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 _ct_bs->initialize();
58 set_bs(_ct_bs);
59 // max_gens is really GenCollectedHeap::heap()->gen_policy()->number_of_generations()
60 // (which always is 2), but GenCollectedHeap has not been initialized yet.
61 int max_gens = 2;
62 _last_cur_val_in_gen = NEW_C_HEAP_ARRAY3(jbyte, max_gens + 1,
63 mtGC, CURRENT_PC, AllocFailStrategy::RETURN_NULL);
64 if (_last_cur_val_in_gen == NULL) {
65 vm_exit_during_initialization("Could not create last_cur_val_in_gen array.");
66 }
67 for (int i = 0; i < max_gens + 1; i++) {
68 _last_cur_val_in_gen[i] = clean_card_val();
69 }
70 _ct_bs->set_CTRS(this);
71 }
72
73 CardTableRS::~CardTableRS() {
74 if (_ct_bs) {
75 delete _ct_bs;
76 _ct_bs = NULL;
77 }
78 if (_last_cur_val_in_gen) {
79 FREE_C_HEAP_ARRAY(jbyte, _last_cur_val_in_gen, mtInternal);
80 }
81 }
82
83 void CardTableRS::resize_covered_region(MemRegion new_region) {
84 _ct_bs->resize_covered_region(new_region);
85 }
86
87 jbyte CardTableRS::find_unused_youngergenP_card_value() {
88 for (jbyte v = youngergenP1_card;
89 v < cur_youngergen_and_prev_nonclean_card;
90 v++) {
91 bool seen = false;
92 for (int g = 0; g < _regions_to_iterate; g++) {
93 if (_last_cur_val_in_gen[g] == v) {
94 seen = true;
95 break;
96 }
97 }
98 if (!seen) return v;
99 }
100 ShouldNotReachHere();
101 return 0;
102 }
103
104 void CardTableRS::prepare_for_younger_refs_iterate(bool parallel) {
105 // Parallel or sequential, we must always set the prev to equal the
106 // last one written.
107 if (parallel) {
108 // Find a parallel value to be used next.
109 jbyte next_val = find_unused_youngergenP_card_value();
110 set_cur_youngergen_card_val(next_val);
111
112 } else {
113 // In an sequential traversal we will always write youngergen, so that
114 // the inline barrier is correct.
115 set_cur_youngergen_card_val(youngergen_card);
116 }
117 }
118
119 void CardTableRS::younger_refs_iterate(Generation* g,
120 OopsInGenClosure* blk) {
121 _last_cur_val_in_gen[2 /* Number of generations */] = cur_youngergen_card_val();
122 g->younger_refs_iterate(blk);
123 }
124
125 inline bool ClearNoncleanCardWrapper::clear_card(jbyte* entry) {
126 if (_is_par) {
127 return clear_card_parallel(entry);
128 } else {
129 return clear_card_serial(entry);
130 }
131 }
132
133 inline bool ClearNoncleanCardWrapper::clear_card_parallel(jbyte* entry) {
134 while (true) {
135 // In the parallel case, we may have to do this several times.
136 jbyte entry_val = *entry;
137 assert(entry_val != CardTableRS::clean_card_val(),
138 "We shouldn't be looking at clean cards, and this should "
139 "be the only place they get cleaned.");
140 if (CardTableRS::card_is_dirty_wrt_gen_iter(entry_val)
141 || _ct->is_prev_youngergen_card_val(entry_val)) {
142 jbyte res =
143 Atomic::cmpxchg(CardTableRS::clean_card_val(), entry, entry_val);
144 if (res == entry_val) {
145 break;
146 } else {
147 assert(res == CardTableRS::cur_youngergen_and_prev_nonclean_card,
148 "The CAS above should only fail if another thread did "
149 "a GC write barrier.");
150 }
151 } else if (entry_val ==
152 CardTableRS::cur_youngergen_and_prev_nonclean_card) {
153 // Parallelism shouldn't matter in this case. Only the thread
154 // assigned to scan the card should change this value.
155 *entry = _ct->cur_youngergen_card_val();
156 break;
157 } else {
158 assert(entry_val == _ct->cur_youngergen_card_val(),
159 "Should be the only possibility.");
160 // In this case, the card was clean before, and become
161 // cur_youngergen only because of processing of a promoted object.
162 // We don't have to look at the card.
163 return false;
164 }
165 }
166 return true;
167 }
168
169
170 inline bool ClearNoncleanCardWrapper::clear_card_serial(jbyte* entry) {
171 jbyte entry_val = *entry;
172 assert(entry_val != CardTableRS::clean_card_val(),
173 "We shouldn't be looking at clean cards, and this should "
174 "be the only place they get cleaned.");
175 assert(entry_val != CardTableRS::cur_youngergen_and_prev_nonclean_card,
176 "This should be possible in the sequential case.");
177 *entry = CardTableRS::clean_card_val();
178 return true;
179 }
180
181 ClearNoncleanCardWrapper::ClearNoncleanCardWrapper(
182 DirtyCardToOopClosure* dirty_card_closure, CardTableRS* ct) :
183 _dirty_card_closure(dirty_card_closure), _ct(ct) {
184 // Cannot yet substitute active_workers for n_par_threads
185 // in the case where parallelism is being turned off by
186 // setting n_par_threads to 0.
187 _is_par = (SharedHeap::heap()->n_par_threads() > 0);
188 assert(!_is_par ||
189 (SharedHeap::heap()->n_par_threads() ==
190 SharedHeap::heap()->workers()->active_workers()), "Mismatch");
191 }
192
193 bool ClearNoncleanCardWrapper::is_word_aligned(jbyte* entry) {
194 return (((intptr_t)entry) & (BytesPerWord-1)) == 0;
195 }
196
197 void ClearNoncleanCardWrapper::do_MemRegion(MemRegion mr) {
198 assert(mr.word_size() > 0, "Error");
199 assert(_ct->is_aligned(mr.start()), "mr.start() should be card aligned");
200 // mr.end() may not necessarily be card aligned.
201 jbyte* cur_entry = _ct->byte_for(mr.last());
202 const jbyte* limit = _ct->byte_for(mr.start());
203 HeapWord* end_of_non_clean = mr.end();
204 HeapWord* start_of_non_clean = end_of_non_clean;
205 while (cur_entry >= limit) {
206 HeapWord* cur_hw = _ct->addr_for(cur_entry);
207 if ((*cur_entry != CardTableRS::clean_card_val()) && clear_card(cur_entry)) {
208 // Continue the dirty range by opening the
209 // dirty window one card to the left.
210 start_of_non_clean = cur_hw;
211 } else {
212 // We hit a "clean" card; process any non-empty
213 // "dirty" range accumulated so far.
214 if (start_of_non_clean < end_of_non_clean) {
215 const MemRegion mrd(start_of_non_clean, end_of_non_clean);
216 _dirty_card_closure->do_MemRegion(mrd);
217 }
218
219 // fast forward through potential continuous whole-word range of clean cards beginning at a word-boundary
220 if (is_word_aligned(cur_entry)) {
221 jbyte* cur_row = cur_entry - BytesPerWord;
222 while (cur_row >= limit && *((intptr_t*)cur_row) == CardTableRS::clean_card_row()) {
223 cur_row -= BytesPerWord;
224 }
225 cur_entry = cur_row + BytesPerWord;
226 cur_hw = _ct->addr_for(cur_entry);
227 }
228
229 // Reset the dirty window, while continuing to look
230 // for the next dirty card that will start a
231 // new dirty window.
232 end_of_non_clean = cur_hw;
233 start_of_non_clean = cur_hw;
234 }
235 // Note that "cur_entry" leads "start_of_non_clean" in
236 // its leftward excursion after this point
237 // in the loop and, when we hit the left end of "mr",
238 // will point off of the left end of the card-table
239 // for "mr".
240 cur_entry--;
241 }
242 // If the first card of "mr" was dirty, we will have
243 // been left with a dirty window, co-initial with "mr",
244 // which we now process.
245 if (start_of_non_clean < end_of_non_clean) {
246 const MemRegion mrd(start_of_non_clean, end_of_non_clean);
247 _dirty_card_closure->do_MemRegion(mrd);
248 }
249 }
250
251 // clean (by dirty->clean before) ==> cur_younger_gen
252 // dirty ==> cur_youngergen_and_prev_nonclean_card
253 // precleaned ==> cur_youngergen_and_prev_nonclean_card
254 // prev-younger-gen ==> cur_youngergen_and_prev_nonclean_card
255 // cur-younger-gen ==> cur_younger_gen
256 // cur_youngergen_and_prev_nonclean_card ==> no change.
257 void CardTableRS::write_ref_field_gc_par(void* field, oop new_val) {
258 jbyte* entry = ct_bs()->byte_for(field);
259 do {
260 jbyte entry_val = *entry;
261 // We put this first because it's probably the most common case.
262 if (entry_val == clean_card_val()) {
263 // No threat of contention with cleaning threads.
264 *entry = cur_youngergen_card_val();
265 return;
266 } else if (card_is_dirty_wrt_gen_iter(entry_val)
267 || is_prev_youngergen_card_val(entry_val)) {
268 // Mark it as both cur and prev youngergen; card cleaning thread will
269 // eventually remove the previous stuff.
270 jbyte new_val = cur_youngergen_and_prev_nonclean_card;
271 jbyte res = Atomic::cmpxchg(new_val, entry, entry_val);
272 // Did the CAS succeed?
273 if (res == entry_val) return;
274 // Otherwise, retry, to see the new value.
275 continue;
276 } else {
277 assert(entry_val == cur_youngergen_and_prev_nonclean_card
278 || entry_val == cur_youngergen_card_val(),
279 "should be only possibilities.");
280 return;
281 }
282 } while (true);
283 }
284
285 void CardTableRS::younger_refs_in_space_iterate(Space* sp,
286 OopsInGenClosure* cl) {
287 const MemRegion urasm = sp->used_region_at_save_marks();
288 #ifdef ASSERT
289 // Convert the assertion check to a warning if we are running
290 // CMS+ParNew until related bug is fixed.
291 MemRegion ur = sp->used_region();
292 assert(ur.contains(urasm) || (UseConcMarkSweepGC && UseParNewGC),
293 err_msg("Did you forget to call save_marks()? "
294 "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in "
295 "[" PTR_FORMAT ", " PTR_FORMAT ")",
296 p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end())));
297 // In the case of CMS+ParNew, issue a warning
298 if (!ur.contains(urasm)) {
299 assert(UseConcMarkSweepGC && UseParNewGC, "Tautology: see assert above");
300 warning("CMS+ParNew: Did you forget to call save_marks()? "
301 "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in "
302 "[" PTR_FORMAT ", " PTR_FORMAT ")",
303 p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end()));
304 MemRegion ur2 = sp->used_region();
305 MemRegion urasm2 = sp->used_region_at_save_marks();
306 if (!ur.equals(ur2)) {
307 warning("CMS+ParNew: Flickering used_region()!!");
308 }
309 if (!urasm.equals(urasm2)) {
310 warning("CMS+ParNew: Flickering used_region_at_save_marks()!!");
311 }
312 ShouldNotReachHere();
313 }
314 #endif
315 _ct_bs->non_clean_card_iterate_possibly_parallel(sp, urasm, cl, this);
316 }
317
318 void CardTableRS::clear_into_younger(Generation* old_gen) {
319 assert(old_gen == GenCollectedHeap::heap()->old_gen(),
320 "Should only be called for the old generation");
321 // The card tables for the youngest gen need never be cleared.
322 // There's a bit of subtlety in the clear() and invalidate()
323 // methods that we exploit here and in invalidate_or_clear()
324 // below to avoid missing cards at the fringes. If clear() or
325 // invalidate() are changed in the future, this code should
326 // be revisited. 20040107.ysr
327 clear(old_gen->prev_used_region());
328 }
329
330 void CardTableRS::invalidate_or_clear(Generation* old_gen) {
331 assert(old_gen == GenCollectedHeap::heap()->old_gen(),
332 "Should only be called for the old generation");
333 // Invalidate the cards for the currently occupied part of
334 // the old generation and clear the cards for the
335 // unoccupied part of the generation (if any, making use
336 // of that generation's prev_used_region to determine that
337 // region). No need to do anything for the youngest
338 // generation. Also see note#20040107.ysr above.
339 MemRegion used_mr = old_gen->used_region();
340 MemRegion to_be_cleared_mr = old_gen->prev_used_region().minus(used_mr);
341 if (!to_be_cleared_mr.is_empty()) {
342 clear(to_be_cleared_mr);
343 }
344 invalidate(used_mr);
345 }
346
347
348 class VerifyCleanCardClosure: public OopClosure {
349 private:
350 HeapWord* _boundary;
351 HeapWord* _begin;
352 HeapWord* _end;
353 protected:
354 template <class T> void do_oop_work(T* p) {
355 HeapWord* jp = (HeapWord*)p;
356 assert(jp >= _begin && jp < _end,
357 err_msg("Error: jp " PTR_FORMAT " should be within "
358 "[_begin, _end) = [" PTR_FORMAT "," PTR_FORMAT ")",
359 p2i(jp), p2i(_begin), p2i(_end)));
360 oop obj = oopDesc::load_decode_heap_oop(p);
361 guarantee(obj == NULL || (HeapWord*)obj >= _boundary,
362 err_msg("pointer " PTR_FORMAT " at " PTR_FORMAT " on "
363 "clean card crosses boundary" PTR_FORMAT,
364 p2i((HeapWord*)obj), p2i(jp), p2i(_boundary)));
365 }
366
367 public:
368 VerifyCleanCardClosure(HeapWord* b, HeapWord* begin, HeapWord* end) :
369 _boundary(b), _begin(begin), _end(end) {
370 assert(b <= begin,
371 err_msg("Error: boundary " PTR_FORMAT " should be at or below begin " PTR_FORMAT,
372 p2i(b), p2i(begin)));
373 assert(begin <= end,
374 err_msg("Error: begin " PTR_FORMAT " should be strictly below end " PTR_FORMAT,
375 p2i(begin), p2i(end)));
376 }
377
378 virtual void do_oop(oop* p) { VerifyCleanCardClosure::do_oop_work(p); }
379 virtual void do_oop(narrowOop* p) { VerifyCleanCardClosure::do_oop_work(p); }
380 };
381
382 class VerifyCTSpaceClosure: public SpaceClosure {
383 private:
384 CardTableRS* _ct;
385 HeapWord* _boundary;
386 public:
387 VerifyCTSpaceClosure(CardTableRS* ct, HeapWord* boundary) :
388 _ct(ct), _boundary(boundary) {}
389 virtual void do_space(Space* s) { _ct->verify_space(s, _boundary); }
390 };
391
392 class VerifyCTGenClosure: public GenCollectedHeap::GenClosure {
393 CardTableRS* _ct;
394 public:
395 VerifyCTGenClosure(CardTableRS* ct) : _ct(ct) {}
396 void do_generation(Generation* gen) {
397 // Skip the youngest generation.
398 if (gen == GenCollectedHeap::heap()->young_gen()) {
399 return;
400 }
401 // Normally, we're interested in pointers to younger generations.
402 VerifyCTSpaceClosure blk(_ct, gen->reserved().start());
403 gen->space_iterate(&blk, true);
404 }
405 };
406
407 void CardTableRS::verify_space(Space* s, HeapWord* gen_boundary) {
408 // We don't need to do young-gen spaces.
409 if (s->end() <= gen_boundary) {
410 return;
411 }
412 MemRegion used = s->used_region();
413
414 jbyte* cur_entry = byte_for(used.start());
415 jbyte* limit = byte_after(used.last());
416 while (cur_entry < limit) {
417 if (*cur_entry == CardTableModRefBS::clean_card) {
418 jbyte* first_dirty = cur_entry+1;
419 while (first_dirty < limit &&
420 *first_dirty == CardTableModRefBS::clean_card) {
421 first_dirty++;
422 }
423 // If the first object is a regular object, and it has a
424 // young-to-old field, that would mark the previous card.
425 HeapWord* boundary = addr_for(cur_entry);
426 HeapWord* end = (first_dirty >= limit) ? used.end() : addr_for(first_dirty);
427 HeapWord* boundary_block = s->block_start(boundary);
428 HeapWord* begin = boundary; // Until proven otherwise.
429 HeapWord* start_block = boundary_block; // Until proven otherwise.
430 if (boundary_block < boundary) {
431 if (s->block_is_obj(boundary_block) && s->obj_is_alive(boundary_block)) {
432 oop boundary_obj = oop(boundary_block);
433 if (!boundary_obj->is_objArray() &&
434 !boundary_obj->is_typeArray()) {
435 guarantee(cur_entry > byte_for(used.start()),
436 "else boundary would be boundary_block");
437 if (*byte_for(boundary_block) != CardTableModRefBS::clean_card) {
438 begin = boundary_block + s->block_size(boundary_block);
439 start_block = begin;
440 }
441 }
442 }
443 }
444 // Now traverse objects until end.
445 if (begin < end) {
446 MemRegion mr(begin, end);
447 VerifyCleanCardClosure verify_blk(gen_boundary, begin, end);
448 for (HeapWord* cur = start_block; cur < end; cur += s->block_size(cur)) {
449 if (s->block_is_obj(cur) && s->obj_is_alive(cur)) {
450 oop(cur)->oop_iterate_no_header(&verify_blk, mr);
451 }
452 }
453 }
454 cur_entry = first_dirty;
455 } else {
456 // We'd normally expect that cur_youngergen_and_prev_nonclean_card
457 // is a transient value, that cannot be in the card table
458 // except during GC, and thus assert that:
459 // guarantee(*cur_entry != cur_youngergen_and_prev_nonclean_card,
460 // "Illegal CT value");
461 // That however, need not hold, as will become clear in the
462 // following...
463
464 // We'd normally expect that if we are in the parallel case,
465 // we can't have left a prev value (which would be different
466 // from the current value) in the card table, and so we'd like to
467 // assert that:
468 // guarantee(cur_youngergen_card_val() == youngergen_card
469 // || !is_prev_youngergen_card_val(*cur_entry),
470 // "Illegal CT value");
471 // That, however, may not hold occasionally, because of
472 // CMS or MSC in the old gen. To wit, consider the
473 // following two simple illustrative scenarios:
474 // (a) CMS: Consider the case where a large object L
475 // spanning several cards is allocated in the old
476 // gen, and has a young gen reference stored in it, dirtying
477 // some interior cards. A young collection scans the card,
478 // finds a young ref and installs a youngergenP_n value.
479 // L then goes dead. Now a CMS collection starts,
480 // finds L dead and sweeps it up. Assume that L is
481 // abutting _unallocated_blk, so _unallocated_blk is
482 // adjusted down to (below) L. Assume further that
483 // no young collection intervenes during this CMS cycle.
484 // The next young gen cycle will not get to look at this
485 // youngergenP_n card since it lies in the unoccupied
486 // part of the space.
487 // Some young collections later the blocks on this
488 // card can be re-allocated either due to direct allocation
489 // or due to absorbing promotions. At this time, the
490 // before-gc verification will fail the above assert.
491 // (b) MSC: In this case, an object L with a young reference
492 // is on a card that (therefore) holds a youngergen_n value.
493 // Suppose also that L lies towards the end of the used
494 // the used space before GC. An MSC collection
495 // occurs that compacts to such an extent that this
496 // card is no longer in the occupied part of the space.
497 // Since current code in MSC does not always clear cards
498 // in the unused part of old gen, this stale youngergen_n
499 // value is left behind and can later be covered by
500 // an object when promotion or direct allocation
501 // re-allocates that part of the heap.
502 //
503 // Fortunately, the presence of such stale card values is
504 // "only" a minor annoyance in that subsequent young collections
505 // might needlessly scan such cards, but would still never corrupt
506 // the heap as a result. However, it's likely not to be a significant
507 // performance inhibitor in practice. For instance,
508 // some recent measurements with unoccupied cards eagerly cleared
509 // out to maintain this invariant, showed next to no
510 // change in young collection times; of course one can construct
511 // degenerate examples where the cost can be significant.)
512 // Note, in particular, that if the "stale" card is modified
513 // after re-allocation, it would be dirty, not "stale". Thus,
514 // we can never have a younger ref in such a card and it is
515 // safe not to scan that card in any collection. [As we see
516 // below, we do some unnecessary scanning
517 // in some cases in the current parallel scanning algorithm.]
518 //
519 // The main point below is that the parallel card scanning code
520 // deals correctly with these stale card values. There are two main
521 // cases to consider where we have a stale "younger gen" value and a
522 // "derivative" case to consider, where we have a stale
523 // "cur_younger_gen_and_prev_non_clean" value, as will become
524 // apparent in the case analysis below.
525 // o Case 1. If the stale value corresponds to a younger_gen_n
526 // value other than the cur_younger_gen value then the code
527 // treats this as being tantamount to a prev_younger_gen
528 // card. This means that the card may be unnecessarily scanned.
529 // There are two sub-cases to consider:
530 // o Case 1a. Let us say that the card is in the occupied part
531 // of the generation at the time the collection begins. In
532 // that case the card will be either cleared when it is scanned
533 // for young pointers, or will be set to cur_younger_gen as a
534 // result of promotion. (We have elided the normal case where
535 // the scanning thread and the promoting thread interleave
536 // possibly resulting in a transient
537 // cur_younger_gen_and_prev_non_clean value before settling
538 // to cur_younger_gen. [End Case 1a.]
539 // o Case 1b. Consider now the case when the card is in the unoccupied
540 // part of the space which becomes occupied because of promotions
541 // into it during the current young GC. In this case the card
542 // will never be scanned for young references. The current
543 // code will set the card value to either
544 // cur_younger_gen_and_prev_non_clean or leave
545 // it with its stale value -- because the promotions didn't
546 // result in any younger refs on that card. Of these two
547 // cases, the latter will be covered in Case 1a during
548 // a subsequent scan. To deal with the former case, we need
549 // to further consider how we deal with a stale value of
550 // cur_younger_gen_and_prev_non_clean in our case analysis
551 // below. This we do in Case 3 below. [End Case 1b]
552 // [End Case 1]
553 // o Case 2. If the stale value corresponds to cur_younger_gen being
554 // a value not necessarily written by a current promotion, the
555 // card will not be scanned by the younger refs scanning code.
556 // (This is OK since as we argued above such cards cannot contain
557 // any younger refs.) The result is that this value will be
558 // treated as a prev_younger_gen value in a subsequent collection,
559 // which is addressed in Case 1 above. [End Case 2]
560 // o Case 3. We here consider the "derivative" case from Case 1b. above
561 // because of which we may find a stale
562 // cur_younger_gen_and_prev_non_clean card value in the table.
563 // Once again, as in Case 1, we consider two subcases, depending
564 // on whether the card lies in the occupied or unoccupied part
565 // of the space at the start of the young collection.
566 // o Case 3a. Let us say the card is in the occupied part of
567 // the old gen at the start of the young collection. In that
568 // case, the card will be scanned by the younger refs scanning
569 // code which will set it to cur_younger_gen. In a subsequent
570 // scan, the card will be considered again and get its final
571 // correct value. [End Case 3a]
572 // o Case 3b. Now consider the case where the card is in the
573 // unoccupied part of the old gen, and is occupied as a result
574 // of promotions during thus young gc. In that case,
575 // the card will not be scanned for younger refs. The presence
576 // of newly promoted objects on the card will then result in
577 // its keeping the value cur_younger_gen_and_prev_non_clean
578 // value, which we have dealt with in Case 3 here. [End Case 3b]
579 // [End Case 3]
580 //
581 // (Please refer to the code in the helper class
582 // ClearNonCleanCardWrapper and in CardTableModRefBS for details.)
583 //
584 // The informal arguments above can be tightened into a formal
585 // correctness proof and it behooves us to write up such a proof,
586 // or to use model checking to prove that there are no lingering
587 // concerns.
588 //
589 // Clearly because of Case 3b one cannot bound the time for
590 // which a card will retain what we have called a "stale" value.
591 // However, one can obtain a Loose upper bound on the redundant
592 // work as a result of such stale values. Note first that any
593 // time a stale card lies in the occupied part of the space at
594 // the start of the collection, it is scanned by younger refs
595 // code and we can define a rank function on card values that
596 // declines when this is so. Note also that when a card does not
597 // lie in the occupied part of the space at the beginning of a
598 // young collection, its rank can either decline or stay unchanged.
599 // In this case, no extra work is done in terms of redundant
600 // younger refs scanning of that card.
601 // Then, the case analysis above reveals that, in the worst case,
602 // any such stale card will be scanned unnecessarily at most twice.
603 //
604 // It is nonetheless advisable to try and get rid of some of this
605 // redundant work in a subsequent (low priority) re-design of
606 // the card-scanning code, if only to simplify the underlying
607 // state machine analysis/proof. ysr 1/28/2002. XXX
608 cur_entry++;
609 }
610 }
611 }
612
613 void CardTableRS::verify() {
614 // At present, we only know how to verify the card table RS for
615 // generational heaps.
616 VerifyCTGenClosure blk(this);
617 CollectedHeap* ch = Universe::heap();
618
619 if (ch->kind() == CollectedHeap::GenCollectedHeap) {
620 GenCollectedHeap::heap()->generation_iterate(&blk, false);
621 _ct_bs->verify();
622 }
623 }
624
625
626 void CardTableRS::verify_aligned_region_empty(MemRegion mr) {
627 if (!mr.is_empty()) {
628 jbyte* cur_entry = byte_for(mr.start());
629 jbyte* limit = byte_after(mr.last());
630 // The region mr may not start on a card boundary so
631 // the first card may reflect a write to the space
632 // just prior to mr.
633 if (!is_aligned(mr.start())) {
634 cur_entry++;
635 }
636 for (;cur_entry < limit; cur_entry++) {
637 guarantee(*cur_entry == CardTableModRefBS::clean_card,
638 "Unexpected dirty card found");
639 }
640 }
641 }