1 /*
2 * Copyright (c) 2001, 2014, 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 "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/atomic.inline.hpp"
33 #include "runtime/java.hpp"
34 #include "runtime/os.hpp"
35 #include "utilities/macros.hpp"
36 #if INCLUDE_ALL_GCS
37 #include "gc_implementation/g1/concurrentMark.hpp"
38 #include "gc_implementation/g1/g1SATBCardTableModRefBS.hpp"
39 #endif // INCLUDE_ALL_GCS
40
41 CardTableRS::CardTableRS(MemRegion whole_heap,
42 int max_covered_regions) :
43 GenRemSet(),
44 _cur_youngergen_card_val(youngergenP1_card),
45 _regions_to_iterate(max_covered_regions - 1)
46 {
47 #if INCLUDE_ALL_GCS
48 if (UseG1GC) {
49 _ct_bs = new G1SATBCardTableLoggingModRefBS(whole_heap,
50 max_covered_regions);
51 } else {
52 _ct_bs = new CardTableModRefBSForCTRS(whole_heap, max_covered_regions);
53 }
54 #else
55 _ct_bs = new CardTableModRefBSForCTRS(whole_heap, max_covered_regions);
56 #endif
57 set_bs(_ct_bs);
58 _last_cur_val_in_gen = NEW_C_HEAP_ARRAY3(jbyte, GenCollectedHeap::max_gens + 1,
59 mtGC, 0, AllocFailStrategy::RETURN_NULL);
60 if (_last_cur_val_in_gen == NULL) {
61 vm_exit_during_initialization("Could not create last_cur_val_in_gen array.");
62 }
63 for (int i = 0; i < GenCollectedHeap::max_gens + 1; i++) {
64 _last_cur_val_in_gen[i] = clean_card_val();
65 }
66 _ct_bs->set_CTRS(this);
67 }
68
69 CardTableRS::~CardTableRS() {
70 if (_ct_bs) {
71 delete _ct_bs;
72 _ct_bs = NULL;
73 }
74 if (_last_cur_val_in_gen) {
75 FREE_C_HEAP_ARRAY(jbyte, _last_cur_val_in_gen, mtInternal);
76 }
77 }
78
79 void CardTableRS::resize_covered_region(MemRegion new_region) {
80 _ct_bs->resize_covered_region(new_region);
81 }
82
83 jbyte CardTableRS::find_unused_youngergenP_card_value() {
84 for (jbyte v = youngergenP1_card;
85 v < cur_youngergen_and_prev_nonclean_card;
86 v++) {
87 bool seen = false;
88 for (int g = 0; g < _regions_to_iterate; g++) {
89 if (_last_cur_val_in_gen[g] == v) {
90 seen = true;
91 break;
92 }
93 }
94 if (!seen) return v;
95 }
96 ShouldNotReachHere();
97 return 0;
98 }
99
100 void CardTableRS::prepare_for_younger_refs_iterate(bool parallel) {
101 // Parallel or sequential, we must always set the prev to equal the
102 // last one written.
103 if (parallel) {
104 // Find a parallel value to be used next.
105 jbyte next_val = find_unused_youngergenP_card_value();
106 set_cur_youngergen_card_val(next_val);
107
108 } else {
109 // In an sequential traversal we will always write youngergen, so that
110 // the inline barrier is correct.
111 set_cur_youngergen_card_val(youngergen_card);
112 }
113 }
114
115 void CardTableRS::younger_refs_iterate(Generation* g,
116 OopsInGenClosure* blk) {
117 _last_cur_val_in_gen[g->level()+1] = cur_youngergen_card_val();
118 g->younger_refs_iterate(blk);
119 }
120
121 inline bool ClearNoncleanCardWrapper::clear_card(jbyte* entry) {
122 if (_is_par) {
123 return clear_card_parallel(entry);
124 } else {
125 return clear_card_serial(entry);
126 }
127 }
128
129 inline bool ClearNoncleanCardWrapper::clear_card_parallel(jbyte* entry) {
130 while (true) {
131 // In the parallel case, we may have to do this several times.
132 jbyte entry_val = *entry;
133 assert(entry_val != CardTableRS::clean_card_val(),
134 "We shouldn't be looking at clean cards, and this should "
135 "be the only place they get cleaned.");
136 if (CardTableRS::card_is_dirty_wrt_gen_iter(entry_val)
137 || _ct->is_prev_youngergen_card_val(entry_val)) {
138 jbyte res =
139 Atomic::cmpxchg(CardTableRS::clean_card_val(), entry, entry_val);
140 if (res == entry_val) {
141 break;
142 } else {
143 assert(res == CardTableRS::cur_youngergen_and_prev_nonclean_card,
144 "The CAS above should only fail if another thread did "
145 "a GC write barrier.");
146 }
147 } else if (entry_val ==
148 CardTableRS::cur_youngergen_and_prev_nonclean_card) {
149 // Parallelism shouldn't matter in this case. Only the thread
150 // assigned to scan the card should change this value.
151 *entry = _ct->cur_youngergen_card_val();
152 break;
153 } else {
154 assert(entry_val == _ct->cur_youngergen_card_val(),
155 "Should be the only possibility.");
156 // In this case, the card was clean before, and become
157 // cur_youngergen only because of processing of a promoted object.
158 // We don't have to look at the card.
159 return false;
160 }
161 }
162 return true;
163 }
164
165
166 inline bool ClearNoncleanCardWrapper::clear_card_serial(jbyte* entry) {
167 jbyte entry_val = *entry;
168 assert(entry_val != CardTableRS::clean_card_val(),
169 "We shouldn't be looking at clean cards, and this should "
170 "be the only place they get cleaned.");
171 assert(entry_val != CardTableRS::cur_youngergen_and_prev_nonclean_card,
172 "This should be possible in the sequential case.");
173 *entry = CardTableRS::clean_card_val();
174 return true;
175 }
176
177 ClearNoncleanCardWrapper::ClearNoncleanCardWrapper(
178 DirtyCardToOopClosure* dirty_card_closure, CardTableRS* ct) :
179 _dirty_card_closure(dirty_card_closure), _ct(ct) {
180 // Cannot yet substitute active_workers for n_par_threads
181 // in the case where parallelism is being turned off by
182 // setting n_par_threads to 0.
183 _is_par = (SharedHeap::heap()->n_par_threads() > 0);
184 assert(!_is_par ||
185 (SharedHeap::heap()->n_par_threads() ==
186 SharedHeap::heap()->workers()->active_workers()), "Mismatch");
187 }
188
189 bool ClearNoncleanCardWrapper::is_word_aligned(jbyte* entry) {
190 return (((intptr_t)entry) & (BytesPerWord-1)) == 0;
191 }
192
193 void ClearNoncleanCardWrapper::do_MemRegion(MemRegion mr) {
194 assert(mr.word_size() > 0, "Error");
195 assert(_ct->is_aligned(mr.start()), "mr.start() should be card aligned");
196 // mr.end() may not necessarily be card aligned.
197 jbyte* cur_entry = _ct->byte_for(mr.last());
198 const jbyte* limit = _ct->byte_for(mr.start());
199 HeapWord* end_of_non_clean = mr.end();
200 HeapWord* start_of_non_clean = end_of_non_clean;
201 while (cur_entry >= limit) {
202 HeapWord* cur_hw = _ct->addr_for(cur_entry);
203 if ((*cur_entry != CardTableRS::clean_card_val()) && clear_card(cur_entry)) {
204 // Continue the dirty range by opening the
205 // dirty window one card to the left.
206 start_of_non_clean = cur_hw;
207 } else {
208 // We hit a "clean" card; process any non-empty
209 // "dirty" range accumulated so far.
210 if (start_of_non_clean < end_of_non_clean) {
211 const MemRegion mrd(start_of_non_clean, end_of_non_clean);
212 _dirty_card_closure->do_MemRegion(mrd);
213 }
214
215 // fast forward through potential continuous whole-word range of clean cards beginning at a word-boundary
216 if (is_word_aligned(cur_entry)) {
217 jbyte* cur_row = cur_entry - BytesPerWord;
218 while (cur_row >= limit && *((intptr_t*)cur_row) == CardTableRS::clean_card_row()) {
219 cur_row -= BytesPerWord;
220 }
221 cur_entry = cur_row + BytesPerWord;
222 cur_hw = _ct->addr_for(cur_entry);
223 }
224
225 // Reset the dirty window, while continuing to look
226 // for the next dirty card that will start a
227 // new dirty window.
228 end_of_non_clean = cur_hw;
229 start_of_non_clean = cur_hw;
230 }
231 // Note that "cur_entry" leads "start_of_non_clean" in
232 // its leftward excursion after this point
233 // in the loop and, when we hit the left end of "mr",
234 // will point off of the left end of the card-table
235 // for "mr".
236 cur_entry--;
237 }
238 // If the first card of "mr" was dirty, we will have
239 // been left with a dirty window, co-initial with "mr",
240 // which we now process.
241 if (start_of_non_clean < end_of_non_clean) {
242 const MemRegion mrd(start_of_non_clean, end_of_non_clean);
243 _dirty_card_closure->do_MemRegion(mrd);
244 }
245 }
246
247 // clean (by dirty->clean before) ==> cur_younger_gen
248 // dirty ==> cur_youngergen_and_prev_nonclean_card
249 // precleaned ==> cur_youngergen_and_prev_nonclean_card
250 // prev-younger-gen ==> cur_youngergen_and_prev_nonclean_card
251 // cur-younger-gen ==> cur_younger_gen
252 // cur_youngergen_and_prev_nonclean_card ==> no change.
253 void CardTableRS::write_ref_field_gc_par(void* field, oop new_val) {
254 jbyte* entry = ct_bs()->byte_for(field);
255 do {
256 jbyte entry_val = *entry;
257 // We put this first because it's probably the most common case.
258 if (entry_val == clean_card_val()) {
259 // No threat of contention with cleaning threads.
260 *entry = cur_youngergen_card_val();
261 return;
262 } else if (card_is_dirty_wrt_gen_iter(entry_val)
263 || is_prev_youngergen_card_val(entry_val)) {
264 // Mark it as both cur and prev youngergen; card cleaning thread will
265 // eventually remove the previous stuff.
266 jbyte new_val = cur_youngergen_and_prev_nonclean_card;
267 jbyte res = Atomic::cmpxchg(new_val, entry, entry_val);
268 // Did the CAS succeed?
269 if (res == entry_val) return;
270 // Otherwise, retry, to see the new value.
271 continue;
272 } else {
273 assert(entry_val == cur_youngergen_and_prev_nonclean_card
274 || entry_val == cur_youngergen_card_val(),
275 "should be only possibilities.");
276 return;
277 }
278 } while (true);
279 }
280
281 void CardTableRS::younger_refs_in_space_iterate(Space* sp,
282 OopsInGenClosure* cl) {
283 const MemRegion urasm = sp->used_region_at_save_marks();
284 #ifdef ASSERT
285 // Convert the assertion check to a warning if we are running
286 // CMS+ParNew until related bug is fixed.
287 MemRegion ur = sp->used_region();
288 assert(ur.contains(urasm) || (UseConcMarkSweepGC && UseParNewGC),
289 err_msg("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 // In the case of CMS+ParNew, issue a warning
294 if (!ur.contains(urasm)) {
295 assert(UseConcMarkSweepGC && UseParNewGC, "Tautology: see assert above");
296 warning("CMS+ParNew: Did you forget to call save_marks()? "
297 "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in "
298 "[" PTR_FORMAT ", " PTR_FORMAT ")",
299 p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end()));
300 MemRegion ur2 = sp->used_region();
301 MemRegion urasm2 = sp->used_region_at_save_marks();
302 if (!ur.equals(ur2)) {
303 warning("CMS+ParNew: Flickering used_region()!!");
304 }
305 if (!urasm.equals(urasm2)) {
306 warning("CMS+ParNew: Flickering used_region_at_save_marks()!!");
307 }
308 ShouldNotReachHere();
309 }
310 #endif
311 _ct_bs->non_clean_card_iterate_possibly_parallel(sp, urasm, cl, this);
312 }
313
314 void CardTableRS::clear_into_younger(Generation* old_gen) {
315 assert(old_gen->level() == 1, "Should only be called for the old generation");
316 // The card tables for the youngest gen need never be cleared.
317 // There's a bit of subtlety in the clear() and invalidate()
318 // methods that we exploit here and in invalidate_or_clear()
319 // below to avoid missing cards at the fringes. If clear() or
320 // invalidate() are changed in the future, this code should
321 // be revisited. 20040107.ysr
322 clear(old_gen->prev_used_region());
323 }
324
325 void CardTableRS::invalidate_or_clear(Generation* old_gen) {
326 assert(old_gen->level() == 1, "Should only be called for the old generation");
327 // Invalidate the cards for the currently occupied part of
328 // the old generation and clear the cards for the
329 // unoccupied part of the generation (if any, making use
330 // of that generation's prev_used_region to determine that
331 // region). No need to do anything for the youngest
332 // generation. Also see note#20040107.ysr above.
333 MemRegion used_mr = old_gen->used_region();
334 MemRegion to_be_cleared_mr = old_gen->prev_used_region().minus(used_mr);
335 if (!to_be_cleared_mr.is_empty()) {
336 clear(to_be_cleared_mr);
337 }
338 invalidate(used_mr);
339 }
340
341
342 class VerifyCleanCardClosure: public OopClosure {
343 private:
344 HeapWord* _boundary;
345 HeapWord* _begin;
346 HeapWord* _end;
347 protected:
348 template <class T> void do_oop_work(T* p) {
349 HeapWord* jp = (HeapWord*)p;
350 assert(jp >= _begin && jp < _end,
351 err_msg("Error: jp " PTR_FORMAT " should be within "
352 "[_begin, _end) = [" PTR_FORMAT "," PTR_FORMAT ")",
353 p2i(jp), p2i(_begin), p2i(_end)));
354 oop obj = oopDesc::load_decode_heap_oop(p);
355 guarantee(obj == NULL || (HeapWord*)obj >= _boundary,
356 err_msg("pointer " PTR_FORMAT " at " PTR_FORMAT " on "
357 "clean card crosses boundary" PTR_FORMAT,
358 p2i((HeapWord*)obj), p2i(jp), p2i(_boundary)));
359 }
360
361 public:
362 VerifyCleanCardClosure(HeapWord* b, HeapWord* begin, HeapWord* end) :
363 _boundary(b), _begin(begin), _end(end) {
364 assert(b <= begin,
365 err_msg("Error: boundary " PTR_FORMAT " should be at or below begin " PTR_FORMAT,
366 p2i(b), p2i(begin)));
367 assert(begin <= end,
368 err_msg("Error: begin " PTR_FORMAT " should be strictly below end " PTR_FORMAT,
369 p2i(begin), p2i(end)));
370 }
371
372 virtual void do_oop(oop* p) { VerifyCleanCardClosure::do_oop_work(p); }
373 virtual void do_oop(narrowOop* p) { VerifyCleanCardClosure::do_oop_work(p); }
374 };
375
376 class VerifyCTSpaceClosure: public SpaceClosure {
377 private:
378 CardTableRS* _ct;
379 HeapWord* _boundary;
380 public:
381 VerifyCTSpaceClosure(CardTableRS* ct, HeapWord* boundary) :
382 _ct(ct), _boundary(boundary) {}
383 virtual void do_space(Space* s) { _ct->verify_space(s, _boundary); }
384 };
385
386 class VerifyCTGenClosure: public GenCollectedHeap::GenClosure {
387 CardTableRS* _ct;
388 public:
389 VerifyCTGenClosure(CardTableRS* ct) : _ct(ct) {}
390 void do_generation(Generation* gen) {
391 // Skip the youngest generation.
392 if (gen->level() == 0) return;
393 // Normally, we're interested in pointers to younger generations.
394 VerifyCTSpaceClosure blk(_ct, gen->reserved().start());
395 gen->space_iterate(&blk, true);
396 }
397 };
398
399 void CardTableRS::verify_space(Space* s, HeapWord* gen_boundary) {
400 // We don't need to do young-gen spaces.
401 if (s->end() <= gen_boundary) return;
402 MemRegion used = s->used_region();
403
404 jbyte* cur_entry = byte_for(used.start());
405 jbyte* limit = byte_after(used.last());
406 while (cur_entry < limit) {
407 if (*cur_entry == CardTableModRefBS::clean_card) {
408 jbyte* first_dirty = cur_entry+1;
409 while (first_dirty < limit &&
410 *first_dirty == CardTableModRefBS::clean_card) {
411 first_dirty++;
412 }
413 // If the first object is a regular object, and it has a
414 // young-to-old field, that would mark the previous card.
415 HeapWord* boundary = addr_for(cur_entry);
416 HeapWord* end = (first_dirty >= limit) ? used.end() : addr_for(first_dirty);
417 HeapWord* boundary_block = s->block_start(boundary);
418 HeapWord* begin = boundary; // Until proven otherwise.
419 HeapWord* start_block = boundary_block; // Until proven otherwise.
420 if (boundary_block < boundary) {
421 if (s->block_is_obj(boundary_block) && s->obj_is_alive(boundary_block)) {
422 oop boundary_obj = oop(boundary_block);
423 if (!boundary_obj->is_objArray() &&
424 !boundary_obj->is_typeArray()) {
425 guarantee(cur_entry > byte_for(used.start()),
426 "else boundary would be boundary_block");
427 if (*byte_for(boundary_block) != CardTableModRefBS::clean_card) {
428 begin = boundary_block + s->block_size(boundary_block);
429 start_block = begin;
430 }
431 }
432 }
433 }
434 // Now traverse objects until end.
435 if (begin < end) {
436 MemRegion mr(begin, end);
437 VerifyCleanCardClosure verify_blk(gen_boundary, begin, end);
438 for (HeapWord* cur = start_block; cur < end; cur += s->block_size(cur)) {
439 if (s->block_is_obj(cur) && s->obj_is_alive(cur)) {
440 oop(cur)->oop_iterate_no_header(&verify_blk, mr);
441 }
442 }
443 }
444 cur_entry = first_dirty;
445 } else {
446 // We'd normally expect that cur_youngergen_and_prev_nonclean_card
447 // is a transient value, that cannot be in the card table
448 // except during GC, and thus assert that:
449 // guarantee(*cur_entry != cur_youngergen_and_prev_nonclean_card,
450 // "Illegal CT value");
451 // That however, need not hold, as will become clear in the
452 // following...
453
454 // We'd normally expect that if we are in the parallel case,
455 // we can't have left a prev value (which would be different
456 // from the current value) in the card table, and so we'd like to
457 // assert that:
458 // guarantee(cur_youngergen_card_val() == youngergen_card
459 // || !is_prev_youngergen_card_val(*cur_entry),
460 // "Illegal CT value");
461 // That, however, may not hold occasionally, because of
462 // CMS or MSC in the old gen. To wit, consider the
463 // following two simple illustrative scenarios:
464 // (a) CMS: Consider the case where a large object L
465 // spanning several cards is allocated in the old
466 // gen, and has a young gen reference stored in it, dirtying
467 // some interior cards. A young collection scans the card,
468 // finds a young ref and installs a youngergenP_n value.
469 // L then goes dead. Now a CMS collection starts,
470 // finds L dead and sweeps it up. Assume that L is
471 // abutting _unallocated_blk, so _unallocated_blk is
472 // adjusted down to (below) L. Assume further that
473 // no young collection intervenes during this CMS cycle.
474 // The next young gen cycle will not get to look at this
475 // youngergenP_n card since it lies in the unoccupied
476 // part of the space.
477 // Some young collections later the blocks on this
478 // card can be re-allocated either due to direct allocation
479 // or due to absorbing promotions. At this time, the
480 // before-gc verification will fail the above assert.
481 // (b) MSC: In this case, an object L with a young reference
482 // is on a card that (therefore) holds a youngergen_n value.
483 // Suppose also that L lies towards the end of the used
484 // the used space before GC. An MSC collection
485 // occurs that compacts to such an extent that this
486 // card is no longer in the occupied part of the space.
487 // Since current code in MSC does not always clear cards
488 // in the unused part of old gen, this stale youngergen_n
489 // value is left behind and can later be covered by
490 // an object when promotion or direct allocation
491 // re-allocates that part of the heap.
492 //
493 // Fortunately, the presence of such stale card values is
494 // "only" a minor annoyance in that subsequent young collections
495 // might needlessly scan such cards, but would still never corrupt
496 // the heap as a result. However, it's likely not to be a significant
497 // performance inhibitor in practice. For instance,
498 // some recent measurements with unoccupied cards eagerly cleared
499 // out to maintain this invariant, showed next to no
500 // change in young collection times; of course one can construct
501 // degenerate examples where the cost can be significant.)
502 // Note, in particular, that if the "stale" card is modified
503 // after re-allocation, it would be dirty, not "stale". Thus,
504 // we can never have a younger ref in such a card and it is
505 // safe not to scan that card in any collection. [As we see
506 // below, we do some unnecessary scanning
507 // in some cases in the current parallel scanning algorithm.]
508 //
509 // The main point below is that the parallel card scanning code
510 // deals correctly with these stale card values. There are two main
511 // cases to consider where we have a stale "younger gen" value and a
512 // "derivative" case to consider, where we have a stale
513 // "cur_younger_gen_and_prev_non_clean" value, as will become
514 // apparent in the case analysis below.
515 // o Case 1. If the stale value corresponds to a younger_gen_n
516 // value other than the cur_younger_gen value then the code
517 // treats this as being tantamount to a prev_younger_gen
518 // card. This means that the card may be unnecessarily scanned.
519 // There are two sub-cases to consider:
520 // o Case 1a. Let us say that the card is in the occupied part
521 // of the generation at the time the collection begins. In
522 // that case the card will be either cleared when it is scanned
523 // for young pointers, or will be set to cur_younger_gen as a
524 // result of promotion. (We have elided the normal case where
525 // the scanning thread and the promoting thread interleave
526 // possibly resulting in a transient
527 // cur_younger_gen_and_prev_non_clean value before settling
528 // to cur_younger_gen. [End Case 1a.]
529 // o Case 1b. Consider now the case when the card is in the unoccupied
530 // part of the space which becomes occupied because of promotions
531 // into it during the current young GC. In this case the card
532 // will never be scanned for young references. The current
533 // code will set the card value to either
534 // cur_younger_gen_and_prev_non_clean or leave
535 // it with its stale value -- because the promotions didn't
536 // result in any younger refs on that card. Of these two
537 // cases, the latter will be covered in Case 1a during
538 // a subsequent scan. To deal with the former case, we need
539 // to further consider how we deal with a stale value of
540 // cur_younger_gen_and_prev_non_clean in our case analysis
541 // below. This we do in Case 3 below. [End Case 1b]
542 // [End Case 1]
543 // o Case 2. If the stale value corresponds to cur_younger_gen being
544 // a value not necessarily written by a current promotion, the
545 // card will not be scanned by the younger refs scanning code.
546 // (This is OK since as we argued above such cards cannot contain
547 // any younger refs.) The result is that this value will be
548 // treated as a prev_younger_gen value in a subsequent collection,
549 // which is addressed in Case 1 above. [End Case 2]
550 // o Case 3. We here consider the "derivative" case from Case 1b. above
551 // because of which we may find a stale
552 // cur_younger_gen_and_prev_non_clean card value in the table.
553 // Once again, as in Case 1, we consider two subcases, depending
554 // on whether the card lies in the occupied or unoccupied part
555 // of the space at the start of the young collection.
556 // o Case 3a. Let us say the card is in the occupied part of
557 // the old gen at the start of the young collection. In that
558 // case, the card will be scanned by the younger refs scanning
559 // code which will set it to cur_younger_gen. In a subsequent
560 // scan, the card will be considered again and get its final
561 // correct value. [End Case 3a]
562 // o Case 3b. Now consider the case where the card is in the
563 // unoccupied part of the old gen, and is occupied as a result
564 // of promotions during thus young gc. In that case,
565 // the card will not be scanned for younger refs. The presence
566 // of newly promoted objects on the card will then result in
567 // its keeping the value cur_younger_gen_and_prev_non_clean
568 // value, which we have dealt with in Case 3 here. [End Case 3b]
569 // [End Case 3]
570 //
571 // (Please refer to the code in the helper class
572 // ClearNonCleanCardWrapper and in CardTableModRefBS for details.)
573 //
574 // The informal arguments above can be tightened into a formal
575 // correctness proof and it behooves us to write up such a proof,
576 // or to use model checking to prove that there are no lingering
577 // concerns.
578 //
579 // Clearly because of Case 3b one cannot bound the time for
580 // which a card will retain what we have called a "stale" value.
581 // However, one can obtain a Loose upper bound on the redundant
582 // work as a result of such stale values. Note first that any
583 // time a stale card lies in the occupied part of the space at
584 // the start of the collection, it is scanned by younger refs
585 // code and we can define a rank function on card values that
586 // declines when this is so. Note also that when a card does not
587 // lie in the occupied part of the space at the beginning of a
588 // young collection, its rank can either decline or stay unchanged.
589 // In this case, no extra work is done in terms of redundant
590 // younger refs scanning of that card.
591 // Then, the case analysis above reveals that, in the worst case,
592 // any such stale card will be scanned unnecessarily at most twice.
593 //
594 // It is nonetheless advisable to try and get rid of some of this
595 // redundant work in a subsequent (low priority) re-design of
596 // the card-scanning code, if only to simplify the underlying
597 // state machine analysis/proof. ysr 1/28/2002. XXX
598 cur_entry++;
599 }
600 }
601 }
602
603 void CardTableRS::verify() {
604 // At present, we only know how to verify the card table RS for
605 // generational heaps.
606 VerifyCTGenClosure blk(this);
607 CollectedHeap* ch = Universe::heap();
608
609 if (ch->kind() == CollectedHeap::GenCollectedHeap) {
610 GenCollectedHeap::heap()->generation_iterate(&blk, false);
611 _ct_bs->verify();
612 }
613 }
614
615
616 void CardTableRS::verify_aligned_region_empty(MemRegion mr) {
617 if (!mr.is_empty()) {
618 jbyte* cur_entry = byte_for(mr.start());
619 jbyte* limit = byte_after(mr.last());
620 // The region mr may not start on a card boundary so
621 // the first card may reflect a write to the space
622 // just prior to mr.
623 if (!is_aligned(mr.start())) {
624 cur_entry++;
625 }
626 for (;cur_entry < limit; cur_entry++) {
627 guarantee(*cur_entry == CardTableModRefBS::clean_card,
628 "Unexpected dirty card found");
629 }
630 }
631 }