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 // The indexing in this array is slightly odd. We want to access 107 // the old generation record here, which is at index 2. 108 _last_cur_val_in_gen[2] = cur_youngergen_card_val(); 109 g->younger_refs_iterate(blk); 110 } 111 112 inline bool ClearNoncleanCardWrapper::clear_card(jbyte* entry) { 113 if (_is_par) { 114 return clear_card_parallel(entry); 115 } else { 116 return clear_card_serial(entry); 117 } 118 } 119 120 inline bool ClearNoncleanCardWrapper::clear_card_parallel(jbyte* entry) { 121 while (true) { 122 // In the parallel case, we may have to do this several times. 123 jbyte entry_val = *entry; 124 assert(entry_val != CardTableRS::clean_card_val(), 125 "We shouldn't be looking at clean cards, and this should " 126 "be the only place they get cleaned."); 127 if (CardTableRS::card_is_dirty_wrt_gen_iter(entry_val) 128 || _ct->is_prev_youngergen_card_val(entry_val)) { 129 jbyte res = 130 Atomic::cmpxchg(CardTableRS::clean_card_val(), entry, entry_val); 131 if (res == entry_val) { 132 break; 133 } else { 134 assert(res == CardTableRS::cur_youngergen_and_prev_nonclean_card, 135 "The CAS above should only fail if another thread did " 136 "a GC write barrier."); 137 } 138 } else if (entry_val == 139 CardTableRS::cur_youngergen_and_prev_nonclean_card) { 140 // Parallelism shouldn't matter in this case. Only the thread 141 // assigned to scan the card should change this value. 142 *entry = _ct->cur_youngergen_card_val(); 143 break; 144 } else { 145 assert(entry_val == _ct->cur_youngergen_card_val(), 146 "Should be the only possibility."); 147 // In this case, the card was clean before, and become 148 // cur_youngergen only because of processing of a promoted object. 149 // We don't have to look at the card. 150 return false; 151 } 152 } 153 return true; 154 } 155 156 157 inline bool ClearNoncleanCardWrapper::clear_card_serial(jbyte* entry) { 158 jbyte entry_val = *entry; 159 assert(entry_val != CardTableRS::clean_card_val(), 160 "We shouldn't be looking at clean cards, and this should " 161 "be the only place they get cleaned."); 162 assert(entry_val != CardTableRS::cur_youngergen_and_prev_nonclean_card, 163 "This should be possible in the sequential case."); 164 *entry = CardTableRS::clean_card_val(); 165 return true; 166 } 167 168 ClearNoncleanCardWrapper::ClearNoncleanCardWrapper( 169 DirtyCardToOopClosure* dirty_card_closure, CardTableRS* ct) : 170 _dirty_card_closure(dirty_card_closure), _ct(ct) { 171 // Cannot yet substitute active_workers for n_par_threads 172 // in the case where parallelism is being turned off by 173 // setting n_par_threads to 0. 174 _is_par = (GenCollectedHeap::heap()->n_par_threads() > 0); 175 assert(!_is_par || 176 (GenCollectedHeap::heap()->n_par_threads() == 177 GenCollectedHeap::heap()->workers()->active_workers()), "Mismatch"); 178 } 179 180 bool ClearNoncleanCardWrapper::is_word_aligned(jbyte* entry) { 181 return (((intptr_t)entry) & (BytesPerWord-1)) == 0; 182 } 183 184 // The regions are visited in *decreasing* address order. 185 // This order aids with imprecise card marking, where a dirty 186 // card may cause scanning, and summarization marking, of objects 187 // that extend onto subsequent cards. 188 void ClearNoncleanCardWrapper::do_MemRegion(MemRegion mr) { 189 assert(mr.word_size() > 0, "Error"); 190 assert(_ct->is_aligned(mr.start()), "mr.start() should be card aligned"); 191 // mr.end() may not necessarily be card aligned. 192 jbyte* cur_entry = _ct->byte_for(mr.last()); 193 const jbyte* limit = _ct->byte_for(mr.start()); 194 HeapWord* end_of_non_clean = mr.end(); 195 HeapWord* start_of_non_clean = end_of_non_clean; 196 while (cur_entry >= limit) { 197 HeapWord* cur_hw = _ct->addr_for(cur_entry); 198 if ((*cur_entry != CardTableRS::clean_card_val()) && clear_card(cur_entry)) { 199 // Continue the dirty range by opening the 200 // dirty window one card to the left. 201 start_of_non_clean = cur_hw; 202 } else { 203 // We hit a "clean" card; process any non-empty 204 // "dirty" range accumulated so far. 205 if (start_of_non_clean < end_of_non_clean) { 206 const MemRegion mrd(start_of_non_clean, end_of_non_clean); 207 _dirty_card_closure->do_MemRegion(mrd); 208 } 209 210 // fast forward through potential continuous whole-word range of clean cards beginning at a word-boundary 211 if (is_word_aligned(cur_entry)) { 212 jbyte* cur_row = cur_entry - BytesPerWord; 213 while (cur_row >= limit && *((intptr_t*)cur_row) == CardTableRS::clean_card_row()) { 214 cur_row -= BytesPerWord; 215 } 216 cur_entry = cur_row + BytesPerWord; 217 cur_hw = _ct->addr_for(cur_entry); 218 } 219 220 // Reset the dirty window, while continuing to look 221 // for the next dirty card that will start a 222 // new dirty window. 223 end_of_non_clean = cur_hw; 224 start_of_non_clean = cur_hw; 225 } 226 // Note that "cur_entry" leads "start_of_non_clean" in 227 // its leftward excursion after this point 228 // in the loop and, when we hit the left end of "mr", 229 // will point off of the left end of the card-table 230 // for "mr". 231 cur_entry--; 232 } 233 // If the first card of "mr" was dirty, we will have 234 // been left with a dirty window, co-initial with "mr", 235 // which we now process. 236 if (start_of_non_clean < end_of_non_clean) { 237 const MemRegion mrd(start_of_non_clean, end_of_non_clean); 238 _dirty_card_closure->do_MemRegion(mrd); 239 } 240 } 241 242 // clean (by dirty->clean before) ==> cur_younger_gen 243 // dirty ==> cur_youngergen_and_prev_nonclean_card 244 // precleaned ==> cur_youngergen_and_prev_nonclean_card 245 // prev-younger-gen ==> cur_youngergen_and_prev_nonclean_card 246 // cur-younger-gen ==> cur_younger_gen 247 // cur_youngergen_and_prev_nonclean_card ==> no change. 248 void CardTableRS::write_ref_field_gc_par(void* field, oop new_val) { 249 jbyte* entry = ct_bs()->byte_for(field); 250 do { 251 jbyte entry_val = *entry; 252 // We put this first because it's probably the most common case. 253 if (entry_val == clean_card_val()) { 254 // No threat of contention with cleaning threads. 255 *entry = cur_youngergen_card_val(); 256 return; 257 } else if (card_is_dirty_wrt_gen_iter(entry_val) 258 || is_prev_youngergen_card_val(entry_val)) { 259 // Mark it as both cur and prev youngergen; card cleaning thread will 260 // eventually remove the previous stuff. 261 jbyte new_val = cur_youngergen_and_prev_nonclean_card; 262 jbyte res = Atomic::cmpxchg(new_val, entry, entry_val); 263 // Did the CAS succeed? 264 if (res == entry_val) return; 265 // Otherwise, retry, to see the new value. 266 continue; 267 } else { 268 assert(entry_val == cur_youngergen_and_prev_nonclean_card 269 || entry_val == cur_youngergen_card_val(), 270 "should be only possibilities."); 271 return; 272 } 273 } while (true); 274 } 275 276 void CardTableRS::younger_refs_in_space_iterate(Space* sp, 277 OopsInGenClosure* cl) { 278 const MemRegion urasm = sp->used_region_at_save_marks(); 279 #ifdef ASSERT 280 // Convert the assertion check to a warning if we are running 281 // CMS+ParNew until related bug is fixed. 282 MemRegion ur = sp->used_region(); 283 assert(ur.contains(urasm) || (UseConcMarkSweepGC), 284 err_msg("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 // In the case of CMS+ParNew, issue a warning 289 if (!ur.contains(urasm)) { 290 assert(UseConcMarkSweepGC, "Tautology: see assert above"); 291 warning("CMS+ParNew: Did you forget to call save_marks()? " 292 "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in " 293 "[" PTR_FORMAT ", " PTR_FORMAT ")", 294 p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end())); 295 MemRegion ur2 = sp->used_region(); 296 MemRegion urasm2 = sp->used_region_at_save_marks(); 297 if (!ur.equals(ur2)) { 298 warning("CMS+ParNew: Flickering used_region()!!"); 299 } 300 if (!urasm.equals(urasm2)) { 301 warning("CMS+ParNew: Flickering used_region_at_save_marks()!!"); 302 } 303 ShouldNotReachHere(); 304 } 305 #endif 306 _ct_bs->non_clean_card_iterate_possibly_parallel(sp, urasm, cl, this); 307 } 308 309 void CardTableRS::clear_into_younger(Generation* old_gen) { 310 assert(old_gen == GenCollectedHeap::heap()->old_gen(), 311 "Should only be called for the old generation"); 312 // The card tables for the youngest gen need never be cleared. 313 // There's a bit of subtlety in the clear() and invalidate() 314 // methods that we exploit here and in invalidate_or_clear() 315 // below to avoid missing cards at the fringes. If clear() or 316 // invalidate() are changed in the future, this code should 317 // be revisited. 20040107.ysr 318 clear(old_gen->prev_used_region()); 319 } 320 321 void CardTableRS::invalidate_or_clear(Generation* old_gen) { 322 assert(old_gen == GenCollectedHeap::heap()->old_gen(), 323 "Should only be called for the old generation"); 324 // Invalidate the cards for the currently occupied part of 325 // the old generation and clear the cards for the 326 // unoccupied part of the generation (if any, making use 327 // of that generation's prev_used_region to determine that 328 // region). No need to do anything for the youngest 329 // generation. Also see note#20040107.ysr above. 330 MemRegion used_mr = old_gen->used_region(); 331 MemRegion to_be_cleared_mr = old_gen->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 } 337 338 339 class VerifyCleanCardClosure: public OopClosure { 340 private: 341 HeapWord* _boundary; 342 HeapWord* _begin; 343 HeapWord* _end; 344 protected: 345 template <class T> void do_oop_work(T* p) { 346 HeapWord* jp = (HeapWord*)p; 347 assert(jp >= _begin && jp < _end, 348 err_msg("Error: jp " PTR_FORMAT " should be within " 349 "[_begin, _end) = [" PTR_FORMAT "," PTR_FORMAT ")", 350 p2i(jp), p2i(_begin), p2i(_end))); 351 oop obj = oopDesc::load_decode_heap_oop(p); 352 guarantee(obj == NULL || (HeapWord*)obj >= _boundary, 353 err_msg("pointer " PTR_FORMAT " at " PTR_FORMAT " on " 354 "clean card crosses boundary" PTR_FORMAT, 355 p2i((HeapWord*)obj), p2i(jp), p2i(_boundary))); 356 } 357 358 public: 359 VerifyCleanCardClosure(HeapWord* b, HeapWord* begin, HeapWord* end) : 360 _boundary(b), _begin(begin), _end(end) { 361 assert(b <= begin, 362 err_msg("Error: boundary " PTR_FORMAT " should be at or below begin " PTR_FORMAT, 363 p2i(b), p2i(begin))); 364 assert(begin <= end, 365 err_msg("Error: begin " PTR_FORMAT " should be strictly below end " PTR_FORMAT, 366 p2i(begin), p2i(end))); 367 } 368 369 virtual void do_oop(oop* p) { VerifyCleanCardClosure::do_oop_work(p); } 370 virtual void do_oop(narrowOop* p) { VerifyCleanCardClosure::do_oop_work(p); } 371 }; 372 373 class VerifyCTSpaceClosure: public SpaceClosure { 374 private: 375 CardTableRS* _ct; 376 HeapWord* _boundary; 377 public: 378 VerifyCTSpaceClosure(CardTableRS* ct, HeapWord* boundary) : 379 _ct(ct), _boundary(boundary) {} 380 virtual void do_space(Space* s) { _ct->verify_space(s, _boundary); } 381 }; 382 383 class VerifyCTGenClosure: public GenCollectedHeap::GenClosure { 384 CardTableRS* _ct; 385 public: 386 VerifyCTGenClosure(CardTableRS* ct) : _ct(ct) {} 387 void do_generation(Generation* gen) { 388 // Skip the youngest generation. 389 if (gen == GenCollectedHeap::heap()->young_gen()) { 390 return; 391 } 392 // Normally, we're interested in pointers to younger generations. 393 VerifyCTSpaceClosure blk(_ct, gen->reserved().start()); 394 gen->space_iterate(&blk, true); 395 } 396 }; 397 398 void CardTableRS::verify_space(Space* s, HeapWord* gen_boundary) { 399 // We don't need to do young-gen spaces. 400 if (s->end() <= gen_boundary) return; 401 MemRegion used = s->used_region(); 402 403 jbyte* cur_entry = byte_for(used.start()); 404 jbyte* limit = byte_after(used.last()); 405 while (cur_entry < limit) { 406 if (*cur_entry == CardTableModRefBS::clean_card) { 407 jbyte* first_dirty = cur_entry+1; 408 while (first_dirty < limit && 409 *first_dirty == CardTableModRefBS::clean_card) { 410 first_dirty++; 411 } 412 // If the first object is a regular object, and it has a 413 // young-to-old field, that would mark the previous card. 414 HeapWord* boundary = addr_for(cur_entry); 415 HeapWord* end = (first_dirty >= limit) ? used.end() : addr_for(first_dirty); 416 HeapWord* boundary_block = s->block_start(boundary); 417 HeapWord* begin = boundary; // Until proven otherwise. 418 HeapWord* start_block = boundary_block; // Until proven otherwise. 419 if (boundary_block < boundary) { 420 if (s->block_is_obj(boundary_block) && s->obj_is_alive(boundary_block)) { 421 oop boundary_obj = oop(boundary_block); 422 if (!boundary_obj->is_objArray() && 423 !boundary_obj->is_typeArray()) { 424 guarantee(cur_entry > byte_for(used.start()), 425 "else boundary would be boundary_block"); 426 if (*byte_for(boundary_block) != CardTableModRefBS::clean_card) { 427 begin = boundary_block + s->block_size(boundary_block); 428 start_block = begin; 429 } 430 } 431 } 432 } 433 // Now traverse objects until end. 434 if (begin < end) { 435 MemRegion mr(begin, end); 436 VerifyCleanCardClosure verify_blk(gen_boundary, begin, end); 437 for (HeapWord* cur = start_block; cur < end; cur += s->block_size(cur)) { 438 if (s->block_is_obj(cur) && s->obj_is_alive(cur)) { 439 oop(cur)->oop_iterate_no_header(&verify_blk, mr); 440 } 441 } 442 } 443 cur_entry = first_dirty; 444 } else { 445 // We'd normally expect that cur_youngergen_and_prev_nonclean_card 446 // is a transient value, that cannot be in the card table 447 // except during GC, and thus assert that: 448 // guarantee(*cur_entry != cur_youngergen_and_prev_nonclean_card, 449 // "Illegal CT value"); 450 // That however, need not hold, as will become clear in the 451 // following... 452 453 // We'd normally expect that if we are in the parallel case, 454 // we can't have left a prev value (which would be different 455 // from the current value) in the card table, and so we'd like to 456 // assert that: 457 // guarantee(cur_youngergen_card_val() == youngergen_card 458 // || !is_prev_youngergen_card_val(*cur_entry), 459 // "Illegal CT value"); 460 // That, however, may not hold occasionally, because of 461 // CMS or MSC in the old gen. To wit, consider the 462 // following two simple illustrative scenarios: 463 // (a) CMS: Consider the case where a large object L 464 // spanning several cards is allocated in the old 465 // gen, and has a young gen reference stored in it, dirtying 466 // some interior cards. A young collection scans the card, 467 // finds a young ref and installs a youngergenP_n value. 468 // L then goes dead. Now a CMS collection starts, 469 // finds L dead and sweeps it up. Assume that L is 470 // abutting _unallocated_blk, so _unallocated_blk is 471 // adjusted down to (below) L. Assume further that 472 // no young collection intervenes during this CMS cycle. 473 // The next young gen cycle will not get to look at this 474 // youngergenP_n card since it lies in the unoccupied 475 // part of the space. 476 // Some young collections later the blocks on this 477 // card can be re-allocated either due to direct allocation 478 // or due to absorbing promotions. At this time, the 479 // before-gc verification will fail the above assert. 480 // (b) MSC: In this case, an object L with a young reference 481 // is on a card that (therefore) holds a youngergen_n value. 482 // Suppose also that L lies towards the end of the used 483 // the used space before GC. An MSC collection 484 // occurs that compacts to such an extent that this 485 // card is no longer in the occupied part of the space. 486 // Since current code in MSC does not always clear cards 487 // in the unused part of old gen, this stale youngergen_n 488 // value is left behind and can later be covered by 489 // an object when promotion or direct allocation 490 // re-allocates that part of the heap. 491 // 492 // Fortunately, the presence of such stale card values is 493 // "only" a minor annoyance in that subsequent young collections 494 // might needlessly scan such cards, but would still never corrupt 495 // the heap as a result. However, it's likely not to be a significant 496 // performance inhibitor in practice. For instance, 497 // some recent measurements with unoccupied cards eagerly cleared 498 // out to maintain this invariant, showed next to no 499 // change in young collection times; of course one can construct 500 // degenerate examples where the cost can be significant.) 501 // Note, in particular, that if the "stale" card is modified 502 // after re-allocation, it would be dirty, not "stale". Thus, 503 // we can never have a younger ref in such a card and it is 504 // safe not to scan that card in any collection. [As we see 505 // below, we do some unnecessary scanning 506 // in some cases in the current parallel scanning algorithm.] 507 // 508 // The main point below is that the parallel card scanning code 509 // deals correctly with these stale card values. There are two main 510 // cases to consider where we have a stale "younger gen" value and a 511 // "derivative" case to consider, where we have a stale 512 // "cur_younger_gen_and_prev_non_clean" value, as will become 513 // apparent in the case analysis below. 514 // o Case 1. If the stale value corresponds to a younger_gen_n 515 // value other than the cur_younger_gen value then the code 516 // treats this as being tantamount to a prev_younger_gen 517 // card. This means that the card may be unnecessarily scanned. 518 // There are two sub-cases to consider: 519 // o Case 1a. Let us say that the card is in the occupied part 520 // of the generation at the time the collection begins. In 521 // that case the card will be either cleared when it is scanned 522 // for young pointers, or will be set to cur_younger_gen as a 523 // result of promotion. (We have elided the normal case where 524 // the scanning thread and the promoting thread interleave 525 // possibly resulting in a transient 526 // cur_younger_gen_and_prev_non_clean value before settling 527 // to cur_younger_gen. [End Case 1a.] 528 // o Case 1b. Consider now the case when the card is in the unoccupied 529 // part of the space which becomes occupied because of promotions 530 // into it during the current young GC. In this case the card 531 // will never be scanned for young references. The current 532 // code will set the card value to either 533 // cur_younger_gen_and_prev_non_clean or leave 534 // it with its stale value -- because the promotions didn't 535 // result in any younger refs on that card. Of these two 536 // cases, the latter will be covered in Case 1a during 537 // a subsequent scan. To deal with the former case, we need 538 // to further consider how we deal with a stale value of 539 // cur_younger_gen_and_prev_non_clean in our case analysis 540 // below. This we do in Case 3 below. [End Case 1b] 541 // [End Case 1] 542 // o Case 2. If the stale value corresponds to cur_younger_gen being 543 // a value not necessarily written by a current promotion, the 544 // card will not be scanned by the younger refs scanning code. 545 // (This is OK since as we argued above such cards cannot contain 546 // any younger refs.) The result is that this value will be 547 // treated as a prev_younger_gen value in a subsequent collection, 548 // which is addressed in Case 1 above. [End Case 2] 549 // o Case 3. We here consider the "derivative" case from Case 1b. above 550 // because of which we may find a stale 551 // cur_younger_gen_and_prev_non_clean card value in the table. 552 // Once again, as in Case 1, we consider two subcases, depending 553 // on whether the card lies in the occupied or unoccupied part 554 // of the space at the start of the young collection. 555 // o Case 3a. Let us say the card is in the occupied part of 556 // the old gen at the start of the young collection. In that 557 // case, the card will be scanned by the younger refs scanning 558 // code which will set it to cur_younger_gen. In a subsequent 559 // scan, the card will be considered again and get its final 560 // correct value. [End Case 3a] 561 // o Case 3b. Now consider the case where the card is in the 562 // unoccupied part of the old gen, and is occupied as a result 563 // of promotions during thus young gc. In that case, 564 // the card will not be scanned for younger refs. The presence 565 // of newly promoted objects on the card will then result in 566 // its keeping the value cur_younger_gen_and_prev_non_clean 567 // value, which we have dealt with in Case 3 here. [End Case 3b] 568 // [End Case 3] 569 // 570 // (Please refer to the code in the helper class 571 // ClearNonCleanCardWrapper and in CardTableModRefBS for details.) 572 // 573 // The informal arguments above can be tightened into a formal 574 // correctness proof and it behooves us to write up such a proof, 575 // or to use model checking to prove that there are no lingering 576 // concerns. 577 // 578 // Clearly because of Case 3b one cannot bound the time for 579 // which a card will retain what we have called a "stale" value. 580 // However, one can obtain a Loose upper bound on the redundant 581 // work as a result of such stale values. Note first that any 582 // time a stale card lies in the occupied part of the space at 583 // the start of the collection, it is scanned by younger refs 584 // code and we can define a rank function on card values that 585 // declines when this is so. Note also that when a card does not 586 // lie in the occupied part of the space at the beginning of a 587 // young collection, its rank can either decline or stay unchanged. 588 // In this case, no extra work is done in terms of redundant 589 // younger refs scanning of that card. 590 // Then, the case analysis above reveals that, in the worst case, 591 // any such stale card will be scanned unnecessarily at most twice. 592 // 593 // It is nonetheless advisable to try and get rid of some of this 594 // redundant work in a subsequent (low priority) re-design of 595 // the card-scanning code, if only to simplify the underlying 596 // state machine analysis/proof. ysr 1/28/2002. XXX 597 cur_entry++; 598 } 599 } 600 } 601 602 void CardTableRS::verify() { 603 // At present, we only know how to verify the card table RS for 604 // generational heaps. 605 VerifyCTGenClosure blk(this); 606 GenCollectedHeap::heap()->generation_iterate(&blk, false); 607 _ct_bs->verify(); 608 }