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 _last_cur_val_in_gen[g->level()+1] = cur_youngergen_card_val(); 107 g->younger_refs_iterate(blk); 108 } 109 110 inline bool ClearNoncleanCardWrapper::clear_card(jbyte* entry) { 111 if (_is_par) { 112 return clear_card_parallel(entry); 113 } else { 114 return clear_card_serial(entry); 115 } 116 } 117 118 inline bool ClearNoncleanCardWrapper::clear_card_parallel(jbyte* entry) { 119 while (true) { 120 // In the parallel case, we may have to do this several times. 121 jbyte entry_val = *entry; 122 assert(entry_val != CardTableRS::clean_card_val(), 123 "We shouldn't be looking at clean cards, and this should " 124 "be the only place they get cleaned."); 125 if (CardTableRS::card_is_dirty_wrt_gen_iter(entry_val) 126 || _ct->is_prev_youngergen_card_val(entry_val)) { 127 jbyte res = 128 Atomic::cmpxchg(CardTableRS::clean_card_val(), entry, entry_val); 129 if (res == entry_val) { 130 break; 131 } else { 132 assert(res == CardTableRS::cur_youngergen_and_prev_nonclean_card, 133 "The CAS above should only fail if another thread did " 134 "a GC write barrier."); 135 } 136 } else if (entry_val == 137 CardTableRS::cur_youngergen_and_prev_nonclean_card) { 138 // Parallelism shouldn't matter in this case. Only the thread 139 // assigned to scan the card should change this value. 140 *entry = _ct->cur_youngergen_card_val(); 141 break; 142 } else { 143 assert(entry_val == _ct->cur_youngergen_card_val(), 144 "Should be the only possibility."); 145 // In this case, the card was clean before, and become 146 // cur_youngergen only because of processing of a promoted object. 147 // We don't have to look at the card. 148 return false; 149 } 150 } 151 return true; 152 } 153 154 155 inline bool ClearNoncleanCardWrapper::clear_card_serial(jbyte* entry) { 156 jbyte entry_val = *entry; 157 assert(entry_val != CardTableRS::clean_card_val(), 158 "We shouldn't be looking at clean cards, and this should " 159 "be the only place they get cleaned."); 160 assert(entry_val != CardTableRS::cur_youngergen_and_prev_nonclean_card, 161 "This should be possible in the sequential case."); 162 *entry = CardTableRS::clean_card_val(); 163 return true; 164 } 165 166 ClearNoncleanCardWrapper::ClearNoncleanCardWrapper( 167 DirtyCardToOopClosure* dirty_card_closure, CardTableRS* ct) : 168 _dirty_card_closure(dirty_card_closure), _ct(ct) { 169 // Cannot yet substitute active_workers for n_par_threads 170 // in the case where parallelism is being turned off by 171 // setting n_par_threads to 0. 172 _is_par = (GenCollectedHeap::heap()->n_par_threads() > 0); 173 assert(!_is_par || 174 (GenCollectedHeap::heap()->n_par_threads() == 175 GenCollectedHeap::heap()->workers()->active_workers()), "Mismatch"); 176 } 177 178 bool ClearNoncleanCardWrapper::is_word_aligned(jbyte* entry) { 179 return (((intptr_t)entry) & (BytesPerWord-1)) == 0; 180 } 181 182 // The regions are visited in *decreasing* address order. 183 // This order aids with imprecise card marking, where a dirty 184 // card may cause scanning, and summarization marking, of objects 185 // that extend onto subsequent cards. 186 void ClearNoncleanCardWrapper::do_MemRegion(MemRegion mr) { 187 assert(mr.word_size() > 0, "Error"); 188 assert(_ct->is_aligned(mr.start()), "mr.start() should be card aligned"); 189 // mr.end() may not necessarily be card aligned. 190 jbyte* cur_entry = _ct->byte_for(mr.last()); 191 const jbyte* limit = _ct->byte_for(mr.start()); 192 HeapWord* end_of_non_clean = mr.end(); 193 HeapWord* start_of_non_clean = end_of_non_clean; 194 while (cur_entry >= limit) { 195 HeapWord* cur_hw = _ct->addr_for(cur_entry); 196 if ((*cur_entry != CardTableRS::clean_card_val()) && clear_card(cur_entry)) { 197 // Continue the dirty range by opening the 198 // dirty window one card to the left. 199 start_of_non_clean = cur_hw; 200 } else { 201 // We hit a "clean" card; process any non-empty 202 // "dirty" range accumulated so far. 203 if (start_of_non_clean < end_of_non_clean) { 204 const MemRegion mrd(start_of_non_clean, end_of_non_clean); 205 _dirty_card_closure->do_MemRegion(mrd); 206 } 207 208 // fast forward through potential continuous whole-word range of clean cards beginning at a word-boundary 209 if (is_word_aligned(cur_entry)) { 210 jbyte* cur_row = cur_entry - BytesPerWord; 211 while (cur_row >= limit && *((intptr_t*)cur_row) == CardTableRS::clean_card_row()) { 212 cur_row -= BytesPerWord; 213 } 214 cur_entry = cur_row + BytesPerWord; 215 cur_hw = _ct->addr_for(cur_entry); 216 } 217 218 // Reset the dirty window, while continuing to look 219 // for the next dirty card that will start a 220 // new dirty window. 221 end_of_non_clean = cur_hw; 222 start_of_non_clean = cur_hw; 223 } 224 // Note that "cur_entry" leads "start_of_non_clean" in 225 // its leftward excursion after this point 226 // in the loop and, when we hit the left end of "mr", 227 // will point off of the left end of the card-table 228 // for "mr". 229 cur_entry--; 230 } 231 // If the first card of "mr" was dirty, we will have 232 // been left with a dirty window, co-initial with "mr", 233 // which we now process. 234 if (start_of_non_clean < end_of_non_clean) { 235 const MemRegion mrd(start_of_non_clean, end_of_non_clean); 236 _dirty_card_closure->do_MemRegion(mrd); 237 } 238 } 239 240 // clean (by dirty->clean before) ==> cur_younger_gen 241 // dirty ==> cur_youngergen_and_prev_nonclean_card 242 // precleaned ==> cur_youngergen_and_prev_nonclean_card 243 // prev-younger-gen ==> cur_youngergen_and_prev_nonclean_card 244 // cur-younger-gen ==> cur_younger_gen 245 // cur_youngergen_and_prev_nonclean_card ==> no change. 246 void CardTableRS::write_ref_field_gc_par(void* field, oop new_val) { 247 jbyte* entry = ct_bs()->byte_for(field); 248 do { 249 jbyte entry_val = *entry; 250 // We put this first because it's probably the most common case. 251 if (entry_val == clean_card_val()) { 252 // No threat of contention with cleaning threads. 253 *entry = cur_youngergen_card_val(); 254 return; 255 } else if (card_is_dirty_wrt_gen_iter(entry_val) 256 || is_prev_youngergen_card_val(entry_val)) { 257 // Mark it as both cur and prev youngergen; card cleaning thread will 258 // eventually remove the previous stuff. 259 jbyte new_val = cur_youngergen_and_prev_nonclean_card; 260 jbyte res = Atomic::cmpxchg(new_val, entry, entry_val); 261 // Did the CAS succeed? 262 if (res == entry_val) return; 263 // Otherwise, retry, to see the new value. 264 continue; 265 } else { 266 assert(entry_val == cur_youngergen_and_prev_nonclean_card 267 || entry_val == cur_youngergen_card_val(), 268 "should be only possibilities."); 269 return; 270 } 271 } while (true); 272 } 273 274 void CardTableRS::younger_refs_in_space_iterate(Space* sp, 275 OopsInGenClosure* cl) { 276 const MemRegion urasm = sp->used_region_at_save_marks(); 277 #ifdef ASSERT 278 // Convert the assertion check to a warning if we are running 279 // CMS+ParNew until related bug is fixed. 280 MemRegion ur = sp->used_region(); 281 assert(ur.contains(urasm) || (UseConcMarkSweepGC), 282 err_msg("Did you forget to call save_marks()? " 283 "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in " 284 "[" PTR_FORMAT ", " PTR_FORMAT ")", 285 p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end()))); 286 // In the case of CMS+ParNew, issue a warning 287 if (!ur.contains(urasm)) { 288 assert(UseConcMarkSweepGC, "Tautology: see assert above"); 289 warning("CMS+ParNew: Did you forget to call save_marks()? " 290 "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in " 291 "[" PTR_FORMAT ", " PTR_FORMAT ")", 292 p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end())); 293 MemRegion ur2 = sp->used_region(); 294 MemRegion urasm2 = sp->used_region_at_save_marks(); 295 if (!ur.equals(ur2)) { 296 warning("CMS+ParNew: Flickering used_region()!!"); 297 } 298 if (!urasm.equals(urasm2)) { 299 warning("CMS+ParNew: Flickering used_region_at_save_marks()!!"); 300 } 301 ShouldNotReachHere(); 302 } 303 #endif 304 _ct_bs->non_clean_card_iterate_possibly_parallel(sp, urasm, cl, this); 305 } 306 307 void CardTableRS::clear_into_younger(Generation* old_gen) { 308 assert(old_gen->level() == 1, "Should only be called for the old generation"); 309 // The card tables for the youngest gen need never be cleared. 310 // There's a bit of subtlety in the clear() and invalidate() 311 // methods that we exploit here and in invalidate_or_clear() 312 // below to avoid missing cards at the fringes. If clear() or 313 // invalidate() are changed in the future, this code should 314 // be revisited. 20040107.ysr 315 clear(old_gen->prev_used_region()); 316 } 317 318 void CardTableRS::invalidate_or_clear(Generation* old_gen) { 319 assert(old_gen->level() == 1, "Should only be called for the old generation"); 320 // Invalidate the cards for the currently occupied part of 321 // the old generation and clear the cards for the 322 // unoccupied part of the generation (if any, making use 323 // of that generation's prev_used_region to determine that 324 // region). No need to do anything for the youngest 325 // generation. Also see note#20040107.ysr above. 326 MemRegion used_mr = old_gen->used_region(); 327 MemRegion to_be_cleared_mr = old_gen->prev_used_region().minus(used_mr); 328 if (!to_be_cleared_mr.is_empty()) { 329 clear(to_be_cleared_mr); 330 } 331 invalidate(used_mr); 332 } 333 334 335 class VerifyCleanCardClosure: public OopClosure { 336 private: 337 HeapWord* _boundary; 338 HeapWord* _begin; 339 HeapWord* _end; 340 protected: 341 template <class T> void do_oop_work(T* p) { 342 HeapWord* jp = (HeapWord*)p; 343 assert(jp >= _begin && jp < _end, 344 err_msg("Error: jp " PTR_FORMAT " should be within " 345 "[_begin, _end) = [" PTR_FORMAT "," PTR_FORMAT ")", 346 p2i(jp), p2i(_begin), p2i(_end))); 347 oop obj = oopDesc::load_decode_heap_oop(p); 348 guarantee(obj == NULL || (HeapWord*)obj >= _boundary, 349 err_msg("pointer " PTR_FORMAT " at " PTR_FORMAT " on " 350 "clean card crosses boundary" PTR_FORMAT, 351 p2i((HeapWord*)obj), p2i(jp), p2i(_boundary))); 352 } 353 354 public: 355 VerifyCleanCardClosure(HeapWord* b, HeapWord* begin, HeapWord* end) : 356 _boundary(b), _begin(begin), _end(end) { 357 assert(b <= begin, 358 err_msg("Error: boundary " PTR_FORMAT " should be at or below begin " PTR_FORMAT, 359 p2i(b), p2i(begin))); 360 assert(begin <= end, 361 err_msg("Error: begin " PTR_FORMAT " should be strictly below end " PTR_FORMAT, 362 p2i(begin), p2i(end))); 363 } 364 365 virtual void do_oop(oop* p) { VerifyCleanCardClosure::do_oop_work(p); } 366 virtual void do_oop(narrowOop* p) { VerifyCleanCardClosure::do_oop_work(p); } 367 }; 368 369 class VerifyCTSpaceClosure: public SpaceClosure { 370 private: 371 CardTableRS* _ct; 372 HeapWord* _boundary; 373 public: 374 VerifyCTSpaceClosure(CardTableRS* ct, HeapWord* boundary) : 375 _ct(ct), _boundary(boundary) {} 376 virtual void do_space(Space* s) { _ct->verify_space(s, _boundary); } 377 }; 378 379 class VerifyCTGenClosure: public GenCollectedHeap::GenClosure { 380 CardTableRS* _ct; 381 public: 382 VerifyCTGenClosure(CardTableRS* ct) : _ct(ct) {} 383 void do_generation(Generation* gen) { 384 // Skip the youngest generation. 385 if (gen->level() == 0) return; 386 // Normally, we're interested in pointers to younger generations. 387 VerifyCTSpaceClosure blk(_ct, gen->reserved().start()); 388 gen->space_iterate(&blk, true); 389 } 390 }; 391 392 void CardTableRS::verify_space(Space* s, HeapWord* gen_boundary) { 393 // We don't need to do young-gen spaces. 394 if (s->end() <= gen_boundary) return; 395 MemRegion used = s->used_region(); 396 397 jbyte* cur_entry = byte_for(used.start()); 398 jbyte* limit = byte_after(used.last()); 399 while (cur_entry < limit) { 400 if (*cur_entry == CardTableModRefBS::clean_card) { 401 jbyte* first_dirty = cur_entry+1; 402 while (first_dirty < limit && 403 *first_dirty == CardTableModRefBS::clean_card) { 404 first_dirty++; 405 } 406 // If the first object is a regular object, and it has a 407 // young-to-old field, that would mark the previous card. 408 HeapWord* boundary = addr_for(cur_entry); 409 HeapWord* end = (first_dirty >= limit) ? used.end() : addr_for(first_dirty); 410 HeapWord* boundary_block = s->block_start(boundary); 411 HeapWord* begin = boundary; // Until proven otherwise. 412 HeapWord* start_block = boundary_block; // Until proven otherwise. 413 if (boundary_block < boundary) { 414 if (s->block_is_obj(boundary_block) && s->obj_is_alive(boundary_block)) { 415 oop boundary_obj = oop(boundary_block); 416 if (!boundary_obj->is_objArray() && 417 !boundary_obj->is_typeArray()) { 418 guarantee(cur_entry > byte_for(used.start()), 419 "else boundary would be boundary_block"); 420 if (*byte_for(boundary_block) != CardTableModRefBS::clean_card) { 421 begin = boundary_block + s->block_size(boundary_block); 422 start_block = begin; 423 } 424 } 425 } 426 } 427 // Now traverse objects until end. 428 if (begin < end) { 429 MemRegion mr(begin, end); 430 VerifyCleanCardClosure verify_blk(gen_boundary, begin, end); 431 for (HeapWord* cur = start_block; cur < end; cur += s->block_size(cur)) { 432 if (s->block_is_obj(cur) && s->obj_is_alive(cur)) { 433 oop(cur)->oop_iterate_no_header(&verify_blk, mr); 434 } 435 } 436 } 437 cur_entry = first_dirty; 438 } else { 439 // We'd normally expect that cur_youngergen_and_prev_nonclean_card 440 // is a transient value, that cannot be in the card table 441 // except during GC, and thus assert that: 442 // guarantee(*cur_entry != cur_youngergen_and_prev_nonclean_card, 443 // "Illegal CT value"); 444 // That however, need not hold, as will become clear in the 445 // following... 446 447 // We'd normally expect that if we are in the parallel case, 448 // we can't have left a prev value (which would be different 449 // from the current value) in the card table, and so we'd like to 450 // assert that: 451 // guarantee(cur_youngergen_card_val() == youngergen_card 452 // || !is_prev_youngergen_card_val(*cur_entry), 453 // "Illegal CT value"); 454 // That, however, may not hold occasionally, because of 455 // CMS or MSC in the old gen. To wit, consider the 456 // following two simple illustrative scenarios: 457 // (a) CMS: Consider the case where a large object L 458 // spanning several cards is allocated in the old 459 // gen, and has a young gen reference stored in it, dirtying 460 // some interior cards. A young collection scans the card, 461 // finds a young ref and installs a youngergenP_n value. 462 // L then goes dead. Now a CMS collection starts, 463 // finds L dead and sweeps it up. Assume that L is 464 // abutting _unallocated_blk, so _unallocated_blk is 465 // adjusted down to (below) L. Assume further that 466 // no young collection intervenes during this CMS cycle. 467 // The next young gen cycle will not get to look at this 468 // youngergenP_n card since it lies in the unoccupied 469 // part of the space. 470 // Some young collections later the blocks on this 471 // card can be re-allocated either due to direct allocation 472 // or due to absorbing promotions. At this time, the 473 // before-gc verification will fail the above assert. 474 // (b) MSC: In this case, an object L with a young reference 475 // is on a card that (therefore) holds a youngergen_n value. 476 // Suppose also that L lies towards the end of the used 477 // the used space before GC. An MSC collection 478 // occurs that compacts to such an extent that this 479 // card is no longer in the occupied part of the space. 480 // Since current code in MSC does not always clear cards 481 // in the unused part of old gen, this stale youngergen_n 482 // value is left behind and can later be covered by 483 // an object when promotion or direct allocation 484 // re-allocates that part of the heap. 485 // 486 // Fortunately, the presence of such stale card values is 487 // "only" a minor annoyance in that subsequent young collections 488 // might needlessly scan such cards, but would still never corrupt 489 // the heap as a result. However, it's likely not to be a significant 490 // performance inhibitor in practice. For instance, 491 // some recent measurements with unoccupied cards eagerly cleared 492 // out to maintain this invariant, showed next to no 493 // change in young collection times; of course one can construct 494 // degenerate examples where the cost can be significant.) 495 // Note, in particular, that if the "stale" card is modified 496 // after re-allocation, it would be dirty, not "stale". Thus, 497 // we can never have a younger ref in such a card and it is 498 // safe not to scan that card in any collection. [As we see 499 // below, we do some unnecessary scanning 500 // in some cases in the current parallel scanning algorithm.] 501 // 502 // The main point below is that the parallel card scanning code 503 // deals correctly with these stale card values. There are two main 504 // cases to consider where we have a stale "younger gen" value and a 505 // "derivative" case to consider, where we have a stale 506 // "cur_younger_gen_and_prev_non_clean" value, as will become 507 // apparent in the case analysis below. 508 // o Case 1. If the stale value corresponds to a younger_gen_n 509 // value other than the cur_younger_gen value then the code 510 // treats this as being tantamount to a prev_younger_gen 511 // card. This means that the card may be unnecessarily scanned. 512 // There are two sub-cases to consider: 513 // o Case 1a. Let us say that the card is in the occupied part 514 // of the generation at the time the collection begins. In 515 // that case the card will be either cleared when it is scanned 516 // for young pointers, or will be set to cur_younger_gen as a 517 // result of promotion. (We have elided the normal case where 518 // the scanning thread and the promoting thread interleave 519 // possibly resulting in a transient 520 // cur_younger_gen_and_prev_non_clean value before settling 521 // to cur_younger_gen. [End Case 1a.] 522 // o Case 1b. Consider now the case when the card is in the unoccupied 523 // part of the space which becomes occupied because of promotions 524 // into it during the current young GC. In this case the card 525 // will never be scanned for young references. The current 526 // code will set the card value to either 527 // cur_younger_gen_and_prev_non_clean or leave 528 // it with its stale value -- because the promotions didn't 529 // result in any younger refs on that card. Of these two 530 // cases, the latter will be covered in Case 1a during 531 // a subsequent scan. To deal with the former case, we need 532 // to further consider how we deal with a stale value of 533 // cur_younger_gen_and_prev_non_clean in our case analysis 534 // below. This we do in Case 3 below. [End Case 1b] 535 // [End Case 1] 536 // o Case 2. If the stale value corresponds to cur_younger_gen being 537 // a value not necessarily written by a current promotion, the 538 // card will not be scanned by the younger refs scanning code. 539 // (This is OK since as we argued above such cards cannot contain 540 // any younger refs.) The result is that this value will be 541 // treated as a prev_younger_gen value in a subsequent collection, 542 // which is addressed in Case 1 above. [End Case 2] 543 // o Case 3. We here consider the "derivative" case from Case 1b. above 544 // because of which we may find a stale 545 // cur_younger_gen_and_prev_non_clean card value in the table. 546 // Once again, as in Case 1, we consider two subcases, depending 547 // on whether the card lies in the occupied or unoccupied part 548 // of the space at the start of the young collection. 549 // o Case 3a. Let us say the card is in the occupied part of 550 // the old gen at the start of the young collection. In that 551 // case, the card will be scanned by the younger refs scanning 552 // code which will set it to cur_younger_gen. In a subsequent 553 // scan, the card will be considered again and get its final 554 // correct value. [End Case 3a] 555 // o Case 3b. Now consider the case where the card is in the 556 // unoccupied part of the old gen, and is occupied as a result 557 // of promotions during thus young gc. In that case, 558 // the card will not be scanned for younger refs. The presence 559 // of newly promoted objects on the card will then result in 560 // its keeping the value cur_younger_gen_and_prev_non_clean 561 // value, which we have dealt with in Case 3 here. [End Case 3b] 562 // [End Case 3] 563 // 564 // (Please refer to the code in the helper class 565 // ClearNonCleanCardWrapper and in CardTableModRefBS for details.) 566 // 567 // The informal arguments above can be tightened into a formal 568 // correctness proof and it behooves us to write up such a proof, 569 // or to use model checking to prove that there are no lingering 570 // concerns. 571 // 572 // Clearly because of Case 3b one cannot bound the time for 573 // which a card will retain what we have called a "stale" value. 574 // However, one can obtain a Loose upper bound on the redundant 575 // work as a result of such stale values. Note first that any 576 // time a stale card lies in the occupied part of the space at 577 // the start of the collection, it is scanned by younger refs 578 // code and we can define a rank function on card values that 579 // declines when this is so. Note also that when a card does not 580 // lie in the occupied part of the space at the beginning of a 581 // young collection, its rank can either decline or stay unchanged. 582 // In this case, no extra work is done in terms of redundant 583 // younger refs scanning of that card. 584 // Then, the case analysis above reveals that, in the worst case, 585 // any such stale card will be scanned unnecessarily at most twice. 586 // 587 // It is nonetheless advisable to try and get rid of some of this 588 // redundant work in a subsequent (low priority) re-design of 589 // the card-scanning code, if only to simplify the underlying 590 // state machine analysis/proof. ysr 1/28/2002. XXX 591 cur_entry++; 592 } 593 } 594 } 595 596 void CardTableRS::verify() { 597 // At present, we only know how to verify the card table RS for 598 // generational heaps. 599 VerifyCTGenClosure blk(this); 600 GenCollectedHeap::heap()->generation_iterate(&blk, false); 601 _ct_bs->verify(); 602 }