Mon, 21 Nov 2011 07:47:34 +0100
7110718: -XX:MarkSweepAlwaysCompactCount=0 crashes the JVM
Summary: Interpret MarkSweepAlwaysCompactCount < 1 as never do full compaction
Reviewed-by: ysr, tonyp, jmasa, johnc
1 /*
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3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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23 */
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
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 }
66 void CardTableRS::resize_covered_region(MemRegion new_region) {
67 _ct_bs->resize_covered_region(new_region);
68 }
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 }
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);
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 }
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 }
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 }
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 }
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 }
164 ClearNoncleanCardWrapper::ClearNoncleanCardWrapper(
165 DirtyCardToOopClosure* dirty_card_closure, CardTableRS* ct) :
166 _dirty_card_closure(dirty_card_closure), _ct(ct) {
167 _is_par = (SharedHeap::heap()->n_par_threads() > 0);
168 }
170 void ClearNoncleanCardWrapper::do_MemRegion(MemRegion mr) {
171 assert(mr.word_size() > 0, "Error");
172 assert(_ct->is_aligned(mr.start()), "mr.start() should be card aligned");
173 // mr.end() may not necessarily be card aligned.
174 jbyte* cur_entry = _ct->byte_for(mr.last());
175 const jbyte* limit = _ct->byte_for(mr.start());
176 HeapWord* end_of_non_clean = mr.end();
177 HeapWord* start_of_non_clean = end_of_non_clean;
178 while (cur_entry >= limit) {
179 HeapWord* cur_hw = _ct->addr_for(cur_entry);
180 if ((*cur_entry != CardTableRS::clean_card_val()) && clear_card(cur_entry)) {
181 // Continue the dirty range by opening the
182 // dirty window one card to the left.
183 start_of_non_clean = cur_hw;
184 } else {
185 // We hit a "clean" card; process any non-empty
186 // "dirty" range accumulated so far.
187 if (start_of_non_clean < end_of_non_clean) {
188 const MemRegion mrd(start_of_non_clean, end_of_non_clean);
189 _dirty_card_closure->do_MemRegion(mrd);
190 }
191 // Reset the dirty window, while continuing to look
192 // for the next dirty card that will start a
193 // new dirty window.
194 end_of_non_clean = cur_hw;
195 start_of_non_clean = cur_hw;
196 }
197 // Note that "cur_entry" leads "start_of_non_clean" in
198 // its leftward excursion after this point
199 // in the loop and, when we hit the left end of "mr",
200 // will point off of the left end of the card-table
201 // for "mr".
202 cur_entry--;
203 }
204 // If the first card of "mr" was dirty, we will have
205 // been left with a dirty window, co-initial with "mr",
206 // which we now process.
207 if (start_of_non_clean < end_of_non_clean) {
208 const MemRegion mrd(start_of_non_clean, end_of_non_clean);
209 _dirty_card_closure->do_MemRegion(mrd);
210 }
211 }
213 // clean (by dirty->clean before) ==> cur_younger_gen
214 // dirty ==> cur_youngergen_and_prev_nonclean_card
215 // precleaned ==> cur_youngergen_and_prev_nonclean_card
216 // prev-younger-gen ==> cur_youngergen_and_prev_nonclean_card
217 // cur-younger-gen ==> cur_younger_gen
218 // cur_youngergen_and_prev_nonclean_card ==> no change.
219 void CardTableRS::write_ref_field_gc_par(void* field, oop new_val) {
220 jbyte* entry = ct_bs()->byte_for(field);
221 do {
222 jbyte entry_val = *entry;
223 // We put this first because it's probably the most common case.
224 if (entry_val == clean_card_val()) {
225 // No threat of contention with cleaning threads.
226 *entry = cur_youngergen_card_val();
227 return;
228 } else if (card_is_dirty_wrt_gen_iter(entry_val)
229 || is_prev_youngergen_card_val(entry_val)) {
230 // Mark it as both cur and prev youngergen; card cleaning thread will
231 // eventually remove the previous stuff.
232 jbyte new_val = cur_youngergen_and_prev_nonclean_card;
233 jbyte res = Atomic::cmpxchg(new_val, entry, entry_val);
234 // Did the CAS succeed?
235 if (res == entry_val) return;
236 // Otherwise, retry, to see the new value.
237 continue;
238 } else {
239 assert(entry_val == cur_youngergen_and_prev_nonclean_card
240 || entry_val == cur_youngergen_card_val(),
241 "should be only possibilities.");
242 return;
243 }
244 } while (true);
245 }
247 void CardTableRS::younger_refs_in_space_iterate(Space* sp,
248 OopsInGenClosure* cl) {
249 const MemRegion urasm = sp->used_region_at_save_marks();
250 #ifdef ASSERT
251 // Convert the assertion check to a warning if we are running
252 // CMS+ParNew until related bug is fixed.
253 MemRegion ur = sp->used_region();
254 assert(ur.contains(urasm) || (UseConcMarkSweepGC && UseParNewGC),
255 err_msg("Did you forget to call save_marks()? "
256 "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in "
257 "[" PTR_FORMAT ", " PTR_FORMAT ")",
258 urasm.start(), urasm.end(), ur.start(), ur.end()));
259 // In the case of CMS+ParNew, issue a warning
260 if (!ur.contains(urasm)) {
261 assert(UseConcMarkSweepGC && UseParNewGC, "Tautology: see assert above");
262 warning("CMS+ParNew: Did you forget to call save_marks()? "
263 "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in "
264 "[" PTR_FORMAT ", " PTR_FORMAT ")",
265 urasm.start(), urasm.end(), ur.start(), ur.end());
266 MemRegion ur2 = sp->used_region();
267 MemRegion urasm2 = sp->used_region_at_save_marks();
268 if (!ur.equals(ur2)) {
269 warning("CMS+ParNew: Flickering used_region()!!");
270 }
271 if (!urasm.equals(urasm2)) {
272 warning("CMS+ParNew: Flickering used_region_at_save_marks()!!");
273 }
274 ShouldNotReachHere();
275 }
276 #endif
277 _ct_bs->non_clean_card_iterate_possibly_parallel(sp, urasm, cl, this);
278 }
280 void CardTableRS::clear_into_younger(Generation* gen, bool clear_perm) {
281 GenCollectedHeap* gch = GenCollectedHeap::heap();
282 // Generations younger than gen have been evacuated. We can clear
283 // card table entries for gen (we know that it has no pointers
284 // to younger gens) and for those below. The card tables for
285 // the youngest gen need never be cleared, and those for perm gen
286 // will be cleared based on the parameter clear_perm.
287 // There's a bit of subtlety in the clear() and invalidate()
288 // methods that we exploit here and in invalidate_or_clear()
289 // below to avoid missing cards at the fringes. If clear() or
290 // invalidate() are changed in the future, this code should
291 // be revisited. 20040107.ysr
292 Generation* g = gen;
293 for(Generation* prev_gen = gch->prev_gen(g);
294 prev_gen != NULL;
295 g = prev_gen, prev_gen = gch->prev_gen(g)) {
296 MemRegion to_be_cleared_mr = g->prev_used_region();
297 clear(to_be_cleared_mr);
298 }
299 // Clear perm gen cards if asked to do so.
300 if (clear_perm) {
301 MemRegion to_be_cleared_mr = gch->perm_gen()->prev_used_region();
302 clear(to_be_cleared_mr);
303 }
304 }
306 void CardTableRS::invalidate_or_clear(Generation* gen, bool younger,
307 bool perm) {
308 GenCollectedHeap* gch = GenCollectedHeap::heap();
309 // For each generation gen (and younger and/or perm)
310 // invalidate the cards for the currently occupied part
311 // of that generation and clear the cards for the
312 // unoccupied part of the generation (if any, making use
313 // of that generation's prev_used_region to determine that
314 // region). No need to do anything for the youngest
315 // generation. Also see note#20040107.ysr above.
316 Generation* g = gen;
317 for(Generation* prev_gen = gch->prev_gen(g); prev_gen != NULL;
318 g = prev_gen, prev_gen = gch->prev_gen(g)) {
319 MemRegion used_mr = g->used_region();
320 MemRegion to_be_cleared_mr = g->prev_used_region().minus(used_mr);
321 if (!to_be_cleared_mr.is_empty()) {
322 clear(to_be_cleared_mr);
323 }
324 invalidate(used_mr);
325 if (!younger) break;
326 }
327 // Clear perm gen cards if asked to do so.
328 if (perm) {
329 g = gch->perm_gen();
330 MemRegion used_mr = g->used_region();
331 MemRegion to_be_cleared_mr = g->prev_used_region().minus(used_mr);
332 if (!to_be_cleared_mr.is_empty()) {
333 clear(to_be_cleared_mr);
334 }
335 invalidate(used_mr);
336 }
337 }
340 class VerifyCleanCardClosure: public OopClosure {
341 private:
342 HeapWord* _boundary;
343 HeapWord* _begin;
344 HeapWord* _end;
345 protected:
346 template <class T> void do_oop_work(T* p) {
347 HeapWord* jp = (HeapWord*)p;
348 assert(jp >= _begin && jp < _end,
349 err_msg("Error: jp " PTR_FORMAT " should be within "
350 "[_begin, _end) = [" PTR_FORMAT "," PTR_FORMAT ")",
351 _begin, _end));
352 oop obj = oopDesc::load_decode_heap_oop(p);
353 guarantee(obj == NULL || (HeapWord*)obj >= _boundary,
354 err_msg("pointer " PTR_FORMAT " at " PTR_FORMAT " on "
355 "clean card crosses boundary" PTR_FORMAT,
356 (HeapWord*)obj, jp, _boundary));
357 }
359 public:
360 VerifyCleanCardClosure(HeapWord* b, HeapWord* begin, HeapWord* end) :
361 _boundary(b), _begin(begin), _end(end) {
362 assert(b <= begin,
363 err_msg("Error: boundary " PTR_FORMAT " should be at or below begin " PTR_FORMAT,
364 b, begin));
365 assert(begin <= end,
366 err_msg("Error: begin " PTR_FORMAT " should be strictly below end " PTR_FORMAT,
367 begin, end));
368 }
370 virtual void do_oop(oop* p) { VerifyCleanCardClosure::do_oop_work(p); }
371 virtual void do_oop(narrowOop* p) { VerifyCleanCardClosure::do_oop_work(p); }
372 };
374 class VerifyCTSpaceClosure: public SpaceClosure {
375 private:
376 CardTableRS* _ct;
377 HeapWord* _boundary;
378 public:
379 VerifyCTSpaceClosure(CardTableRS* ct, HeapWord* boundary) :
380 _ct(ct), _boundary(boundary) {}
381 virtual void do_space(Space* s) { _ct->verify_space(s, _boundary); }
382 };
384 class VerifyCTGenClosure: public GenCollectedHeap::GenClosure {
385 CardTableRS* _ct;
386 public:
387 VerifyCTGenClosure(CardTableRS* ct) : _ct(ct) {}
388 void do_generation(Generation* gen) {
389 // Skip the youngest generation.
390 if (gen->level() == 0) return;
391 // Normally, we're interested in pointers to younger generations.
392 VerifyCTSpaceClosure blk(_ct, gen->reserved().start());
393 gen->space_iterate(&blk, true);
394 }
395 };
397 void CardTableRS::verify_space(Space* s, HeapWord* gen_boundary) {
398 // We don't need to do young-gen spaces.
399 if (s->end() <= gen_boundary) return;
400 MemRegion used = s->used_region();
402 jbyte* cur_entry = byte_for(used.start());
403 jbyte* limit = byte_after(used.last());
404 while (cur_entry < limit) {
405 if (*cur_entry == CardTableModRefBS::clean_card) {
406 jbyte* first_dirty = cur_entry+1;
407 while (first_dirty < limit &&
408 *first_dirty == CardTableModRefBS::clean_card) {
409 first_dirty++;
410 }
411 // If the first object is a regular object, and it has a
412 // young-to-old field, that would mark the previous card.
413 HeapWord* boundary = addr_for(cur_entry);
414 HeapWord* end = (first_dirty >= limit) ? used.end() : addr_for(first_dirty);
415 HeapWord* boundary_block = s->block_start(boundary);
416 HeapWord* begin = boundary; // Until proven otherwise.
417 HeapWord* start_block = boundary_block; // Until proven otherwise.
418 if (boundary_block < boundary) {
419 if (s->block_is_obj(boundary_block) && s->obj_is_alive(boundary_block)) {
420 oop boundary_obj = oop(boundary_block);
421 if (!boundary_obj->is_objArray() &&
422 !boundary_obj->is_typeArray()) {
423 guarantee(cur_entry > byte_for(used.start()),
424 "else boundary would be boundary_block");
425 if (*byte_for(boundary_block) != CardTableModRefBS::clean_card) {
426 begin = boundary_block + s->block_size(boundary_block);
427 start_block = begin;
428 }
429 }
430 }
431 }
432 // Now traverse objects until end.
433 if (begin < end) {
434 MemRegion mr(begin, end);
435 VerifyCleanCardClosure verify_blk(gen_boundary, begin, end);
436 for (HeapWord* cur = start_block; cur < end; cur += s->block_size(cur)) {
437 if (s->block_is_obj(cur) && s->obj_is_alive(cur)) {
438 oop(cur)->oop_iterate(&verify_blk, mr);
439 }
440 }
441 }
442 cur_entry = first_dirty;
443 } else {
444 // We'd normally expect that cur_youngergen_and_prev_nonclean_card
445 // is a transient value, that cannot be in the card table
446 // except during GC, and thus assert that:
447 // guarantee(*cur_entry != cur_youngergen_and_prev_nonclean_card,
448 // "Illegal CT value");
449 // That however, need not hold, as will become clear in the
450 // following...
452 // We'd normally expect that if we are in the parallel case,
453 // we can't have left a prev value (which would be different
454 // from the current value) in the card table, and so we'd like to
455 // assert that:
456 // guarantee(cur_youngergen_card_val() == youngergen_card
457 // || !is_prev_youngergen_card_val(*cur_entry),
458 // "Illegal CT value");
459 // That, however, may not hold occasionally, because of
460 // CMS or MSC in the old gen. To wit, consider the
461 // following two simple illustrative scenarios:
462 // (a) CMS: Consider the case where a large object L
463 // spanning several cards is allocated in the old
464 // gen, and has a young gen reference stored in it, dirtying
465 // some interior cards. A young collection scans the card,
466 // finds a young ref and installs a youngergenP_n value.
467 // L then goes dead. Now a CMS collection starts,
468 // finds L dead and sweeps it up. Assume that L is
469 // abutting _unallocated_blk, so _unallocated_blk is
470 // adjusted down to (below) L. Assume further that
471 // no young collection intervenes during this CMS cycle.
472 // The next young gen cycle will not get to look at this
473 // youngergenP_n card since it lies in the unoccupied
474 // part of the space.
475 // Some young collections later the blocks on this
476 // card can be re-allocated either due to direct allocation
477 // or due to absorbing promotions. At this time, the
478 // before-gc verification will fail the above assert.
479 // (b) MSC: In this case, an object L with a young reference
480 // is on a card that (therefore) holds a youngergen_n value.
481 // Suppose also that L lies towards the end of the used
482 // the used space before GC. An MSC collection
483 // occurs that compacts to such an extent that this
484 // card is no longer in the occupied part of the space.
485 // Since current code in MSC does not always clear cards
486 // in the unused part of old gen, this stale youngergen_n
487 // value is left behind and can later be covered by
488 // an object when promotion or direct allocation
489 // re-allocates that part of the heap.
490 //
491 // Fortunately, the presence of such stale card values is
492 // "only" a minor annoyance in that subsequent young collections
493 // might needlessly scan such cards, but would still never corrupt
494 // the heap as a result. However, it's likely not to be a significant
495 // performance inhibitor in practice. For instance,
496 // some recent measurements with unoccupied cards eagerly cleared
497 // out to maintain this invariant, showed next to no
498 // change in young collection times; of course one can construct
499 // degenerate examples where the cost can be significant.)
500 // Note, in particular, that if the "stale" card is modified
501 // after re-allocation, it would be dirty, not "stale". Thus,
502 // we can never have a younger ref in such a card and it is
503 // safe not to scan that card in any collection. [As we see
504 // below, we do some unnecessary scanning
505 // in some cases in the current parallel scanning algorithm.]
506 //
507 // The main point below is that the parallel card scanning code
508 // deals correctly with these stale card values. There are two main
509 // cases to consider where we have a stale "younger gen" value and a
510 // "derivative" case to consider, where we have a stale
511 // "cur_younger_gen_and_prev_non_clean" value, as will become
512 // apparent in the case analysis below.
513 // o Case 1. If the stale value corresponds to a younger_gen_n
514 // value other than the cur_younger_gen value then the code
515 // treats this as being tantamount to a prev_younger_gen
516 // card. This means that the card may be unnecessarily scanned.
517 // There are two sub-cases to consider:
518 // o Case 1a. Let us say that the card is in the occupied part
519 // of the generation at the time the collection begins. In
520 // that case the card will be either cleared when it is scanned
521 // for young pointers, or will be set to cur_younger_gen as a
522 // result of promotion. (We have elided the normal case where
523 // the scanning thread and the promoting thread interleave
524 // possibly resulting in a transient
525 // cur_younger_gen_and_prev_non_clean value before settling
526 // to cur_younger_gen. [End Case 1a.]
527 // o Case 1b. Consider now the case when the card is in the unoccupied
528 // part of the space which becomes occupied because of promotions
529 // into it during the current young GC. In this case the card
530 // will never be scanned for young references. The current
531 // code will set the card value to either
532 // cur_younger_gen_and_prev_non_clean or leave
533 // it with its stale value -- because the promotions didn't
534 // result in any younger refs on that card. Of these two
535 // cases, the latter will be covered in Case 1a during
536 // a subsequent scan. To deal with the former case, we need
537 // to further consider how we deal with a stale value of
538 // cur_younger_gen_and_prev_non_clean in our case analysis
539 // below. This we do in Case 3 below. [End Case 1b]
540 // [End Case 1]
541 // o Case 2. If the stale value corresponds to cur_younger_gen being
542 // a value not necessarily written by a current promotion, the
543 // card will not be scanned by the younger refs scanning code.
544 // (This is OK since as we argued above such cards cannot contain
545 // any younger refs.) The result is that this value will be
546 // treated as a prev_younger_gen value in a subsequent collection,
547 // which is addressed in Case 1 above. [End Case 2]
548 // o Case 3. We here consider the "derivative" case from Case 1b. above
549 // because of which we may find a stale
550 // cur_younger_gen_and_prev_non_clean card value in the table.
551 // Once again, as in Case 1, we consider two subcases, depending
552 // on whether the card lies in the occupied or unoccupied part
553 // of the space at the start of the young collection.
554 // o Case 3a. Let us say the card is in the occupied part of
555 // the old gen at the start of the young collection. In that
556 // case, the card will be scanned by the younger refs scanning
557 // code which will set it to cur_younger_gen. In a subsequent
558 // scan, the card will be considered again and get its final
559 // correct value. [End Case 3a]
560 // o Case 3b. Now consider the case where the card is in the
561 // unoccupied part of the old gen, and is occupied as a result
562 // of promotions during thus young gc. In that case,
563 // the card will not be scanned for younger refs. The presence
564 // of newly promoted objects on the card will then result in
565 // its keeping the value cur_younger_gen_and_prev_non_clean
566 // value, which we have dealt with in Case 3 here. [End Case 3b]
567 // [End Case 3]
568 //
569 // (Please refer to the code in the helper class
570 // ClearNonCleanCardWrapper and in CardTableModRefBS for details.)
571 //
572 // The informal arguments above can be tightened into a formal
573 // correctness proof and it behooves us to write up such a proof,
574 // or to use model checking to prove that there are no lingering
575 // concerns.
576 //
577 // Clearly because of Case 3b one cannot bound the time for
578 // which a card will retain what we have called a "stale" value.
579 // However, one can obtain a Loose upper bound on the redundant
580 // work as a result of such stale values. Note first that any
581 // time a stale card lies in the occupied part of the space at
582 // the start of the collection, it is scanned by younger refs
583 // code and we can define a rank function on card values that
584 // declines when this is so. Note also that when a card does not
585 // lie in the occupied part of the space at the beginning of a
586 // young collection, its rank can either decline or stay unchanged.
587 // In this case, no extra work is done in terms of redundant
588 // younger refs scanning of that card.
589 // Then, the case analysis above reveals that, in the worst case,
590 // any such stale card will be scanned unnecessarily at most twice.
591 //
592 // It is nonethelss advisable to try and get rid of some of this
593 // redundant work in a subsequent (low priority) re-design of
594 // the card-scanning code, if only to simplify the underlying
595 // state machine analysis/proof. ysr 1/28/2002. XXX
596 cur_entry++;
597 }
598 }
599 }
601 void CardTableRS::verify() {
602 // At present, we only know how to verify the card table RS for
603 // generational heaps.
604 VerifyCTGenClosure blk(this);
605 CollectedHeap* ch = Universe::heap();
606 // We will do the perm-gen portion of the card table, too.
607 Generation* pg = SharedHeap::heap()->perm_gen();
608 HeapWord* pg_boundary = pg->reserved().start();
610 if (ch->kind() == CollectedHeap::GenCollectedHeap) {
611 GenCollectedHeap::heap()->generation_iterate(&blk, false);
612 _ct_bs->verify();
614 // If the old gen collections also collect perm, then we are only
615 // interested in perm-to-young pointers, not perm-to-old pointers.
616 GenCollectedHeap* gch = GenCollectedHeap::heap();
617 CollectorPolicy* cp = gch->collector_policy();
618 if (cp->is_mark_sweep_policy() || cp->is_concurrent_mark_sweep_policy()) {
619 pg_boundary = gch->get_gen(1)->reserved().start();
620 }
621 }
622 VerifyCTSpaceClosure perm_space_blk(this, pg_boundary);
623 SharedHeap::heap()->perm_gen()->space_iterate(&perm_space_blk, true);
624 }
627 void CardTableRS::verify_aligned_region_empty(MemRegion mr) {
628 if (!mr.is_empty()) {
629 jbyte* cur_entry = byte_for(mr.start());
630 jbyte* limit = byte_after(mr.last());
631 // The region mr may not start on a card boundary so
632 // the first card may reflect a write to the space
633 // just prior to mr.
634 if (!is_aligned(mr.start())) {
635 cur_entry++;
636 }
637 for (;cur_entry < limit; cur_entry++) {
638 guarantee(*cur_entry == CardTableModRefBS::clean_card,
639 "Unexpected dirty card found");
640 }
641 }
642 }