always set the prev to equal the
90 // last one written.
91 if (parallel) {
92 // Find a parallel value to be used next.
93 jbyte next_val = find_unused_youngergenP_card_value();
94 set_cur_youngergen_card_val(next_val);
96 } else {
97 // In an sequential traversal we will always write youngergen, so that
98 // the inline barrier is correct.
99 set_cur_youngergen_card_val(youngergen_card);
100 }
101 }
103 void CardTableRS::younger_refs_iterate(Generation* g,
104 OopsInGenClosure* blk) {
105 _last_cur_val_in_gen[g->level()+1] = cur_youngergen_card_val();
106 g->younger_refs_iterate(blk);
107 }
109 inline bool ClearNoncleanCardWrapper::clear_card(jbyte* entry) {
110 if (_is_par) {
111 return clear_card_parallel(entry);
112 } else {
113 return clear_card_serial(entry);
114 }
115 }
117 inline bool ClearNoncleanCardWrapper::clear_card_parallel(jbyte* entry) {
118 while (true) {
119 // In the parallel case, we may have to do this several times.
120 jbyte entry_val = *entry;
121 assert(entry_val != CardTableRS::clean_card_val(),
122 "We shouldn't be looking at clean cards, and this should "
123 "be the only place they get cleaned.");
124 if (CardTableRS::card_is_dirty_wrt_gen_iter(entry_val)
125 || _ct->is_prev_youngergen_card_val(entry_val)) {
126 jbyte res =
127 Atomic::cmpxchg(CardTableRS::clean_card_val(), entry, entry_val);
128 if (res == entry_val) {
129 break;
130 } else {
131 assert(res == CardTableRS::cur_youngergen_and_prev_nonclean_card,
132 "The CAS above should only fail if another thread did "
133 "a GC write barrier.");
134 }
135 } else if (entry_val ==
136 CardTableRS::cur_youngergen_and_prev_nonclean_card) {
137 // Parallelism shouldn't matter in this case. Only the thread
138 // assigned to scan the card should change this value.
139 *entry = _ct->cur_youngergen_card_val();
140 break;
141 } else {
142 assert(entry_val == _ct->cur_youngergen_card_val(),
143 "Should be the only possibility.");
144 // In this case, the card was clean before, and become
145 // cur_youngergen only because of processing of a promoted object.
146 // We don't have to look at the card.
147 return false;
148 }
149 }
150 return true;
151 }
154 inline bool ClearNoncleanCardWrapper::clear_card_serial(jbyte* entry) {
155 jbyte entry_val = *entry;
156 assert(entry_val != CardTableRS::clean_card_val(),
157 "We shouldn't be looking at clean cards, and this should "
158 "be the only place they get cleaned.");
159 assert(entry_val != CardTableRS::cur_youngergen_and_prev_nonclean_card,
160 "This should be possible in the sequential case.");
161 *entry = CardTableRS::clean_card_val();
162 return true;
163 }
165 ClearNoncleanCardWrapper::ClearNoncleanCardWrapper(
166 DirtyCardToOopClosure* dirty_card_closure, CardTableRS* ct) :
167 _dirty_card_closure(dirty_card_closure), _ct(ct) {
168 // Cannot yet substitute active_workers for n_par_threads
169 // in the case where parallelism is being turned off by
170 // setting n_par_threads to 0.
171 _is_par = (SharedHeap::heap()->n_par_threads() > 0);
172 assert(!_is_par ||
173 (SharedHeap::heap()->n_par_threads() ==
174 SharedHeap::heap()->workers()->active_workers()), "Mismatch");
175 }
177 bool ClearNoncleanCardWrapper::is_word_aligned(jbyte* entry) {
178 return (((intptr_t)entry) & (BytesPerWord-1)) == 0;
179 }
181 void ClearNoncleanCardWrapper::do_MemRegion(MemRegion mr) {
182 assert(mr.word_size() > 0, "Error");
183 assert(_ct->is_aligned(mr.start()), "mr.start() should be card aligned");
184 // mr.end() may not necessarily be card aligned.
185 jbyte* cur_entry = _ct->byte_for(mr.last());
186 const jbyte* limit = _ct->byte_for(mr.start());
187 HeapWord* end_of_non_clean = mr.end();
188 HeapWord* start_of_non_clean = end_of_non_clean;
189 while (cur_entry >= limit) {
190 HeapWord* cur_hw = _ct->addr_for(cur_entry);
191 if ((*cur_entry != CardTableRS::clean_card_val()) && clear_card(cur_entry)) {
192 // Continue the dirty range by opening the
193 // dirty window one card to the left.
194 start_of_non_clean = cur_hw;
195 } else {
196 // We hit a "clean" card; process any non-empty
197 // "dirty" range accumulated so far.
198 if (start_of_non_clean < end_of_non_clean) {
199 const MemRegion mrd(start_of_non_clean, end_of_non_clean);
200 _dirty_card_closure->do_MemRegion(mrd);
201 }
203 // fast forward through potential continuous whole-word range of clean cards beginning at a word-boundary
204 if (is_word_aligned(cur_entry)) {
205 jbyte* cur_row = cur_entry - BytesPerWord;
206 while (cur_row >= limit && *((intptr_t*)cur_row) == CardTableRS::clean_card_row()) {
207 cur_row -= BytesPerWord;
208 }
209 cur_entry = cur_row + BytesPerWord;
210 cur_hw = _ct->addr_for(cur_entry);
211 }
213 // Reset the dirty window, while continuing to look
214 // for the next dirty card that will start a
215 // new dirty window.
216 end_of_non_clean = cur_hw;
217 start_of_non_clean = cur_hw;
218 }
219 // Note that "cur_entry" leads "start_of_non_clean" in
220 // its leftward excursion after this point
221 // in the loop and, when we hit the left end of "mr",
222 // will point off of the left end of the card-table
223 // for "mr".
224 cur_entry--;
225 }
226 // If the first card of "mr" was dirty, we will have
227 // been left with a dirty window, co-initial with "mr",
228 // which we now process.
229 if (start_of_non_clean < end_of_non_clean) {
230 const MemRegion mrd(start_of_non_clean, end_of_non_clean);
231 _dirty_card_closure->do_MemRegion(mrd);
232 }
233 }
235 // clean (by dirty->clean before) ==> cur_younger_gen
236 // dirty ==> cur_youngergen_and_prev_nonclean_card
237 // precleaned ==> cur_youngergen_and_prev_nonclean_card
238 // prev-younger-gen ==> cur_youngergen_and_prev_nonclean_card
239 // cur-younger-gen ==> cur_younger_gen
240 // cur_youngergen_and_prev_nonclean_card ==> no change.
241 void CardTableRS::write_ref_field_gc_par(void* field, oop new_val) {
242 jbyte* entry = ct_bs()->byte_for(field);
243 do {
244 jbyte entry_val = *entry;
245 // We put this first because it's probably the most common case.
246 if (entry_val == clean_card_val()) {
247 // No threat of contention with cleaning threads.
248 *entry = cur_youngergen_card_val();
249 return;
250 } else if (card_is_dirty_wrt_gen_iter(entry_val)
251 || is_prev_youngergen_card_val(entry_val)) {
252 // Mark it as both cur and prev youngergen; card cleaning thread will
253 // eventually remove the previous stuff.
254 jbyte new_val = cur_youngergen_and_prev_nonclean_card;
255 jbyte res = Atomic::cmpxchg(new_val, entry, entry_val);
256 // Did the CAS succeed?
257 if (res == entry_val) return;
258 // Otherwise, retry, to see the new value.
259 continue;
260 } else {
261 assert(entry_val == cur_youngergen_and_prev_nonclean_card
262 || entry_val == cur_youngergen_card_val(),
263 "should be only possibilities.");
264 return;
265 }
266 } while (true);
267 }
269 void CardTableRS::younger_refs_in_space_iterate(Space* sp,
270 OopsInGenClosure* cl) {
271 const MemRegion urasm = sp->used_region_at_save_marks();
272 #ifdef ASSERT
273 // Convert the assertion check to a warning if we are running
274 // CMS+ParNew until related bug is fixed.
275 MemRegion ur = sp->used_region();
276 assert(ur.contains(urasm) || (UseConcMarkSweepGC && UseParNewGC),
277 err_msg("Did you forget to call save_marks()? "
278 "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in "
279 "[" PTR_FORMAT ", " PTR_FORMAT ")",
280 urasm.start(), urasm.end(), ur.start(), ur.end()));
281 // In the case of CMS+ParNew, issue a warning
282 if (!ur.contains(urasm)) {
283 assert(UseConcMarkSweepGC && UseParNewGC, "Tautology: see assert above");
284 warning("CMS+ParNew: Did you forget to call save_marks()? "
285 "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in "
286 "[" PTR_FORMAT ", " PTR_FORMAT ")",
287 urasm.start(), urasm.end(), ur.start(), ur.end());
288 MemRegion ur2 = sp->used_region();
289 MemRegion urasm2 = sp->used_region_at_save_marks();
290 if (!ur.equals(ur2)) {
291 warning("CMS+ParNew: Flickering used_region()!!");
292 }
293 if (!urasm.equals(urasm2)) {
294 warning("CMS+ParNew: Flickering used_region_at_save_marks()!!");
295 }
296 ShouldNotReachHere();
297 }
298 #endif
299 _ct_bs->non_clean_card_iterate_possibly_parallel(sp, urasm, cl, this);
300 }
302 void CardTableRS::clear_into_younger(Generation* gen) {
303 GenCollectedHeap* gch = GenCollectedHeap::heap();
304 // Generations younger than gen have been evacuated. We can clear
305 // card table entries for gen (we know that it has no pointers
306 // to younger gens) and for those below. The card tables for
307 // the youngest gen need never be cleared.
308 // There's a bit of subtlety in the clear() and invalidate()
309 // methods that we exploit here and in invalidate_or_clear()
310 // below to avoid missing cards at the fringes. If clear() or
311 // invalidate() are changed in the future, this code should
312 // be revisited. 20040107.ysr
313 Generation* g = gen;
314 for(Generation* prev_gen = gch->prev_gen(g);
315 prev_gen != NULL;
316 g = prev_gen, prev_gen = gch->prev_gen(g)) {
317 MemRegion to_be_cleared_mr = g->prev_used_region();
318 clear(to_be_cleared_mr);
319 }
320 }
322 void CardTableRS::invalidate_or_clear(Generation* gen, bool younger) {
323 GenCollectedHeap* gch = GenCollectedHeap::heap();
324 // For each generation gen (and younger)
325 // invalidate the cards for the currently occupied part
326 // of that generation and clear the cards for the
327 // unoccupied part of the generation (if any, making use
328 // of that generation's prev_used_region to determine that
329 // region). No need to do anything for the youngest
330 // generation. Also see note#20040107.ysr above.
331 Generation* g = gen;
332 for(Generation* prev_gen = gch->prev_gen(g); prev_gen != NULL;
333 g = prev_gen, prev_gen = gch->prev_gen(g)) {
334 MemRegion used_mr = g->used_region();
335 MemRegion to_be_cleared_mr = g->prev_used_region().minus(used_mr);
336 if (!to_be_cleared_mr.is_empty()) {
337 clear(to_be_cleared_mr);
338 }
339 invalidate(used_mr);
340 if (!younger) break;
341 }
342 }
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 _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 }
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 }
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 };
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 };
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 };
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();
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...
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 }
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();
612 if (ch->kind() == CollectedHeap::GenCollectedHeap) {
613 GenCollectedHeap::heap()->generation_iterate(&blk, false);
614 _ct_bs->verify();
615 }
616 }
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 }