1 /*
2 * Copyright (c) 2001, 2012, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
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23 */
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 bool ClearNoncleanCardWrapper::is_word_aligned(jbyte* entry) {
177 return (((intptr_t)entry) & (BytesPerWord-1)) == 0;
178 }
179
180 void ClearNoncleanCardWrapper::do_MemRegion(MemRegion mr) {
181 assert(mr.word_size() > 0, "Error");
182 assert(_ct->is_aligned(mr.start()), "mr.start() should be card aligned");
183 // mr.end() may not necessarily be card aligned.
184 jbyte* cur_entry = _ct->byte_for(mr.last());
185 const jbyte* limit = _ct->byte_for(mr.start());
186 HeapWord* end_of_non_clean = mr.end();
187 HeapWord* start_of_non_clean = end_of_non_clean;
188 while (cur_entry >= limit) {
189 HeapWord* cur_hw = _ct->addr_for(cur_entry);
190 if ((*cur_entry != CardTableRS::clean_card_val()) && clear_card(cur_entry)) {
191 // Continue the dirty range by opening the
192 // dirty window one card to the left.
193 start_of_non_clean = cur_hw;
194
195 cur_entry--;
196 } else {
197 // We hit a "clean" card; process any non-empty
198 // "dirty" range accumulated so far.
199 if (start_of_non_clean < end_of_non_clean) {
200 const MemRegion mrd(start_of_non_clean, end_of_non_clean);
201 _dirty_card_closure->do_MemRegion(mrd);
202 }
203
204 // fast forward through potential continuous range of clean cards
205 if (is_word_aligned(cur_entry)) {
206 jbyte* cur_row = cur_entry - BytesPerWord;
207 while(cur_row >= limit) {
208 if (*((intptr_t*)cur_row) == CardTableRS::clean_card_row()) {
209 cur_row -= BytesPerWord;
210 }
211 else {
212 break;
213 }
214 }
215 cur_entry = cur_row + (BytesPerWord - 1);
216 HeapWord* last_hw = _ct->addr_for(cur_row + BytesPerWord);
217 end_of_non_clean = last_hw;
218 start_of_non_clean = last_hw;
219 } else {
220 // Reset the dirty window, while continuing to look
221 // for the next dirty card that will start a
222 // new dirty window.
223 end_of_non_clean = cur_hw;
224 start_of_non_clean = cur_hw;
225 cur_entry--;
226 }
227 }
228 // Note that "cur_entry" leads "start_of_non_clean" in
229 // its leftward excursion after this point
230 // in the loop and, when we hit the left end of "mr",
231 // will point off of the left end of the card-table
232 // for "mr".
233 }
234 // If the first card of "mr" was dirty, we will have
235 // been left with a dirty window, co-initial with "mr",
236 // which we now process.
237 if (start_of_non_clean < end_of_non_clean) {
238 const MemRegion mrd(start_of_non_clean, end_of_non_clean);
239 _dirty_card_closure->do_MemRegion(mrd);
240 }
241 }
242
243 // clean (by dirty->clean before) ==> cur_younger_gen
244 // dirty ==> cur_youngergen_and_prev_nonclean_card
245 // precleaned ==> cur_youngergen_and_prev_nonclean_card
246 // prev-younger-gen ==> cur_youngergen_and_prev_nonclean_card
247 // cur-younger-gen ==> cur_younger_gen
248 // cur_youngergen_and_prev_nonclean_card ==> no change.
249 void CardTableRS::write_ref_field_gc_par(void* field, oop new_val) {
250 jbyte* entry = ct_bs()->byte_for(field);
251 do {
252 jbyte entry_val = *entry;
253 // We put this first because it's probably the most common case.
254 if (entry_val == clean_card_val()) {
255 // No threat of contention with cleaning threads.
256 *entry = cur_youngergen_card_val();
257 return;
258 } else if (card_is_dirty_wrt_gen_iter(entry_val)
259 || is_prev_youngergen_card_val(entry_val)) {
260 // Mark it as both cur and prev youngergen; card cleaning thread will
261 // eventually remove the previous stuff.
262 jbyte new_val = cur_youngergen_and_prev_nonclean_card;
263 jbyte res = Atomic::cmpxchg(new_val, entry, entry_val);
264 // Did the CAS succeed?
265 if (res == entry_val) return;
266 // Otherwise, retry, to see the new value.
267 continue;
268 } else {
269 assert(entry_val == cur_youngergen_and_prev_nonclean_card
270 || entry_val == cur_youngergen_card_val(),
271 "should be only possibilities.");
272 return;
273 }
274 } while (true);
275 }
276
277 void CardTableRS::younger_refs_in_space_iterate(Space* sp,
278 OopsInGenClosure* cl) {
279 const MemRegion urasm = sp->used_region_at_save_marks();
280 #ifdef ASSERT
281 // Convert the assertion check to a warning if we are running
282 // CMS+ParNew until related bug is fixed.
283 MemRegion ur = sp->used_region();
284 assert(ur.contains(urasm) || (UseConcMarkSweepGC && UseParNewGC),
285 err_msg("Did you forget to call save_marks()? "
286 "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in "
287 "[" PTR_FORMAT ", " PTR_FORMAT ")",
288 urasm.start(), urasm.end(), ur.start(), ur.end()));
289 // In the case of CMS+ParNew, issue a warning
290 if (!ur.contains(urasm)) {
291 assert(UseConcMarkSweepGC && UseParNewGC, "Tautology: see assert above");
292 warning("CMS+ParNew: Did you forget to call save_marks()? "
293 "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in "
294 "[" PTR_FORMAT ", " PTR_FORMAT ")",
295 urasm.start(), urasm.end(), ur.start(), ur.end());
296 MemRegion ur2 = sp->used_region();
297 MemRegion urasm2 = sp->used_region_at_save_marks();
298 if (!ur.equals(ur2)) {
299 warning("CMS+ParNew: Flickering used_region()!!");
300 }
301 if (!urasm.equals(urasm2)) {
302 warning("CMS+ParNew: Flickering used_region_at_save_marks()!!");
303 }
304 ShouldNotReachHere();
305 }
306 #endif
307 _ct_bs->non_clean_card_iterate_possibly_parallel(sp, urasm, cl, this);
308 }
309
310 void CardTableRS::clear_into_younger(Generation* gen, bool clear_perm) {
311 GenCollectedHeap* gch = GenCollectedHeap::heap();
312 // Generations younger than gen have been evacuated. We can clear
313 // card table entries for gen (we know that it has no pointers
314 // to younger gens) and for those below. The card tables for
315 // the youngest gen need never be cleared, and those for perm gen
316 // will be cleared based on the parameter clear_perm.
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 Generation* g = gen;
323 for(Generation* prev_gen = gch->prev_gen(g);
324 prev_gen != NULL;
325 g = prev_gen, prev_gen = gch->prev_gen(g)) {
326 MemRegion to_be_cleared_mr = g->prev_used_region();
327 clear(to_be_cleared_mr);
328 }
329 // Clear perm gen cards if asked to do so.
330 if (clear_perm) {
331 MemRegion to_be_cleared_mr = gch->perm_gen()->prev_used_region();
332 clear(to_be_cleared_mr);
333 }
334 }
335
336 void CardTableRS::invalidate_or_clear(Generation* gen, bool younger,
337 bool perm) {
338 GenCollectedHeap* gch = GenCollectedHeap::heap();
339 // For each generation gen (and younger and/or perm)
340 // invalidate the cards for the currently occupied part
341 // of that generation and clear the cards for the
342 // unoccupied part of the generation (if any, making use
343 // of that generation's prev_used_region to determine that
344 // region). No need to do anything for the youngest
345 // generation. Also see note#20040107.ysr above.
346 Generation* g = gen;
347 for(Generation* prev_gen = gch->prev_gen(g); prev_gen != NULL;
348 g = prev_gen, prev_gen = gch->prev_gen(g)) {
349 MemRegion used_mr = g->used_region();
350 MemRegion to_be_cleared_mr = g->prev_used_region().minus(used_mr);
351 if (!to_be_cleared_mr.is_empty()) {
352 clear(to_be_cleared_mr);
353 }
354 invalidate(used_mr);
355 if (!younger) break;
356 }
357 // Clear perm gen cards if asked to do so.
358 if (perm) {
359 g = gch->perm_gen();
360 MemRegion used_mr = g->used_region();
361 MemRegion to_be_cleared_mr = g->prev_used_region().minus(used_mr);
362 if (!to_be_cleared_mr.is_empty()) {
363 clear(to_be_cleared_mr);
364 }
365 invalidate(used_mr);
366 }
367 }
368
369
370 class VerifyCleanCardClosure: public OopClosure {
371 private:
372 HeapWord* _boundary;
373 HeapWord* _begin;
374 HeapWord* _end;
375 protected:
376 template <class T> void do_oop_work(T* p) {
377 HeapWord* jp = (HeapWord*)p;
378 assert(jp >= _begin && jp < _end,
379 err_msg("Error: jp " PTR_FORMAT " should be within "
380 "[_begin, _end) = [" PTR_FORMAT "," PTR_FORMAT ")",
381 _begin, _end));
382 oop obj = oopDesc::load_decode_heap_oop(p);
383 guarantee(obj == NULL || (HeapWord*)obj >= _boundary,
384 err_msg("pointer " PTR_FORMAT " at " PTR_FORMAT " on "
385 "clean card crosses boundary" PTR_FORMAT,
386 (HeapWord*)obj, jp, _boundary));
387 }
388
389 public:
390 VerifyCleanCardClosure(HeapWord* b, HeapWord* begin, HeapWord* end) :
391 _boundary(b), _begin(begin), _end(end) {
392 assert(b <= begin,
393 err_msg("Error: boundary " PTR_FORMAT " should be at or below begin " PTR_FORMAT,
394 b, begin));
395 assert(begin <= end,
396 err_msg("Error: begin " PTR_FORMAT " should be strictly below end " PTR_FORMAT,
397 begin, end));
398 }
399
400 virtual void do_oop(oop* p) { VerifyCleanCardClosure::do_oop_work(p); }
401 virtual void do_oop(narrowOop* p) { VerifyCleanCardClosure::do_oop_work(p); }
402 };
403
404 class VerifyCTSpaceClosure: public SpaceClosure {
405 private:
406 CardTableRS* _ct;
407 HeapWord* _boundary;
408 public:
409 VerifyCTSpaceClosure(CardTableRS* ct, HeapWord* boundary) :
410 _ct(ct), _boundary(boundary) {}
411 virtual void do_space(Space* s) { _ct->verify_space(s, _boundary); }
412 };
413
414 class VerifyCTGenClosure: public GenCollectedHeap::GenClosure {
415 CardTableRS* _ct;
416 public:
417 VerifyCTGenClosure(CardTableRS* ct) : _ct(ct) {}
418 void do_generation(Generation* gen) {
419 // Skip the youngest generation.
420 if (gen->level() == 0) return;
421 // Normally, we're interested in pointers to younger generations.
422 VerifyCTSpaceClosure blk(_ct, gen->reserved().start());
423 gen->space_iterate(&blk, true);
424 }
425 };
426
427 void CardTableRS::verify_space(Space* s, HeapWord* gen_boundary) {
428 // We don't need to do young-gen spaces.
429 if (s->end() <= gen_boundary) return;
430 MemRegion used = s->used_region();
431
432 jbyte* cur_entry = byte_for(used.start());
433 jbyte* limit = byte_after(used.last());
434 while (cur_entry < limit) {
435 if (*cur_entry == CardTableModRefBS::clean_card) {
436 jbyte* first_dirty = cur_entry+1;
437 while (first_dirty < limit &&
438 *first_dirty == CardTableModRefBS::clean_card) {
439 first_dirty++;
440 }
441 // If the first object is a regular object, and it has a
442 // young-to-old field, that would mark the previous card.
443 HeapWord* boundary = addr_for(cur_entry);
444 HeapWord* end = (first_dirty >= limit) ? used.end() : addr_for(first_dirty);
445 HeapWord* boundary_block = s->block_start(boundary);
446 HeapWord* begin = boundary; // Until proven otherwise.
447 HeapWord* start_block = boundary_block; // Until proven otherwise.
448 if (boundary_block < boundary) {
449 if (s->block_is_obj(boundary_block) && s->obj_is_alive(boundary_block)) {
450 oop boundary_obj = oop(boundary_block);
451 if (!boundary_obj->is_objArray() &&
452 !boundary_obj->is_typeArray()) {
453 guarantee(cur_entry > byte_for(used.start()),
454 "else boundary would be boundary_block");
455 if (*byte_for(boundary_block) != CardTableModRefBS::clean_card) {
456 begin = boundary_block + s->block_size(boundary_block);
457 start_block = begin;
458 }
459 }
460 }
461 }
462 // Now traverse objects until end.
463 if (begin < end) {
464 MemRegion mr(begin, end);
465 VerifyCleanCardClosure verify_blk(gen_boundary, begin, end);
466 for (HeapWord* cur = start_block; cur < end; cur += s->block_size(cur)) {
467 if (s->block_is_obj(cur) && s->obj_is_alive(cur)) {
468 oop(cur)->oop_iterate(&verify_blk, mr);
469 }
470 }
471 }
472 cur_entry = first_dirty;
473 } else {
474 // We'd normally expect that cur_youngergen_and_prev_nonclean_card
475 // is a transient value, that cannot be in the card table
476 // except during GC, and thus assert that:
477 // guarantee(*cur_entry != cur_youngergen_and_prev_nonclean_card,
478 // "Illegal CT value");
479 // That however, need not hold, as will become clear in the
480 // following...
481
482 // We'd normally expect that if we are in the parallel case,
483 // we can't have left a prev value (which would be different
484 // from the current value) in the card table, and so we'd like to
485 // assert that:
486 // guarantee(cur_youngergen_card_val() == youngergen_card
487 // || !is_prev_youngergen_card_val(*cur_entry),
488 // "Illegal CT value");
489 // That, however, may not hold occasionally, because of
490 // CMS or MSC in the old gen. To wit, consider the
491 // following two simple illustrative scenarios:
492 // (a) CMS: Consider the case where a large object L
493 // spanning several cards is allocated in the old
494 // gen, and has a young gen reference stored in it, dirtying
495 // some interior cards. A young collection scans the card,
496 // finds a young ref and installs a youngergenP_n value.
497 // L then goes dead. Now a CMS collection starts,
498 // finds L dead and sweeps it up. Assume that L is
499 // abutting _unallocated_blk, so _unallocated_blk is
500 // adjusted down to (below) L. Assume further that
501 // no young collection intervenes during this CMS cycle.
502 // The next young gen cycle will not get to look at this
503 // youngergenP_n card since it lies in the unoccupied
504 // part of the space.
505 // Some young collections later the blocks on this
506 // card can be re-allocated either due to direct allocation
507 // or due to absorbing promotions. At this time, the
508 // before-gc verification will fail the above assert.
509 // (b) MSC: In this case, an object L with a young reference
510 // is on a card that (therefore) holds a youngergen_n value.
511 // Suppose also that L lies towards the end of the used
512 // the used space before GC. An MSC collection
513 // occurs that compacts to such an extent that this
514 // card is no longer in the occupied part of the space.
515 // Since current code in MSC does not always clear cards
516 // in the unused part of old gen, this stale youngergen_n
517 // value is left behind and can later be covered by
518 // an object when promotion or direct allocation
519 // re-allocates that part of the heap.
520 //
521 // Fortunately, the presence of such stale card values is
522 // "only" a minor annoyance in that subsequent young collections
523 // might needlessly scan such cards, but would still never corrupt
524 // the heap as a result. However, it's likely not to be a significant
525 // performance inhibitor in practice. For instance,
526 // some recent measurements with unoccupied cards eagerly cleared
527 // out to maintain this invariant, showed next to no
528 // change in young collection times; of course one can construct
529 // degenerate examples where the cost can be significant.)
530 // Note, in particular, that if the "stale" card is modified
531 // after re-allocation, it would be dirty, not "stale". Thus,
532 // we can never have a younger ref in such a card and it is
533 // safe not to scan that card in any collection. [As we see
534 // below, we do some unnecessary scanning
535 // in some cases in the current parallel scanning algorithm.]
536 //
537 // The main point below is that the parallel card scanning code
538 // deals correctly with these stale card values. There are two main
539 // cases to consider where we have a stale "younger gen" value and a
540 // "derivative" case to consider, where we have a stale
541 // "cur_younger_gen_and_prev_non_clean" value, as will become
542 // apparent in the case analysis below.
543 // o Case 1. If the stale value corresponds to a younger_gen_n
544 // value other than the cur_younger_gen value then the code
545 // treats this as being tantamount to a prev_younger_gen
546 // card. This means that the card may be unnecessarily scanned.
547 // There are two sub-cases to consider:
548 // o Case 1a. Let us say that the card is in the occupied part
549 // of the generation at the time the collection begins. In
550 // that case the card will be either cleared when it is scanned
551 // for young pointers, or will be set to cur_younger_gen as a
552 // result of promotion. (We have elided the normal case where
553 // the scanning thread and the promoting thread interleave
554 // possibly resulting in a transient
555 // cur_younger_gen_and_prev_non_clean value before settling
556 // to cur_younger_gen. [End Case 1a.]
557 // o Case 1b. Consider now the case when the card is in the unoccupied
558 // part of the space which becomes occupied because of promotions
559 // into it during the current young GC. In this case the card
560 // will never be scanned for young references. The current
561 // code will set the card value to either
562 // cur_younger_gen_and_prev_non_clean or leave
563 // it with its stale value -- because the promotions didn't
564 // result in any younger refs on that card. Of these two
565 // cases, the latter will be covered in Case 1a during
566 // a subsequent scan. To deal with the former case, we need
567 // to further consider how we deal with a stale value of
568 // cur_younger_gen_and_prev_non_clean in our case analysis
569 // below. This we do in Case 3 below. [End Case 1b]
570 // [End Case 1]
571 // o Case 2. If the stale value corresponds to cur_younger_gen being
572 // a value not necessarily written by a current promotion, the
573 // card will not be scanned by the younger refs scanning code.
574 // (This is OK since as we argued above such cards cannot contain
575 // any younger refs.) The result is that this value will be
576 // treated as a prev_younger_gen value in a subsequent collection,
577 // which is addressed in Case 1 above. [End Case 2]
578 // o Case 3. We here consider the "derivative" case from Case 1b. above
579 // because of which we may find a stale
580 // cur_younger_gen_and_prev_non_clean card value in the table.
581 // Once again, as in Case 1, we consider two subcases, depending
582 // on whether the card lies in the occupied or unoccupied part
583 // of the space at the start of the young collection.
584 // o Case 3a. Let us say the card is in the occupied part of
585 // the old gen at the start of the young collection. In that
586 // case, the card will be scanned by the younger refs scanning
587 // code which will set it to cur_younger_gen. In a subsequent
588 // scan, the card will be considered again and get its final
589 // correct value. [End Case 3a]
590 // o Case 3b. Now consider the case where the card is in the
591 // unoccupied part of the old gen, and is occupied as a result
592 // of promotions during thus young gc. In that case,
593 // the card will not be scanned for younger refs. The presence
594 // of newly promoted objects on the card will then result in
595 // its keeping the value cur_younger_gen_and_prev_non_clean
596 // value, which we have dealt with in Case 3 here. [End Case 3b]
597 // [End Case 3]
598 //
599 // (Please refer to the code in the helper class
600 // ClearNonCleanCardWrapper and in CardTableModRefBS for details.)
601 //
602 // The informal arguments above can be tightened into a formal
603 // correctness proof and it behooves us to write up such a proof,
604 // or to use model checking to prove that there are no lingering
605 // concerns.
606 //
607 // Clearly because of Case 3b one cannot bound the time for
608 // which a card will retain what we have called a "stale" value.
609 // However, one can obtain a Loose upper bound on the redundant
610 // work as a result of such stale values. Note first that any
611 // time a stale card lies in the occupied part of the space at
612 // the start of the collection, it is scanned by younger refs
613 // code and we can define a rank function on card values that
614 // declines when this is so. Note also that when a card does not
615 // lie in the occupied part of the space at the beginning of a
616 // young collection, its rank can either decline or stay unchanged.
617 // In this case, no extra work is done in terms of redundant
618 // younger refs scanning of that card.
619 // Then, the case analysis above reveals that, in the worst case,
620 // any such stale card will be scanned unnecessarily at most twice.
621 //
622 // It is nonethelss advisable to try and get rid of some of this
623 // redundant work in a subsequent (low priority) re-design of
624 // the card-scanning code, if only to simplify the underlying
625 // state machine analysis/proof. ysr 1/28/2002. XXX
626 cur_entry++;
627 }
628 }
629 }
630
631 void CardTableRS::verify() {
632 // At present, we only know how to verify the card table RS for
633 // generational heaps.
634 VerifyCTGenClosure blk(this);
635 CollectedHeap* ch = Universe::heap();
636 // We will do the perm-gen portion of the card table, too.
637 Generation* pg = SharedHeap::heap()->perm_gen();
638 HeapWord* pg_boundary = pg->reserved().start();
639
640 if (ch->kind() == CollectedHeap::GenCollectedHeap) {
641 GenCollectedHeap::heap()->generation_iterate(&blk, false);
642 _ct_bs->verify();
643
644 // If the old gen collections also collect perm, then we are only
645 // interested in perm-to-young pointers, not perm-to-old pointers.
646 GenCollectedHeap* gch = GenCollectedHeap::heap();
647 CollectorPolicy* cp = gch->collector_policy();
648 if (cp->is_mark_sweep_policy() || cp->is_concurrent_mark_sweep_policy()) {
649 pg_boundary = gch->get_gen(1)->reserved().start();
650 }
651 }
652 VerifyCTSpaceClosure perm_space_blk(this, pg_boundary);
653 SharedHeap::heap()->perm_gen()->space_iterate(&perm_space_blk, true);
654 }
655
656
657 void CardTableRS::verify_aligned_region_empty(MemRegion mr) {
658 if (!mr.is_empty()) {
659 jbyte* cur_entry = byte_for(mr.start());
660 jbyte* limit = byte_after(mr.last());
661 // The region mr may not start on a card boundary so
662 // the first card may reflect a write to the space
663 // just prior to mr.
664 if (!is_aligned(mr.start())) {
665 cur_entry++;
666 }
667 for (;cur_entry < limit; cur_entry++) {
668 guarantee(*cur_entry == CardTableModRefBS::clean_card,
669 "Unexpected dirty card found");
670 }
671 }
672 }