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.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 guarantee(Universe::heap()->kind() == CollectedHeap::GenCollectedHeap, "sanity"); 42 _ct_bs = new CardTableModRefBSForCTRS(whole_heap); 43 _ct_bs->initialize(); 44 set_bs(_ct_bs); 45 _last_cur_val_in_gen = NEW_C_HEAP_ARRAY3(jbyte, GenCollectedHeap::max_gens + 1, 46 mtGC, CURRENT_PC, AllocFailStrategy::RETURN_NULL); 47 if (_last_cur_val_in_gen == NULL) { 48 vm_exit_during_initialization("Could not create last_cur_val_in_gen array."); 49 } 50 for (int i = 0; i < GenCollectedHeap::max_gens + 1; i++) { 51 _last_cur_val_in_gen[i] = clean_card_val(); 52 } 53 _ct_bs->set_CTRS(this); 54 } 55 56 CardTableRS::~CardTableRS() { 57 if (_ct_bs) { 58 delete _ct_bs; 59 _ct_bs = NULL; 60 } 61 if (_last_cur_val_in_gen) { 62 FREE_C_HEAP_ARRAY(jbyte, _last_cur_val_in_gen); 63 } 64 } 65 66 void CardTableRS::resize_covered_region(MemRegion new_region) { 67 _ct_bs->resize_covered_region(new_region); 68 } 69 70 jbyte CardTableRS::find_unused_youngergenP_card_value() { 71 for (jbyte v = youngergenP1_card; 72 v < cur_youngergen_and_prev_nonclean_card; 73 v++) { 74 bool seen = false; 75 for (int g = 0; g < _regions_to_iterate; g++) { 76 if (_last_cur_val_in_gen[g] == v) { 77 seen = true; 78 break; 79 } 80 } 81 if (!seen) return v; 82 } 83 ShouldNotReachHere(); 84 return 0; 85 } 86 87 void CardTableRS::prepare_for_younger_refs_iterate(bool parallel) { 88 // Parallel or sequential, we must always set the prev to equal the 89 // last one written. 90 if (parallel) { 91 // Find a parallel value to be used next. 92 jbyte next_val = find_unused_youngergenP_card_value(); 93 set_cur_youngergen_card_val(next_val); 94 95 } else { 96 // In an sequential traversal we will always write youngergen, so that 97 // the inline barrier is correct. 98 set_cur_youngergen_card_val(youngergen_card); 99 } 100 } 101 102 void CardTableRS::younger_refs_iterate(Generation* g, 103 OopsInGenClosure* blk) { 104 _last_cur_val_in_gen[g->level()+1] = cur_youngergen_card_val(); 105 g->younger_refs_iterate(blk); 106 } 107 108 inline bool ClearNoncleanCardWrapper::clear_card(jbyte* entry) { 109 if (_is_par) { 110 return clear_card_parallel(entry); 111 } else { 112 return clear_card_serial(entry); 113 } 114 } 115 116 inline bool ClearNoncleanCardWrapper::clear_card_parallel(jbyte* entry) { 117 while (true) { 118 // In the parallel case, we may have to do this several times. 119 jbyte entry_val = *entry; 120 assert(entry_val != CardTableRS::clean_card_val(), 121 "We shouldn't be looking at clean cards, and this should " 122 "be the only place they get cleaned."); 123 if (CardTableRS::card_is_dirty_wrt_gen_iter(entry_val) 124 || _ct->is_prev_youngergen_card_val(entry_val)) { 125 jbyte res = 126 Atomic::cmpxchg(CardTableRS::clean_card_val(), entry, entry_val); 127 if (res == entry_val) { 128 break; 129 } else { 130 assert(res == CardTableRS::cur_youngergen_and_prev_nonclean_card, 131 "The CAS above should only fail if another thread did " 132 "a GC write barrier."); 133 } 134 } else if (entry_val == 135 CardTableRS::cur_youngergen_and_prev_nonclean_card) { 136 // Parallelism shouldn't matter in this case. Only the thread 137 // assigned to scan the card should change this value. 138 *entry = _ct->cur_youngergen_card_val(); 139 break; 140 } else { 141 assert(entry_val == _ct->cur_youngergen_card_val(), 142 "Should be the only possibility."); 143 // In this case, the card was clean before, and become 144 // cur_youngergen only because of processing of a promoted object. 145 // We don't have to look at the card. 146 return false; 147 } 148 } 149 return true; 150 } 151 152 153 inline bool ClearNoncleanCardWrapper::clear_card_serial(jbyte* entry) { 154 jbyte entry_val = *entry; 155 assert(entry_val != CardTableRS::clean_card_val(), 156 "We shouldn't be looking at clean cards, and this should " 157 "be the only place they get cleaned."); 158 assert(entry_val != CardTableRS::cur_youngergen_and_prev_nonclean_card, 159 "This should be possible in the sequential case."); 160 *entry = CardTableRS::clean_card_val(); 161 return true; 162 } 163 164 ClearNoncleanCardWrapper::ClearNoncleanCardWrapper( 165 DirtyCardToOopClosure* dirty_card_closure, CardTableRS* ct) : 166 _dirty_card_closure(dirty_card_closure), _ct(ct) { 167 // Cannot yet substitute active_workers for n_par_threads 168 // in the case where parallelism is being turned off by 169 // setting n_par_threads to 0. 170 _is_par = (SharedHeap::heap()->n_par_threads() > 0); 171 assert(!_is_par || 172 (SharedHeap::heap()->n_par_threads() == 173 SharedHeap::heap()->workers()->active_workers()), "Mismatch"); 174 } 175 176 bool ClearNoncleanCardWrapper::is_word_aligned(jbyte* entry) { 177 return (((intptr_t)entry) & (BytesPerWord-1)) == 0; 178 } 179 180 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 const MemRegion urasm = sp->used_region_at_save_marks(); 271 #ifdef ASSERT 272 // Convert the assertion check to a warning if we are running 273 // CMS+ParNew until related bug is fixed. 274 MemRegion ur = sp->used_region(); 275 assert(ur.contains(urasm) || (UseConcMarkSweepGC), 276 err_msg("Did you forget to call save_marks()? " 277 "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in " 278 "[" PTR_FORMAT ", " PTR_FORMAT ")", 279 p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end()))); 280 // In the case of CMS+ParNew, issue a warning 281 if (!ur.contains(urasm)) { 282 assert(UseConcMarkSweepGC, "Tautology: see assert above"); 283 warning("CMS+ParNew: Did you forget to call save_marks()? " 284 "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in " 285 "[" PTR_FORMAT ", " PTR_FORMAT ")", 286 p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end())); 287 MemRegion ur2 = sp->used_region(); 288 MemRegion urasm2 = sp->used_region_at_save_marks(); 289 if (!ur.equals(ur2)) { 290 warning("CMS+ParNew: Flickering used_region()!!"); 291 } 292 if (!urasm.equals(urasm2)) { 293 warning("CMS+ParNew: Flickering used_region_at_save_marks()!!"); 294 } 295 ShouldNotReachHere(); 296 } 297 #endif 298 _ct_bs->non_clean_card_iterate_possibly_parallel(sp, urasm, cl, this); 299 } 300 301 void CardTableRS::clear_into_younger(Generation* old_gen) { 302 assert(old_gen->level() == 1, "Should only be called for the old generation"); 303 // The card tables for the youngest gen need never be cleared. 304 // There's a bit of subtlety in the clear() and invalidate() 305 // methods that we exploit here and in invalidate_or_clear() 306 // below to avoid missing cards at the fringes. If clear() or 307 // invalidate() are changed in the future, this code should 308 // be revisited. 20040107.ysr 309 clear(old_gen->prev_used_region()); 310 } 311 312 void CardTableRS::invalidate_or_clear(Generation* old_gen) { 313 assert(old_gen->level() == 1, "Should only be called for the old generation"); 314 // Invalidate the cards for the currently occupied part of 315 // the old generation and clear the cards for the 316 // unoccupied part of the generation (if any, making use 317 // of that generation's prev_used_region to determine that 318 // region). No need to do anything for the youngest 319 // generation. Also see note#20040107.ysr above. 320 MemRegion used_mr = old_gen->used_region(); 321 MemRegion to_be_cleared_mr = old_gen->prev_used_region().minus(used_mr); 322 if (!to_be_cleared_mr.is_empty()) { 323 clear(to_be_cleared_mr); 324 } 325 invalidate(used_mr); 326 } 327 328 329 class VerifyCleanCardClosure: public OopClosure { 330 private: 331 HeapWord* _boundary; 332 HeapWord* _begin; 333 HeapWord* _end; 334 protected: 335 template <class T> void do_oop_work(T* p) { 336 HeapWord* jp = (HeapWord*)p; 337 assert(jp >= _begin && jp < _end, 338 err_msg("Error: jp " PTR_FORMAT " should be within " 339 "[_begin, _end) = [" PTR_FORMAT "," PTR_FORMAT ")", 340 p2i(jp), p2i(_begin), p2i(_end))); 341 oop obj = oopDesc::load_decode_heap_oop(p); 342 guarantee(obj == NULL || (HeapWord*)obj >= _boundary, 343 err_msg("pointer " PTR_FORMAT " at " PTR_FORMAT " on " 344 "clean card crosses boundary" PTR_FORMAT, 345 p2i((HeapWord*)obj), p2i(jp), p2i(_boundary))); 346 } 347 348 public: 349 VerifyCleanCardClosure(HeapWord* b, HeapWord* begin, HeapWord* end) : 350 _boundary(b), _begin(begin), _end(end) { 351 assert(b <= begin, 352 err_msg("Error: boundary " PTR_FORMAT " should be at or below begin " PTR_FORMAT, 353 p2i(b), p2i(begin))); 354 assert(begin <= end, 355 err_msg("Error: begin " PTR_FORMAT " should be strictly below end " PTR_FORMAT, 356 p2i(begin), p2i(end))); 357 } 358 359 virtual void do_oop(oop* p) { VerifyCleanCardClosure::do_oop_work(p); } 360 virtual void do_oop(narrowOop* p) { VerifyCleanCardClosure::do_oop_work(p); } 361 }; 362 363 class VerifyCTSpaceClosure: public SpaceClosure { 364 private: 365 CardTableRS* _ct; 366 HeapWord* _boundary; 367 public: 368 VerifyCTSpaceClosure(CardTableRS* ct, HeapWord* boundary) : 369 _ct(ct), _boundary(boundary) {} 370 virtual void do_space(Space* s) { _ct->verify_space(s, _boundary); } 371 }; 372 373 class VerifyCTGenClosure: public GenCollectedHeap::GenClosure { 374 CardTableRS* _ct; 375 public: 376 VerifyCTGenClosure(CardTableRS* ct) : _ct(ct) {} 377 void do_generation(Generation* gen) { 378 // Skip the youngest generation. 379 if (gen->level() == 0) return; 380 // Normally, we're interested in pointers to younger generations. 381 VerifyCTSpaceClosure blk(_ct, gen->reserved().start()); 382 gen->space_iterate(&blk, true); 383 } 384 }; 385 386 void CardTableRS::verify_space(Space* s, HeapWord* gen_boundary) { 387 // We don't need to do young-gen spaces. 388 if (s->end() <= gen_boundary) return; 389 MemRegion used = s->used_region(); 390 391 jbyte* cur_entry = byte_for(used.start()); 392 jbyte* limit = byte_after(used.last()); 393 while (cur_entry < limit) { 394 if (*cur_entry == CardTableModRefBS::clean_card) { 395 jbyte* first_dirty = cur_entry+1; 396 while (first_dirty < limit && 397 *first_dirty == CardTableModRefBS::clean_card) { 398 first_dirty++; 399 } 400 // If the first object is a regular object, and it has a 401 // young-to-old field, that would mark the previous card. 402 HeapWord* boundary = addr_for(cur_entry); 403 HeapWord* end = (first_dirty >= limit) ? used.end() : addr_for(first_dirty); 404 HeapWord* boundary_block = s->block_start(boundary); 405 HeapWord* begin = boundary; // Until proven otherwise. 406 HeapWord* start_block = boundary_block; // Until proven otherwise. 407 if (boundary_block < boundary) { 408 if (s->block_is_obj(boundary_block) && s->obj_is_alive(boundary_block)) { 409 oop boundary_obj = oop(boundary_block); 410 if (!boundary_obj->is_objArray() && 411 !boundary_obj->is_typeArray()) { 412 guarantee(cur_entry > byte_for(used.start()), 413 "else boundary would be boundary_block"); 414 if (*byte_for(boundary_block) != CardTableModRefBS::clean_card) { 415 begin = boundary_block + s->block_size(boundary_block); 416 start_block = begin; 417 } 418 } 419 } 420 } 421 // Now traverse objects until end. 422 if (begin < end) { 423 MemRegion mr(begin, end); 424 VerifyCleanCardClosure verify_blk(gen_boundary, begin, end); 425 for (HeapWord* cur = start_block; cur < end; cur += s->block_size(cur)) { 426 if (s->block_is_obj(cur) && s->obj_is_alive(cur)) { 427 oop(cur)->oop_iterate_no_header(&verify_blk, mr); 428 } 429 } 430 } 431 cur_entry = first_dirty; 432 } else { 433 // We'd normally expect that cur_youngergen_and_prev_nonclean_card 434 // is a transient value, that cannot be in the card table 435 // except during GC, and thus assert that: 436 // guarantee(*cur_entry != cur_youngergen_and_prev_nonclean_card, 437 // "Illegal CT value"); 438 // That however, need not hold, as will become clear in the 439 // following... 440 441 // We'd normally expect that if we are in the parallel case, 442 // we can't have left a prev value (which would be different 443 // from the current value) in the card table, and so we'd like to 444 // assert that: 445 // guarantee(cur_youngergen_card_val() == youngergen_card 446 // || !is_prev_youngergen_card_val(*cur_entry), 447 // "Illegal CT value"); 448 // That, however, may not hold occasionally, because of 449 // CMS or MSC in the old gen. To wit, consider the 450 // following two simple illustrative scenarios: 451 // (a) CMS: Consider the case where a large object L 452 // spanning several cards is allocated in the old 453 // gen, and has a young gen reference stored in it, dirtying 454 // some interior cards. A young collection scans the card, 455 // finds a young ref and installs a youngergenP_n value. 456 // L then goes dead. Now a CMS collection starts, 457 // finds L dead and sweeps it up. Assume that L is 458 // abutting _unallocated_blk, so _unallocated_blk is 459 // adjusted down to (below) L. Assume further that 460 // no young collection intervenes during this CMS cycle. 461 // The next young gen cycle will not get to look at this 462 // youngergenP_n card since it lies in the unoccupied 463 // part of the space. 464 // Some young collections later the blocks on this 465 // card can be re-allocated either due to direct allocation 466 // or due to absorbing promotions. At this time, the 467 // before-gc verification will fail the above assert. 468 // (b) MSC: In this case, an object L with a young reference 469 // is on a card that (therefore) holds a youngergen_n value. 470 // Suppose also that L lies towards the end of the used 471 // the used space before GC. An MSC collection 472 // occurs that compacts to such an extent that this 473 // card is no longer in the occupied part of the space. 474 // Since current code in MSC does not always clear cards 475 // in the unused part of old gen, this stale youngergen_n 476 // value is left behind and can later be covered by 477 // an object when promotion or direct allocation 478 // re-allocates that part of the heap. 479 // 480 // Fortunately, the presence of such stale card values is 481 // "only" a minor annoyance in that subsequent young collections 482 // might needlessly scan such cards, but would still never corrupt 483 // the heap as a result. However, it's likely not to be a significant 484 // performance inhibitor in practice. For instance, 485 // some recent measurements with unoccupied cards eagerly cleared 486 // out to maintain this invariant, showed next to no 487 // change in young collection times; of course one can construct 488 // degenerate examples where the cost can be significant.) 489 // Note, in particular, that if the "stale" card is modified 490 // after re-allocation, it would be dirty, not "stale". Thus, 491 // we can never have a younger ref in such a card and it is 492 // safe not to scan that card in any collection. [As we see 493 // below, we do some unnecessary scanning 494 // in some cases in the current parallel scanning algorithm.] 495 // 496 // The main point below is that the parallel card scanning code 497 // deals correctly with these stale card values. There are two main 498 // cases to consider where we have a stale "younger gen" value and a 499 // "derivative" case to consider, where we have a stale 500 // "cur_younger_gen_and_prev_non_clean" value, as will become 501 // apparent in the case analysis below. 502 // o Case 1. If the stale value corresponds to a younger_gen_n 503 // value other than the cur_younger_gen value then the code 504 // treats this as being tantamount to a prev_younger_gen 505 // card. This means that the card may be unnecessarily scanned. 506 // There are two sub-cases to consider: 507 // o Case 1a. Let us say that the card is in the occupied part 508 // of the generation at the time the collection begins. In 509 // that case the card will be either cleared when it is scanned 510 // for young pointers, or will be set to cur_younger_gen as a 511 // result of promotion. (We have elided the normal case where 512 // the scanning thread and the promoting thread interleave 513 // possibly resulting in a transient 514 // cur_younger_gen_and_prev_non_clean value before settling 515 // to cur_younger_gen. [End Case 1a.] 516 // o Case 1b. Consider now the case when the card is in the unoccupied 517 // part of the space which becomes occupied because of promotions 518 // into it during the current young GC. In this case the card 519 // will never be scanned for young references. The current 520 // code will set the card value to either 521 // cur_younger_gen_and_prev_non_clean or leave 522 // it with its stale value -- because the promotions didn't 523 // result in any younger refs on that card. Of these two 524 // cases, the latter will be covered in Case 1a during 525 // a subsequent scan. To deal with the former case, we need 526 // to further consider how we deal with a stale value of 527 // cur_younger_gen_and_prev_non_clean in our case analysis 528 // below. This we do in Case 3 below. [End Case 1b] 529 // [End Case 1] 530 // o Case 2. If the stale value corresponds to cur_younger_gen being 531 // a value not necessarily written by a current promotion, the 532 // card will not be scanned by the younger refs scanning code. 533 // (This is OK since as we argued above such cards cannot contain 534 // any younger refs.) The result is that this value will be 535 // treated as a prev_younger_gen value in a subsequent collection, 536 // which is addressed in Case 1 above. [End Case 2] 537 // o Case 3. We here consider the "derivative" case from Case 1b. above 538 // because of which we may find a stale 539 // cur_younger_gen_and_prev_non_clean card value in the table. 540 // Once again, as in Case 1, we consider two subcases, depending 541 // on whether the card lies in the occupied or unoccupied part 542 // of the space at the start of the young collection. 543 // o Case 3a. Let us say the card is in the occupied part of 544 // the old gen at the start of the young collection. In that 545 // case, the card will be scanned by the younger refs scanning 546 // code which will set it to cur_younger_gen. In a subsequent 547 // scan, the card will be considered again and get its final 548 // correct value. [End Case 3a] 549 // o Case 3b. Now consider the case where the card is in the 550 // unoccupied part of the old gen, and is occupied as a result 551 // of promotions during thus young gc. In that case, 552 // the card will not be scanned for younger refs. The presence 553 // of newly promoted objects on the card will then result in 554 // its keeping the value cur_younger_gen_and_prev_non_clean 555 // value, which we have dealt with in Case 3 here. [End Case 3b] 556 // [End Case 3] 557 // 558 // (Please refer to the code in the helper class 559 // ClearNonCleanCardWrapper and in CardTableModRefBS for details.) 560 // 561 // The informal arguments above can be tightened into a formal 562 // correctness proof and it behooves us to write up such a proof, 563 // or to use model checking to prove that there are no lingering 564 // concerns. 565 // 566 // Clearly because of Case 3b one cannot bound the time for 567 // which a card will retain what we have called a "stale" value. 568 // However, one can obtain a Loose upper bound on the redundant 569 // work as a result of such stale values. Note first that any 570 // time a stale card lies in the occupied part of the space at 571 // the start of the collection, it is scanned by younger refs 572 // code and we can define a rank function on card values that 573 // declines when this is so. Note also that when a card does not 574 // lie in the occupied part of the space at the beginning of a 575 // young collection, its rank can either decline or stay unchanged. 576 // In this case, no extra work is done in terms of redundant 577 // younger refs scanning of that card. 578 // Then, the case analysis above reveals that, in the worst case, 579 // any such stale card will be scanned unnecessarily at most twice. 580 // 581 // It is nonetheless advisable to try and get rid of some of this 582 // redundant work in a subsequent (low priority) re-design of 583 // the card-scanning code, if only to simplify the underlying 584 // state machine analysis/proof. ysr 1/28/2002. XXX 585 cur_entry++; 586 } 587 } 588 } 589 590 void CardTableRS::verify() { 591 // At present, we only know how to verify the card table RS for 592 // generational heaps. 593 VerifyCTGenClosure blk(this); 594 CollectedHeap* ch = Universe::heap(); 595 596 if (ch->kind() == CollectedHeap::GenCollectedHeap) { 597 GenCollectedHeap::heap()->generation_iterate(&blk, false); 598 _ct_bs->verify(); 599 } 600 } --- EOF ---