1 /* 2 * Copyright (c) 2001, 2014, Oracle and/or its affiliates. All rights reserved. 3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 4 * 5 * This code is free software; you can redistribute it and/or modify it 6 * under the terms of the GNU General Public License version 2 only, as 7 * published by the Free Software Foundation. 8 * 9 * This code is distributed in the hope that it will be useful, but WITHOUT 10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 12 * version 2 for more details (a copy is included in the LICENSE file that 13 * accompanied this code). 14 * 15 * You should have received a copy of the GNU General Public License version 16 * 2 along with this work; if not, write to the Free Software Foundation, 17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 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.hpp" 31 #include "oops/oop.inline.hpp" 32 #include "runtime/atomic.inline.hpp" 33 #include "runtime/java.hpp" 34 #include "runtime/os.hpp" 35 #include "utilities/macros.hpp" 36 #if INCLUDE_ALL_GCS 37 #include "gc_implementation/g1/concurrentMark.hpp" 38 #include "gc_implementation/g1/g1SATBCardTableModRefBS.hpp" 39 #endif // INCLUDE_ALL_GCS 40 41 CardTableRS::CardTableRS(MemRegion whole_heap) : 42 GenRemSet(), 43 _cur_youngergen_card_val(youngergenP1_card) 44 { 45 #if INCLUDE_ALL_GCS 46 if (UseG1GC) { 47 _ct_bs = new G1SATBCardTableLoggingModRefBS(whole_heap); 48 } else { 49 _ct_bs = new CardTableModRefBSForCTRS(whole_heap); 50 } 51 #else 52 _ct_bs = new CardTableModRefBSForCTRS(whole_heap); 53 #endif 54 _ct_bs->initialize(); 55 set_bs(_ct_bs); 56 _last_cur_val_in_gen = NEW_C_HEAP_ARRAY3(jbyte, GenCollectedHeap::max_gens + 1, 57 mtGC, CURRENT_PC, AllocFailStrategy::RETURN_NULL); 58 if (_last_cur_val_in_gen == NULL) { 59 vm_exit_during_initialization("Could not create last_cur_val_in_gen array."); 60 } 61 for (int i = 0; i < GenCollectedHeap::max_gens + 1; i++) { 62 _last_cur_val_in_gen[i] = clean_card_val(); 63 } 64 _ct_bs->set_CTRS(this); 65 } 66 67 CardTableRS::~CardTableRS() { 68 if (_ct_bs) { 69 delete _ct_bs; 70 _ct_bs = NULL; 71 } 72 if (_last_cur_val_in_gen) { 73 FREE_C_HEAP_ARRAY(jbyte, _last_cur_val_in_gen); 74 } 75 } 76 77 void CardTableRS::resize_covered_region(MemRegion new_region) { 78 _ct_bs->resize_covered_region(new_region); 79 } 80 81 jbyte CardTableRS::find_unused_youngergenP_card_value() { 82 for (jbyte v = youngergenP1_card; 83 v < cur_youngergen_and_prev_nonclean_card; 84 v++) { 85 bool seen = false; 86 for (int g = 0; g < _regions_to_iterate; g++) { 87 if (_last_cur_val_in_gen[g] == v) { 88 seen = true; 89 break; 90 } 91 } 92 if (!seen) return v; 93 } 94 ShouldNotReachHere(); 95 return 0; 96 } 97 98 void CardTableRS::prepare_for_younger_refs_iterate(bool parallel) { 99 // Parallel or sequential, we must always set the prev to equal the 100 // last one written. 101 if (parallel) { 102 // Find a parallel value to be used next. 103 jbyte next_val = find_unused_youngergenP_card_value(); 104 set_cur_youngergen_card_val(next_val); 105 106 } else { 107 // In an sequential traversal we will always write youngergen, so that 108 // the inline barrier is correct. 109 set_cur_youngergen_card_val(youngergen_card); 110 } 111 } 112 113 void CardTableRS::younger_refs_iterate(Generation* g, 114 OopsInGenClosure* blk) { 115 _last_cur_val_in_gen[g->level()+1] = cur_youngergen_card_val(); 116 g->younger_refs_iterate(blk); 117 } 118 119 inline bool ClearNoncleanCardWrapper::clear_card(jbyte* entry) { 120 if (_is_par) { 121 return clear_card_parallel(entry); 122 } else { 123 return clear_card_serial(entry); 124 } 125 } 126 127 inline bool ClearNoncleanCardWrapper::clear_card_parallel(jbyte* entry) { 128 while (true) { 129 // In the parallel case, we may have to do this several times. 130 jbyte entry_val = *entry; 131 assert(entry_val != CardTableRS::clean_card_val(), 132 "We shouldn't be looking at clean cards, and this should " 133 "be the only place they get cleaned."); 134 if (CardTableRS::card_is_dirty_wrt_gen_iter(entry_val) 135 || _ct->is_prev_youngergen_card_val(entry_val)) { 136 jbyte res = 137 Atomic::cmpxchg(CardTableRS::clean_card_val(), entry, entry_val); 138 if (res == entry_val) { 139 break; 140 } else { 141 assert(res == CardTableRS::cur_youngergen_and_prev_nonclean_card, 142 "The CAS above should only fail if another thread did " 143 "a GC write barrier."); 144 } 145 } else if (entry_val == 146 CardTableRS::cur_youngergen_and_prev_nonclean_card) { 147 // Parallelism shouldn't matter in this case. Only the thread 148 // assigned to scan the card should change this value. 149 *entry = _ct->cur_youngergen_card_val(); 150 break; 151 } else { 152 assert(entry_val == _ct->cur_youngergen_card_val(), 153 "Should be the only possibility."); 154 // In this case, the card was clean before, and become 155 // cur_youngergen only because of processing of a promoted object. 156 // We don't have to look at the card. 157 return false; 158 } 159 } 160 return true; 161 } 162 163 164 inline bool ClearNoncleanCardWrapper::clear_card_serial(jbyte* entry) { 165 jbyte entry_val = *entry; 166 assert(entry_val != CardTableRS::clean_card_val(), 167 "We shouldn't be looking at clean cards, and this should " 168 "be the only place they get cleaned."); 169 assert(entry_val != CardTableRS::cur_youngergen_and_prev_nonclean_card, 170 "This should be possible in the sequential case."); 171 *entry = CardTableRS::clean_card_val(); 172 return true; 173 } 174 175 ClearNoncleanCardWrapper::ClearNoncleanCardWrapper( 176 DirtyCardToOopClosure* dirty_card_closure, CardTableRS* ct) : 177 _dirty_card_closure(dirty_card_closure), _ct(ct) { 178 // Cannot yet substitute active_workers for n_par_threads 179 // in the case where parallelism is being turned off by 180 // setting n_par_threads to 0. 181 _is_par = (SharedHeap::heap()->n_par_threads() > 0); 182 assert(!_is_par || 183 (SharedHeap::heap()->n_par_threads() == 184 SharedHeap::heap()->workers()->active_workers()), "Mismatch"); 185 } 186 187 bool ClearNoncleanCardWrapper::is_word_aligned(jbyte* entry) { 188 return (((intptr_t)entry) & (BytesPerWord-1)) == 0; 189 } 190 191 void ClearNoncleanCardWrapper::do_MemRegion(MemRegion mr) { 192 assert(mr.word_size() > 0, "Error"); 193 assert(_ct->is_aligned(mr.start()), "mr.start() should be card aligned"); 194 // mr.end() may not necessarily be card aligned. 195 jbyte* cur_entry = _ct->byte_for(mr.last()); 196 const jbyte* limit = _ct->byte_for(mr.start()); 197 HeapWord* end_of_non_clean = mr.end(); 198 HeapWord* start_of_non_clean = end_of_non_clean; 199 while (cur_entry >= limit) { 200 HeapWord* cur_hw = _ct->addr_for(cur_entry); 201 if ((*cur_entry != CardTableRS::clean_card_val()) && clear_card(cur_entry)) { 202 // Continue the dirty range by opening the 203 // dirty window one card to the left. 204 start_of_non_clean = cur_hw; 205 } else { 206 // We hit a "clean" card; process any non-empty 207 // "dirty" range accumulated so far. 208 if (start_of_non_clean < end_of_non_clean) { 209 const MemRegion mrd(start_of_non_clean, end_of_non_clean); 210 _dirty_card_closure->do_MemRegion(mrd); 211 } 212 213 // fast forward through potential continuous whole-word range of clean cards beginning at a word-boundary 214 if (is_word_aligned(cur_entry)) { 215 jbyte* cur_row = cur_entry - BytesPerWord; 216 while (cur_row >= limit && *((intptr_t*)cur_row) == CardTableRS::clean_card_row()) { 217 cur_row -= BytesPerWord; 218 } 219 cur_entry = cur_row + BytesPerWord; 220 cur_hw = _ct->addr_for(cur_entry); 221 } 222 223 // Reset the dirty window, while continuing to look 224 // for the next dirty card that will start a 225 // new dirty window. 226 end_of_non_clean = cur_hw; 227 start_of_non_clean = cur_hw; 228 } 229 // Note that "cur_entry" leads "start_of_non_clean" in 230 // its leftward excursion after this point 231 // in the loop and, when we hit the left end of "mr", 232 // will point off of the left end of the card-table 233 // for "mr". 234 cur_entry--; 235 } 236 // If the first card of "mr" was dirty, we will have 237 // been left with a dirty window, co-initial with "mr", 238 // which we now process. 239 if (start_of_non_clean < end_of_non_clean) { 240 const MemRegion mrd(start_of_non_clean, end_of_non_clean); 241 _dirty_card_closure->do_MemRegion(mrd); 242 } 243 } 244 245 // clean (by dirty->clean before) ==> cur_younger_gen 246 // dirty ==> cur_youngergen_and_prev_nonclean_card 247 // precleaned ==> cur_youngergen_and_prev_nonclean_card 248 // prev-younger-gen ==> cur_youngergen_and_prev_nonclean_card 249 // cur-younger-gen ==> cur_younger_gen 250 // cur_youngergen_and_prev_nonclean_card ==> no change. 251 void CardTableRS::write_ref_field_gc_par(void* field, oop new_val) { 252 jbyte* entry = ct_bs()->byte_for(field); 253 do { 254 jbyte entry_val = *entry; 255 // We put this first because it's probably the most common case. 256 if (entry_val == clean_card_val()) { 257 // No threat of contention with cleaning threads. 258 *entry = cur_youngergen_card_val(); 259 return; 260 } else if (card_is_dirty_wrt_gen_iter(entry_val) 261 || is_prev_youngergen_card_val(entry_val)) { 262 // Mark it as both cur and prev youngergen; card cleaning thread will 263 // eventually remove the previous stuff. 264 jbyte new_val = cur_youngergen_and_prev_nonclean_card; 265 jbyte res = Atomic::cmpxchg(new_val, entry, entry_val); 266 // Did the CAS succeed? 267 if (res == entry_val) return; 268 // Otherwise, retry, to see the new value. 269 continue; 270 } else { 271 assert(entry_val == cur_youngergen_and_prev_nonclean_card 272 || entry_val == cur_youngergen_card_val(), 273 "should be only possibilities."); 274 return; 275 } 276 } while (true); 277 } 278 279 void CardTableRS::younger_refs_in_space_iterate(Space* sp, 280 OopsInGenClosure* cl) { 281 const MemRegion urasm = sp->used_region_at_save_marks(); 282 #ifdef ASSERT 283 // Convert the assertion check to a warning if we are running 284 // CMS+ParNew until related bug is fixed. 285 MemRegion ur = sp->used_region(); 286 assert(ur.contains(urasm) || (UseConcMarkSweepGC), 287 err_msg("Did you forget to call save_marks()? " 288 "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in " 289 "[" PTR_FORMAT ", " PTR_FORMAT ")", 290 p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end()))); 291 // In the case of CMS+ParNew, issue a warning 292 if (!ur.contains(urasm)) { 293 assert(UseConcMarkSweepGC, "Tautology: see assert above"); 294 warning("CMS+ParNew: Did you forget to call save_marks()? " 295 "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in " 296 "[" PTR_FORMAT ", " PTR_FORMAT ")", 297 p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end())); 298 MemRegion ur2 = sp->used_region(); 299 MemRegion urasm2 = sp->used_region_at_save_marks(); 300 if (!ur.equals(ur2)) { 301 warning("CMS+ParNew: Flickering used_region()!!"); 302 } 303 if (!urasm.equals(urasm2)) { 304 warning("CMS+ParNew: Flickering used_region_at_save_marks()!!"); 305 } 306 ShouldNotReachHere(); 307 } 308 #endif 309 _ct_bs->non_clean_card_iterate_possibly_parallel(sp, urasm, cl, this); 310 } 311 312 void CardTableRS::clear_into_younger(Generation* old_gen) { 313 assert(old_gen->level() == 1, "Should only be called for the old generation"); 314 // The card tables for the youngest gen need never be cleared. 315 // There's a bit of subtlety in the clear() and invalidate() 316 // methods that we exploit here and in invalidate_or_clear() 317 // below to avoid missing cards at the fringes. If clear() or 318 // invalidate() are changed in the future, this code should 319 // be revisited. 20040107.ysr 320 clear(old_gen->prev_used_region()); 321 } 322 323 void CardTableRS::invalidate_or_clear(Generation* old_gen) { 324 assert(old_gen->level() == 1, "Should only be called for the old generation"); 325 // Invalidate the cards for the currently occupied part of 326 // the old generation and clear the cards for the 327 // unoccupied part of the generation (if any, making use 328 // of that generation's prev_used_region to determine that 329 // region). No need to do anything for the youngest 330 // generation. Also see note#20040107.ysr above. 331 MemRegion used_mr = old_gen->used_region(); 332 MemRegion to_be_cleared_mr = old_gen->prev_used_region().minus(used_mr); 333 if (!to_be_cleared_mr.is_empty()) { 334 clear(to_be_cleared_mr); 335 } 336 invalidate(used_mr); 337 } 338 339 340 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 p2i(jp), p2i(_begin), p2i(_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 p2i((HeapWord*)obj), p2i(jp), p2i(_boundary))); 357 } 358 359 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 p2i(b), p2i(begin))); 365 assert(begin <= end, 366 err_msg("Error: begin " PTR_FORMAT " should be strictly below end " PTR_FORMAT, 367 p2i(begin), p2i(end))); 368 } 369 370 virtual void do_oop(oop* p) { VerifyCleanCardClosure::do_oop_work(p); } 371 virtual void do_oop(narrowOop* p) { VerifyCleanCardClosure::do_oop_work(p); } 372 }; 373 374 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 }; 383 384 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 }; 396 397 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(); 401 402 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_no_header(&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... 451 452 // 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 nonetheless 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 } 600 601 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(); 606 607 if (ch->kind() == CollectedHeap::GenCollectedHeap) { 608 GenCollectedHeap::heap()->generation_iterate(&blk, false); 609 _ct_bs->verify(); 610 } 611 }