1 /* 2 * Copyright (c) 2001, 2014, Oracle and/or its affiliates. All rights reserved. 3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 4 * 5 * This code is free software; you can redistribute it and/or modify it 6 * under the terms of the GNU General Public License version 2 only, as 7 * published by the Free Software Foundation. 8 * 9 * This code is distributed in the hope that it will be useful, but WITHOUT 10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 12 * version 2 for more details (a copy is included in the LICENSE file that 13 * accompanied this code). 14 * 15 * You should have received a copy of the GNU General Public License version 16 * 2 along with this work; if not, write to the Free Software Foundation, 17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 18 * 19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 20 * or visit www.oracle.com if you need additional information or have any 21 * questions. 22 * 23 */ 24 25 #include "precompiled.hpp" 26 #include "memory/allocation.inline.hpp" 27 #include "memory/cardTableRS.hpp" 28 #include "memory/genCollectedHeap.hpp" 29 #include "memory/generation.hpp" 30 #include "memory/space.hpp" 31 #include "oops/oop.inline.hpp" 32 #include "runtime/atomic.inline.hpp" 33 #include "runtime/java.hpp" 34 #include "runtime/os.hpp" 35 #include "utilities/macros.hpp" 36 #if INCLUDE_ALL_GCS 37 #include "gc_implementation/g1/concurrentMark.hpp" 38 #include "gc_implementation/g1/g1SATBCardTableModRefBS.hpp" 39 #endif // INCLUDE_ALL_GCS 40 41 CardTableRS::CardTableRS(MemRegion whole_heap, 42 int max_covered_regions) : 43 GenRemSet(), 44 _cur_youngergen_card_val(youngergenP1_card), 45 _regions_to_iterate(max_covered_regions - 1) 46 { 47 #if INCLUDE_ALL_GCS 48 if (UseG1GC) { 49 _ct_bs = new G1SATBCardTableLoggingModRefBS(whole_heap, 50 max_covered_regions); 51 } else { 52 _ct_bs = new CardTableModRefBSForCTRS(whole_heap, max_covered_regions); 53 } 54 #else 55 _ct_bs = new CardTableModRefBSForCTRS(whole_heap, max_covered_regions); 56 #endif 57 _ct_bs->initialize(); 58 set_bs(_ct_bs); 59 _last_cur_val_in_gen = NEW_C_HEAP_ARRAY3(jbyte, GenCollectedHeap::max_gens + 1, 60 mtGC, 0, AllocFailStrategy::RETURN_NULL); 61 if (_last_cur_val_in_gen == NULL) { 62 vm_exit_during_initialization("Could not create last_cur_val_in_gen array."); 63 } 64 for (int i = 0; i < GenCollectedHeap::max_gens + 1; i++) { 65 _last_cur_val_in_gen[i] = clean_card_val(); 66 } 67 _ct_bs->set_CTRS(this); 68 } 69 70 CardTableRS::~CardTableRS() { 71 if (_ct_bs) { 72 delete _ct_bs; 73 _ct_bs = NULL; 74 } 75 if (_last_cur_val_in_gen) { 76 FREE_C_HEAP_ARRAY(jbyte, _last_cur_val_in_gen, mtInternal); 77 } 78 } 79 80 void CardTableRS::resize_covered_region(MemRegion new_region) { 81 _ct_bs->resize_covered_region(new_region); 82 } 83 84 jbyte CardTableRS::find_unused_youngergenP_card_value() { 85 for (jbyte v = youngergenP1_card; 86 v < cur_youngergen_and_prev_nonclean_card; 87 v++) { 88 bool seen = false; 89 for (int g = 0; g < _regions_to_iterate; g++) { 90 if (_last_cur_val_in_gen[g] == v) { 91 seen = true; 92 break; 93 } 94 } 95 if (!seen) return v; 96 } 97 ShouldNotReachHere(); 98 return 0; 99 } 100 101 void CardTableRS::prepare_for_younger_refs_iterate(bool parallel) { 102 // Parallel or sequential, we must always set the prev to equal the 103 // last one written. 104 if (parallel) { 105 // Find a parallel value to be used next. 106 jbyte next_val = find_unused_youngergenP_card_value(); 107 set_cur_youngergen_card_val(next_val); 108 109 } else { 110 // In an sequential traversal we will always write youngergen, so that 111 // the inline barrier is correct. 112 set_cur_youngergen_card_val(youngergen_card); 113 } 114 } 115 116 void CardTableRS::younger_refs_iterate(Generation* g, 117 OopsInGenClosure* blk) { 118 _last_cur_val_in_gen[g->level()+1] = cur_youngergen_card_val(); 119 g->younger_refs_iterate(blk); 120 } 121 122 inline bool ClearNoncleanCardWrapper::clear_card(jbyte* entry) { 123 if (_is_par) { 124 return clear_card_parallel(entry); 125 } else { 126 return clear_card_serial(entry); 127 } 128 } 129 130 inline bool ClearNoncleanCardWrapper::clear_card_parallel(jbyte* entry) { 131 while (true) { 132 // In the parallel case, we may have to do this several times. 133 jbyte entry_val = *entry; 134 assert(entry_val != CardTableRS::clean_card_val(), 135 "We shouldn't be looking at clean cards, and this should " 136 "be the only place they get cleaned."); 137 if (CardTableRS::card_is_dirty_wrt_gen_iter(entry_val) 138 || _ct->is_prev_youngergen_card_val(entry_val)) { 139 jbyte res = 140 Atomic::cmpxchg(CardTableRS::clean_card_val(), entry, entry_val); 141 if (res == entry_val) { 142 break; 143 } else { 144 assert(res == CardTableRS::cur_youngergen_and_prev_nonclean_card, 145 "The CAS above should only fail if another thread did " 146 "a GC write barrier."); 147 } 148 } else if (entry_val == 149 CardTableRS::cur_youngergen_and_prev_nonclean_card) { 150 // Parallelism shouldn't matter in this case. Only the thread 151 // assigned to scan the card should change this value. 152 *entry = _ct->cur_youngergen_card_val(); 153 break; 154 } else { 155 assert(entry_val == _ct->cur_youngergen_card_val(), 156 "Should be the only possibility."); 157 // In this case, the card was clean before, and become 158 // cur_youngergen only because of processing of a promoted object. 159 // We don't have to look at the card. 160 return false; 161 } 162 } 163 return true; 164 } 165 166 167 inline bool ClearNoncleanCardWrapper::clear_card_serial(jbyte* entry) { 168 jbyte entry_val = *entry; 169 assert(entry_val != CardTableRS::clean_card_val(), 170 "We shouldn't be looking at clean cards, and this should " 171 "be the only place they get cleaned."); 172 assert(entry_val != CardTableRS::cur_youngergen_and_prev_nonclean_card, 173 "This should be possible in the sequential case."); 174 *entry = CardTableRS::clean_card_val(); 175 return true; 176 } 177 178 ClearNoncleanCardWrapper::ClearNoncleanCardWrapper( 179 DirtyCardToOopClosure* dirty_card_closure, CardTableRS* ct) : 180 _dirty_card_closure(dirty_card_closure), _ct(ct) { 181 // Cannot yet substitute active_workers for n_par_threads 182 // in the case where parallelism is being turned off by 183 // setting n_par_threads to 0. 184 _is_par = (SharedHeap::heap()->n_par_threads() > 0); 185 assert(!_is_par || 186 (SharedHeap::heap()->n_par_threads() == 187 SharedHeap::heap()->workers()->active_workers()), "Mismatch"); 188 } 189 190 bool ClearNoncleanCardWrapper::is_word_aligned(jbyte* entry) { 191 return (((intptr_t)entry) & (BytesPerWord-1)) == 0; 192 } 193 194 void ClearNoncleanCardWrapper::do_MemRegion(MemRegion mr) { 195 assert(mr.word_size() > 0, "Error"); 196 assert(_ct->is_aligned(mr.start()), "mr.start() should be card aligned"); 197 // mr.end() may not necessarily be card aligned. 198 jbyte* cur_entry = _ct->byte_for(mr.last()); 199 const jbyte* limit = _ct->byte_for(mr.start()); 200 HeapWord* end_of_non_clean = mr.end(); 201 HeapWord* start_of_non_clean = end_of_non_clean; 202 while (cur_entry >= limit) { 203 HeapWord* cur_hw = _ct->addr_for(cur_entry); 204 if ((*cur_entry != CardTableRS::clean_card_val()) && clear_card(cur_entry)) { 205 // Continue the dirty range by opening the 206 // dirty window one card to the left. 207 start_of_non_clean = cur_hw; 208 } else { 209 // We hit a "clean" card; process any non-empty 210 // "dirty" range accumulated so far. 211 if (start_of_non_clean < end_of_non_clean) { 212 const MemRegion mrd(start_of_non_clean, end_of_non_clean); 213 _dirty_card_closure->do_MemRegion(mrd); 214 } 215 216 // fast forward through potential continuous whole-word range of clean cards beginning at a word-boundary 217 if (is_word_aligned(cur_entry)) { 218 jbyte* cur_row = cur_entry - BytesPerWord; 219 while (cur_row >= limit && *((intptr_t*)cur_row) == CardTableRS::clean_card_row()) { 220 cur_row -= BytesPerWord; 221 } 222 cur_entry = cur_row + BytesPerWord; 223 cur_hw = _ct->addr_for(cur_entry); 224 } 225 226 // Reset the dirty window, while continuing to look 227 // for the next dirty card that will start a 228 // new dirty window. 229 end_of_non_clean = cur_hw; 230 start_of_non_clean = cur_hw; 231 } 232 // Note that "cur_entry" leads "start_of_non_clean" in 233 // its leftward excursion after this point 234 // in the loop and, when we hit the left end of "mr", 235 // will point off of the left end of the card-table 236 // for "mr". 237 cur_entry--; 238 } 239 // If the first card of "mr" was dirty, we will have 240 // been left with a dirty window, co-initial with "mr", 241 // which we now process. 242 if (start_of_non_clean < end_of_non_clean) { 243 const MemRegion mrd(start_of_non_clean, end_of_non_clean); 244 _dirty_card_closure->do_MemRegion(mrd); 245 } 246 } 247 248 // clean (by dirty->clean before) ==> cur_younger_gen 249 // dirty ==> cur_youngergen_and_prev_nonclean_card 250 // precleaned ==> cur_youngergen_and_prev_nonclean_card 251 // prev-younger-gen ==> cur_youngergen_and_prev_nonclean_card 252 // cur-younger-gen ==> cur_younger_gen 253 // cur_youngergen_and_prev_nonclean_card ==> no change. 254 void CardTableRS::write_ref_field_gc_par(void* field, oop new_val) { 255 jbyte* entry = ct_bs()->byte_for(field); 256 do { 257 jbyte entry_val = *entry; 258 // We put this first because it's probably the most common case. 259 if (entry_val == clean_card_val()) { 260 // No threat of contention with cleaning threads. 261 *entry = cur_youngergen_card_val(); 262 return; 263 } else if (card_is_dirty_wrt_gen_iter(entry_val) 264 || is_prev_youngergen_card_val(entry_val)) { 265 // Mark it as both cur and prev youngergen; card cleaning thread will 266 // eventually remove the previous stuff. 267 jbyte new_val = cur_youngergen_and_prev_nonclean_card; 268 jbyte res = Atomic::cmpxchg(new_val, entry, entry_val); 269 // Did the CAS succeed? 270 if (res == entry_val) return; 271 // Otherwise, retry, to see the new value. 272 continue; 273 } else { 274 assert(entry_val == cur_youngergen_and_prev_nonclean_card 275 || entry_val == cur_youngergen_card_val(), 276 "should be only possibilities."); 277 return; 278 } 279 } while (true); 280 } 281 282 void CardTableRS::younger_refs_in_space_iterate(Space* sp, 283 OopsInGenClosure* cl) { 284 const MemRegion urasm = sp->used_region_at_save_marks(); 285 #ifdef ASSERT 286 // Convert the assertion check to a warning if we are running 287 // CMS+ParNew until related bug is fixed. 288 MemRegion ur = sp->used_region(); 289 assert(ur.contains(urasm) || (UseConcMarkSweepGC && UseParNewGC), 290 err_msg("Did you forget to call save_marks()? " 291 "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in " 292 "[" PTR_FORMAT ", " PTR_FORMAT ")", 293 p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end()))); 294 // In the case of CMS+ParNew, issue a warning 295 if (!ur.contains(urasm)) { 296 assert(UseConcMarkSweepGC && UseParNewGC, "Tautology: see assert above"); 297 warning("CMS+ParNew: Did you forget to call save_marks()? " 298 "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in " 299 "[" PTR_FORMAT ", " PTR_FORMAT ")", 300 p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end())); 301 MemRegion ur2 = sp->used_region(); 302 MemRegion urasm2 = sp->used_region_at_save_marks(); 303 if (!ur.equals(ur2)) { 304 warning("CMS+ParNew: Flickering used_region()!!"); 305 } 306 if (!urasm.equals(urasm2)) { 307 warning("CMS+ParNew: Flickering used_region_at_save_marks()!!"); 308 } 309 ShouldNotReachHere(); 310 } 311 #endif 312 _ct_bs->non_clean_card_iterate_possibly_parallel(sp, urasm, cl, this); 313 } 314 315 void CardTableRS::clear_into_younger(Generation* old_gen) { 316 assert(old_gen->level() == 1, "Should only be called for the old generation"); 317 // The card tables for the youngest gen need never be cleared. 318 // There's a bit of subtlety in the clear() and invalidate() 319 // methods that we exploit here and in invalidate_or_clear() 320 // below to avoid missing cards at the fringes. If clear() or 321 // invalidate() are changed in the future, this code should 322 // be revisited. 20040107.ysr 323 clear(old_gen->prev_used_region()); 324 } 325 326 void CardTableRS::invalidate_or_clear(Generation* old_gen) { 327 assert(old_gen->level() == 1, "Should only be called for the old generation"); 328 // Invalidate the cards for the currently occupied part of 329 // the old generation and clear the cards for the 330 // unoccupied part of the generation (if any, making use 331 // of that generation's prev_used_region to determine that 332 // region). No need to do anything for the youngest 333 // generation. Also see note#20040107.ysr above. 334 MemRegion used_mr = old_gen->used_region(); 335 MemRegion to_be_cleared_mr = old_gen->prev_used_region().minus(used_mr); 336 if (!to_be_cleared_mr.is_empty()) { 337 clear(to_be_cleared_mr); 338 } 339 invalidate(used_mr); 340 } 341 342 343 class VerifyCleanCardClosure: public OopClosure { 344 private: 345 HeapWord* _boundary; 346 HeapWord* _begin; 347 HeapWord* _end; 348 protected: 349 template <class T> void do_oop_work(T* p) { 350 HeapWord* jp = (HeapWord*)p; 351 assert(jp >= _begin && jp < _end, 352 err_msg("Error: jp " PTR_FORMAT " should be within " 353 "[_begin, _end) = [" PTR_FORMAT "," PTR_FORMAT ")", 354 p2i(jp), p2i(_begin), p2i(_end))); 355 oop obj = oopDesc::load_decode_heap_oop(p); 356 guarantee(obj == NULL || (HeapWord*)obj >= _boundary, 357 err_msg("pointer " PTR_FORMAT " at " PTR_FORMAT " on " 358 "clean card crosses boundary" PTR_FORMAT, 359 p2i((HeapWord*)obj), p2i(jp), p2i(_boundary))); 360 } 361 362 public: 363 VerifyCleanCardClosure(HeapWord* b, HeapWord* begin, HeapWord* end) : 364 _boundary(b), _begin(begin), _end(end) { 365 assert(b <= begin, 366 err_msg("Error: boundary " PTR_FORMAT " should be at or below begin " PTR_FORMAT, 367 p2i(b), p2i(begin))); 368 assert(begin <= end, 369 err_msg("Error: begin " PTR_FORMAT " should be strictly below end " PTR_FORMAT, 370 p2i(begin), p2i(end))); 371 } 372 373 virtual void do_oop(oop* p) { VerifyCleanCardClosure::do_oop_work(p); } 374 virtual void do_oop(narrowOop* p) { VerifyCleanCardClosure::do_oop_work(p); } 375 }; 376 377 class VerifyCTSpaceClosure: public SpaceClosure { 378 private: 379 CardTableRS* _ct; 380 HeapWord* _boundary; 381 public: 382 VerifyCTSpaceClosure(CardTableRS* ct, HeapWord* boundary) : 383 _ct(ct), _boundary(boundary) {} 384 virtual void do_space(Space* s) { _ct->verify_space(s, _boundary); } 385 }; 386 387 class VerifyCTGenClosure: public GenCollectedHeap::GenClosure { 388 CardTableRS* _ct; 389 public: 390 VerifyCTGenClosure(CardTableRS* ct) : _ct(ct) {} 391 void do_generation(Generation* gen) { 392 // Skip the youngest generation. 393 if (gen->level() == 0) return; 394 // Normally, we're interested in pointers to younger generations. 395 VerifyCTSpaceClosure blk(_ct, gen->reserved().start()); 396 gen->space_iterate(&blk, true); 397 } 398 }; 399 400 void CardTableRS::verify_space(Space* s, HeapWord* gen_boundary) { 401 // We don't need to do young-gen spaces. 402 if (s->end() <= gen_boundary) return; 403 MemRegion used = s->used_region(); 404 405 jbyte* cur_entry = byte_for(used.start()); 406 jbyte* limit = byte_after(used.last()); 407 while (cur_entry < limit) { 408 if (*cur_entry == CardTableModRefBS::clean_card) { 409 jbyte* first_dirty = cur_entry+1; 410 while (first_dirty < limit && 411 *first_dirty == CardTableModRefBS::clean_card) { 412 first_dirty++; 413 } 414 // If the first object is a regular object, and it has a 415 // young-to-old field, that would mark the previous card. 416 HeapWord* boundary = addr_for(cur_entry); 417 HeapWord* end = (first_dirty >= limit) ? used.end() : addr_for(first_dirty); 418 HeapWord* boundary_block = s->block_start(boundary); 419 HeapWord* begin = boundary; // Until proven otherwise. 420 HeapWord* start_block = boundary_block; // Until proven otherwise. 421 if (boundary_block < boundary) { 422 if (s->block_is_obj(boundary_block) && s->obj_is_alive(boundary_block)) { 423 oop boundary_obj = oop(boundary_block); 424 if (!boundary_obj->is_objArray() && 425 !boundary_obj->is_typeArray()) { 426 guarantee(cur_entry > byte_for(used.start()), 427 "else boundary would be boundary_block"); 428 if (*byte_for(boundary_block) != CardTableModRefBS::clean_card) { 429 begin = boundary_block + s->block_size(boundary_block); 430 start_block = begin; 431 } 432 } 433 } 434 } 435 // Now traverse objects until end. 436 if (begin < end) { 437 MemRegion mr(begin, end); 438 VerifyCleanCardClosure verify_blk(gen_boundary, begin, end); 439 for (HeapWord* cur = start_block; cur < end; cur += s->block_size(cur)) { 440 if (s->block_is_obj(cur) && s->obj_is_alive(cur)) { 441 oop(cur)->oop_iterate_no_header(&verify_blk, mr); 442 } 443 } 444 } 445 cur_entry = first_dirty; 446 } else { 447 // We'd normally expect that cur_youngergen_and_prev_nonclean_card 448 // is a transient value, that cannot be in the card table 449 // except during GC, and thus assert that: 450 // guarantee(*cur_entry != cur_youngergen_and_prev_nonclean_card, 451 // "Illegal CT value"); 452 // That however, need not hold, as will become clear in the 453 // following... 454 455 // We'd normally expect that if we are in the parallel case, 456 // we can't have left a prev value (which would be different 457 // from the current value) in the card table, and so we'd like to 458 // assert that: 459 // guarantee(cur_youngergen_card_val() == youngergen_card 460 // || !is_prev_youngergen_card_val(*cur_entry), 461 // "Illegal CT value"); 462 // That, however, may not hold occasionally, because of 463 // CMS or MSC in the old gen. To wit, consider the 464 // following two simple illustrative scenarios: 465 // (a) CMS: Consider the case where a large object L 466 // spanning several cards is allocated in the old 467 // gen, and has a young gen reference stored in it, dirtying 468 // some interior cards. A young collection scans the card, 469 // finds a young ref and installs a youngergenP_n value. 470 // L then goes dead. Now a CMS collection starts, 471 // finds L dead and sweeps it up. Assume that L is 472 // abutting _unallocated_blk, so _unallocated_blk is 473 // adjusted down to (below) L. Assume further that 474 // no young collection intervenes during this CMS cycle. 475 // The next young gen cycle will not get to look at this 476 // youngergenP_n card since it lies in the unoccupied 477 // part of the space. 478 // Some young collections later the blocks on this 479 // card can be re-allocated either due to direct allocation 480 // or due to absorbing promotions. At this time, the 481 // before-gc verification will fail the above assert. 482 // (b) MSC: In this case, an object L with a young reference 483 // is on a card that (therefore) holds a youngergen_n value. 484 // Suppose also that L lies towards the end of the used 485 // the used space before GC. An MSC collection 486 // occurs that compacts to such an extent that this 487 // card is no longer in the occupied part of the space. 488 // Since current code in MSC does not always clear cards 489 // in the unused part of old gen, this stale youngergen_n 490 // value is left behind and can later be covered by 491 // an object when promotion or direct allocation 492 // re-allocates that part of the heap. 493 // 494 // Fortunately, the presence of such stale card values is 495 // "only" a minor annoyance in that subsequent young collections 496 // might needlessly scan such cards, but would still never corrupt 497 // the heap as a result. However, it's likely not to be a significant 498 // performance inhibitor in practice. For instance, 499 // some recent measurements with unoccupied cards eagerly cleared 500 // out to maintain this invariant, showed next to no 501 // change in young collection times; of course one can construct 502 // degenerate examples where the cost can be significant.) 503 // Note, in particular, that if the "stale" card is modified 504 // after re-allocation, it would be dirty, not "stale". Thus, 505 // we can never have a younger ref in such a card and it is 506 // safe not to scan that card in any collection. [As we see 507 // below, we do some unnecessary scanning 508 // in some cases in the current parallel scanning algorithm.] 509 // 510 // The main point below is that the parallel card scanning code 511 // deals correctly with these stale card values. There are two main 512 // cases to consider where we have a stale "younger gen" value and a 513 // "derivative" case to consider, where we have a stale 514 // "cur_younger_gen_and_prev_non_clean" value, as will become 515 // apparent in the case analysis below. 516 // o Case 1. If the stale value corresponds to a younger_gen_n 517 // value other than the cur_younger_gen value then the code 518 // treats this as being tantamount to a prev_younger_gen 519 // card. This means that the card may be unnecessarily scanned. 520 // There are two sub-cases to consider: 521 // o Case 1a. Let us say that the card is in the occupied part 522 // of the generation at the time the collection begins. In 523 // that case the card will be either cleared when it is scanned 524 // for young pointers, or will be set to cur_younger_gen as a 525 // result of promotion. (We have elided the normal case where 526 // the scanning thread and the promoting thread interleave 527 // possibly resulting in a transient 528 // cur_younger_gen_and_prev_non_clean value before settling 529 // to cur_younger_gen. [End Case 1a.] 530 // o Case 1b. Consider now the case when the card is in the unoccupied 531 // part of the space which becomes occupied because of promotions 532 // into it during the current young GC. In this case the card 533 // will never be scanned for young references. The current 534 // code will set the card value to either 535 // cur_younger_gen_and_prev_non_clean or leave 536 // it with its stale value -- because the promotions didn't 537 // result in any younger refs on that card. Of these two 538 // cases, the latter will be covered in Case 1a during 539 // a subsequent scan. To deal with the former case, we need 540 // to further consider how we deal with a stale value of 541 // cur_younger_gen_and_prev_non_clean in our case analysis 542 // below. This we do in Case 3 below. [End Case 1b] 543 // [End Case 1] 544 // o Case 2. If the stale value corresponds to cur_younger_gen being 545 // a value not necessarily written by a current promotion, the 546 // card will not be scanned by the younger refs scanning code. 547 // (This is OK since as we argued above such cards cannot contain 548 // any younger refs.) The result is that this value will be 549 // treated as a prev_younger_gen value in a subsequent collection, 550 // which is addressed in Case 1 above. [End Case 2] 551 // o Case 3. We here consider the "derivative" case from Case 1b. above 552 // because of which we may find a stale 553 // cur_younger_gen_and_prev_non_clean card value in the table. 554 // Once again, as in Case 1, we consider two subcases, depending 555 // on whether the card lies in the occupied or unoccupied part 556 // of the space at the start of the young collection. 557 // o Case 3a. Let us say the card is in the occupied part of 558 // the old gen at the start of the young collection. In that 559 // case, the card will be scanned by the younger refs scanning 560 // code which will set it to cur_younger_gen. In a subsequent 561 // scan, the card will be considered again and get its final 562 // correct value. [End Case 3a] 563 // o Case 3b. Now consider the case where the card is in the 564 // unoccupied part of the old gen, and is occupied as a result 565 // of promotions during thus young gc. In that case, 566 // the card will not be scanned for younger refs. The presence 567 // of newly promoted objects on the card will then result in 568 // its keeping the value cur_younger_gen_and_prev_non_clean 569 // value, which we have dealt with in Case 3 here. [End Case 3b] 570 // [End Case 3] 571 // 572 // (Please refer to the code in the helper class 573 // ClearNonCleanCardWrapper and in CardTableModRefBS for details.) 574 // 575 // The informal arguments above can be tightened into a formal 576 // correctness proof and it behooves us to write up such a proof, 577 // or to use model checking to prove that there are no lingering 578 // concerns. 579 // 580 // Clearly because of Case 3b one cannot bound the time for 581 // which a card will retain what we have called a "stale" value. 582 // However, one can obtain a Loose upper bound on the redundant 583 // work as a result of such stale values. Note first that any 584 // time a stale card lies in the occupied part of the space at 585 // the start of the collection, it is scanned by younger refs 586 // code and we can define a rank function on card values that 587 // declines when this is so. Note also that when a card does not 588 // lie in the occupied part of the space at the beginning of a 589 // young collection, its rank can either decline or stay unchanged. 590 // In this case, no extra work is done in terms of redundant 591 // younger refs scanning of that card. 592 // Then, the case analysis above reveals that, in the worst case, 593 // any such stale card will be scanned unnecessarily at most twice. 594 // 595 // It is nonetheless advisable to try and get rid of some of this 596 // redundant work in a subsequent (low priority) re-design of 597 // the card-scanning code, if only to simplify the underlying 598 // state machine analysis/proof. ysr 1/28/2002. XXX 599 cur_entry++; 600 } 601 } 602 } 603 604 void CardTableRS::verify() { 605 // At present, we only know how to verify the card table RS for 606 // generational heaps. 607 VerifyCTGenClosure blk(this); 608 CollectedHeap* ch = Universe::heap(); 609 610 if (ch->kind() == CollectedHeap::GenCollectedHeap) { 611 GenCollectedHeap::heap()->generation_iterate(&blk, false); 612 _ct_bs->verify(); 613 } 614 } 615 616 617 void CardTableRS::verify_aligned_region_empty(MemRegion mr) { 618 if (!mr.is_empty()) { 619 jbyte* cur_entry = byte_for(mr.start()); 620 jbyte* limit = byte_after(mr.last()); 621 // The region mr may not start on a card boundary so 622 // the first card may reflect a write to the space 623 // just prior to mr. 624 if (!is_aligned(mr.start())) { 625 cur_entry++; 626 } 627 for (;cur_entry < limit; cur_entry++) { 628 guarantee(*cur_entry == CardTableModRefBS::clean_card, 629 "Unexpected dirty card found"); 630 } 631 } 632 }