1 /* 2 * Copyright (c) 2001, 2013, 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/java.hpp" 33 #include "runtime/os.hpp" 34 #include "utilities/macros.hpp" 35 #if INCLUDE_ALL_GCS 36 #include "gc_implementation/g1/concurrentMark.hpp" 37 #include "gc_implementation/g1/g1SATBCardTableModRefBS.hpp" 38 #endif // INCLUDE_ALL_GCS 39 40 CardTableRS::CardTableRS(MemRegion whole_heap, 41 int max_covered_regions) : 42 GenRemSet(), 43 _cur_youngergen_card_val(youngergenP1_card), 44 _regions_to_iterate(max_covered_regions - 1) 45 { 46 #if INCLUDE_ALL_GCS 47 if (UseG1GC) { 48 _ct_bs = new G1SATBCardTableLoggingModRefBS(whole_heap, 49 max_covered_regions); 50 } else { 51 _ct_bs = new CardTableModRefBSForCTRS(whole_heap, max_covered_regions); 52 } 53 #else 54 _ct_bs = new CardTableModRefBSForCTRS(whole_heap, max_covered_regions); 55 #endif 56 set_bs(_ct_bs); 57 _last_cur_val_in_gen = NEW_C_HEAP_ARRAY3(jbyte, GenCollectedHeap::max_gens + 1, 58 mtGC, 0, AllocFailStrategy::RETURN_NULL); 59 if (_last_cur_val_in_gen == NULL) { 60 vm_exit_during_initialization("Could not create last_cur_val_in_gen array."); 61 } 62 for (int i = 0; i < GenCollectedHeap::max_gens + 1; i++) { 63 _last_cur_val_in_gen[i] = clean_card_val(); 64 } 65 _ct_bs->set_CTRS(this); 66 } 67 68 CardTableRS::~CardTableRS() { 69 if (_ct_bs) { 70 delete _ct_bs; 71 _ct_bs = NULL; 72 } 73 if (_last_cur_val_in_gen) { 74 FREE_C_HEAP_ARRAY(jbyte, _last_cur_val_in_gen, mtInternal); 75 } 76 } 77 78 void CardTableRS::resize_covered_region(MemRegion new_region) { 79 _ct_bs->resize_covered_region(new_region); 80 } 81 82 jbyte CardTableRS::find_unused_youngergenP_card_value() { 83 for (jbyte v = youngergenP1_card; 84 v < cur_youngergen_and_prev_nonclean_card; 85 v++) { 86 bool seen = false; 87 for (int g = 0; g < _regions_to_iterate; g++) { 88 if (_last_cur_val_in_gen[g] == v) { 89 seen = true; 90 break; 91 } 92 } 93 if (!seen) return v; 94 } 95 ShouldNotReachHere(); 96 return 0; 97 } 98 99 void CardTableRS::prepare_for_younger_refs_iterate(bool parallel) { 100 // Parallel or sequential, we must always set the prev to equal the 101 // last one written. 102 if (parallel) { 103 // Find a parallel value to be used next. 104 jbyte next_val = find_unused_youngergenP_card_value(); 105 set_cur_youngergen_card_val(next_val); 106 107 } else { 108 // In an sequential traversal we will always write youngergen, so that 109 // the inline barrier is correct. 110 set_cur_youngergen_card_val(youngergen_card); 111 } 112 } 113 114 void CardTableRS::younger_refs_iterate(Generation* g, 115 OopsInGenClosure* blk) { 116 _last_cur_val_in_gen[g->level()+1] = cur_youngergen_card_val(); 117 g->younger_refs_iterate(blk); 118 } 119 120 inline bool ClearNoncleanCardWrapper::clear_card(jbyte* entry) { 121 if (_is_par) { 122 return clear_card_parallel(entry); 123 } else { 124 return clear_card_serial(entry); 125 } 126 } 127 128 inline bool ClearNoncleanCardWrapper::clear_card_parallel(jbyte* entry) { 129 while (true) { 130 // In the parallel case, we may have to do this several times. 131 jbyte entry_val = *entry; 132 assert(entry_val != CardTableRS::clean_card_val(), 133 "We shouldn't be looking at clean cards, and this should " 134 "be the only place they get cleaned."); 135 if (CardTableRS::card_is_dirty_wrt_gen_iter(entry_val) 136 || _ct->is_prev_youngergen_card_val(entry_val)) { 137 jbyte res = 138 Atomic::cmpxchg(CardTableRS::clean_card_val(), entry, entry_val); 139 if (res == entry_val) { 140 break; 141 } else { 142 assert(res == CardTableRS::cur_youngergen_and_prev_nonclean_card, 143 "The CAS above should only fail if another thread did " 144 "a GC write barrier."); 145 } 146 } else if (entry_val == 147 CardTableRS::cur_youngergen_and_prev_nonclean_card) { 148 // Parallelism shouldn't matter in this case. Only the thread 149 // assigned to scan the card should change this value. 150 *entry = _ct->cur_youngergen_card_val(); 151 break; 152 } else { 153 assert(entry_val == _ct->cur_youngergen_card_val(), 154 "Should be the only possibility."); 155 // In this case, the card was clean before, and become 156 // cur_youngergen only because of processing of a promoted object. 157 // We don't have to look at the card. 158 return false; 159 } 160 } 161 return true; 162 } 163 164 165 inline bool ClearNoncleanCardWrapper::clear_card_serial(jbyte* entry) { 166 jbyte entry_val = *entry; 167 assert(entry_val != CardTableRS::clean_card_val(), 168 "We shouldn't be looking at clean cards, and this should " 169 "be the only place they get cleaned."); 170 assert(entry_val != CardTableRS::cur_youngergen_and_prev_nonclean_card, 171 "This should be possible in the sequential case."); 172 *entry = CardTableRS::clean_card_val(); 173 return true; 174 } 175 176 ClearNoncleanCardWrapper::ClearNoncleanCardWrapper( 177 DirtyCardToOopClosure* dirty_card_closure, CardTableRS* ct) : 178 _dirty_card_closure(dirty_card_closure), _ct(ct) { 179 // Cannot yet substitute active_workers for n_par_threads 180 // in the case where parallelism is being turned off by 181 // setting n_par_threads to 0. 182 _is_par = (SharedHeap::heap()->n_par_threads() > 0); 183 assert(!_is_par || 184 (SharedHeap::heap()->n_par_threads() == 185 SharedHeap::heap()->workers()->active_workers()), "Mismatch"); 186 } 187 188 bool ClearNoncleanCardWrapper::is_word_aligned(jbyte* entry) { 189 return (((intptr_t)entry) & (BytesPerWord-1)) == 0; 190 } 191 192 void ClearNoncleanCardWrapper::do_MemRegion(MemRegion mr) { 193 assert(mr.word_size() > 0, "Error"); 194 assert(_ct->is_aligned(mr.start()), "mr.start() should be card aligned"); 195 // mr.end() may not necessarily be card aligned. 196 jbyte* cur_entry = _ct->byte_for(mr.last()); 197 const jbyte* limit = _ct->byte_for(mr.start()); 198 HeapWord* end_of_non_clean = mr.end(); 199 HeapWord* start_of_non_clean = end_of_non_clean; 200 while (cur_entry >= limit) { 201 HeapWord* cur_hw = _ct->addr_for(cur_entry); 202 if ((*cur_entry != CardTableRS::clean_card_val()) && clear_card(cur_entry)) { 203 // Continue the dirty range by opening the 204 // dirty window one card to the left. 205 start_of_non_clean = cur_hw; 206 } else { 207 // We hit a "clean" card; process any non-empty 208 // "dirty" range accumulated so far. 209 if (start_of_non_clean < end_of_non_clean) { 210 const MemRegion mrd(start_of_non_clean, end_of_non_clean); 211 _dirty_card_closure->do_MemRegion(mrd); 212 } 213 214 // fast forward through potential continuous whole-word range of clean cards beginning at a word-boundary 215 if (is_word_aligned(cur_entry)) { 216 jbyte* cur_row = cur_entry - BytesPerWord; 217 while (cur_row >= limit && *((intptr_t*)cur_row) == CardTableRS::clean_card_row()) { 218 cur_row -= BytesPerWord; 219 } 220 cur_entry = cur_row + BytesPerWord; 221 cur_hw = _ct->addr_for(cur_entry); 222 } 223 224 // Reset the dirty window, while continuing to look 225 // for the next dirty card that will start a 226 // new dirty window. 227 end_of_non_clean = cur_hw; 228 start_of_non_clean = cur_hw; 229 } 230 // Note that "cur_entry" leads "start_of_non_clean" in 231 // its leftward excursion after this point 232 // in the loop and, when we hit the left end of "mr", 233 // will point off of the left end of the card-table 234 // for "mr". 235 cur_entry--; 236 } 237 // If the first card of "mr" was dirty, we will have 238 // been left with a dirty window, co-initial with "mr", 239 // which we now process. 240 if (start_of_non_clean < end_of_non_clean) { 241 const MemRegion mrd(start_of_non_clean, end_of_non_clean); 242 _dirty_card_closure->do_MemRegion(mrd); 243 } 244 } 245 246 // clean (by dirty->clean before) ==> cur_younger_gen 247 // dirty ==> cur_youngergen_and_prev_nonclean_card 248 // precleaned ==> cur_youngergen_and_prev_nonclean_card 249 // prev-younger-gen ==> cur_youngergen_and_prev_nonclean_card 250 // cur-younger-gen ==> cur_younger_gen 251 // cur_youngergen_and_prev_nonclean_card ==> no change. 252 void CardTableRS::write_ref_field_gc_par(void* field, oop new_val) { 253 jbyte* entry = ct_bs()->byte_for(field); 254 do { 255 jbyte entry_val = *entry; 256 // We put this first because it's probably the most common case. 257 if (entry_val == clean_card_val()) { 258 // No threat of contention with cleaning threads. 259 *entry = cur_youngergen_card_val(); 260 return; 261 } else if (card_is_dirty_wrt_gen_iter(entry_val) 262 || is_prev_youngergen_card_val(entry_val)) { 263 // Mark it as both cur and prev youngergen; card cleaning thread will 264 // eventually remove the previous stuff. 265 jbyte new_val = cur_youngergen_and_prev_nonclean_card; 266 jbyte res = Atomic::cmpxchg(new_val, entry, entry_val); 267 // Did the CAS succeed? 268 if (res == entry_val) return; 269 // Otherwise, retry, to see the new value. 270 continue; 271 } else { 272 assert(entry_val == cur_youngergen_and_prev_nonclean_card 273 || entry_val == cur_youngergen_card_val(), 274 "should be only possibilities."); 275 return; 276 } 277 } while (true); 278 } 279 280 void CardTableRS::younger_refs_in_space_iterate(Space* sp, 281 OopsInGenClosure* cl) { 282 const MemRegion urasm = sp->used_region_at_save_marks(); 283 #ifdef ASSERT 284 // Convert the assertion check to a warning if we are running 285 // CMS+ParNew until related bug is fixed. 286 MemRegion ur = sp->used_region(); 287 assert(ur.contains(urasm) || (UseConcMarkSweepGC && UseParNewGC), 288 err_msg("Did you forget to call save_marks()? " 289 "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in " 290 "[" PTR_FORMAT ", " PTR_FORMAT ")", 291 urasm.start(), urasm.end(), ur.start(), ur.end())); 292 // In the case of CMS+ParNew, issue a warning 293 if (!ur.contains(urasm)) { 294 assert(UseConcMarkSweepGC && UseParNewGC, "Tautology: see assert above"); 295 warning("CMS+ParNew: Did you forget to call save_marks()? " 296 "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in " 297 "[" PTR_FORMAT ", " PTR_FORMAT ")", 298 urasm.start(), urasm.end(), ur.start(), ur.end()); 299 MemRegion ur2 = sp->used_region(); 300 MemRegion urasm2 = sp->used_region_at_save_marks(); 301 if (!ur.equals(ur2)) { 302 warning("CMS+ParNew: Flickering used_region()!!"); 303 } 304 if (!urasm.equals(urasm2)) { 305 warning("CMS+ParNew: Flickering used_region_at_save_marks()!!"); 306 } 307 ShouldNotReachHere(); 308 } 309 #endif 310 _ct_bs->non_clean_card_iterate_possibly_parallel(sp, urasm, cl, this); 311 } 312 313 void CardTableRS::clear_into_younger(Generation* gen) { 314 GenCollectedHeap* gch = GenCollectedHeap::heap(); 315 // Generations younger than gen have been evacuated. We can clear 316 // card table entries for gen (we know that it has no pointers 317 // to younger gens) and for those below. The card tables for 318 // the youngest gen need never be cleared. 319 // There's a bit of subtlety in the clear() and invalidate() 320 // methods that we exploit here and in invalidate_or_clear() 321 // below to avoid missing cards at the fringes. If clear() or 322 // invalidate() are changed in the future, this code should 323 // be revisited. 20040107.ysr 324 Generation* old_gen = gen; 325 clear(old_gen->prev_used_region()); 326 Generation* young_gen = gch->prev_gen(old_gen); 327 clear(young_gen->prev_used_region()); 328 } 329 330 void CardTableRS::invalidate_or_clear(Generation* gen) { 331 // For generation gen invalidate the cards for the currently 332 // occupied part of that generation and clear the cards for the 333 // unoccupied part of the generation (if any, making use 334 // of that generation's prev_used_region to determine that 335 // region). No need to do anything for the youngest 336 // generation. Also see note#20040107.ysr above. 337 MemRegion used_mr = gen->used_region(); 338 MemRegion to_be_cleared_mr = gen->prev_used_region().minus(used_mr); 339 if (!to_be_cleared_mr.is_empty()) { 340 clear(to_be_cleared_mr); 341 } 342 invalidate(used_mr); 343 } 344 345 346 class VerifyCleanCardClosure: public OopClosure { 347 private: 348 HeapWord* _boundary; 349 HeapWord* _begin; 350 HeapWord* _end; 351 protected: 352 template <class T> void do_oop_work(T* p) { 353 HeapWord* jp = (HeapWord*)p; 354 assert(jp >= _begin && jp < _end, 355 err_msg("Error: jp " PTR_FORMAT " should be within " 356 "[_begin, _end) = [" PTR_FORMAT "," PTR_FORMAT ")", 357 jp, _begin, _end)); 358 oop obj = oopDesc::load_decode_heap_oop(p); 359 guarantee(obj == NULL || (HeapWord*)obj >= _boundary, 360 err_msg("pointer " PTR_FORMAT " at " PTR_FORMAT " on " 361 "clean card crosses boundary" PTR_FORMAT, 362 (HeapWord*)obj, jp, _boundary)); 363 } 364 365 public: 366 VerifyCleanCardClosure(HeapWord* b, HeapWord* begin, HeapWord* end) : 367 _boundary(b), _begin(begin), _end(end) { 368 assert(b <= begin, 369 err_msg("Error: boundary " PTR_FORMAT " should be at or below begin " PTR_FORMAT, 370 b, begin)); 371 assert(begin <= end, 372 err_msg("Error: begin " PTR_FORMAT " should be strictly below end " PTR_FORMAT, 373 begin, end)); 374 } 375 376 virtual void do_oop(oop* p) { VerifyCleanCardClosure::do_oop_work(p); } 377 virtual void do_oop(narrowOop* p) { VerifyCleanCardClosure::do_oop_work(p); } 378 }; 379 380 class VerifyCTSpaceClosure: public SpaceClosure { 381 private: 382 CardTableRS* _ct; 383 HeapWord* _boundary; 384 public: 385 VerifyCTSpaceClosure(CardTableRS* ct, HeapWord* boundary) : 386 _ct(ct), _boundary(boundary) {} 387 virtual void do_space(Space* s) { _ct->verify_space(s, _boundary); } 388 }; 389 390 class VerifyCTGenClosure: public GenCollectedHeap::GenClosure { 391 CardTableRS* _ct; 392 public: 393 VerifyCTGenClosure(CardTableRS* ct) : _ct(ct) {} 394 void do_generation(Generation* gen) { 395 // Skip the youngest generation. 396 if (gen->level() == 0) return; 397 // Normally, we're interested in pointers to younger generations. 398 VerifyCTSpaceClosure blk(_ct, gen->reserved().start()); 399 gen->space_iterate(&blk, true); 400 } 401 }; 402 403 void CardTableRS::verify_space(Space* s, HeapWord* gen_boundary) { 404 // We don't need to do young-gen spaces. 405 if (s->end() <= gen_boundary) return; 406 MemRegion used = s->used_region(); 407 408 jbyte* cur_entry = byte_for(used.start()); 409 jbyte* limit = byte_after(used.last()); 410 while (cur_entry < limit) { 411 if (*cur_entry == CardTableModRefBS::clean_card) { 412 jbyte* first_dirty = cur_entry+1; 413 while (first_dirty < limit && 414 *first_dirty == CardTableModRefBS::clean_card) { 415 first_dirty++; 416 } 417 // If the first object is a regular object, and it has a 418 // young-to-old field, that would mark the previous card. 419 HeapWord* boundary = addr_for(cur_entry); 420 HeapWord* end = (first_dirty >= limit) ? used.end() : addr_for(first_dirty); 421 HeapWord* boundary_block = s->block_start(boundary); 422 HeapWord* begin = boundary; // Until proven otherwise. 423 HeapWord* start_block = boundary_block; // Until proven otherwise. 424 if (boundary_block < boundary) { 425 if (s->block_is_obj(boundary_block) && s->obj_is_alive(boundary_block)) { 426 oop boundary_obj = oop(boundary_block); 427 if (!boundary_obj->is_objArray() && 428 !boundary_obj->is_typeArray()) { 429 guarantee(cur_entry > byte_for(used.start()), 430 "else boundary would be boundary_block"); 431 if (*byte_for(boundary_block) != CardTableModRefBS::clean_card) { 432 begin = boundary_block + s->block_size(boundary_block); 433 start_block = begin; 434 } 435 } 436 } 437 } 438 // Now traverse objects until end. 439 if (begin < end) { 440 MemRegion mr(begin, end); 441 VerifyCleanCardClosure verify_blk(gen_boundary, begin, end); 442 for (HeapWord* cur = start_block; cur < end; cur += s->block_size(cur)) { 443 if (s->block_is_obj(cur) && s->obj_is_alive(cur)) { 444 oop(cur)->oop_iterate_no_header(&verify_blk, mr); 445 } 446 } 447 } 448 cur_entry = first_dirty; 449 } else { 450 // We'd normally expect that cur_youngergen_and_prev_nonclean_card 451 // is a transient value, that cannot be in the card table 452 // except during GC, and thus assert that: 453 // guarantee(*cur_entry != cur_youngergen_and_prev_nonclean_card, 454 // "Illegal CT value"); 455 // That however, need not hold, as will become clear in the 456 // following... 457 458 // We'd normally expect that if we are in the parallel case, 459 // we can't have left a prev value (which would be different 460 // from the current value) in the card table, and so we'd like to 461 // assert that: 462 // guarantee(cur_youngergen_card_val() == youngergen_card 463 // || !is_prev_youngergen_card_val(*cur_entry), 464 // "Illegal CT value"); 465 // That, however, may not hold occasionally, because of 466 // CMS or MSC in the old gen. To wit, consider the 467 // following two simple illustrative scenarios: 468 // (a) CMS: Consider the case where a large object L 469 // spanning several cards is allocated in the old 470 // gen, and has a young gen reference stored in it, dirtying 471 // some interior cards. A young collection scans the card, 472 // finds a young ref and installs a youngergenP_n value. 473 // L then goes dead. Now a CMS collection starts, 474 // finds L dead and sweeps it up. Assume that L is 475 // abutting _unallocated_blk, so _unallocated_blk is 476 // adjusted down to (below) L. Assume further that 477 // no young collection intervenes during this CMS cycle. 478 // The next young gen cycle will not get to look at this 479 // youngergenP_n card since it lies in the unoccupied 480 // part of the space. 481 // Some young collections later the blocks on this 482 // card can be re-allocated either due to direct allocation 483 // or due to absorbing promotions. At this time, the 484 // before-gc verification will fail the above assert. 485 // (b) MSC: In this case, an object L with a young reference 486 // is on a card that (therefore) holds a youngergen_n value. 487 // Suppose also that L lies towards the end of the used 488 // the used space before GC. An MSC collection 489 // occurs that compacts to such an extent that this 490 // card is no longer in the occupied part of the space. 491 // Since current code in MSC does not always clear cards 492 // in the unused part of old gen, this stale youngergen_n 493 // value is left behind and can later be covered by 494 // an object when promotion or direct allocation 495 // re-allocates that part of the heap. 496 // 497 // Fortunately, the presence of such stale card values is 498 // "only" a minor annoyance in that subsequent young collections 499 // might needlessly scan such cards, but would still never corrupt 500 // the heap as a result. However, it's likely not to be a significant 501 // performance inhibitor in practice. For instance, 502 // some recent measurements with unoccupied cards eagerly cleared 503 // out to maintain this invariant, showed next to no 504 // change in young collection times; of course one can construct 505 // degenerate examples where the cost can be significant.) 506 // Note, in particular, that if the "stale" card is modified 507 // after re-allocation, it would be dirty, not "stale". Thus, 508 // we can never have a younger ref in such a card and it is 509 // safe not to scan that card in any collection. [As we see 510 // below, we do some unnecessary scanning 511 // in some cases in the current parallel scanning algorithm.] 512 // 513 // The main point below is that the parallel card scanning code 514 // deals correctly with these stale card values. There are two main 515 // cases to consider where we have a stale "younger gen" value and a 516 // "derivative" case to consider, where we have a stale 517 // "cur_younger_gen_and_prev_non_clean" value, as will become 518 // apparent in the case analysis below. 519 // o Case 1. If the stale value corresponds to a younger_gen_n 520 // value other than the cur_younger_gen value then the code 521 // treats this as being tantamount to a prev_younger_gen 522 // card. This means that the card may be unnecessarily scanned. 523 // There are two sub-cases to consider: 524 // o Case 1a. Let us say that the card is in the occupied part 525 // of the generation at the time the collection begins. In 526 // that case the card will be either cleared when it is scanned 527 // for young pointers, or will be set to cur_younger_gen as a 528 // result of promotion. (We have elided the normal case where 529 // the scanning thread and the promoting thread interleave 530 // possibly resulting in a transient 531 // cur_younger_gen_and_prev_non_clean value before settling 532 // to cur_younger_gen. [End Case 1a.] 533 // o Case 1b. Consider now the case when the card is in the unoccupied 534 // part of the space which becomes occupied because of promotions 535 // into it during the current young GC. In this case the card 536 // will never be scanned for young references. The current 537 // code will set the card value to either 538 // cur_younger_gen_and_prev_non_clean or leave 539 // it with its stale value -- because the promotions didn't 540 // result in any younger refs on that card. Of these two 541 // cases, the latter will be covered in Case 1a during 542 // a subsequent scan. To deal with the former case, we need 543 // to further consider how we deal with a stale value of 544 // cur_younger_gen_and_prev_non_clean in our case analysis 545 // below. This we do in Case 3 below. [End Case 1b] 546 // [End Case 1] 547 // o Case 2. If the stale value corresponds to cur_younger_gen being 548 // a value not necessarily written by a current promotion, the 549 // card will not be scanned by the younger refs scanning code. 550 // (This is OK since as we argued above such cards cannot contain 551 // any younger refs.) The result is that this value will be 552 // treated as a prev_younger_gen value in a subsequent collection, 553 // which is addressed in Case 1 above. [End Case 2] 554 // o Case 3. We here consider the "derivative" case from Case 1b. above 555 // because of which we may find a stale 556 // cur_younger_gen_and_prev_non_clean card value in the table. 557 // Once again, as in Case 1, we consider two subcases, depending 558 // on whether the card lies in the occupied or unoccupied part 559 // of the space at the start of the young collection. 560 // o Case 3a. Let us say the card is in the occupied part of 561 // the old gen at the start of the young collection. In that 562 // case, the card will be scanned by the younger refs scanning 563 // code which will set it to cur_younger_gen. In a subsequent 564 // scan, the card will be considered again and get its final 565 // correct value. [End Case 3a] 566 // o Case 3b. Now consider the case where the card is in the 567 // unoccupied part of the old gen, and is occupied as a result 568 // of promotions during thus young gc. In that case, 569 // the card will not be scanned for younger refs. The presence 570 // of newly promoted objects on the card will then result in 571 // its keeping the value cur_younger_gen_and_prev_non_clean 572 // value, which we have dealt with in Case 3 here. [End Case 3b] 573 // [End Case 3] 574 // 575 // (Please refer to the code in the helper class 576 // ClearNonCleanCardWrapper and in CardTableModRefBS for details.) 577 // 578 // The informal arguments above can be tightened into a formal 579 // correctness proof and it behooves us to write up such a proof, 580 // or to use model checking to prove that there are no lingering 581 // concerns. 582 // 583 // Clearly because of Case 3b one cannot bound the time for 584 // which a card will retain what we have called a "stale" value. 585 // However, one can obtain a Loose upper bound on the redundant 586 // work as a result of such stale values. Note first that any 587 // time a stale card lies in the occupied part of the space at 588 // the start of the collection, it is scanned by younger refs 589 // code and we can define a rank function on card values that 590 // declines when this is so. Note also that when a card does not 591 // lie in the occupied part of the space at the beginning of a 592 // young collection, its rank can either decline or stay unchanged. 593 // In this case, no extra work is done in terms of redundant 594 // younger refs scanning of that card. 595 // Then, the case analysis above reveals that, in the worst case, 596 // any such stale card will be scanned unnecessarily at most twice. 597 // 598 // It is nonethelss advisable to try and get rid of some of this 599 // redundant work in a subsequent (low priority) re-design of 600 // the card-scanning code, if only to simplify the underlying 601 // state machine analysis/proof. ysr 1/28/2002. XXX 602 cur_entry++; 603 } 604 } 605 } 606 607 void CardTableRS::verify() { 608 // At present, we only know how to verify the card table RS for 609 // generational heaps. 610 VerifyCTGenClosure blk(this); 611 CollectedHeap* ch = Universe::heap(); 612 613 if (ch->kind() == CollectedHeap::GenCollectedHeap) { 614 GenCollectedHeap::heap()->generation_iterate(&blk, false); 615 _ct_bs->verify(); 616 } 617 } 618 619 620 void CardTableRS::verify_aligned_region_empty(MemRegion mr) { 621 if (!mr.is_empty()) { 622 jbyte* cur_entry = byte_for(mr.start()); 623 jbyte* limit = byte_after(mr.last()); 624 // The region mr may not start on a card boundary so 625 // the first card may reflect a write to the space 626 // just prior to mr. 627 if (!is_aligned(mr.start())) { 628 cur_entry++; 629 } 630 for (;cur_entry < limit; cur_entry++) { 631 guarantee(*cur_entry == CardTableModRefBS::clean_card, 632 "Unexpected dirty card found"); 633 } 634 } 635 }