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* gen) {
314 GenCollectedHeap* gch = GenCollectedHeap::heap();
315 // Generations younger than gen have been evacuated. We can clear
316 // card table entries for gen (we know that it has no pointers
317 // to younger gens) and for those below. The card tables for
318 // the youngest gen need never be cleared.
319 // There's a bit of subtlety in the clear() and invalidate()
320 // methods that we exploit here and in invalidate_or_clear()
321 // below to avoid missing cards at the fringes. If clear() or
322 // invalidate() are changed in the future, this code should
323 // be revisited. 20040107.ysr
324 Generation* g = gen;
325 for(Generation* prev_gen = gch->prev_gen(g);
326 prev_gen != NULL;
327 g = prev_gen, prev_gen = gch->prev_gen(g)) {
328 MemRegion to_be_cleared_mr = g->prev_used_region();
329 clear(to_be_cleared_mr);
330 }
331 }
332
333 void CardTableRS::invalidate_or_clear(Generation* gen, bool younger) {
334 GenCollectedHeap* gch = GenCollectedHeap::heap();
335 // For each generation gen (and younger)
336 // invalidate the cards for the currently occupied part
337 // of that generation and clear the cards for the
338 // unoccupied part of the generation (if any, making use
339 // of that generation's prev_used_region to determine that
340 // region). No need to do anything for the youngest
341 // generation. Also see note#20040107.ysr above.
342 Generation* g = gen;
343 for(Generation* prev_gen = gch->prev_gen(g); prev_gen != NULL;
344 g = prev_gen, prev_gen = gch->prev_gen(g)) {
345 MemRegion used_mr = g->used_region();
346 MemRegion to_be_cleared_mr = g->prev_used_region().minus(used_mr);
347 if (!to_be_cleared_mr.is_empty()) {
348 clear(to_be_cleared_mr);
349 }
350 invalidate(used_mr);
351 if (!younger) break;
352 }
353 }
354
355
356 class VerifyCleanCardClosure: public OopClosure {
357 private:
358 HeapWord* _boundary;
359 HeapWord* _begin;
360 HeapWord* _end;
361 protected:
362 template <class T> void do_oop_work(T* p) {
363 HeapWord* jp = (HeapWord*)p;
364 assert(jp >= _begin && jp < _end,
365 err_msg("Error: jp " PTR_FORMAT " should be within "
366 "[_begin, _end) = [" PTR_FORMAT "," PTR_FORMAT ")",
367 jp, _begin, _end));
368 oop obj = oopDesc::load_decode_heap_oop(p);
369 guarantee(obj == NULL || (HeapWord*)obj >= _boundary,
370 err_msg("pointer " PTR_FORMAT " at " PTR_FORMAT " on "
371 "clean card crosses boundary" PTR_FORMAT,
372 (HeapWord*)obj, jp, _boundary));
373 }
374
375 public:
376 VerifyCleanCardClosure(HeapWord* b, HeapWord* begin, HeapWord* end) :
377 _boundary(b), _begin(begin), _end(end) {
378 assert(b <= begin,
379 err_msg("Error: boundary " PTR_FORMAT " should be at or below begin " PTR_FORMAT,
380 b, begin));
381 assert(begin <= end,
382 err_msg("Error: begin " PTR_FORMAT " should be strictly below end " PTR_FORMAT,
383 begin, end));
384 }
385
386 virtual void do_oop(oop* p) { VerifyCleanCardClosure::do_oop_work(p); }
387 virtual void do_oop(narrowOop* p) { VerifyCleanCardClosure::do_oop_work(p); }
388 };
389
390 class VerifyCTSpaceClosure: public SpaceClosure {
391 private:
392 CardTableRS* _ct;
393 HeapWord* _boundary;
394 public:
395 VerifyCTSpaceClosure(CardTableRS* ct, HeapWord* boundary) :
396 _ct(ct), _boundary(boundary) {}
397 virtual void do_space(Space* s) { _ct->verify_space(s, _boundary); }
398 };
399
400 class VerifyCTGenClosure: public GenCollectedHeap::GenClosure {
401 CardTableRS* _ct;
402 public:
403 VerifyCTGenClosure(CardTableRS* ct) : _ct(ct) {}
404 void do_generation(Generation* gen) {
405 // Skip the youngest generation.
406 if (gen->level() == 0) return;
407 // Normally, we're interested in pointers to younger generations.
408 VerifyCTSpaceClosure blk(_ct, gen->reserved().start());
409 gen->space_iterate(&blk, true);
410 }
411 };
412
413 void CardTableRS::verify_space(Space* s, HeapWord* gen_boundary) {
414 // We don't need to do young-gen spaces.
415 if (s->end() <= gen_boundary) return;
416 MemRegion used = s->used_region();
417
418 jbyte* cur_entry = byte_for(used.start());
419 jbyte* limit = byte_after(used.last());
420 while (cur_entry < limit) {
421 if (*cur_entry == CardTableModRefBS::clean_card) {
422 jbyte* first_dirty = cur_entry+1;
423 while (first_dirty < limit &&
424 *first_dirty == CardTableModRefBS::clean_card) {
425 first_dirty++;
426 }
427 // If the first object is a regular object, and it has a
428 // young-to-old field, that would mark the previous card.
429 HeapWord* boundary = addr_for(cur_entry);
430 HeapWord* end = (first_dirty >= limit) ? used.end() : addr_for(first_dirty);
431 HeapWord* boundary_block = s->block_start(boundary);
432 HeapWord* begin = boundary; // Until proven otherwise.
433 HeapWord* start_block = boundary_block; // Until proven otherwise.
434 if (boundary_block < boundary) {
435 if (s->block_is_obj(boundary_block) && s->obj_is_alive(boundary_block)) {
436 oop boundary_obj = oop(boundary_block);
437 if (!boundary_obj->is_objArray() &&
438 !boundary_obj->is_typeArray()) {
439 guarantee(cur_entry > byte_for(used.start()),
440 "else boundary would be boundary_block");
441 if (*byte_for(boundary_block) != CardTableModRefBS::clean_card) {
442 begin = boundary_block + s->block_size(boundary_block);
443 start_block = begin;
444 }
445 }
446 }
447 }
448 // Now traverse objects until end.
449 if (begin < end) {
450 MemRegion mr(begin, end);
451 VerifyCleanCardClosure verify_blk(gen_boundary, begin, end);
452 for (HeapWord* cur = start_block; cur < end; cur += s->block_size(cur)) {
453 if (s->block_is_obj(cur) && s->obj_is_alive(cur)) {
454 oop(cur)->oop_iterate_no_header(&verify_blk, mr);
455 }
456 }
457 }
458 cur_entry = first_dirty;
459 } else {
460 // We'd normally expect that cur_youngergen_and_prev_nonclean_card
461 // is a transient value, that cannot be in the card table
462 // except during GC, and thus assert that:
463 // guarantee(*cur_entry != cur_youngergen_and_prev_nonclean_card,
464 // "Illegal CT value");
465 // That however, need not hold, as will become clear in the
466 // following...
467
468 // We'd normally expect that if we are in the parallel case,
469 // we can't have left a prev value (which would be different
470 // from the current value) in the card table, and so we'd like to
471 // assert that:
472 // guarantee(cur_youngergen_card_val() == youngergen_card
473 // || !is_prev_youngergen_card_val(*cur_entry),
474 // "Illegal CT value");
475 // That, however, may not hold occasionally, because of
476 // CMS or MSC in the old gen. To wit, consider the
477 // following two simple illustrative scenarios:
478 // (a) CMS: Consider the case where a large object L
479 // spanning several cards is allocated in the old
480 // gen, and has a young gen reference stored in it, dirtying
481 // some interior cards. A young collection scans the card,
482 // finds a young ref and installs a youngergenP_n value.
483 // L then goes dead. Now a CMS collection starts,
484 // finds L dead and sweeps it up. Assume that L is
485 // abutting _unallocated_blk, so _unallocated_blk is
486 // adjusted down to (below) L. Assume further that
487 // no young collection intervenes during this CMS cycle.
488 // The next young gen cycle will not get to look at this
489 // youngergenP_n card since it lies in the unoccupied
490 // part of the space.
491 // Some young collections later the blocks on this
492 // card can be re-allocated either due to direct allocation
493 // or due to absorbing promotions. At this time, the
494 // before-gc verification will fail the above assert.
495 // (b) MSC: In this case, an object L with a young reference
496 // is on a card that (therefore) holds a youngergen_n value.
497 // Suppose also that L lies towards the end of the used
498 // the used space before GC. An MSC collection
499 // occurs that compacts to such an extent that this
500 // card is no longer in the occupied part of the space.
501 // Since current code in MSC does not always clear cards
502 // in the unused part of old gen, this stale youngergen_n
503 // value is left behind and can later be covered by
504 // an object when promotion or direct allocation
505 // re-allocates that part of the heap.
506 //
507 // Fortunately, the presence of such stale card values is
508 // "only" a minor annoyance in that subsequent young collections
509 // might needlessly scan such cards, but would still never corrupt
510 // the heap as a result. However, it's likely not to be a significant
511 // performance inhibitor in practice. For instance,
512 // some recent measurements with unoccupied cards eagerly cleared
513 // out to maintain this invariant, showed next to no
514 // change in young collection times; of course one can construct
515 // degenerate examples where the cost can be significant.)
516 // Note, in particular, that if the "stale" card is modified
517 // after re-allocation, it would be dirty, not "stale". Thus,
518 // we can never have a younger ref in such a card and it is
519 // safe not to scan that card in any collection. [As we see
520 // below, we do some unnecessary scanning
521 // in some cases in the current parallel scanning algorithm.]
522 //
523 // The main point below is that the parallel card scanning code
524 // deals correctly with these stale card values. There are two main
525 // cases to consider where we have a stale "younger gen" value and a
526 // "derivative" case to consider, where we have a stale
527 // "cur_younger_gen_and_prev_non_clean" value, as will become
528 // apparent in the case analysis below.
529 // o Case 1. If the stale value corresponds to a younger_gen_n
530 // value other than the cur_younger_gen value then the code
531 // treats this as being tantamount to a prev_younger_gen
532 // card. This means that the card may be unnecessarily scanned.
533 // There are two sub-cases to consider:
534 // o Case 1a. Let us say that the card is in the occupied part
535 // of the generation at the time the collection begins. In
536 // that case the card will be either cleared when it is scanned
537 // for young pointers, or will be set to cur_younger_gen as a
538 // result of promotion. (We have elided the normal case where
539 // the scanning thread and the promoting thread interleave
540 // possibly resulting in a transient
541 // cur_younger_gen_and_prev_non_clean value before settling
542 // to cur_younger_gen. [End Case 1a.]
543 // o Case 1b. Consider now the case when the card is in the unoccupied
544 // part of the space which becomes occupied because of promotions
545 // into it during the current young GC. In this case the card
546 // will never be scanned for young references. The current
547 // code will set the card value to either
548 // cur_younger_gen_and_prev_non_clean or leave
549 // it with its stale value -- because the promotions didn't
550 // result in any younger refs on that card. Of these two
551 // cases, the latter will be covered in Case 1a during
552 // a subsequent scan. To deal with the former case, we need
553 // to further consider how we deal with a stale value of
554 // cur_younger_gen_and_prev_non_clean in our case analysis
555 // below. This we do in Case 3 below. [End Case 1b]
556 // [End Case 1]
557 // o Case 2. If the stale value corresponds to cur_younger_gen being
558 // a value not necessarily written by a current promotion, the
559 // card will not be scanned by the younger refs scanning code.
560 // (This is OK since as we argued above such cards cannot contain
561 // any younger refs.) The result is that this value will be
562 // treated as a prev_younger_gen value in a subsequent collection,
563 // which is addressed in Case 1 above. [End Case 2]
564 // o Case 3. We here consider the "derivative" case from Case 1b. above
565 // because of which we may find a stale
566 // cur_younger_gen_and_prev_non_clean card value in the table.
567 // Once again, as in Case 1, we consider two subcases, depending
568 // on whether the card lies in the occupied or unoccupied part
569 // of the space at the start of the young collection.
570 // o Case 3a. Let us say the card is in the occupied part of
571 // the old gen at the start of the young collection. In that
572 // case, the card will be scanned by the younger refs scanning
573 // code which will set it to cur_younger_gen. In a subsequent
574 // scan, the card will be considered again and get its final
575 // correct value. [End Case 3a]
576 // o Case 3b. Now consider the case where the card is in the
577 // unoccupied part of the old gen, and is occupied as a result
578 // of promotions during thus young gc. In that case,
579 // the card will not be scanned for younger refs. The presence
580 // of newly promoted objects on the card will then result in
581 // its keeping the value cur_younger_gen_and_prev_non_clean
582 // value, which we have dealt with in Case 3 here. [End Case 3b]
583 // [End Case 3]
584 //
585 // (Please refer to the code in the helper class
586 // ClearNonCleanCardWrapper and in CardTableModRefBS for details.)
587 //
588 // The informal arguments above can be tightened into a formal
589 // correctness proof and it behooves us to write up such a proof,
590 // or to use model checking to prove that there are no lingering
591 // concerns.
592 //
593 // Clearly because of Case 3b one cannot bound the time for
594 // which a card will retain what we have called a "stale" value.
595 // However, one can obtain a Loose upper bound on the redundant
596 // work as a result of such stale values. Note first that any
597 // time a stale card lies in the occupied part of the space at
598 // the start of the collection, it is scanned by younger refs
599 // code and we can define a rank function on card values that
600 // declines when this is so. Note also that when a card does not
601 // lie in the occupied part of the space at the beginning of a
602 // young collection, its rank can either decline or stay unchanged.
603 // In this case, no extra work is done in terms of redundant
604 // younger refs scanning of that card.
605 // Then, the case analysis above reveals that, in the worst case,
606 // any such stale card will be scanned unnecessarily at most twice.
607 //
608 // It is nonethelss advisable to try and get rid of some of this
609 // redundant work in a subsequent (low priority) re-design of
610 // the card-scanning code, if only to simplify the underlying
611 // state machine analysis/proof. ysr 1/28/2002. XXX
612 cur_entry++;
613 }
614 }
615 }
616
617 void CardTableRS::verify() {
618 // At present, we only know how to verify the card table RS for
619 // generational heaps.
620 VerifyCTGenClosure blk(this);
621 CollectedHeap* ch = Universe::heap();
622
623 if (ch->kind() == CollectedHeap::GenCollectedHeap) {
624 GenCollectedHeap::heap()->generation_iterate(&blk, false);
625 _ct_bs->verify();
626 }
627 }
628
629
630 void CardTableRS::verify_aligned_region_empty(MemRegion mr) {
631 if (!mr.is_empty()) {
632 jbyte* cur_entry = byte_for(mr.start());
633 jbyte* limit = byte_after(mr.last());
634 // The region mr may not start on a card boundary so
635 // the first card may reflect a write to the space
636 // just prior to mr.
637 if (!is_aligned(mr.start())) {
638 cur_entry++;
639 }
640 for (;cur_entry < limit; cur_entry++) {
641 guarantee(*cur_entry == CardTableModRefBS::clean_card,
642 "Unexpected dirty card found");
643 }
644 }
645 }