1 /* 2 * Copyright (c) 2001, 2015, 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. 22 * 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.inline.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 37 CardTableRS::CardTableRS(MemRegion whole_heap) : 38 GenRemSet(), 39 _cur_youngergen_card_val(youngergenP1_card) 40 { 41 _ct_bs = new CardTableModRefBSForCTRS(whole_heap); 42 _ct_bs->initialize(); 43 set_bs(_ct_bs); 44 // max_gens is really GenCollectedHeap::heap()->gen_policy()->number_of_generations() 45 // (which is always 2, young & old), but GenCollectedHeap has not been initialized yet. 46 uint max_gens = 2; 47 _last_cur_val_in_gen = NEW_C_HEAP_ARRAY3(jbyte, max_gens + 1, 48 mtGC, CURRENT_PC, AllocFailStrategy::RETURN_NULL); 49 if (_last_cur_val_in_gen == NULL) { 50 vm_exit_during_initialization("Could not create last_cur_val_in_gen array."); 51 } 52 for (uint i = 0; i < max_gens + 1; i++) { 53 _last_cur_val_in_gen[i] = clean_card_val(); 54 } 55 _ct_bs->set_CTRS(this); 56 } 57 58 CardTableRS::~CardTableRS() { 59 if (_ct_bs) { 60 delete _ct_bs; 61 _ct_bs = NULL; 62 } 63 if (_last_cur_val_in_gen) { 64 FREE_C_HEAP_ARRAY(jbyte, _last_cur_val_in_gen); 65 } 66 } 67 68 void CardTableRS::resize_covered_region(MemRegion new_region) { 69 _ct_bs->resize_covered_region(new_region); 70 } 71 72 jbyte CardTableRS::find_unused_youngergenP_card_value() { 73 for (jbyte v = youngergenP1_card; 74 v < cur_youngergen_and_prev_nonclean_card; 75 v++) { 76 bool seen = false; 77 for (int g = 0; g < _regions_to_iterate; g++) { 78 if (_last_cur_val_in_gen[g] == v) { 79 seen = true; 80 break; 81 } 82 } 83 if (!seen) return v; 84 } 85 ShouldNotReachHere(); 86 return 0; 87 } 88 89 void CardTableRS::prepare_for_younger_refs_iterate(bool parallel) { 90 // Parallel or sequential, we must always set the prev to equal the 91 // last one written. 92 if (parallel) { 93 // Find a parallel value to be used next. 94 jbyte next_val = find_unused_youngergenP_card_value(); 95 set_cur_youngergen_card_val(next_val); 96 97 } else { 98 // In an sequential traversal we will always write youngergen, so that 99 // the inline barrier is correct. 100 set_cur_youngergen_card_val(youngergen_card); 101 } 102 } 103 104 void CardTableRS::younger_refs_iterate(Generation* g, 105 OopsInGenClosure* blk, 106 uint n_threads) { 107 _last_cur_val_in_gen[g->level()+1] = cur_youngergen_card_val(); 108 g->younger_refs_iterate(blk, n_threads); 109 } 110 111 inline bool ClearNoncleanCardWrapper::clear_card(jbyte* entry) { 112 if (_is_par) { 113 return clear_card_parallel(entry); 114 } else { 115 return clear_card_serial(entry); 116 } 117 } 118 119 inline bool ClearNoncleanCardWrapper::clear_card_parallel(jbyte* entry) { 120 while (true) { 121 // In the parallel case, we may have to do this several times. 122 jbyte entry_val = *entry; 123 assert(entry_val != CardTableRS::clean_card_val(), 124 "We shouldn't be looking at clean cards, and this should " 125 "be the only place they get cleaned."); 126 if (CardTableRS::card_is_dirty_wrt_gen_iter(entry_val) 127 || _ct->is_prev_youngergen_card_val(entry_val)) { 128 jbyte res = 129 Atomic::cmpxchg(CardTableRS::clean_card_val(), entry, entry_val); 130 if (res == entry_val) { 131 break; 132 } else { 133 assert(res == CardTableRS::cur_youngergen_and_prev_nonclean_card, 134 "The CAS above should only fail if another thread did " 135 "a GC write barrier."); 136 } 137 } else if (entry_val == 138 CardTableRS::cur_youngergen_and_prev_nonclean_card) { 139 // Parallelism shouldn't matter in this case. Only the thread 140 // assigned to scan the card should change this value. 141 *entry = _ct->cur_youngergen_card_val(); 142 break; 143 } else { 144 assert(entry_val == _ct->cur_youngergen_card_val(), 145 "Should be the only possibility."); 146 // In this case, the card was clean before, and become 147 // cur_youngergen only because of processing of a promoted object. 148 // We don't have to look at the card. 149 return false; 150 } 151 } 152 return true; 153 } 154 155 156 inline bool ClearNoncleanCardWrapper::clear_card_serial(jbyte* entry) { 157 jbyte entry_val = *entry; 158 assert(entry_val != CardTableRS::clean_card_val(), 159 "We shouldn't be looking at clean cards, and this should " 160 "be the only place they get cleaned."); 161 assert(entry_val != CardTableRS::cur_youngergen_and_prev_nonclean_card, 162 "This should be possible in the sequential case."); 163 *entry = CardTableRS::clean_card_val(); 164 return true; 165 } 166 167 ClearNoncleanCardWrapper::ClearNoncleanCardWrapper( 168 DirtyCardToOopClosure* dirty_card_closure, CardTableRS* ct, bool is_par) : 169 _dirty_card_closure(dirty_card_closure), _ct(ct), _is_par(is_par) { 170 } 171 172 bool ClearNoncleanCardWrapper::is_word_aligned(jbyte* entry) { 173 return (((intptr_t)entry) & (BytesPerWord-1)) == 0; 174 } 175 176 // The regions are visited in *decreasing* address order. 177 // This order aids with imprecise card marking, where a dirty 178 // card may cause scanning, and summarization marking, of objects 179 // that extend onto subsequent cards. 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 } else { 195 // We hit a "clean" card; process any non-empty 196 // "dirty" range accumulated so far. 197 if (start_of_non_clean < end_of_non_clean) { 198 const MemRegion mrd(start_of_non_clean, end_of_non_clean); 199 _dirty_card_closure->do_MemRegion(mrd); 200 } 201 202 // fast forward through potential continuous whole-word range of clean cards beginning at a word-boundary 203 if (is_word_aligned(cur_entry)) { 204 jbyte* cur_row = cur_entry - BytesPerWord; 205 while (cur_row >= limit && *((intptr_t*)cur_row) == CardTableRS::clean_card_row()) { 206 cur_row -= BytesPerWord; 207 } 208 cur_entry = cur_row + BytesPerWord; 209 cur_hw = _ct->addr_for(cur_entry); 210 } 211 212 // Reset the dirty window, while continuing to look 213 // for the next dirty card that will start a 214 // new dirty window. 215 end_of_non_clean = cur_hw; 216 start_of_non_clean = cur_hw; 217 } 218 // Note that "cur_entry" leads "start_of_non_clean" in 219 // its leftward excursion after this point 220 // in the loop and, when we hit the left end of "mr", 221 // will point off of the left end of the card-table 222 // for "mr". 223 cur_entry--; 224 } 225 // If the first card of "mr" was dirty, we will have 226 // been left with a dirty window, co-initial with "mr", 227 // which we now process. 228 if (start_of_non_clean < end_of_non_clean) { 229 const MemRegion mrd(start_of_non_clean, end_of_non_clean); 230 _dirty_card_closure->do_MemRegion(mrd); 231 } 232 } 233 234 // clean (by dirty->clean before) ==> cur_younger_gen 235 // dirty ==> cur_youngergen_and_prev_nonclean_card 236 // precleaned ==> cur_youngergen_and_prev_nonclean_card 237 // prev-younger-gen ==> cur_youngergen_and_prev_nonclean_card 238 // cur-younger-gen ==> cur_younger_gen 239 // cur_youngergen_and_prev_nonclean_card ==> no change. 240 void CardTableRS::write_ref_field_gc_par(void* field, oop new_val) { 241 jbyte* entry = ct_bs()->byte_for(field); 242 do { 243 jbyte entry_val = *entry; 244 // We put this first because it's probably the most common case. 245 if (entry_val == clean_card_val()) { 246 // No threat of contention with cleaning threads. 247 *entry = cur_youngergen_card_val(); 248 return; 249 } else if (card_is_dirty_wrt_gen_iter(entry_val) 250 || is_prev_youngergen_card_val(entry_val)) { 251 // Mark it as both cur and prev youngergen; card cleaning thread will 252 // eventually remove the previous stuff. 253 jbyte new_val = cur_youngergen_and_prev_nonclean_card; 254 jbyte res = Atomic::cmpxchg(new_val, entry, entry_val); 255 // Did the CAS succeed? 256 if (res == entry_val) return; 257 // Otherwise, retry, to see the new value. 258 continue; 259 } else { 260 assert(entry_val == cur_youngergen_and_prev_nonclean_card 261 || entry_val == cur_youngergen_card_val(), 262 "should be only possibilities."); 263 return; 264 } 265 } while (true); 266 } 267 268 void CardTableRS::younger_refs_in_space_iterate(Space* sp, 269 OopsInGenClosure* cl, 270 uint n_threads) { 271 const MemRegion urasm = sp->used_region_at_save_marks(); 272 #ifdef ASSERT 273 // Convert the assertion check to a warning if we are running 274 // CMS+ParNew until related bug is fixed. 275 MemRegion ur = sp->used_region(); 276 assert(ur.contains(urasm) || (UseConcMarkSweepGC), 277 err_msg("Did you forget to call save_marks()? " 278 "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in " 279 "[" PTR_FORMAT ", " PTR_FORMAT ")", 280 p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end()))); 281 // In the case of CMS+ParNew, issue a warning 282 if (!ur.contains(urasm)) { 283 assert(UseConcMarkSweepGC, "Tautology: see assert above"); 284 warning("CMS+ParNew: Did you forget to call save_marks()? " 285 "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in " 286 "[" PTR_FORMAT ", " PTR_FORMAT ")", 287 p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end())); 288 MemRegion ur2 = sp->used_region(); 289 MemRegion urasm2 = sp->used_region_at_save_marks(); 290 if (!ur.equals(ur2)) { 291 warning("CMS+ParNew: Flickering used_region()!!"); 292 } 293 if (!urasm.equals(urasm2)) { 294 warning("CMS+ParNew: Flickering used_region_at_save_marks()!!"); 295 } 296 ShouldNotReachHere(); 297 } 298 #endif 299 _ct_bs->non_clean_card_iterate_possibly_parallel(sp, urasm, cl, this, n_threads); 300 } 301 302 void CardTableRS::clear_into_younger(Generation* old_gen) { 303 assert(old_gen->level() == 1, "Should only be called for the old generation"); 304 // The card tables for the youngest gen need never be cleared. 305 // There's a bit of subtlety in the clear() and invalidate() 306 // methods that we exploit here and in invalidate_or_clear() 307 // below to avoid missing cards at the fringes. If clear() or 308 // invalidate() are changed in the future, this code should 309 // be revisited. 20040107.ysr 310 clear(old_gen->prev_used_region()); 311 } 312 313 void CardTableRS::invalidate_or_clear(Generation* old_gen) { 314 assert(old_gen->level() == 1, "Should only be called for the old generation"); 315 // Invalidate the cards for the currently occupied part of 316 // the old generation and clear the cards for the 317 // unoccupied part of the generation (if any, making use 318 // of that generation's prev_used_region to determine that 319 // region). No need to do anything for the youngest 320 // generation. Also see note#20040107.ysr above. 321 MemRegion used_mr = old_gen->used_region(); 322 MemRegion to_be_cleared_mr = old_gen->prev_used_region().minus(used_mr); 323 if (!to_be_cleared_mr.is_empty()) { 324 clear(to_be_cleared_mr); 325 } 326 invalidate(used_mr); 327 } 328 329 330 class VerifyCleanCardClosure: public OopClosure { 331 private: 332 HeapWord* _boundary; 333 HeapWord* _begin; 334 HeapWord* _end; 335 protected: 336 template <class T> void do_oop_work(T* p) { 337 HeapWord* jp = (HeapWord*)p; 338 assert(jp >= _begin && jp < _end, 339 err_msg("Error: jp " PTR_FORMAT " should be within " 340 "[_begin, _end) = [" PTR_FORMAT "," PTR_FORMAT ")", 341 p2i(jp), p2i(_begin), p2i(_end))); 342 oop obj = oopDesc::load_decode_heap_oop(p); 343 guarantee(obj == NULL || (HeapWord*)obj >= _boundary, 344 err_msg("pointer " PTR_FORMAT " at " PTR_FORMAT " on " 345 "clean card crosses boundary" PTR_FORMAT, 346 p2i((HeapWord*)obj), p2i(jp), p2i(_boundary))); 347 } 348 349 public: 350 VerifyCleanCardClosure(HeapWord* b, HeapWord* begin, HeapWord* end) : 351 _boundary(b), _begin(begin), _end(end) { 352 assert(b <= begin, 353 err_msg("Error: boundary " PTR_FORMAT " should be at or below begin " PTR_FORMAT, 354 p2i(b), p2i(begin))); 355 assert(begin <= end, 356 err_msg("Error: begin " PTR_FORMAT " should be strictly below end " PTR_FORMAT, 357 p2i(begin), p2i(end))); 358 } 359 360 virtual void do_oop(oop* p) { VerifyCleanCardClosure::do_oop_work(p); } 361 virtual void do_oop(narrowOop* p) { VerifyCleanCardClosure::do_oop_work(p); } 362 }; 363 364 class VerifyCTSpaceClosure: public SpaceClosure { 365 private: 366 CardTableRS* _ct; 367 HeapWord* _boundary; 368 public: 369 VerifyCTSpaceClosure(CardTableRS* ct, HeapWord* boundary) : 370 _ct(ct), _boundary(boundary) {} 371 virtual void do_space(Space* s) { _ct->verify_space(s, _boundary); } 372 }; 373 374 class VerifyCTGenClosure: public GenCollectedHeap::GenClosure { 375 CardTableRS* _ct; 376 public: 377 VerifyCTGenClosure(CardTableRS* ct) : _ct(ct) {} 378 void do_generation(Generation* gen) { 379 // Skip the youngest generation. 380 if (gen->level() == 0) return; 381 // Normally, we're interested in pointers to younger generations. 382 VerifyCTSpaceClosure blk(_ct, gen->reserved().start()); 383 gen->space_iterate(&blk, true); 384 } 385 }; 386 387 void CardTableRS::verify_space(Space* s, HeapWord* gen_boundary) { 388 // We don't need to do young-gen spaces. 389 if (s->end() <= gen_boundary) return; 390 MemRegion used = s->used_region(); 391 392 jbyte* cur_entry = byte_for(used.start()); 393 jbyte* limit = byte_after(used.last()); 394 while (cur_entry < limit) { 395 if (*cur_entry == CardTableModRefBS::clean_card) { 396 jbyte* first_dirty = cur_entry+1; 397 while (first_dirty < limit && 398 *first_dirty == CardTableModRefBS::clean_card) { 399 first_dirty++; 400 } 401 // If the first object is a regular object, and it has a 402 // young-to-old field, that would mark the previous card. 403 HeapWord* boundary = addr_for(cur_entry); 404 HeapWord* end = (first_dirty >= limit) ? used.end() : addr_for(first_dirty); 405 HeapWord* boundary_block = s->block_start(boundary); 406 HeapWord* begin = boundary; // Until proven otherwise. 407 HeapWord* start_block = boundary_block; // Until proven otherwise. 408 if (boundary_block < boundary) { 409 if (s->block_is_obj(boundary_block) && s->obj_is_alive(boundary_block)) { 410 oop boundary_obj = oop(boundary_block); 411 if (!boundary_obj->is_objArray() && 412 !boundary_obj->is_typeArray()) { 413 guarantee(cur_entry > byte_for(used.start()), 414 "else boundary would be boundary_block"); 415 if (*byte_for(boundary_block) != CardTableModRefBS::clean_card) { 416 begin = boundary_block + s->block_size(boundary_block); 417 start_block = begin; 418 } 419 } 420 } 421 } 422 // Now traverse objects until end. 423 if (begin < end) { 424 MemRegion mr(begin, end); 425 VerifyCleanCardClosure verify_blk(gen_boundary, begin, end); 426 for (HeapWord* cur = start_block; cur < end; cur += s->block_size(cur)) { 427 if (s->block_is_obj(cur) && s->obj_is_alive(cur)) { 428 oop(cur)->oop_iterate_no_header(&verify_blk, mr); 429 } 430 } 431 } 432 cur_entry = first_dirty; 433 } else { 434 // We'd normally expect that cur_youngergen_and_prev_nonclean_card 435 // is a transient value, that cannot be in the card table 436 // except during GC, and thus assert that: 437 // guarantee(*cur_entry != cur_youngergen_and_prev_nonclean_card, 438 // "Illegal CT value"); 439 // That however, need not hold, as will become clear in the 440 // following... 441 442 // We'd normally expect that if we are in the parallel case, 443 // we can't have left a prev value (which would be different 444 // from the current value) in the card table, and so we'd like to 445 // assert that: 446 // guarantee(cur_youngergen_card_val() == youngergen_card 447 // || !is_prev_youngergen_card_val(*cur_entry), 448 // "Illegal CT value"); 449 // That, however, may not hold occasionally, because of 450 // CMS or MSC in the old gen. To wit, consider the 451 // following two simple illustrative scenarios: 452 // (a) CMS: Consider the case where a large object L 453 // spanning several cards is allocated in the old 454 // gen, and has a young gen reference stored in it, dirtying 455 // some interior cards. A young collection scans the card, 456 // finds a young ref and installs a youngergenP_n value. 457 // L then goes dead. Now a CMS collection starts, 458 // finds L dead and sweeps it up. Assume that L is 459 // abutting _unallocated_blk, so _unallocated_blk is 460 // adjusted down to (below) L. Assume further that 461 // no young collection intervenes during this CMS cycle. 462 // The next young gen cycle will not get to look at this 463 // youngergenP_n card since it lies in the unoccupied 464 // part of the space. 465 // Some young collections later the blocks on this 466 // card can be re-allocated either due to direct allocation 467 // or due to absorbing promotions. At this time, the 468 // before-gc verification will fail the above assert. 469 // (b) MSC: In this case, an object L with a young reference 470 // is on a card that (therefore) holds a youngergen_n value. 471 // Suppose also that L lies towards the end of the used 472 // the used space before GC. An MSC collection 473 // occurs that compacts to such an extent that this 474 // card is no longer in the occupied part of the space. 475 // Since current code in MSC does not always clear cards 476 // in the unused part of old gen, this stale youngergen_n 477 // value is left behind and can later be covered by 478 // an object when promotion or direct allocation 479 // re-allocates that part of the heap. 480 // 481 // Fortunately, the presence of such stale card values is 482 // "only" a minor annoyance in that subsequent young collections 483 // might needlessly scan such cards, but would still never corrupt 484 // the heap as a result. However, it's likely not to be a significant 485 // performance inhibitor in practice. For instance, 486 // some recent measurements with unoccupied cards eagerly cleared 487 // out to maintain this invariant, showed next to no 488 // change in young collection times; of course one can construct 489 // degenerate examples where the cost can be significant.) 490 // Note, in particular, that if the "stale" card is modified 491 // after re-allocation, it would be dirty, not "stale". Thus, 492 // we can never have a younger ref in such a card and it is 493 // safe not to scan that card in any collection. [As we see 494 // below, we do some unnecessary scanning 495 // in some cases in the current parallel scanning algorithm.] 496 // 497 // The main point below is that the parallel card scanning code 498 // deals correctly with these stale card values. There are two main 499 // cases to consider where we have a stale "younger gen" value and a 500 // "derivative" case to consider, where we have a stale 501 // "cur_younger_gen_and_prev_non_clean" value, as will become 502 // apparent in the case analysis below. 503 // o Case 1. If the stale value corresponds to a younger_gen_n 504 // value other than the cur_younger_gen value then the code 505 // treats this as being tantamount to a prev_younger_gen 506 // card. This means that the card may be unnecessarily scanned. 507 // There are two sub-cases to consider: 508 // o Case 1a. Let us say that the card is in the occupied part 509 // of the generation at the time the collection begins. In 510 // that case the card will be either cleared when it is scanned 511 // for young pointers, or will be set to cur_younger_gen as a 512 // result of promotion. (We have elided the normal case where 513 // the scanning thread and the promoting thread interleave 514 // possibly resulting in a transient 515 // cur_younger_gen_and_prev_non_clean value before settling 516 // to cur_younger_gen. [End Case 1a.] 517 // o Case 1b. Consider now the case when the card is in the unoccupied 518 // part of the space which becomes occupied because of promotions 519 // into it during the current young GC. In this case the card 520 // will never be scanned for young references. The current 521 // code will set the card value to either 522 // cur_younger_gen_and_prev_non_clean or leave 523 // it with its stale value -- because the promotions didn't 524 // result in any younger refs on that card. Of these two 525 // cases, the latter will be covered in Case 1a during 526 // a subsequent scan. To deal with the former case, we need 527 // to further consider how we deal with a stale value of 528 // cur_younger_gen_and_prev_non_clean in our case analysis 529 // below. This we do in Case 3 below. [End Case 1b] 530 // [End Case 1] 531 // o Case 2. If the stale value corresponds to cur_younger_gen being 532 // a value not necessarily written by a current promotion, the 533 // card will not be scanned by the younger refs scanning code. 534 // (This is OK since as we argued above such cards cannot contain 535 // any younger refs.) The result is that this value will be 536 // treated as a prev_younger_gen value in a subsequent collection, 537 // which is addressed in Case 1 above. [End Case 2] 538 // o Case 3. We here consider the "derivative" case from Case 1b. above 539 // because of which we may find a stale 540 // cur_younger_gen_and_prev_non_clean card value in the table. 541 // Once again, as in Case 1, we consider two subcases, depending 542 // on whether the card lies in the occupied or unoccupied part 543 // of the space at the start of the young collection. 544 // o Case 3a. Let us say the card is in the occupied part of 545 // the old gen at the start of the young collection. In that 546 // case, the card will be scanned by the younger refs scanning 547 // code which will set it to cur_younger_gen. In a subsequent 548 // scan, the card will be considered again and get its final 549 // correct value. [End Case 3a] 550 // o Case 3b. Now consider the case where the card is in the 551 // unoccupied part of the old gen, and is occupied as a result 552 // of promotions during thus young gc. In that case, 553 // the card will not be scanned for younger refs. The presence 554 // of newly promoted objects on the card will then result in 555 // its keeping the value cur_younger_gen_and_prev_non_clean 556 // value, which we have dealt with in Case 3 here. [End Case 3b] 557 // [End Case 3] 558 // 559 // (Please refer to the code in the helper class 560 // ClearNonCleanCardWrapper and in CardTableModRefBS for details.) 561 // 562 // The informal arguments above can be tightened into a formal 563 // correctness proof and it behooves us to write up such a proof, 564 // or to use model checking to prove that there are no lingering 565 // concerns. 566 // 567 // Clearly because of Case 3b one cannot bound the time for 568 // which a card will retain what we have called a "stale" value. 569 // However, one can obtain a Loose upper bound on the redundant 570 // work as a result of such stale values. Note first that any 571 // time a stale card lies in the occupied part of the space at 572 // the start of the collection, it is scanned by younger refs 573 // code and we can define a rank function on card values that 574 // declines when this is so. Note also that when a card does not 575 // lie in the occupied part of the space at the beginning of a 576 // young collection, its rank can either decline or stay unchanged. 577 // In this case, no extra work is done in terms of redundant 578 // younger refs scanning of that card. 579 // Then, the case analysis above reveals that, in the worst case, 580 // any such stale card will be scanned unnecessarily at most twice. 581 // 582 // It is nonetheless advisable to try and get rid of some of this 583 // redundant work in a subsequent (low priority) re-design of 584 // the card-scanning code, if only to simplify the underlying 585 // state machine analysis/proof. ysr 1/28/2002. XXX 586 cur_entry++; 587 } 588 } 589 } 590 591 void CardTableRS::verify() { 592 // At present, we only know how to verify the card table RS for 593 // generational heaps. 594 VerifyCTGenClosure blk(this); 595 GenCollectedHeap::heap()->generation_iterate(&blk, false); 596 _ct_bs->verify(); 597 }