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annotate src/share/vm/memory/cardTableRS.cpp @ 1552:c18cbe5936b8
6941466: Oracle rebranding changes for Hotspot repositories
Summary: Change all the Sun copyrights to Oracle copyright
Reviewed-by: ohair
| author | trims |
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| date | Thu, 27 May 2010 19:08:38 -0700 |
| parents | bd02caa94611 |
| children | f95d63e2154a |
| rev | line source |
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| 0 | 1 /* |
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2 * Copyright (c) 2001, 2009, Oracle and/or its affiliates. All rights reserved. |
| 0 | 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 * | |
|
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19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA |
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20 * or visit www.oracle.com if you need additional information or have any |
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21 * questions. |
| 0 | 22 * |
| 23 */ | |
| 24 | |
| 25 # include "incls/_precompiled.incl" | |
| 26 # include "incls/_cardTableRS.cpp.incl" | |
| 27 | |
| 28 CardTableRS::CardTableRS(MemRegion whole_heap, | |
| 29 int max_covered_regions) : | |
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30 GenRemSet(), |
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31 _cur_youngergen_card_val(youngergenP1_card), |
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32 _regions_to_iterate(max_covered_regions - 1) |
| 0 | 33 { |
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34 #ifndef SERIALGC |
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35 if (UseG1GC) { |
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36 _ct_bs = new G1SATBCardTableLoggingModRefBS(whole_heap, |
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37 max_covered_regions); |
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38 } else { |
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39 _ct_bs = new CardTableModRefBSForCTRS(whole_heap, max_covered_regions); |
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40 } |
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41 #else |
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42 _ct_bs = new CardTableModRefBSForCTRS(whole_heap, max_covered_regions); |
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43 #endif |
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44 set_bs(_ct_bs); |
| 0 | 45 _last_cur_val_in_gen = new jbyte[GenCollectedHeap::max_gens + 1]; |
| 46 if (_last_cur_val_in_gen == NULL) { | |
| 47 vm_exit_during_initialization("Could not last_cur_val_in_gen array."); | |
| 48 } | |
| 49 for (int i = 0; i < GenCollectedHeap::max_gens + 1; i++) { | |
| 50 _last_cur_val_in_gen[i] = clean_card_val(); | |
| 51 } | |
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52 _ct_bs->set_CTRS(this); |
| 0 | 53 } |
| 54 | |
| 55 void CardTableRS::resize_covered_region(MemRegion new_region) { | |
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56 _ct_bs->resize_covered_region(new_region); |
| 0 | 57 } |
| 58 | |
| 59 jbyte CardTableRS::find_unused_youngergenP_card_value() { | |
| 60 for (jbyte v = youngergenP1_card; | |
| 61 v < cur_youngergen_and_prev_nonclean_card; | |
| 62 v++) { | |
| 63 bool seen = false; | |
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64 for (int g = 0; g < _regions_to_iterate; g++) { |
| 0 | 65 if (_last_cur_val_in_gen[g] == v) { |
| 66 seen = true; | |
| 67 break; | |
| 68 } | |
| 69 } | |
| 70 if (!seen) return v; | |
| 71 } | |
| 72 ShouldNotReachHere(); | |
| 73 return 0; | |
| 74 } | |
| 75 | |
| 76 void CardTableRS::prepare_for_younger_refs_iterate(bool parallel) { | |
| 77 // Parallel or sequential, we must always set the prev to equal the | |
| 78 // last one written. | |
| 79 if (parallel) { | |
| 80 // Find a parallel value to be used next. | |
| 81 jbyte next_val = find_unused_youngergenP_card_value(); | |
| 82 set_cur_youngergen_card_val(next_val); | |
| 83 | |
| 84 } else { | |
| 85 // In an sequential traversal we will always write youngergen, so that | |
| 86 // the inline barrier is correct. | |
| 87 set_cur_youngergen_card_val(youngergen_card); | |
| 88 } | |
| 89 } | |
| 90 | |
| 91 void CardTableRS::younger_refs_iterate(Generation* g, | |
| 92 OopsInGenClosure* blk) { | |
| 93 _last_cur_val_in_gen[g->level()+1] = cur_youngergen_card_val(); | |
| 94 g->younger_refs_iterate(blk); | |
| 95 } | |
| 96 | |
| 97 class ClearNoncleanCardWrapper: public MemRegionClosure { | |
| 98 MemRegionClosure* _dirty_card_closure; | |
| 99 CardTableRS* _ct; | |
| 100 bool _is_par; | |
| 101 private: | |
| 102 // Clears the given card, return true if the corresponding card should be | |
| 103 // processed. | |
| 104 bool clear_card(jbyte* entry) { | |
| 105 if (_is_par) { | |
| 106 while (true) { | |
| 107 // In the parallel case, we may have to do this several times. | |
| 108 jbyte entry_val = *entry; | |
| 109 assert(entry_val != CardTableRS::clean_card_val(), | |
| 110 "We shouldn't be looking at clean cards, and this should " | |
| 111 "be the only place they get cleaned."); | |
| 112 if (CardTableRS::card_is_dirty_wrt_gen_iter(entry_val) | |
| 113 || _ct->is_prev_youngergen_card_val(entry_val)) { | |
| 114 jbyte res = | |
| 115 Atomic::cmpxchg(CardTableRS::clean_card_val(), entry, entry_val); | |
| 116 if (res == entry_val) { | |
| 117 break; | |
| 118 } else { | |
| 119 assert(res == CardTableRS::cur_youngergen_and_prev_nonclean_card, | |
| 120 "The CAS above should only fail if another thread did " | |
| 121 "a GC write barrier."); | |
| 122 } | |
| 123 } else if (entry_val == | |
| 124 CardTableRS::cur_youngergen_and_prev_nonclean_card) { | |
| 125 // Parallelism shouldn't matter in this case. Only the thread | |
| 126 // assigned to scan the card should change this value. | |
| 127 *entry = _ct->cur_youngergen_card_val(); | |
| 128 break; | |
| 129 } else { | |
| 130 assert(entry_val == _ct->cur_youngergen_card_val(), | |
| 131 "Should be the only possibility."); | |
| 132 // In this case, the card was clean before, and become | |
| 133 // cur_youngergen only because of processing of a promoted object. | |
| 134 // We don't have to look at the card. | |
| 135 return false; | |
| 136 } | |
| 137 } | |
| 138 return true; | |
| 139 } else { | |
| 140 jbyte entry_val = *entry; | |
| 141 assert(entry_val != CardTableRS::clean_card_val(), | |
| 142 "We shouldn't be looking at clean cards, and this should " | |
| 143 "be the only place they get cleaned."); | |
| 144 assert(entry_val != CardTableRS::cur_youngergen_and_prev_nonclean_card, | |
| 145 "This should be possible in the sequential case."); | |
| 146 *entry = CardTableRS::clean_card_val(); | |
| 147 return true; | |
| 148 } | |
| 149 } | |
| 150 | |
| 151 public: | |
| 152 ClearNoncleanCardWrapper(MemRegionClosure* dirty_card_closure, | |
| 153 CardTableRS* ct) : | |
| 154 _dirty_card_closure(dirty_card_closure), _ct(ct) { | |
| 155 _is_par = (SharedHeap::heap()->n_par_threads() > 0); | |
| 156 } | |
| 157 void do_MemRegion(MemRegion mr) { | |
| 158 // We start at the high end of "mr", walking backwards | |
| 159 // while accumulating a contiguous dirty range of cards in | |
| 160 // [start_of_non_clean, end_of_non_clean) which we then | |
| 161 // process en masse. | |
| 162 HeapWord* end_of_non_clean = mr.end(); | |
| 163 HeapWord* start_of_non_clean = end_of_non_clean; | |
| 164 jbyte* entry = _ct->byte_for(mr.last()); | |
| 165 const jbyte* first_entry = _ct->byte_for(mr.start()); | |
| 166 while (entry >= first_entry) { | |
| 167 HeapWord* cur = _ct->addr_for(entry); | |
| 168 if (!clear_card(entry)) { | |
| 169 // We hit a clean card; process any non-empty | |
| 170 // dirty range accumulated so far. | |
| 171 if (start_of_non_clean < end_of_non_clean) { | |
| 172 MemRegion mr2(start_of_non_clean, end_of_non_clean); | |
| 173 _dirty_card_closure->do_MemRegion(mr2); | |
| 174 } | |
| 175 // Reset the dirty window while continuing to | |
| 176 // look for the next dirty window to process. | |
| 177 end_of_non_clean = cur; | |
| 178 start_of_non_clean = end_of_non_clean; | |
| 179 } | |
| 180 // Open the left end of the window one card to the left. | |
| 181 start_of_non_clean = cur; | |
| 182 // Note that "entry" leads "start_of_non_clean" in | |
| 183 // its leftward excursion after this point | |
| 184 // in the loop and, when we hit the left end of "mr", | |
| 185 // will point off of the left end of the card-table | |
| 186 // for "mr". | |
| 187 entry--; | |
| 188 } | |
| 189 // If the first card of "mr" was dirty, we will have | |
| 190 // been left with a dirty window, co-initial with "mr", | |
| 191 // which we now process. | |
| 192 if (start_of_non_clean < end_of_non_clean) { | |
| 193 MemRegion mr2(start_of_non_clean, end_of_non_clean); | |
| 194 _dirty_card_closure->do_MemRegion(mr2); | |
| 195 } | |
| 196 } | |
| 197 }; | |
| 198 // clean (by dirty->clean before) ==> cur_younger_gen | |
| 199 // dirty ==> cur_youngergen_and_prev_nonclean_card | |
| 200 // precleaned ==> cur_youngergen_and_prev_nonclean_card | |
| 201 // prev-younger-gen ==> cur_youngergen_and_prev_nonclean_card | |
| 202 // cur-younger-gen ==> cur_younger_gen | |
| 203 // cur_youngergen_and_prev_nonclean_card ==> no change. | |
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204 void CardTableRS::write_ref_field_gc_par(void* field, oop new_val) { |
| 0 | 205 jbyte* entry = ct_bs()->byte_for(field); |
| 206 do { | |
| 207 jbyte entry_val = *entry; | |
| 208 // We put this first because it's probably the most common case. | |
| 209 if (entry_val == clean_card_val()) { | |
| 210 // No threat of contention with cleaning threads. | |
| 211 *entry = cur_youngergen_card_val(); | |
| 212 return; | |
| 213 } else if (card_is_dirty_wrt_gen_iter(entry_val) | |
| 214 || is_prev_youngergen_card_val(entry_val)) { | |
| 215 // Mark it as both cur and prev youngergen; card cleaning thread will | |
| 216 // eventually remove the previous stuff. | |
| 217 jbyte new_val = cur_youngergen_and_prev_nonclean_card; | |
| 218 jbyte res = Atomic::cmpxchg(new_val, entry, entry_val); | |
| 219 // Did the CAS succeed? | |
| 220 if (res == entry_val) return; | |
| 221 // Otherwise, retry, to see the new value. | |
| 222 continue; | |
| 223 } else { | |
| 224 assert(entry_val == cur_youngergen_and_prev_nonclean_card | |
| 225 || entry_val == cur_youngergen_card_val(), | |
| 226 "should be only possibilities."); | |
| 227 return; | |
| 228 } | |
| 229 } while (true); | |
| 230 } | |
| 231 | |
| 232 void CardTableRS::younger_refs_in_space_iterate(Space* sp, | |
| 233 OopsInGenClosure* cl) { | |
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234 DirtyCardToOopClosure* dcto_cl = sp->new_dcto_cl(cl, _ct_bs->precision(), |
| 0 | 235 cl->gen_boundary()); |
| 236 ClearNoncleanCardWrapper clear_cl(dcto_cl, this); | |
| 237 | |
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238 _ct_bs->non_clean_card_iterate(sp, sp->used_region_at_save_marks(), |
| 0 | 239 dcto_cl, &clear_cl, false); |
| 240 } | |
| 241 | |
| 242 void CardTableRS::clear_into_younger(Generation* gen, bool clear_perm) { | |
| 243 GenCollectedHeap* gch = GenCollectedHeap::heap(); | |
| 244 // Generations younger than gen have been evacuated. We can clear | |
| 245 // card table entries for gen (we know that it has no pointers | |
| 246 // to younger gens) and for those below. The card tables for | |
| 247 // the youngest gen need never be cleared, and those for perm gen | |
| 248 // will be cleared based on the parameter clear_perm. | |
| 249 // There's a bit of subtlety in the clear() and invalidate() | |
| 250 // methods that we exploit here and in invalidate_or_clear() | |
| 251 // below to avoid missing cards at the fringes. If clear() or | |
| 252 // invalidate() are changed in the future, this code should | |
| 253 // be revisited. 20040107.ysr | |
| 254 Generation* g = gen; | |
| 255 for(Generation* prev_gen = gch->prev_gen(g); | |
| 256 prev_gen != NULL; | |
| 257 g = prev_gen, prev_gen = gch->prev_gen(g)) { | |
| 258 MemRegion to_be_cleared_mr = g->prev_used_region(); | |
| 259 clear(to_be_cleared_mr); | |
| 260 } | |
| 261 // Clear perm gen cards if asked to do so. | |
| 262 if (clear_perm) { | |
| 263 MemRegion to_be_cleared_mr = gch->perm_gen()->prev_used_region(); | |
| 264 clear(to_be_cleared_mr); | |
| 265 } | |
| 266 } | |
| 267 | |
| 268 void CardTableRS::invalidate_or_clear(Generation* gen, bool younger, | |
| 269 bool perm) { | |
| 270 GenCollectedHeap* gch = GenCollectedHeap::heap(); | |
| 271 // For each generation gen (and younger and/or perm) | |
| 272 // invalidate the cards for the currently occupied part | |
| 273 // of that generation and clear the cards for the | |
| 274 // unoccupied part of the generation (if any, making use | |
| 275 // of that generation's prev_used_region to determine that | |
| 276 // region). No need to do anything for the youngest | |
| 277 // generation. Also see note#20040107.ysr above. | |
| 278 Generation* g = gen; | |
| 279 for(Generation* prev_gen = gch->prev_gen(g); prev_gen != NULL; | |
| 280 g = prev_gen, prev_gen = gch->prev_gen(g)) { | |
| 281 MemRegion used_mr = g->used_region(); | |
| 282 MemRegion to_be_cleared_mr = g->prev_used_region().minus(used_mr); | |
| 283 if (!to_be_cleared_mr.is_empty()) { | |
| 284 clear(to_be_cleared_mr); | |
| 285 } | |
| 286 invalidate(used_mr); | |
| 287 if (!younger) break; | |
| 288 } | |
| 289 // Clear perm gen cards if asked to do so. | |
| 290 if (perm) { | |
| 291 g = gch->perm_gen(); | |
| 292 MemRegion used_mr = g->used_region(); | |
| 293 MemRegion to_be_cleared_mr = g->prev_used_region().minus(used_mr); | |
| 294 if (!to_be_cleared_mr.is_empty()) { | |
| 295 clear(to_be_cleared_mr); | |
| 296 } | |
| 297 invalidate(used_mr); | |
| 298 } | |
| 299 } | |
| 300 | |
| 301 | |
| 302 class VerifyCleanCardClosure: public OopClosure { | |
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303 private: |
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304 HeapWord* _boundary; |
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305 HeapWord* _begin; |
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306 HeapWord* _end; |
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307 protected: |
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308 template <class T> void do_oop_work(T* p) { |
| 0 | 309 HeapWord* jp = (HeapWord*)p; |
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310 if (jp >= _begin && jp < _end) { |
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311 oop obj = oopDesc::load_decode_heap_oop(p); |
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312 guarantee(obj == NULL || |
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313 (HeapWord*)p < _boundary || |
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314 (HeapWord*)obj >= _boundary, |
| 0 | 315 "pointer on clean card crosses boundary"); |
| 316 } | |
| 317 } | |
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318 public: |
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319 VerifyCleanCardClosure(HeapWord* b, HeapWord* begin, HeapWord* end) : |
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320 _boundary(b), _begin(begin), _end(end) {} |
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321 virtual void do_oop(oop* p) { VerifyCleanCardClosure::do_oop_work(p); } |
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322 virtual void do_oop(narrowOop* p) { VerifyCleanCardClosure::do_oop_work(p); } |
| 0 | 323 }; |
| 324 | |
| 325 class VerifyCTSpaceClosure: public SpaceClosure { | |
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326 private: |
| 0 | 327 CardTableRS* _ct; |
| 328 HeapWord* _boundary; | |
| 329 public: | |
| 330 VerifyCTSpaceClosure(CardTableRS* ct, HeapWord* boundary) : | |
| 331 _ct(ct), _boundary(boundary) {} | |
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332 virtual void do_space(Space* s) { _ct->verify_space(s, _boundary); } |
| 0 | 333 }; |
| 334 | |
| 335 class VerifyCTGenClosure: public GenCollectedHeap::GenClosure { | |
| 336 CardTableRS* _ct; | |
| 337 public: | |
| 338 VerifyCTGenClosure(CardTableRS* ct) : _ct(ct) {} | |
| 339 void do_generation(Generation* gen) { | |
| 340 // Skip the youngest generation. | |
| 341 if (gen->level() == 0) return; | |
| 342 // Normally, we're interested in pointers to younger generations. | |
| 343 VerifyCTSpaceClosure blk(_ct, gen->reserved().start()); | |
| 344 gen->space_iterate(&blk, true); | |
| 345 } | |
| 346 }; | |
| 347 | |
| 348 void CardTableRS::verify_space(Space* s, HeapWord* gen_boundary) { | |
| 349 // We don't need to do young-gen spaces. | |
| 350 if (s->end() <= gen_boundary) return; | |
| 351 MemRegion used = s->used_region(); | |
| 352 | |
| 353 jbyte* cur_entry = byte_for(used.start()); | |
| 354 jbyte* limit = byte_after(used.last()); | |
| 355 while (cur_entry < limit) { | |
| 356 if (*cur_entry == CardTableModRefBS::clean_card) { | |
| 357 jbyte* first_dirty = cur_entry+1; | |
| 358 while (first_dirty < limit && | |
| 359 *first_dirty == CardTableModRefBS::clean_card) { | |
| 360 first_dirty++; | |
| 361 } | |
| 362 // If the first object is a regular object, and it has a | |
| 363 // young-to-old field, that would mark the previous card. | |
| 364 HeapWord* boundary = addr_for(cur_entry); | |
| 365 HeapWord* end = (first_dirty >= limit) ? used.end() : addr_for(first_dirty); | |
| 366 HeapWord* boundary_block = s->block_start(boundary); | |
| 367 HeapWord* begin = boundary; // Until proven otherwise. | |
| 368 HeapWord* start_block = boundary_block; // Until proven otherwise. | |
| 369 if (boundary_block < boundary) { | |
| 370 if (s->block_is_obj(boundary_block) && s->obj_is_alive(boundary_block)) { | |
| 371 oop boundary_obj = oop(boundary_block); | |
| 372 if (!boundary_obj->is_objArray() && | |
| 373 !boundary_obj->is_typeArray()) { | |
| 374 guarantee(cur_entry > byte_for(used.start()), | |
| 375 "else boundary would be boundary_block"); | |
| 376 if (*byte_for(boundary_block) != CardTableModRefBS::clean_card) { | |
| 377 begin = boundary_block + s->block_size(boundary_block); | |
| 378 start_block = begin; | |
| 379 } | |
| 380 } | |
| 381 } | |
| 382 } | |
| 383 // Now traverse objects until end. | |
| 384 HeapWord* cur = start_block; | |
| 385 VerifyCleanCardClosure verify_blk(gen_boundary, begin, end); | |
| 386 while (cur < end) { | |
| 387 if (s->block_is_obj(cur) && s->obj_is_alive(cur)) { | |
| 388 oop(cur)->oop_iterate(&verify_blk); | |
| 389 } | |
| 390 cur += s->block_size(cur); | |
| 391 } | |
| 392 cur_entry = first_dirty; | |
| 393 } else { | |
| 394 // We'd normally expect that cur_youngergen_and_prev_nonclean_card | |
| 395 // is a transient value, that cannot be in the card table | |
| 396 // except during GC, and thus assert that: | |
| 397 // guarantee(*cur_entry != cur_youngergen_and_prev_nonclean_card, | |
| 398 // "Illegal CT value"); | |
| 399 // That however, need not hold, as will become clear in the | |
| 400 // following... | |
| 401 | |
| 402 // We'd normally expect that if we are in the parallel case, | |
| 403 // we can't have left a prev value (which would be different | |
| 404 // from the current value) in the card table, and so we'd like to | |
| 405 // assert that: | |
| 406 // guarantee(cur_youngergen_card_val() == youngergen_card | |
| 407 // || !is_prev_youngergen_card_val(*cur_entry), | |
| 408 // "Illegal CT value"); | |
| 409 // That, however, may not hold occasionally, because of | |
| 410 // CMS or MSC in the old gen. To wit, consider the | |
| 411 // following two simple illustrative scenarios: | |
| 412 // (a) CMS: Consider the case where a large object L | |
| 413 // spanning several cards is allocated in the old | |
| 414 // gen, and has a young gen reference stored in it, dirtying | |
| 415 // some interior cards. A young collection scans the card, | |
| 416 // finds a young ref and installs a youngergenP_n value. | |
| 417 // L then goes dead. Now a CMS collection starts, | |
| 418 // finds L dead and sweeps it up. Assume that L is | |
| 419 // abutting _unallocated_blk, so _unallocated_blk is | |
| 420 // adjusted down to (below) L. Assume further that | |
| 421 // no young collection intervenes during this CMS cycle. | |
| 422 // The next young gen cycle will not get to look at this | |
| 423 // youngergenP_n card since it lies in the unoccupied | |
| 424 // part of the space. | |
| 425 // Some young collections later the blocks on this | |
| 426 // card can be re-allocated either due to direct allocation | |
| 427 // or due to absorbing promotions. At this time, the | |
| 428 // before-gc verification will fail the above assert. | |
| 429 // (b) MSC: In this case, an object L with a young reference | |
| 430 // is on a card that (therefore) holds a youngergen_n value. | |
| 431 // Suppose also that L lies towards the end of the used | |
| 432 // the used space before GC. An MSC collection | |
| 433 // occurs that compacts to such an extent that this | |
| 434 // card is no longer in the occupied part of the space. | |
| 435 // Since current code in MSC does not always clear cards | |
| 436 // in the unused part of old gen, this stale youngergen_n | |
| 437 // value is left behind and can later be covered by | |
| 438 // an object when promotion or direct allocation | |
| 439 // re-allocates that part of the heap. | |
| 440 // | |
| 441 // Fortunately, the presence of such stale card values is | |
| 442 // "only" a minor annoyance in that subsequent young collections | |
| 443 // might needlessly scan such cards, but would still never corrupt | |
| 444 // the heap as a result. However, it's likely not to be a significant | |
| 445 // performance inhibitor in practice. For instance, | |
| 446 // some recent measurements with unoccupied cards eagerly cleared | |
| 447 // out to maintain this invariant, showed next to no | |
| 448 // change in young collection times; of course one can construct | |
| 449 // degenerate examples where the cost can be significant.) | |
| 450 // Note, in particular, that if the "stale" card is modified | |
| 451 // after re-allocation, it would be dirty, not "stale". Thus, | |
| 452 // we can never have a younger ref in such a card and it is | |
| 453 // safe not to scan that card in any collection. [As we see | |
| 454 // below, we do some unnecessary scanning | |
| 455 // in some cases in the current parallel scanning algorithm.] | |
| 456 // | |
| 457 // The main point below is that the parallel card scanning code | |
| 458 // deals correctly with these stale card values. There are two main | |
| 459 // cases to consider where we have a stale "younger gen" value and a | |
| 460 // "derivative" case to consider, where we have a stale | |
| 461 // "cur_younger_gen_and_prev_non_clean" value, as will become | |
| 462 // apparent in the case analysis below. | |
| 463 // o Case 1. If the stale value corresponds to a younger_gen_n | |
| 464 // value other than the cur_younger_gen value then the code | |
| 465 // treats this as being tantamount to a prev_younger_gen | |
| 466 // card. This means that the card may be unnecessarily scanned. | |
| 467 // There are two sub-cases to consider: | |
| 468 // o Case 1a. Let us say that the card is in the occupied part | |
| 469 // of the generation at the time the collection begins. In | |
| 470 // that case the card will be either cleared when it is scanned | |
| 471 // for young pointers, or will be set to cur_younger_gen as a | |
| 472 // result of promotion. (We have elided the normal case where | |
| 473 // the scanning thread and the promoting thread interleave | |
| 474 // possibly resulting in a transient | |
| 475 // cur_younger_gen_and_prev_non_clean value before settling | |
| 476 // to cur_younger_gen. [End Case 1a.] | |
| 477 // o Case 1b. Consider now the case when the card is in the unoccupied | |
| 478 // part of the space which becomes occupied because of promotions | |
| 479 // into it during the current young GC. In this case the card | |
| 480 // will never be scanned for young references. The current | |
| 481 // code will set the card value to either | |
| 482 // cur_younger_gen_and_prev_non_clean or leave | |
| 483 // it with its stale value -- because the promotions didn't | |
| 484 // result in any younger refs on that card. Of these two | |
| 485 // cases, the latter will be covered in Case 1a during | |
| 486 // a subsequent scan. To deal with the former case, we need | |
| 487 // to further consider how we deal with a stale value of | |
| 488 // cur_younger_gen_and_prev_non_clean in our case analysis | |
| 489 // below. This we do in Case 3 below. [End Case 1b] | |
| 490 // [End Case 1] | |
| 491 // o Case 2. If the stale value corresponds to cur_younger_gen being | |
| 492 // a value not necessarily written by a current promotion, the | |
| 493 // card will not be scanned by the younger refs scanning code. | |
| 494 // (This is OK since as we argued above such cards cannot contain | |
| 495 // any younger refs.) The result is that this value will be | |
| 496 // treated as a prev_younger_gen value in a subsequent collection, | |
| 497 // which is addressed in Case 1 above. [End Case 2] | |
| 498 // o Case 3. We here consider the "derivative" case from Case 1b. above | |
| 499 // because of which we may find a stale | |
| 500 // cur_younger_gen_and_prev_non_clean card value in the table. | |
| 501 // Once again, as in Case 1, we consider two subcases, depending | |
| 502 // on whether the card lies in the occupied or unoccupied part | |
| 503 // of the space at the start of the young collection. | |
| 504 // o Case 3a. Let us say the card is in the occupied part of | |
| 505 // the old gen at the start of the young collection. In that | |
| 506 // case, the card will be scanned by the younger refs scanning | |
| 507 // code which will set it to cur_younger_gen. In a subsequent | |
| 508 // scan, the card will be considered again and get its final | |
| 509 // correct value. [End Case 3a] | |
| 510 // o Case 3b. Now consider the case where the card is in the | |
| 511 // unoccupied part of the old gen, and is occupied as a result | |
| 512 // of promotions during thus young gc. In that case, | |
| 513 // the card will not be scanned for younger refs. The presence | |
| 514 // of newly promoted objects on the card will then result in | |
| 515 // its keeping the value cur_younger_gen_and_prev_non_clean | |
| 516 // value, which we have dealt with in Case 3 here. [End Case 3b] | |
| 517 // [End Case 3] | |
| 518 // | |
| 519 // (Please refer to the code in the helper class | |
| 520 // ClearNonCleanCardWrapper and in CardTableModRefBS for details.) | |
| 521 // | |
| 522 // The informal arguments above can be tightened into a formal | |
| 523 // correctness proof and it behooves us to write up such a proof, | |
| 524 // or to use model checking to prove that there are no lingering | |
| 525 // concerns. | |
| 526 // | |
| 527 // Clearly because of Case 3b one cannot bound the time for | |
| 528 // which a card will retain what we have called a "stale" value. | |
| 529 // However, one can obtain a Loose upper bound on the redundant | |
| 530 // work as a result of such stale values. Note first that any | |
| 531 // time a stale card lies in the occupied part of the space at | |
| 532 // the start of the collection, it is scanned by younger refs | |
| 533 // code and we can define a rank function on card values that | |
| 534 // declines when this is so. Note also that when a card does not | |
| 535 // lie in the occupied part of the space at the beginning of a | |
| 536 // young collection, its rank can either decline or stay unchanged. | |
| 537 // In this case, no extra work is done in terms of redundant | |
| 538 // younger refs scanning of that card. | |
| 539 // Then, the case analysis above reveals that, in the worst case, | |
| 540 // any such stale card will be scanned unnecessarily at most twice. | |
| 541 // | |
| 542 // It is nonethelss advisable to try and get rid of some of this | |
| 543 // redundant work in a subsequent (low priority) re-design of | |
| 544 // the card-scanning code, if only to simplify the underlying | |
| 545 // state machine analysis/proof. ysr 1/28/2002. XXX | |
| 546 cur_entry++; | |
| 547 } | |
| 548 } | |
| 549 } | |
| 550 | |
| 551 void CardTableRS::verify() { | |
| 552 // At present, we only know how to verify the card table RS for | |
| 553 // generational heaps. | |
| 554 VerifyCTGenClosure blk(this); | |
| 555 CollectedHeap* ch = Universe::heap(); | |
| 556 // We will do the perm-gen portion of the card table, too. | |
| 557 Generation* pg = SharedHeap::heap()->perm_gen(); | |
| 558 HeapWord* pg_boundary = pg->reserved().start(); | |
| 559 | |
| 560 if (ch->kind() == CollectedHeap::GenCollectedHeap) { | |
| 561 GenCollectedHeap::heap()->generation_iterate(&blk, false); | |
|
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562 _ct_bs->verify(); |
| 0 | 563 |
| 564 // If the old gen collections also collect perm, then we are only | |
| 565 // interested in perm-to-young pointers, not perm-to-old pointers. | |
| 566 GenCollectedHeap* gch = GenCollectedHeap::heap(); | |
| 567 CollectorPolicy* cp = gch->collector_policy(); | |
| 568 if (cp->is_mark_sweep_policy() || cp->is_concurrent_mark_sweep_policy()) { | |
| 569 pg_boundary = gch->get_gen(1)->reserved().start(); | |
| 570 } | |
| 571 } | |
| 572 VerifyCTSpaceClosure perm_space_blk(this, pg_boundary); | |
| 573 SharedHeap::heap()->perm_gen()->space_iterate(&perm_space_blk, true); | |
| 574 } | |
| 575 | |
| 576 | |
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577 void CardTableRS::verify_aligned_region_empty(MemRegion mr) { |
| 0 | 578 if (!mr.is_empty()) { |
| 579 jbyte* cur_entry = byte_for(mr.start()); | |
| 580 jbyte* limit = byte_after(mr.last()); | |
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581 // The region mr may not start on a card boundary so |
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582 // the first card may reflect a write to the space |
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583 // just prior to mr. |
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584 if (!is_aligned(mr.start())) { |
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585 cur_entry++; |
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586 } |
| 0 | 587 for (;cur_entry < limit; cur_entry++) { |
| 588 guarantee(*cur_entry == CardTableModRefBS::clean_card, | |
| 589 "Unexpected dirty card found"); | |
| 590 } | |
| 591 } | |
| 592 } |