Mercurial > hg > truffle
annotate src/share/vm/memory/cardTableRS.cpp @ 3357:fc2b798ab316
6883834: ParNew: assert(!_g->to()->is_in_reserved(obj),"Scanning field twice?") with LargeObjects tests
Summary: Fixed process_chunk_boundaries(), used for parallel card scanning when using ParNew/CMS, so as to prevent double-scanning, or worse, non-scanning of imprecisely marked objects exceeding parallel chunk size. Made some sizing parameters for parallel card scanning diagnostic, disabled ParallelGCRetainPLAB, and elaborated and clarified some comments.
Reviewed-by: stefank, johnc
| author | ysr |
|---|---|
| date | Tue, 10 May 2011 00:33:21 -0700 |
| parents | 1f4413413144 |
| children | bca17e38de00 |
| rev | line source |
|---|---|
| 0 | 1 /* |
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2 * Copyright (c) 2001, 2011, 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 | |
| 1972 | 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 #ifndef SERIALGC | |
| 35 #include "gc_implementation/g1/concurrentMark.hpp" | |
| 36 #include "gc_implementation/g1/g1SATBCardTableModRefBS.hpp" | |
| 37 #endif | |
| 0 | 38 |
| 39 CardTableRS::CardTableRS(MemRegion whole_heap, | |
| 40 int max_covered_regions) : | |
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41 GenRemSet(), |
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42 _cur_youngergen_card_val(youngergenP1_card), |
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43 _regions_to_iterate(max_covered_regions - 1) |
| 0 | 44 { |
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45 #ifndef SERIALGC |
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46 if (UseG1GC) { |
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47 _ct_bs = new G1SATBCardTableLoggingModRefBS(whole_heap, |
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48 max_covered_regions); |
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49 } else { |
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50 _ct_bs = new CardTableModRefBSForCTRS(whole_heap, max_covered_regions); |
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51 } |
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52 #else |
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53 _ct_bs = new CardTableModRefBSForCTRS(whole_heap, max_covered_regions); |
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54 #endif |
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55 set_bs(_ct_bs); |
| 0 | 56 _last_cur_val_in_gen = new jbyte[GenCollectedHeap::max_gens + 1]; |
| 57 if (_last_cur_val_in_gen == NULL) { | |
| 58 vm_exit_during_initialization("Could not last_cur_val_in_gen array."); | |
| 59 } | |
| 60 for (int i = 0; i < GenCollectedHeap::max_gens + 1; i++) { | |
| 61 _last_cur_val_in_gen[i] = clean_card_val(); | |
| 62 } | |
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63 _ct_bs->set_CTRS(this); |
| 0 | 64 } |
| 65 | |
| 66 void CardTableRS::resize_covered_region(MemRegion new_region) { | |
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67 _ct_bs->resize_covered_region(new_region); |
| 0 | 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; | |
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75 for (int g = 0; g < _regions_to_iterate; g++) { |
| 0 | 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 | |
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108 inline bool ClearNoncleanCardWrapper::clear_card(jbyte* entry) { |
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109 if (_is_par) { |
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110 return clear_card_parallel(entry); |
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111 } else { |
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112 return clear_card_serial(entry); |
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113 } |
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114 } |
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115 |
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116 inline bool ClearNoncleanCardWrapper::clear_card_parallel(jbyte* entry) { |
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117 while (true) { |
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118 // In the parallel case, we may have to do this several times. |
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119 jbyte entry_val = *entry; |
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120 assert(entry_val != CardTableRS::clean_card_val(), |
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121 "We shouldn't be looking at clean cards, and this should " |
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122 "be the only place they get cleaned."); |
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123 if (CardTableRS::card_is_dirty_wrt_gen_iter(entry_val) |
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124 || _ct->is_prev_youngergen_card_val(entry_val)) { |
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125 jbyte res = |
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126 Atomic::cmpxchg(CardTableRS::clean_card_val(), entry, entry_val); |
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127 if (res == entry_val) { |
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128 break; |
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129 } else { |
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130 assert(res == CardTableRS::cur_youngergen_and_prev_nonclean_card, |
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131 "The CAS above should only fail if another thread did " |
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132 "a GC write barrier."); |
| 0 | 133 } |
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134 } else if (entry_val == |
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135 CardTableRS::cur_youngergen_and_prev_nonclean_card) { |
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136 // Parallelism shouldn't matter in this case. Only the thread |
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137 // assigned to scan the card should change this value. |
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138 *entry = _ct->cur_youngergen_card_val(); |
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139 break; |
| 0 | 140 } else { |
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141 assert(entry_val == _ct->cur_youngergen_card_val(), |
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142 "Should be the only possibility."); |
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143 // In this case, the card was clean before, and become |
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144 // cur_youngergen only because of processing of a promoted object. |
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145 // We don't have to look at the card. |
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146 return false; |
| 0 | 147 } |
| 148 } | |
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149 return true; |
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150 } |
| 0 | 151 |
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152 |
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153 inline bool ClearNoncleanCardWrapper::clear_card_serial(jbyte* entry) { |
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154 jbyte entry_val = *entry; |
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155 assert(entry_val != CardTableRS::clean_card_val(), |
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156 "We shouldn't be looking at clean cards, and this should " |
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157 "be the only place they get cleaned."); |
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158 assert(entry_val != CardTableRS::cur_youngergen_and_prev_nonclean_card, |
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159 "This should be possible in the sequential case."); |
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160 *entry = CardTableRS::clean_card_val(); |
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161 return true; |
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162 } |
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163 |
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164 ClearNoncleanCardWrapper::ClearNoncleanCardWrapper( |
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165 DirtyCardToOopClosure* dirty_card_closure, CardTableRS* ct) : |
| 0 | 166 _dirty_card_closure(dirty_card_closure), _ct(ct) { |
| 167 _is_par = (SharedHeap::heap()->n_par_threads() > 0); | |
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168 } |
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169 |
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170 void ClearNoncleanCardWrapper::do_MemRegion(MemRegion mr) { |
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171 assert(mr.word_size() > 0, "Error"); |
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172 assert(_ct->is_aligned(mr.start()), "mr.start() should be card aligned"); |
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173 // mr.end() may not necessarily be card aligned. |
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174 jbyte* cur_entry = _ct->byte_for(mr.last()); |
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175 const jbyte* limit = _ct->byte_for(mr.start()); |
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176 HeapWord* end_of_non_clean = mr.end(); |
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177 HeapWord* start_of_non_clean = end_of_non_clean; |
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178 while (cur_entry >= limit) { |
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179 HeapWord* cur_hw = _ct->addr_for(cur_entry); |
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180 if ((*cur_entry != CardTableRS::clean_card_val()) && clear_card(cur_entry)) { |
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181 // Continue the dirty range by opening the |
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182 // dirty window one card to the left. |
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183 start_of_non_clean = cur_hw; |
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184 } else { |
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185 // We hit a "clean" card; process any non-empty |
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186 // "dirty" range accumulated so far. |
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187 if (start_of_non_clean < end_of_non_clean) { |
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188 const MemRegion mrd(start_of_non_clean, end_of_non_clean); |
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189 _dirty_card_closure->do_MemRegion(mrd); |
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190 } |
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191 // Reset the dirty window, while continuing to look |
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192 // for the next dirty card that will start a |
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193 // new dirty window. |
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194 end_of_non_clean = cur_hw; |
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195 start_of_non_clean = cur_hw; |
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196 } |
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197 // Note that "cur_entry" leads "start_of_non_clean" in |
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198 // its leftward excursion after this point |
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199 // in the loop and, when we hit the left end of "mr", |
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200 // will point off of the left end of the card-table |
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201 // for "mr". |
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202 cur_entry--; |
| 0 | 203 } |
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204 // If the first card of "mr" was dirty, we will have |
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205 // been left with a dirty window, co-initial with "mr", |
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206 // which we now process. |
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207 if (start_of_non_clean < end_of_non_clean) { |
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208 const MemRegion mrd(start_of_non_clean, end_of_non_clean); |
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209 _dirty_card_closure->do_MemRegion(mrd); |
| 0 | 210 } |
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211 } |
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212 |
| 0 | 213 // clean (by dirty->clean before) ==> cur_younger_gen |
| 214 // dirty ==> cur_youngergen_and_prev_nonclean_card | |
| 215 // precleaned ==> cur_youngergen_and_prev_nonclean_card | |
| 216 // prev-younger-gen ==> cur_youngergen_and_prev_nonclean_card | |
| 217 // cur-younger-gen ==> cur_younger_gen | |
| 218 // cur_youngergen_and_prev_nonclean_card ==> no change. | |
|
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219 void CardTableRS::write_ref_field_gc_par(void* field, oop new_val) { |
| 0 | 220 jbyte* entry = ct_bs()->byte_for(field); |
| 221 do { | |
| 222 jbyte entry_val = *entry; | |
| 223 // We put this first because it's probably the most common case. | |
| 224 if (entry_val == clean_card_val()) { | |
| 225 // No threat of contention with cleaning threads. | |
| 226 *entry = cur_youngergen_card_val(); | |
| 227 return; | |
| 228 } else if (card_is_dirty_wrt_gen_iter(entry_val) | |
| 229 || is_prev_youngergen_card_val(entry_val)) { | |
| 230 // Mark it as both cur and prev youngergen; card cleaning thread will | |
| 231 // eventually remove the previous stuff. | |
| 232 jbyte new_val = cur_youngergen_and_prev_nonclean_card; | |
| 233 jbyte res = Atomic::cmpxchg(new_val, entry, entry_val); | |
| 234 // Did the CAS succeed? | |
| 235 if (res == entry_val) return; | |
| 236 // Otherwise, retry, to see the new value. | |
| 237 continue; | |
| 238 } else { | |
| 239 assert(entry_val == cur_youngergen_and_prev_nonclean_card | |
| 240 || entry_val == cur_youngergen_card_val(), | |
| 241 "should be only possibilities."); | |
| 242 return; | |
| 243 } | |
| 244 } while (true); | |
| 245 } | |
| 246 | |
| 247 void CardTableRS::younger_refs_in_space_iterate(Space* sp, | |
| 248 OopsInGenClosure* cl) { | |
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249 const MemRegion urasm = sp->used_region_at_save_marks(); |
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250 #ifdef ASSERT |
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251 // Convert the assertion check to a warning if we are running |
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252 // CMS+ParNew until related bug is fixed. |
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253 MemRegion ur = sp->used_region(); |
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254 assert(ur.contains(urasm) || (UseConcMarkSweepGC && UseParNewGC), |
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255 err_msg("Did you forget to call save_marks()? " |
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256 "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in " |
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257 "[" PTR_FORMAT ", " PTR_FORMAT ")", |
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258 urasm.start(), urasm.end(), ur.start(), ur.end())); |
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259 // In the case of CMS+ParNew, issue a warning |
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260 if (!ur.contains(urasm)) { |
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261 assert(UseConcMarkSweepGC && UseParNewGC, "Tautology: see assert above"); |
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262 warning("CMS+ParNew: Did you forget to call save_marks()? " |
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263 "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in " |
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264 "[" PTR_FORMAT ", " PTR_FORMAT ")", |
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265 urasm.start(), urasm.end(), ur.start(), ur.end()); |
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266 MemRegion ur2 = sp->used_region(); |
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267 MemRegion urasm2 = sp->used_region_at_save_marks(); |
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268 if (!ur.equals(ur2)) { |
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269 warning("CMS+ParNew: Flickering used_region()!!"); |
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270 } |
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271 if (!urasm.equals(urasm2)) { |
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272 warning("CMS+ParNew: Flickering used_region_at_save_marks()!!"); |
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273 } |
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274 ShouldNotReachHere(); |
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275 } |
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276 #endif |
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277 _ct_bs->non_clean_card_iterate_possibly_parallel(sp, urasm, cl, this); |
| 0 | 278 } |
| 279 | |
| 280 void CardTableRS::clear_into_younger(Generation* gen, bool clear_perm) { | |
| 281 GenCollectedHeap* gch = GenCollectedHeap::heap(); | |
| 282 // Generations younger than gen have been evacuated. We can clear | |
| 283 // card table entries for gen (we know that it has no pointers | |
| 284 // to younger gens) and for those below. The card tables for | |
| 285 // the youngest gen need never be cleared, and those for perm gen | |
| 286 // will be cleared based on the parameter clear_perm. | |
| 287 // There's a bit of subtlety in the clear() and invalidate() | |
| 288 // methods that we exploit here and in invalidate_or_clear() | |
| 289 // below to avoid missing cards at the fringes. If clear() or | |
| 290 // invalidate() are changed in the future, this code should | |
| 291 // be revisited. 20040107.ysr | |
| 292 Generation* g = gen; | |
| 293 for(Generation* prev_gen = gch->prev_gen(g); | |
| 294 prev_gen != NULL; | |
| 295 g = prev_gen, prev_gen = gch->prev_gen(g)) { | |
| 296 MemRegion to_be_cleared_mr = g->prev_used_region(); | |
| 297 clear(to_be_cleared_mr); | |
| 298 } | |
| 299 // Clear perm gen cards if asked to do so. | |
| 300 if (clear_perm) { | |
| 301 MemRegion to_be_cleared_mr = gch->perm_gen()->prev_used_region(); | |
| 302 clear(to_be_cleared_mr); | |
| 303 } | |
| 304 } | |
| 305 | |
| 306 void CardTableRS::invalidate_or_clear(Generation* gen, bool younger, | |
| 307 bool perm) { | |
| 308 GenCollectedHeap* gch = GenCollectedHeap::heap(); | |
| 309 // For each generation gen (and younger and/or perm) | |
| 310 // invalidate the cards for the currently occupied part | |
| 311 // of that generation and clear the cards for the | |
| 312 // unoccupied part of the generation (if any, making use | |
| 313 // of that generation's prev_used_region to determine that | |
| 314 // region). No need to do anything for the youngest | |
| 315 // generation. Also see note#20040107.ysr above. | |
| 316 Generation* g = gen; | |
| 317 for(Generation* prev_gen = gch->prev_gen(g); prev_gen != NULL; | |
| 318 g = prev_gen, prev_gen = gch->prev_gen(g)) { | |
| 319 MemRegion used_mr = g->used_region(); | |
| 320 MemRegion to_be_cleared_mr = g->prev_used_region().minus(used_mr); | |
| 321 if (!to_be_cleared_mr.is_empty()) { | |
| 322 clear(to_be_cleared_mr); | |
| 323 } | |
| 324 invalidate(used_mr); | |
| 325 if (!younger) break; | |
| 326 } | |
| 327 // Clear perm gen cards if asked to do so. | |
| 328 if (perm) { | |
| 329 g = gch->perm_gen(); | |
| 330 MemRegion used_mr = g->used_region(); | |
| 331 MemRegion to_be_cleared_mr = g->prev_used_region().minus(used_mr); | |
| 332 if (!to_be_cleared_mr.is_empty()) { | |
| 333 clear(to_be_cleared_mr); | |
| 334 } | |
| 335 invalidate(used_mr); | |
| 336 } | |
| 337 } | |
| 338 | |
| 339 | |
| 340 class VerifyCleanCardClosure: public OopClosure { | |
|
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341 private: |
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342 HeapWord* _boundary; |
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343 HeapWord* _begin; |
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344 HeapWord* _end; |
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345 protected: |
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346 template <class T> void do_oop_work(T* p) { |
| 0 | 347 HeapWord* jp = (HeapWord*)p; |
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348 assert(jp >= _begin && jp < _end, |
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349 err_msg("Error: jp " PTR_FORMAT " should be within " |
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350 "[_begin, _end) = [" PTR_FORMAT "," PTR_FORMAT ")", |
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351 _begin, _end)); |
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352 oop obj = oopDesc::load_decode_heap_oop(p); |
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353 guarantee(obj == NULL || (HeapWord*)obj >= _boundary, |
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354 err_msg("pointer " PTR_FORMAT " at " PTR_FORMAT " on " |
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355 "clean card crosses boundary" PTR_FORMAT, |
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356 (HeapWord*)obj, jp, _boundary)); |
| 0 | 357 } |
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358 |
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359 public: |
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360 VerifyCleanCardClosure(HeapWord* b, HeapWord* begin, HeapWord* end) : |
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361 _boundary(b), _begin(begin), _end(end) { |
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362 assert(b <= begin, |
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363 err_msg("Error: boundary " PTR_FORMAT " should be at or below begin " PTR_FORMAT, |
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364 b, begin)); |
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365 assert(begin <= end, |
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366 err_msg("Error: begin " PTR_FORMAT " should be strictly below end " PTR_FORMAT, |
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367 begin, end)); |
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368 } |
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369 |
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370 virtual void do_oop(oop* p) { VerifyCleanCardClosure::do_oop_work(p); } |
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371 virtual void do_oop(narrowOop* p) { VerifyCleanCardClosure::do_oop_work(p); } |
| 0 | 372 }; |
| 373 | |
| 374 class VerifyCTSpaceClosure: public SpaceClosure { | |
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375 private: |
| 0 | 376 CardTableRS* _ct; |
| 377 HeapWord* _boundary; | |
| 378 public: | |
| 379 VerifyCTSpaceClosure(CardTableRS* ct, HeapWord* boundary) : | |
| 380 _ct(ct), _boundary(boundary) {} | |
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381 virtual void do_space(Space* s) { _ct->verify_space(s, _boundary); } |
| 0 | 382 }; |
| 383 | |
| 384 class VerifyCTGenClosure: public GenCollectedHeap::GenClosure { | |
| 385 CardTableRS* _ct; | |
| 386 public: | |
| 387 VerifyCTGenClosure(CardTableRS* ct) : _ct(ct) {} | |
| 388 void do_generation(Generation* gen) { | |
| 389 // Skip the youngest generation. | |
| 390 if (gen->level() == 0) return; | |
| 391 // Normally, we're interested in pointers to younger generations. | |
| 392 VerifyCTSpaceClosure blk(_ct, gen->reserved().start()); | |
| 393 gen->space_iterate(&blk, true); | |
| 394 } | |
| 395 }; | |
| 396 | |
| 397 void CardTableRS::verify_space(Space* s, HeapWord* gen_boundary) { | |
| 398 // We don't need to do young-gen spaces. | |
| 399 if (s->end() <= gen_boundary) return; | |
| 400 MemRegion used = s->used_region(); | |
| 401 | |
| 402 jbyte* cur_entry = byte_for(used.start()); | |
| 403 jbyte* limit = byte_after(used.last()); | |
| 404 while (cur_entry < limit) { | |
| 405 if (*cur_entry == CardTableModRefBS::clean_card) { | |
| 406 jbyte* first_dirty = cur_entry+1; | |
| 407 while (first_dirty < limit && | |
| 408 *first_dirty == CardTableModRefBS::clean_card) { | |
| 409 first_dirty++; | |
| 410 } | |
| 411 // If the first object is a regular object, and it has a | |
| 412 // young-to-old field, that would mark the previous card. | |
| 413 HeapWord* boundary = addr_for(cur_entry); | |
| 414 HeapWord* end = (first_dirty >= limit) ? used.end() : addr_for(first_dirty); | |
| 415 HeapWord* boundary_block = s->block_start(boundary); | |
| 416 HeapWord* begin = boundary; // Until proven otherwise. | |
| 417 HeapWord* start_block = boundary_block; // Until proven otherwise. | |
| 418 if (boundary_block < boundary) { | |
| 419 if (s->block_is_obj(boundary_block) && s->obj_is_alive(boundary_block)) { | |
| 420 oop boundary_obj = oop(boundary_block); | |
| 421 if (!boundary_obj->is_objArray() && | |
| 422 !boundary_obj->is_typeArray()) { | |
| 423 guarantee(cur_entry > byte_for(used.start()), | |
| 424 "else boundary would be boundary_block"); | |
| 425 if (*byte_for(boundary_block) != CardTableModRefBS::clean_card) { | |
| 426 begin = boundary_block + s->block_size(boundary_block); | |
| 427 start_block = begin; | |
| 428 } | |
| 429 } | |
| 430 } | |
| 431 } | |
| 432 // Now traverse objects until end. | |
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433 if (begin < end) { |
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434 MemRegion mr(begin, end); |
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435 VerifyCleanCardClosure verify_blk(gen_boundary, begin, end); |
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436 for (HeapWord* cur = start_block; cur < end; cur += s->block_size(cur)) { |
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437 if (s->block_is_obj(cur) && s->obj_is_alive(cur)) { |
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438 oop(cur)->oop_iterate(&verify_blk, mr); |
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439 } |
| 0 | 440 } |
| 441 } | |
| 442 cur_entry = first_dirty; | |
| 443 } else { | |
| 444 // We'd normally expect that cur_youngergen_and_prev_nonclean_card | |
| 445 // is a transient value, that cannot be in the card table | |
| 446 // except during GC, and thus assert that: | |
| 447 // guarantee(*cur_entry != cur_youngergen_and_prev_nonclean_card, | |
| 448 // "Illegal CT value"); | |
| 449 // That however, need not hold, as will become clear in the | |
| 450 // following... | |
| 451 | |
| 452 // We'd normally expect that if we are in the parallel case, | |
| 453 // we can't have left a prev value (which would be different | |
| 454 // from the current value) in the card table, and so we'd like to | |
| 455 // assert that: | |
| 456 // guarantee(cur_youngergen_card_val() == youngergen_card | |
| 457 // || !is_prev_youngergen_card_val(*cur_entry), | |
| 458 // "Illegal CT value"); | |
| 459 // That, however, may not hold occasionally, because of | |
| 460 // CMS or MSC in the old gen. To wit, consider the | |
| 461 // following two simple illustrative scenarios: | |
| 462 // (a) CMS: Consider the case where a large object L | |
| 463 // spanning several cards is allocated in the old | |
| 464 // gen, and has a young gen reference stored in it, dirtying | |
| 465 // some interior cards. A young collection scans the card, | |
| 466 // finds a young ref and installs a youngergenP_n value. | |
| 467 // L then goes dead. Now a CMS collection starts, | |
| 468 // finds L dead and sweeps it up. Assume that L is | |
| 469 // abutting _unallocated_blk, so _unallocated_blk is | |
| 470 // adjusted down to (below) L. Assume further that | |
| 471 // no young collection intervenes during this CMS cycle. | |
| 472 // The next young gen cycle will not get to look at this | |
| 473 // youngergenP_n card since it lies in the unoccupied | |
| 474 // part of the space. | |
| 475 // Some young collections later the blocks on this | |
| 476 // card can be re-allocated either due to direct allocation | |
| 477 // or due to absorbing promotions. At this time, the | |
| 478 // before-gc verification will fail the above assert. | |
| 479 // (b) MSC: In this case, an object L with a young reference | |
| 480 // is on a card that (therefore) holds a youngergen_n value. | |
| 481 // Suppose also that L lies towards the end of the used | |
| 482 // the used space before GC. An MSC collection | |
| 483 // occurs that compacts to such an extent that this | |
| 484 // card is no longer in the occupied part of the space. | |
| 485 // Since current code in MSC does not always clear cards | |
| 486 // in the unused part of old gen, this stale youngergen_n | |
| 487 // value is left behind and can later be covered by | |
| 488 // an object when promotion or direct allocation | |
| 489 // re-allocates that part of the heap. | |
| 490 // | |
| 491 // Fortunately, the presence of such stale card values is | |
| 492 // "only" a minor annoyance in that subsequent young collections | |
| 493 // might needlessly scan such cards, but would still never corrupt | |
| 494 // the heap as a result. However, it's likely not to be a significant | |
| 495 // performance inhibitor in practice. For instance, | |
| 496 // some recent measurements with unoccupied cards eagerly cleared | |
| 497 // out to maintain this invariant, showed next to no | |
| 498 // change in young collection times; of course one can construct | |
| 499 // degenerate examples where the cost can be significant.) | |
| 500 // Note, in particular, that if the "stale" card is modified | |
| 501 // after re-allocation, it would be dirty, not "stale". Thus, | |
| 502 // we can never have a younger ref in such a card and it is | |
| 503 // safe not to scan that card in any collection. [As we see | |
| 504 // below, we do some unnecessary scanning | |
| 505 // in some cases in the current parallel scanning algorithm.] | |
| 506 // | |
| 507 // The main point below is that the parallel card scanning code | |
| 508 // deals correctly with these stale card values. There are two main | |
| 509 // cases to consider where we have a stale "younger gen" value and a | |
| 510 // "derivative" case to consider, where we have a stale | |
| 511 // "cur_younger_gen_and_prev_non_clean" value, as will become | |
| 512 // apparent in the case analysis below. | |
| 513 // o Case 1. If the stale value corresponds to a younger_gen_n | |
| 514 // value other than the cur_younger_gen value then the code | |
| 515 // treats this as being tantamount to a prev_younger_gen | |
| 516 // card. This means that the card may be unnecessarily scanned. | |
| 517 // There are two sub-cases to consider: | |
| 518 // o Case 1a. Let us say that the card is in the occupied part | |
| 519 // of the generation at the time the collection begins. In | |
| 520 // that case the card will be either cleared when it is scanned | |
| 521 // for young pointers, or will be set to cur_younger_gen as a | |
| 522 // result of promotion. (We have elided the normal case where | |
| 523 // the scanning thread and the promoting thread interleave | |
| 524 // possibly resulting in a transient | |
| 525 // cur_younger_gen_and_prev_non_clean value before settling | |
| 526 // to cur_younger_gen. [End Case 1a.] | |
| 527 // o Case 1b. Consider now the case when the card is in the unoccupied | |
| 528 // part of the space which becomes occupied because of promotions | |
| 529 // into it during the current young GC. In this case the card | |
| 530 // will never be scanned for young references. The current | |
| 531 // code will set the card value to either | |
| 532 // cur_younger_gen_and_prev_non_clean or leave | |
| 533 // it with its stale value -- because the promotions didn't | |
| 534 // result in any younger refs on that card. Of these two | |
| 535 // cases, the latter will be covered in Case 1a during | |
| 536 // a subsequent scan. To deal with the former case, we need | |
| 537 // to further consider how we deal with a stale value of | |
| 538 // cur_younger_gen_and_prev_non_clean in our case analysis | |
| 539 // below. This we do in Case 3 below. [End Case 1b] | |
| 540 // [End Case 1] | |
| 541 // o Case 2. If the stale value corresponds to cur_younger_gen being | |
| 542 // a value not necessarily written by a current promotion, the | |
| 543 // card will not be scanned by the younger refs scanning code. | |
| 544 // (This is OK since as we argued above such cards cannot contain | |
| 545 // any younger refs.) The result is that this value will be | |
| 546 // treated as a prev_younger_gen value in a subsequent collection, | |
| 547 // which is addressed in Case 1 above. [End Case 2] | |
| 548 // o Case 3. We here consider the "derivative" case from Case 1b. above | |
| 549 // because of which we may find a stale | |
| 550 // cur_younger_gen_and_prev_non_clean card value in the table. | |
| 551 // Once again, as in Case 1, we consider two subcases, depending | |
| 552 // on whether the card lies in the occupied or unoccupied part | |
| 553 // of the space at the start of the young collection. | |
| 554 // o Case 3a. Let us say the card is in the occupied part of | |
| 555 // the old gen at the start of the young collection. In that | |
| 556 // case, the card will be scanned by the younger refs scanning | |
| 557 // code which will set it to cur_younger_gen. In a subsequent | |
| 558 // scan, the card will be considered again and get its final | |
| 559 // correct value. [End Case 3a] | |
| 560 // o Case 3b. Now consider the case where the card is in the | |
| 561 // unoccupied part of the old gen, and is occupied as a result | |
| 562 // of promotions during thus young gc. In that case, | |
| 563 // the card will not be scanned for younger refs. The presence | |
| 564 // of newly promoted objects on the card will then result in | |
| 565 // its keeping the value cur_younger_gen_and_prev_non_clean | |
| 566 // value, which we have dealt with in Case 3 here. [End Case 3b] | |
| 567 // [End Case 3] | |
| 568 // | |
| 569 // (Please refer to the code in the helper class | |
| 570 // ClearNonCleanCardWrapper and in CardTableModRefBS for details.) | |
| 571 // | |
| 572 // The informal arguments above can be tightened into a formal | |
| 573 // correctness proof and it behooves us to write up such a proof, | |
| 574 // or to use model checking to prove that there are no lingering | |
| 575 // concerns. | |
| 576 // | |
| 577 // Clearly because of Case 3b one cannot bound the time for | |
| 578 // which a card will retain what we have called a "stale" value. | |
| 579 // However, one can obtain a Loose upper bound on the redundant | |
| 580 // work as a result of such stale values. Note first that any | |
| 581 // time a stale card lies in the occupied part of the space at | |
| 582 // the start of the collection, it is scanned by younger refs | |
| 583 // code and we can define a rank function on card values that | |
| 584 // declines when this is so. Note also that when a card does not | |
| 585 // lie in the occupied part of the space at the beginning of a | |
| 586 // young collection, its rank can either decline or stay unchanged. | |
| 587 // In this case, no extra work is done in terms of redundant | |
| 588 // younger refs scanning of that card. | |
| 589 // Then, the case analysis above reveals that, in the worst case, | |
| 590 // any such stale card will be scanned unnecessarily at most twice. | |
| 591 // | |
| 592 // It is nonethelss advisable to try and get rid of some of this | |
| 593 // redundant work in a subsequent (low priority) re-design of | |
| 594 // the card-scanning code, if only to simplify the underlying | |
| 595 // state machine analysis/proof. ysr 1/28/2002. XXX | |
| 596 cur_entry++; | |
| 597 } | |
| 598 } | |
| 599 } | |
| 600 | |
| 601 void CardTableRS::verify() { | |
| 602 // At present, we only know how to verify the card table RS for | |
| 603 // generational heaps. | |
| 604 VerifyCTGenClosure blk(this); | |
| 605 CollectedHeap* ch = Universe::heap(); | |
| 606 // We will do the perm-gen portion of the card table, too. | |
| 607 Generation* pg = SharedHeap::heap()->perm_gen(); | |
| 608 HeapWord* pg_boundary = pg->reserved().start(); | |
| 609 | |
| 610 if (ch->kind() == CollectedHeap::GenCollectedHeap) { | |
| 611 GenCollectedHeap::heap()->generation_iterate(&blk, false); | |
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612 _ct_bs->verify(); |
| 0 | 613 |
| 614 // If the old gen collections also collect perm, then we are only | |
| 615 // interested in perm-to-young pointers, not perm-to-old pointers. | |
| 616 GenCollectedHeap* gch = GenCollectedHeap::heap(); | |
| 617 CollectorPolicy* cp = gch->collector_policy(); | |
| 618 if (cp->is_mark_sweep_policy() || cp->is_concurrent_mark_sweep_policy()) { | |
| 619 pg_boundary = gch->get_gen(1)->reserved().start(); | |
| 620 } | |
| 621 } | |
| 622 VerifyCTSpaceClosure perm_space_blk(this, pg_boundary); | |
| 623 SharedHeap::heap()->perm_gen()->space_iterate(&perm_space_blk, true); | |
| 624 } | |
| 625 | |
| 626 | |
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627 void CardTableRS::verify_aligned_region_empty(MemRegion mr) { |
| 0 | 628 if (!mr.is_empty()) { |
| 629 jbyte* cur_entry = byte_for(mr.start()); | |
| 630 jbyte* limit = byte_after(mr.last()); | |
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631 // The region mr may not start on a card boundary so |
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632 // the first card may reflect a write to the space |
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633 // just prior to mr. |
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634 if (!is_aligned(mr.start())) { |
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635 cur_entry++; |
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636 } |
| 0 | 637 for (;cur_entry < limit; cur_entry++) { |
| 638 guarantee(*cur_entry == CardTableModRefBS::clean_card, | |
| 639 "Unexpected dirty card found"); | |
| 640 } | |
| 641 } | |
| 642 } |