1 /*
2 * Copyright (c) 2001, 2011, 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/java.hpp"
33 #include "runtime/os.hpp"
34 #ifndef SERIALGC
35 #include "gc_implementation/g1/concurrentMark.hpp"
36 #include "gc_implementation/g1/g1SATBCardTableModRefBS.hpp"
37 #endif
38
39 CardTableRS::CardTableRS(MemRegion whole_heap,
40 int max_covered_regions) :
41 GenRemSet(),
42 _cur_youngergen_card_val(youngergenP1_card),
43 _regions_to_iterate(max_covered_regions - 1)
44 {
45 #ifndef SERIALGC
46 if (UseG1GC) {
47 _ct_bs = new G1SATBCardTableLoggingModRefBS(whole_heap,
48 max_covered_regions);
49 } else {
50 _ct_bs = new CardTableModRefBSForCTRS(whole_heap, max_covered_regions);
51 }
52 #else
53 _ct_bs = new CardTableModRefBSForCTRS(whole_heap, max_covered_regions);
54 #endif
55 set_bs(_ct_bs);
56 _last_cur_val_in_gen = new jbyte[GenCollectedHeap::max_gens + 1];
57 if (_last_cur_val_in_gen == NULL) {
58 vm_exit_during_initialization("Could not last_cur_val_in_gen array.");
59 }
60 for (int i = 0; i < GenCollectedHeap::max_gens + 1; i++) {
61 _last_cur_val_in_gen[i] = clean_card_val();
62 }
63 _ct_bs->set_CTRS(this);
64 }
65
66 void CardTableRS::resize_covered_region(MemRegion new_region) {
67 _ct_bs->resize_covered_region(new_region);
68 }
69
70 jbyte CardTableRS::find_unused_youngergenP_card_value() {
71 for (jbyte v = youngergenP1_card;
72 v < cur_youngergen_and_prev_nonclean_card;
73 v++) {
74 bool seen = false;
75 for (int g = 0; g < _regions_to_iterate; g++) {
76 if (_last_cur_val_in_gen[g] == v) {
77 seen = true;
78 break;
79 }
80 }
81 if (!seen) return v;
82 }
83 ShouldNotReachHere();
84 return 0;
85 }
86
87 void CardTableRS::prepare_for_younger_refs_iterate(bool parallel) {
88 // Parallel or sequential, we must always set the prev to equal the
89 // last one written.
90 if (parallel) {
91 // Find a parallel value to be used next.
92 jbyte next_val = find_unused_youngergenP_card_value();
93 set_cur_youngergen_card_val(next_val);
94
95 } else {
96 // In an sequential traversal we will always write youngergen, so that
97 // the inline barrier is correct.
98 set_cur_youngergen_card_val(youngergen_card);
99 }
100 }
101
102 void CardTableRS::younger_refs_iterate(Generation* g,
103 OopsInGenClosure* blk) {
104 _last_cur_val_in_gen[g->level()+1] = cur_youngergen_card_val();
105 g->younger_refs_iterate(blk);
106 }
107
108 inline bool ClearNoncleanCardWrapper::clear_card(jbyte* entry) {
109 if (_is_par) {
110 return clear_card_parallel(entry);
111 } else {
112 return clear_card_serial(entry);
113 }
114 }
115
116 inline bool ClearNoncleanCardWrapper::clear_card_parallel(jbyte* entry) {
117 while (true) {
118 // In the parallel case, we may have to do this several times.
119 jbyte entry_val = *entry;
120 assert(entry_val != CardTableRS::clean_card_val(),
121 "We shouldn't be looking at clean cards, and this should "
122 "be the only place they get cleaned.");
123 if (CardTableRS::card_is_dirty_wrt_gen_iter(entry_val)
124 || _ct->is_prev_youngergen_card_val(entry_val)) {
125 jbyte res =
126 Atomic::cmpxchg(CardTableRS::clean_card_val(), entry, entry_val);
127 if (res == entry_val) {
128 break;
129 } else {
130 assert(res == CardTableRS::cur_youngergen_and_prev_nonclean_card,
131 "The CAS above should only fail if another thread did "
132 "a GC write barrier.");
133 }
134 } else if (entry_val ==
135 CardTableRS::cur_youngergen_and_prev_nonclean_card) {
136 // Parallelism shouldn't matter in this case. Only the thread
137 // assigned to scan the card should change this value.
138 *entry = _ct->cur_youngergen_card_val();
139 break;
140 } else {
141 assert(entry_val == _ct->cur_youngergen_card_val(),
142 "Should be the only possibility.");
143 // In this case, the card was clean before, and become
144 // cur_youngergen only because of processing of a promoted object.
145 // We don't have to look at the card.
146 return false;
147 }
148 }
149 return true;
150 }
151
152
153 inline bool ClearNoncleanCardWrapper::clear_card_serial(jbyte* entry) {
154 jbyte entry_val = *entry;
155 assert(entry_val != CardTableRS::clean_card_val(),
156 "We shouldn't be looking at clean cards, and this should "
157 "be the only place they get cleaned.");
158 assert(entry_val != CardTableRS::cur_youngergen_and_prev_nonclean_card,
159 "This should be possible in the sequential case.");
160 *entry = CardTableRS::clean_card_val();
161 return true;
162 }
163
164 ClearNoncleanCardWrapper::ClearNoncleanCardWrapper(
165 DirtyCardToOopClosure* dirty_card_closure, CardTableRS* ct) :
166 _dirty_card_closure(dirty_card_closure), _ct(ct) {
167 // Cannot yet substitute active_workers for n_par_threads
168 // in the case where parallelism is being turned off by
169 // setting n_par_threads to 0.
170 _is_par = (SharedHeap::heap()->n_par_threads() > 0);
171 assert(!_is_par ||
172 (SharedHeap::heap()->n_par_threads() ==
173 SharedHeap::heap()->workers()->active_workers()), "Mismatch");
174 }
175
176 void ClearNoncleanCardWrapper::do_MemRegion(MemRegion mr) {
177 assert(mr.word_size() > 0, "Error");
178 assert(_ct->is_aligned(mr.start()), "mr.start() should be card aligned");
179 // mr.end() may not necessarily be card aligned.
180 jbyte* cur_entry = _ct->byte_for(mr.last());
181 const jbyte* limit = _ct->byte_for(mr.start());
182 HeapWord* end_of_non_clean = mr.end();
183 HeapWord* start_of_non_clean = end_of_non_clean;
184 while (cur_entry >= limit) {
185 HeapWord* cur_hw = _ct->addr_for(cur_entry);
186 if ((*cur_entry != CardTableRS::clean_card_val()) && clear_card(cur_entry)) {
187 // Continue the dirty range by opening the
188 // dirty window one card to the left.
189 start_of_non_clean = cur_hw;
190 } else {
191 // We hit a "clean" card; process any non-empty
192 // "dirty" range accumulated so far.
193 if (start_of_non_clean < end_of_non_clean) {
194 const MemRegion mrd(start_of_non_clean, end_of_non_clean);
195 _dirty_card_closure->do_MemRegion(mrd);
196 }
197 // Reset the dirty window, while continuing to look
198 // for the next dirty card that will start a
199 // new dirty window.
200 end_of_non_clean = cur_hw;
201 start_of_non_clean = cur_hw;
202 }
203 // Note that "cur_entry" leads "start_of_non_clean" in
204 // its leftward excursion after this point
205 // in the loop and, when we hit the left end of "mr",
206 // will point off of the left end of the card-table
207 // for "mr".
208 cur_entry--;
209 }
210 // If the first card of "mr" was dirty, we will have
211 // been left with a dirty window, co-initial with "mr",
212 // which we now process.
213 if (start_of_non_clean < end_of_non_clean) {
214 const MemRegion mrd(start_of_non_clean, end_of_non_clean);
215 _dirty_card_closure->do_MemRegion(mrd);
216 }
217 }
218
219 // clean (by dirty->clean before) ==> cur_younger_gen
220 // dirty ==> cur_youngergen_and_prev_nonclean_card
221 // precleaned ==> cur_youngergen_and_prev_nonclean_card
222 // prev-younger-gen ==> cur_youngergen_and_prev_nonclean_card
223 // cur-younger-gen ==> cur_younger_gen
224 // cur_youngergen_and_prev_nonclean_card ==> no change.
225 void CardTableRS::write_ref_field_gc_par(void* field, oop new_val) {
226 jbyte* entry = ct_bs()->byte_for(field);
227 do {
228 jbyte entry_val = *entry;
229 // We put this first because it's probably the most common case.
230 if (entry_val == clean_card_val()) {
231 // No threat of contention with cleaning threads.
232 *entry = cur_youngergen_card_val();
233 return;
234 } else if (card_is_dirty_wrt_gen_iter(entry_val)
235 || is_prev_youngergen_card_val(entry_val)) {
236 // Mark it as both cur and prev youngergen; card cleaning thread will
237 // eventually remove the previous stuff.
238 jbyte new_val = cur_youngergen_and_prev_nonclean_card;
239 jbyte res = Atomic::cmpxchg(new_val, entry, entry_val);
240 // Did the CAS succeed?
241 if (res == entry_val) return;
242 // Otherwise, retry, to see the new value.
243 continue;
244 } else {
245 assert(entry_val == cur_youngergen_and_prev_nonclean_card
246 || entry_val == cur_youngergen_card_val(),
247 "should be only possibilities.");
248 return;
249 }
250 } while (true);
251 }
252
253 void CardTableRS::younger_refs_in_space_iterate(Space* sp,
254 OopsInGenClosure* cl) {
255 const MemRegion urasm = sp->used_region_at_save_marks();
256 #ifdef ASSERT
257 // Convert the assertion check to a warning if we are running
258 // CMS+ParNew until related bug is fixed.
259 MemRegion ur = sp->used_region();
260 assert(ur.contains(urasm) || (UseConcMarkSweepGC && UseParNewGC),
261 err_msg("Did you forget to call save_marks()? "
262 "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in "
263 "[" PTR_FORMAT ", " PTR_FORMAT ")",
264 urasm.start(), urasm.end(), ur.start(), ur.end()));
265 // In the case of CMS+ParNew, issue a warning
266 if (!ur.contains(urasm)) {
267 assert(UseConcMarkSweepGC && UseParNewGC, "Tautology: see assert above");
268 warning("CMS+ParNew: Did you forget to call save_marks()? "
269 "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in "
270 "[" PTR_FORMAT ", " PTR_FORMAT ")",
271 urasm.start(), urasm.end(), ur.start(), ur.end());
272 MemRegion ur2 = sp->used_region();
273 MemRegion urasm2 = sp->used_region_at_save_marks();
274 if (!ur.equals(ur2)) {
275 warning("CMS+ParNew: Flickering used_region()!!");
276 }
277 if (!urasm.equals(urasm2)) {
278 warning("CMS+ParNew: Flickering used_region_at_save_marks()!!");
279 }
280 ShouldNotReachHere();
281 }
282 #endif
283 _ct_bs->non_clean_card_iterate_possibly_parallel(sp, urasm, cl, this);
284 }
285
286 void CardTableRS::clear_into_younger(Generation* gen, bool clear_perm) {
287 GenCollectedHeap* gch = GenCollectedHeap::heap();
288 // Generations younger than gen have been evacuated. We can clear
289 // card table entries for gen (we know that it has no pointers
290 // to younger gens) and for those below. The card tables for
291 // the youngest gen need never be cleared, and those for perm gen
292 // will be cleared based on the parameter clear_perm.
293 // There's a bit of subtlety in the clear() and invalidate()
294 // methods that we exploit here and in invalidate_or_clear()
295 // below to avoid missing cards at the fringes. If clear() or
296 // invalidate() are changed in the future, this code should
297 // be revisited. 20040107.ysr
298 Generation* g = gen;
299 for(Generation* prev_gen = gch->prev_gen(g);
300 prev_gen != NULL;
301 g = prev_gen, prev_gen = gch->prev_gen(g)) {
302 MemRegion to_be_cleared_mr = g->prev_used_region();
303 clear(to_be_cleared_mr);
304 }
305 // Clear perm gen cards if asked to do so.
306 if (clear_perm) {
307 MemRegion to_be_cleared_mr = gch->perm_gen()->prev_used_region();
308 clear(to_be_cleared_mr);
309 }
310 }
311
312 void CardTableRS::invalidate_or_clear(Generation* gen, bool younger,
313 bool perm) {
314 GenCollectedHeap* gch = GenCollectedHeap::heap();
315 // For each generation gen (and younger and/or perm)
316 // invalidate the cards for the currently occupied part
317 // of that generation and clear the cards for the
318 // unoccupied part of the generation (if any, making use
319 // of that generation's prev_used_region to determine that
320 // region). No need to do anything for the youngest
321 // generation. Also see note#20040107.ysr above.
322 Generation* g = gen;
323 for(Generation* prev_gen = gch->prev_gen(g); prev_gen != NULL;
324 g = prev_gen, prev_gen = gch->prev_gen(g)) {
325 MemRegion used_mr = g->used_region();
326 MemRegion to_be_cleared_mr = g->prev_used_region().minus(used_mr);
327 if (!to_be_cleared_mr.is_empty()) {
328 clear(to_be_cleared_mr);
329 }
330 invalidate(used_mr);
331 if (!younger) break;
332 }
333 // Clear perm gen cards if asked to do so.
334 if (perm) {
335 g = gch->perm_gen();
336 MemRegion used_mr = g->used_region();
337 MemRegion to_be_cleared_mr = g->prev_used_region().minus(used_mr);
338 if (!to_be_cleared_mr.is_empty()) {
339 clear(to_be_cleared_mr);
340 }
341 invalidate(used_mr);
342 }
343 }
344
345
346 class VerifyCleanCardClosure: public OopClosure {
347 private:
348 HeapWord* _boundary;
349 HeapWord* _begin;
350 HeapWord* _end;
351 protected:
352 template <class T> void do_oop_work(T* p) {
353 HeapWord* jp = (HeapWord*)p;
354 assert(jp >= _begin && jp < _end,
355 err_msg("Error: jp " PTR_FORMAT " should be within "
356 "[_begin, _end) = [" PTR_FORMAT "," PTR_FORMAT ")",
357 _begin, _end));
358 oop obj = oopDesc::load_decode_heap_oop(p);
359 guarantee(obj == NULL || (HeapWord*)obj >= _boundary,
360 err_msg("pointer " PTR_FORMAT " at " PTR_FORMAT " on "
361 "clean card crosses boundary" PTR_FORMAT,
362 (HeapWord*)obj, jp, _boundary));
363 }
364
365 public:
366 VerifyCleanCardClosure(HeapWord* b, HeapWord* begin, HeapWord* end) :
367 _boundary(b), _begin(begin), _end(end) {
368 assert(b <= begin,
369 err_msg("Error: boundary " PTR_FORMAT " should be at or below begin " PTR_FORMAT,
370 b, begin));
371 assert(begin <= end,
372 err_msg("Error: begin " PTR_FORMAT " should be strictly below end " PTR_FORMAT,
373 begin, end));
374 }
375
376 virtual void do_oop(oop* p) { VerifyCleanCardClosure::do_oop_work(p); }
377 virtual void do_oop(narrowOop* p) { VerifyCleanCardClosure::do_oop_work(p); }
378 };
379
380 class VerifyCTSpaceClosure: public SpaceClosure {
381 private:
382 CardTableRS* _ct;
383 HeapWord* _boundary;
384 public:
385 VerifyCTSpaceClosure(CardTableRS* ct, HeapWord* boundary) :
386 _ct(ct), _boundary(boundary) {}
387 virtual void do_space(Space* s) { _ct->verify_space(s, _boundary); }
388 };
389
390 class VerifyCTGenClosure: public GenCollectedHeap::GenClosure {
391 CardTableRS* _ct;
392 public:
393 VerifyCTGenClosure(CardTableRS* ct) : _ct(ct) {}
394 void do_generation(Generation* gen) {
395 // Skip the youngest generation.
396 if (gen->level() == 0) return;
397 // Normally, we're interested in pointers to younger generations.
398 VerifyCTSpaceClosure blk(_ct, gen->reserved().start());
399 gen->space_iterate(&blk, true);
400 }
401 };
402
403 void CardTableRS::verify_space(Space* s, HeapWord* gen_boundary) {
404 // We don't need to do young-gen spaces.
405 if (s->end() <= gen_boundary) return;
406 MemRegion used = s->used_region();
407
408 jbyte* cur_entry = byte_for(used.start());
409 jbyte* limit = byte_after(used.last());
410 while (cur_entry < limit) {
411 if (*cur_entry == CardTableModRefBS::clean_card) {
412 jbyte* first_dirty = cur_entry+1;
413 while (first_dirty < limit &&
414 *first_dirty == CardTableModRefBS::clean_card) {
415 first_dirty++;
416 }
417 // If the first object is a regular object, and it has a
418 // young-to-old field, that would mark the previous card.
419 HeapWord* boundary = addr_for(cur_entry);
420 HeapWord* end = (first_dirty >= limit) ? used.end() : addr_for(first_dirty);
421 HeapWord* boundary_block = s->block_start(boundary);
422 HeapWord* begin = boundary; // Until proven otherwise.
423 HeapWord* start_block = boundary_block; // Until proven otherwise.
424 if (boundary_block < boundary) {
425 if (s->block_is_obj(boundary_block) && s->obj_is_alive(boundary_block)) {
426 oop boundary_obj = oop(boundary_block);
427 if (!boundary_obj->is_objArray() &&
428 !boundary_obj->is_typeArray()) {
429 guarantee(cur_entry > byte_for(used.start()),
430 "else boundary would be boundary_block");
431 if (*byte_for(boundary_block) != CardTableModRefBS::clean_card) {
432 begin = boundary_block + s->block_size(boundary_block);
433 start_block = begin;
434 }
435 }
436 }
437 }
438 // Now traverse objects until end.
439 if (begin < end) {
440 MemRegion mr(begin, end);
441 VerifyCleanCardClosure verify_blk(gen_boundary, begin, end);
442 for (HeapWord* cur = start_block; cur < end; cur += s->block_size(cur)) {
443 if (s->block_is_obj(cur) && s->obj_is_alive(cur)) {
444 oop(cur)->oop_iterate(&verify_blk, mr);
445 }
446 }
447 }
448 cur_entry = first_dirty;
449 } else {
450 // We'd normally expect that cur_youngergen_and_prev_nonclean_card
451 // is a transient value, that cannot be in the card table
452 // except during GC, and thus assert that:
453 // guarantee(*cur_entry != cur_youngergen_and_prev_nonclean_card,
454 // "Illegal CT value");
455 // That however, need not hold, as will become clear in the
456 // following...
457
458 // We'd normally expect that if we are in the parallel case,
459 // we can't have left a prev value (which would be different
460 // from the current value) in the card table, and so we'd like to
461 // assert that:
462 // guarantee(cur_youngergen_card_val() == youngergen_card
463 // || !is_prev_youngergen_card_val(*cur_entry),
464 // "Illegal CT value");
465 // That, however, may not hold occasionally, because of
466 // CMS or MSC in the old gen. To wit, consider the
467 // following two simple illustrative scenarios:
468 // (a) CMS: Consider the case where a large object L
469 // spanning several cards is allocated in the old
470 // gen, and has a young gen reference stored in it, dirtying
471 // some interior cards. A young collection scans the card,
472 // finds a young ref and installs a youngergenP_n value.
473 // L then goes dead. Now a CMS collection starts,
474 // finds L dead and sweeps it up. Assume that L is
475 // abutting _unallocated_blk, so _unallocated_blk is
476 // adjusted down to (below) L. Assume further that
477 // no young collection intervenes during this CMS cycle.
478 // The next young gen cycle will not get to look at this
479 // youngergenP_n card since it lies in the unoccupied
480 // part of the space.
481 // Some young collections later the blocks on this
482 // card can be re-allocated either due to direct allocation
483 // or due to absorbing promotions. At this time, the
484 // before-gc verification will fail the above assert.
485 // (b) MSC: In this case, an object L with a young reference
486 // is on a card that (therefore) holds a youngergen_n value.
487 // Suppose also that L lies towards the end of the used
488 // the used space before GC. An MSC collection
489 // occurs that compacts to such an extent that this
490 // card is no longer in the occupied part of the space.
491 // Since current code in MSC does not always clear cards
492 // in the unused part of old gen, this stale youngergen_n
493 // value is left behind and can later be covered by
494 // an object when promotion or direct allocation
495 // re-allocates that part of the heap.
496 //
497 // Fortunately, the presence of such stale card values is
498 // "only" a minor annoyance in that subsequent young collections
499 // might needlessly scan such cards, but would still never corrupt
500 // the heap as a result. However, it's likely not to be a significant
501 // performance inhibitor in practice. For instance,
502 // some recent measurements with unoccupied cards eagerly cleared
503 // out to maintain this invariant, showed next to no
504 // change in young collection times; of course one can construct
505 // degenerate examples where the cost can be significant.)
506 // Note, in particular, that if the "stale" card is modified
507 // after re-allocation, it would be dirty, not "stale". Thus,
508 // we can never have a younger ref in such a card and it is
509 // safe not to scan that card in any collection. [As we see
510 // below, we do some unnecessary scanning
511 // in some cases in the current parallel scanning algorithm.]
512 //
513 // The main point below is that the parallel card scanning code
514 // deals correctly with these stale card values. There are two main
515 // cases to consider where we have a stale "younger gen" value and a
516 // "derivative" case to consider, where we have a stale
517 // "cur_younger_gen_and_prev_non_clean" value, as will become
518 // apparent in the case analysis below.
519 // o Case 1. If the stale value corresponds to a younger_gen_n
520 // value other than the cur_younger_gen value then the code
521 // treats this as being tantamount to a prev_younger_gen
522 // card. This means that the card may be unnecessarily scanned.
523 // There are two sub-cases to consider:
524 // o Case 1a. Let us say that the card is in the occupied part
525 // of the generation at the time the collection begins. In
526 // that case the card will be either cleared when it is scanned
527 // for young pointers, or will be set to cur_younger_gen as a
528 // result of promotion. (We have elided the normal case where
529 // the scanning thread and the promoting thread interleave
530 // possibly resulting in a transient
531 // cur_younger_gen_and_prev_non_clean value before settling
532 // to cur_younger_gen. [End Case 1a.]
533 // o Case 1b. Consider now the case when the card is in the unoccupied
534 // part of the space which becomes occupied because of promotions
535 // into it during the current young GC. In this case the card
536 // will never be scanned for young references. The current
537 // code will set the card value to either
538 // cur_younger_gen_and_prev_non_clean or leave
539 // it with its stale value -- because the promotions didn't
540 // result in any younger refs on that card. Of these two
541 // cases, the latter will be covered in Case 1a during
542 // a subsequent scan. To deal with the former case, we need
543 // to further consider how we deal with a stale value of
544 // cur_younger_gen_and_prev_non_clean in our case analysis
545 // below. This we do in Case 3 below. [End Case 1b]
546 // [End Case 1]
547 // o Case 2. If the stale value corresponds to cur_younger_gen being
548 // a value not necessarily written by a current promotion, the
549 // card will not be scanned by the younger refs scanning code.
550 // (This is OK since as we argued above such cards cannot contain
551 // any younger refs.) The result is that this value will be
552 // treated as a prev_younger_gen value in a subsequent collection,
553 // which is addressed in Case 1 above. [End Case 2]
554 // o Case 3. We here consider the "derivative" case from Case 1b. above
555 // because of which we may find a stale
556 // cur_younger_gen_and_prev_non_clean card value in the table.
557 // Once again, as in Case 1, we consider two subcases, depending
558 // on whether the card lies in the occupied or unoccupied part
559 // of the space at the start of the young collection.
560 // o Case 3a. Let us say the card is in the occupied part of
561 // the old gen at the start of the young collection. In that
562 // case, the card will be scanned by the younger refs scanning
563 // code which will set it to cur_younger_gen. In a subsequent
564 // scan, the card will be considered again and get its final
565 // correct value. [End Case 3a]
566 // o Case 3b. Now consider the case where the card is in the
567 // unoccupied part of the old gen, and is occupied as a result
568 // of promotions during thus young gc. In that case,
569 // the card will not be scanned for younger refs. The presence
570 // of newly promoted objects on the card will then result in
571 // its keeping the value cur_younger_gen_and_prev_non_clean
572 // value, which we have dealt with in Case 3 here. [End Case 3b]
573 // [End Case 3]
574 //
575 // (Please refer to the code in the helper class
576 // ClearNonCleanCardWrapper and in CardTableModRefBS for details.)
577 //
578 // The informal arguments above can be tightened into a formal
579 // correctness proof and it behooves us to write up such a proof,
580 // or to use model checking to prove that there are no lingering
581 // concerns.
582 //
583 // Clearly because of Case 3b one cannot bound the time for
584 // which a card will retain what we have called a "stale" value.
585 // However, one can obtain a Loose upper bound on the redundant
586 // work as a result of such stale values. Note first that any
587 // time a stale card lies in the occupied part of the space at
588 // the start of the collection, it is scanned by younger refs
589 // code and we can define a rank function on card values that
590 // declines when this is so. Note also that when a card does not
591 // lie in the occupied part of the space at the beginning of a
592 // young collection, its rank can either decline or stay unchanged.
593 // In this case, no extra work is done in terms of redundant
594 // younger refs scanning of that card.
595 // Then, the case analysis above reveals that, in the worst case,
596 // any such stale card will be scanned unnecessarily at most twice.
597 //
598 // It is nonethelss advisable to try and get rid of some of this
599 // redundant work in a subsequent (low priority) re-design of
600 // the card-scanning code, if only to simplify the underlying
601 // state machine analysis/proof. ysr 1/28/2002. XXX
602 cur_entry++;
603 }
604 }
605 }
606
607 void CardTableRS::verify() {
608 // At present, we only know how to verify the card table RS for
609 // generational heaps.
610 VerifyCTGenClosure blk(this);
611 CollectedHeap* ch = Universe::heap();
612 // We will do the perm-gen portion of the card table, too.
613 Generation* pg = SharedHeap::heap()->perm_gen();
614 HeapWord* pg_boundary = pg->reserved().start();
615
616 if (ch->kind() == CollectedHeap::GenCollectedHeap) {
617 GenCollectedHeap::heap()->generation_iterate(&blk, false);
618 _ct_bs->verify();
619
620 // If the old gen collections also collect perm, then we are only
621 // interested in perm-to-young pointers, not perm-to-old pointers.
622 GenCollectedHeap* gch = GenCollectedHeap::heap();
623 CollectorPolicy* cp = gch->collector_policy();
624 if (cp->is_mark_sweep_policy() || cp->is_concurrent_mark_sweep_policy()) {
625 pg_boundary = gch->get_gen(1)->reserved().start();
626 }
627 }
628 VerifyCTSpaceClosure perm_space_blk(this, pg_boundary);
629 SharedHeap::heap()->perm_gen()->space_iterate(&perm_space_blk, true);
630 }
631
632
633 void CardTableRS::verify_aligned_region_empty(MemRegion mr) {
634 if (!mr.is_empty()) {
635 jbyte* cur_entry = byte_for(mr.start());
636 jbyte* limit = byte_after(mr.last());
637 // The region mr may not start on a card boundary so
638 // the first card may reflect a write to the space
639 // just prior to mr.
640 if (!is_aligned(mr.start())) {
641 cur_entry++;
642 }
643 for (;cur_entry < limit; cur_entry++) {
644 guarantee(*cur_entry == CardTableModRefBS::clean_card,
645 "Unexpected dirty card found");
646 }
647 }
648 }