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annotate src/share/vm/memory/cardTableRS.cpp @ 11722:ff05c78a7f64
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author | Christian Wirth <christian.wirth@oracle.com> |
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date | Thu, 19 Sep 2013 10:36:56 +0200 |
parents | f9be75d21404 |
children | 330dfb0476f4 |
rev | line source |
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0 | 1 /* |
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2 * Copyright (c) 2001, 2013, 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" | |
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34 #include "utilities/macros.hpp" |
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35 #if INCLUDE_ALL_GCS |
1972 | 36 #include "gc_implementation/g1/concurrentMark.hpp" |
37 #include "gc_implementation/g1/g1SATBCardTableModRefBS.hpp" | |
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38 #endif // INCLUDE_ALL_GCS |
0 | 39 |
40 CardTableRS::CardTableRS(MemRegion whole_heap, | |
41 int max_covered_regions) : | |
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42 GenRemSet(), |
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43 _cur_youngergen_card_val(youngergenP1_card), |
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44 _regions_to_iterate(max_covered_regions - 1) |
0 | 45 { |
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46 #if INCLUDE_ALL_GCS |
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47 if (UseG1GC) { |
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48 _ct_bs = new G1SATBCardTableLoggingModRefBS(whole_heap, |
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49 max_covered_regions); |
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50 } else { |
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51 _ct_bs = new CardTableModRefBSForCTRS(whole_heap, max_covered_regions); |
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52 } |
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53 #else |
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54 _ct_bs = new CardTableModRefBSForCTRS(whole_heap, max_covered_regions); |
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55 #endif |
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56 set_bs(_ct_bs); |
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57 _last_cur_val_in_gen = NEW_C_HEAP_ARRAY3(jbyte, GenCollectedHeap::max_gens + 1, |
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58 mtGC, 0, AllocFailStrategy::RETURN_NULL); |
0 | 59 if (_last_cur_val_in_gen == NULL) { |
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60 vm_exit_during_initialization("Could not create last_cur_val_in_gen array."); |
0 | 61 } |
62 for (int i = 0; i < GenCollectedHeap::max_gens + 1; i++) { | |
63 _last_cur_val_in_gen[i] = clean_card_val(); | |
64 } | |
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65 _ct_bs->set_CTRS(this); |
0 | 66 } |
67 | |
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68 CardTableRS::~CardTableRS() { |
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69 if (_ct_bs) { |
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70 delete _ct_bs; |
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71 _ct_bs = NULL; |
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72 } |
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73 if (_last_cur_val_in_gen) { |
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74 FREE_C_HEAP_ARRAY(jbyte, _last_cur_val_in_gen, mtInternal); |
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75 } |
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76 } |
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77 |
0 | 78 void CardTableRS::resize_covered_region(MemRegion new_region) { |
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79 _ct_bs->resize_covered_region(new_region); |
0 | 80 } |
81 | |
82 jbyte CardTableRS::find_unused_youngergenP_card_value() { | |
83 for (jbyte v = youngergenP1_card; | |
84 v < cur_youngergen_and_prev_nonclean_card; | |
85 v++) { | |
86 bool seen = false; | |
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87 for (int g = 0; g < _regions_to_iterate; g++) { |
0 | 88 if (_last_cur_val_in_gen[g] == v) { |
89 seen = true; | |
90 break; | |
91 } | |
92 } | |
93 if (!seen) return v; | |
94 } | |
95 ShouldNotReachHere(); | |
96 return 0; | |
97 } | |
98 | |
99 void CardTableRS::prepare_for_younger_refs_iterate(bool parallel) { | |
100 // Parallel or sequential, we must always set the prev to equal the | |
101 // last one written. | |
102 if (parallel) { | |
103 // Find a parallel value to be used next. | |
104 jbyte next_val = find_unused_youngergenP_card_value(); | |
105 set_cur_youngergen_card_val(next_val); | |
106 | |
107 } else { | |
108 // In an sequential traversal we will always write youngergen, so that | |
109 // the inline barrier is correct. | |
110 set_cur_youngergen_card_val(youngergen_card); | |
111 } | |
112 } | |
113 | |
114 void CardTableRS::younger_refs_iterate(Generation* g, | |
115 OopsInGenClosure* blk) { | |
116 _last_cur_val_in_gen[g->level()+1] = cur_youngergen_card_val(); | |
117 g->younger_refs_iterate(blk); | |
118 } | |
119 | |
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120 inline bool ClearNoncleanCardWrapper::clear_card(jbyte* entry) { |
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121 if (_is_par) { |
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122 return clear_card_parallel(entry); |
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123 } else { |
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124 return clear_card_serial(entry); |
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125 } |
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126 } |
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127 |
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128 inline bool ClearNoncleanCardWrapper::clear_card_parallel(jbyte* entry) { |
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129 while (true) { |
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130 // In the parallel case, we may have to do this several times. |
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131 jbyte entry_val = *entry; |
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132 assert(entry_val != CardTableRS::clean_card_val(), |
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133 "We shouldn't be looking at clean cards, and this should " |
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134 "be the only place they get cleaned."); |
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135 if (CardTableRS::card_is_dirty_wrt_gen_iter(entry_val) |
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136 || _ct->is_prev_youngergen_card_val(entry_val)) { |
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137 jbyte res = |
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138 Atomic::cmpxchg(CardTableRS::clean_card_val(), entry, entry_val); |
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139 if (res == entry_val) { |
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140 break; |
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141 } else { |
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142 assert(res == CardTableRS::cur_youngergen_and_prev_nonclean_card, |
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143 "The CAS above should only fail if another thread did " |
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144 "a GC write barrier."); |
0 | 145 } |
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146 } else if (entry_val == |
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147 CardTableRS::cur_youngergen_and_prev_nonclean_card) { |
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148 // Parallelism shouldn't matter in this case. Only the thread |
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149 // assigned to scan the card should change this value. |
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150 *entry = _ct->cur_youngergen_card_val(); |
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151 break; |
0 | 152 } else { |
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153 assert(entry_val == _ct->cur_youngergen_card_val(), |
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154 "Should be the only possibility."); |
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155 // In this case, the card was clean before, and become |
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156 // cur_youngergen only because of processing of a promoted object. |
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157 // We don't have to look at the card. |
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158 return false; |
0 | 159 } |
160 } | |
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161 return true; |
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162 } |
0 | 163 |
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164 |
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165 inline bool ClearNoncleanCardWrapper::clear_card_serial(jbyte* entry) { |
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166 jbyte entry_val = *entry; |
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167 assert(entry_val != CardTableRS::clean_card_val(), |
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168 "We shouldn't be looking at clean cards, and this should " |
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169 "be the only place they get cleaned."); |
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170 assert(entry_val != CardTableRS::cur_youngergen_and_prev_nonclean_card, |
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171 "This should be possible in the sequential case."); |
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172 *entry = CardTableRS::clean_card_val(); |
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173 return true; |
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174 } |
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175 |
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176 ClearNoncleanCardWrapper::ClearNoncleanCardWrapper( |
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177 DirtyCardToOopClosure* dirty_card_closure, CardTableRS* ct) : |
0 | 178 _dirty_card_closure(dirty_card_closure), _ct(ct) { |
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179 // Cannot yet substitute active_workers for n_par_threads |
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180 // in the case where parallelism is being turned off by |
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181 // setting n_par_threads to 0. |
0 | 182 _is_par = (SharedHeap::heap()->n_par_threads() > 0); |
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183 assert(!_is_par || |
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184 (SharedHeap::heap()->n_par_threads() == |
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185 SharedHeap::heap()->workers()->active_workers()), "Mismatch"); |
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186 } |
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187 |
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188 bool ClearNoncleanCardWrapper::is_word_aligned(jbyte* entry) { |
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189 return (((intptr_t)entry) & (BytesPerWord-1)) == 0; |
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190 } |
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191 |
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192 void ClearNoncleanCardWrapper::do_MemRegion(MemRegion mr) { |
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193 assert(mr.word_size() > 0, "Error"); |
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194 assert(_ct->is_aligned(mr.start()), "mr.start() should be card aligned"); |
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195 // mr.end() may not necessarily be card aligned. |
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196 jbyte* cur_entry = _ct->byte_for(mr.last()); |
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197 const jbyte* limit = _ct->byte_for(mr.start()); |
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198 HeapWord* end_of_non_clean = mr.end(); |
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199 HeapWord* start_of_non_clean = end_of_non_clean; |
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200 while (cur_entry >= limit) { |
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201 HeapWord* cur_hw = _ct->addr_for(cur_entry); |
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202 if ((*cur_entry != CardTableRS::clean_card_val()) && clear_card(cur_entry)) { |
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203 // Continue the dirty range by opening the |
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204 // dirty window one card to the left. |
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205 start_of_non_clean = cur_hw; |
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206 } else { |
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207 // We hit a "clean" card; process any non-empty |
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208 // "dirty" range accumulated so far. |
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209 if (start_of_non_clean < end_of_non_clean) { |
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210 const MemRegion mrd(start_of_non_clean, end_of_non_clean); |
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211 _dirty_card_closure->do_MemRegion(mrd); |
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212 } |
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213 |
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214 // fast forward through potential continuous whole-word range of clean cards beginning at a word-boundary |
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215 if (is_word_aligned(cur_entry)) { |
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216 jbyte* cur_row = cur_entry - BytesPerWord; |
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217 while (cur_row >= limit && *((intptr_t*)cur_row) == CardTableRS::clean_card_row()) { |
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218 cur_row -= BytesPerWord; |
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219 } |
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220 cur_entry = cur_row + BytesPerWord; |
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221 cur_hw = _ct->addr_for(cur_entry); |
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222 } |
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223 |
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224 // Reset the dirty window, while continuing to look |
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225 // for the next dirty card that will start a |
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226 // new dirty window. |
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227 end_of_non_clean = cur_hw; |
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228 start_of_non_clean = cur_hw; |
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229 } |
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230 // Note that "cur_entry" leads "start_of_non_clean" in |
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231 // its leftward excursion after this point |
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232 // in the loop and, when we hit the left end of "mr", |
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233 // will point off of the left end of the card-table |
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234 // for "mr". |
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235 cur_entry--; |
0 | 236 } |
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237 // If the first card of "mr" was dirty, we will have |
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238 // been left with a dirty window, co-initial with "mr", |
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239 // which we now process. |
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240 if (start_of_non_clean < end_of_non_clean) { |
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241 const MemRegion mrd(start_of_non_clean, end_of_non_clean); |
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242 _dirty_card_closure->do_MemRegion(mrd); |
0 | 243 } |
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244 } |
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245 |
0 | 246 // clean (by dirty->clean before) ==> cur_younger_gen |
247 // dirty ==> cur_youngergen_and_prev_nonclean_card | |
248 // precleaned ==> cur_youngergen_and_prev_nonclean_card | |
249 // prev-younger-gen ==> cur_youngergen_and_prev_nonclean_card | |
250 // cur-younger-gen ==> cur_younger_gen | |
251 // cur_youngergen_and_prev_nonclean_card ==> no change. | |
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252 void CardTableRS::write_ref_field_gc_par(void* field, oop new_val) { |
0 | 253 jbyte* entry = ct_bs()->byte_for(field); |
254 do { | |
255 jbyte entry_val = *entry; | |
256 // We put this first because it's probably the most common case. | |
257 if (entry_val == clean_card_val()) { | |
258 // No threat of contention with cleaning threads. | |
259 *entry = cur_youngergen_card_val(); | |
260 return; | |
261 } else if (card_is_dirty_wrt_gen_iter(entry_val) | |
262 || is_prev_youngergen_card_val(entry_val)) { | |
263 // Mark it as both cur and prev youngergen; card cleaning thread will | |
264 // eventually remove the previous stuff. | |
265 jbyte new_val = cur_youngergen_and_prev_nonclean_card; | |
266 jbyte res = Atomic::cmpxchg(new_val, entry, entry_val); | |
267 // Did the CAS succeed? | |
268 if (res == entry_val) return; | |
269 // Otherwise, retry, to see the new value. | |
270 continue; | |
271 } else { | |
272 assert(entry_val == cur_youngergen_and_prev_nonclean_card | |
273 || entry_val == cur_youngergen_card_val(), | |
274 "should be only possibilities."); | |
275 return; | |
276 } | |
277 } while (true); | |
278 } | |
279 | |
280 void CardTableRS::younger_refs_in_space_iterate(Space* sp, | |
281 OopsInGenClosure* cl) { | |
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282 const MemRegion urasm = sp->used_region_at_save_marks(); |
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283 #ifdef ASSERT |
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284 // Convert the assertion check to a warning if we are running |
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285 // CMS+ParNew until related bug is fixed. |
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286 MemRegion ur = sp->used_region(); |
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287 assert(ur.contains(urasm) || (UseConcMarkSweepGC && UseParNewGC), |
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288 err_msg("Did you forget to call save_marks()? " |
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289 "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in " |
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290 "[" PTR_FORMAT ", " PTR_FORMAT ")", |
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291 urasm.start(), urasm.end(), ur.start(), ur.end())); |
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292 // In the case of CMS+ParNew, issue a warning |
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293 if (!ur.contains(urasm)) { |
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294 assert(UseConcMarkSweepGC && UseParNewGC, "Tautology: see assert above"); |
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295 warning("CMS+ParNew: Did you forget to call save_marks()? " |
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296 "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in " |
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297 "[" PTR_FORMAT ", " PTR_FORMAT ")", |
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298 urasm.start(), urasm.end(), ur.start(), ur.end()); |
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299 MemRegion ur2 = sp->used_region(); |
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300 MemRegion urasm2 = sp->used_region_at_save_marks(); |
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301 if (!ur.equals(ur2)) { |
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302 warning("CMS+ParNew: Flickering used_region()!!"); |
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303 } |
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304 if (!urasm.equals(urasm2)) { |
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305 warning("CMS+ParNew: Flickering used_region_at_save_marks()!!"); |
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306 } |
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307 ShouldNotReachHere(); |
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308 } |
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309 #endif |
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310 _ct_bs->non_clean_card_iterate_possibly_parallel(sp, urasm, cl, this); |
0 | 311 } |
312 | |
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313 void CardTableRS::clear_into_younger(Generation* gen) { |
0 | 314 GenCollectedHeap* gch = GenCollectedHeap::heap(); |
315 // Generations younger than gen have been evacuated. We can clear | |
316 // card table entries for gen (we know that it has no pointers | |
317 // to younger gens) and for those below. The card tables for | |
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318 // the youngest gen need never be cleared. |
0 | 319 // There's a bit of subtlety in the clear() and invalidate() |
320 // methods that we exploit here and in invalidate_or_clear() | |
321 // below to avoid missing cards at the fringes. If clear() or | |
322 // invalidate() are changed in the future, this code should | |
323 // be revisited. 20040107.ysr | |
324 Generation* g = gen; | |
325 for(Generation* prev_gen = gch->prev_gen(g); | |
326 prev_gen != NULL; | |
327 g = prev_gen, prev_gen = gch->prev_gen(g)) { | |
328 MemRegion to_be_cleared_mr = g->prev_used_region(); | |
329 clear(to_be_cleared_mr); | |
330 } | |
331 } | |
332 | |
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333 void CardTableRS::invalidate_or_clear(Generation* gen, bool younger) { |
0 | 334 GenCollectedHeap* gch = GenCollectedHeap::heap(); |
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335 // For each generation gen (and younger) |
0 | 336 // invalidate the cards for the currently occupied part |
337 // of that generation and clear the cards for the | |
338 // unoccupied part of the generation (if any, making use | |
339 // of that generation's prev_used_region to determine that | |
340 // region). No need to do anything for the youngest | |
341 // generation. Also see note#20040107.ysr above. | |
342 Generation* g = gen; | |
343 for(Generation* prev_gen = gch->prev_gen(g); prev_gen != NULL; | |
344 g = prev_gen, prev_gen = gch->prev_gen(g)) { | |
345 MemRegion used_mr = g->used_region(); | |
346 MemRegion to_be_cleared_mr = g->prev_used_region().minus(used_mr); | |
347 if (!to_be_cleared_mr.is_empty()) { | |
348 clear(to_be_cleared_mr); | |
349 } | |
350 invalidate(used_mr); | |
351 if (!younger) break; | |
352 } | |
353 } | |
354 | |
355 | |
356 class VerifyCleanCardClosure: public OopClosure { | |
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357 private: |
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358 HeapWord* _boundary; |
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359 HeapWord* _begin; |
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360 HeapWord* _end; |
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361 protected: |
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362 template <class T> void do_oop_work(T* p) { |
0 | 363 HeapWord* jp = (HeapWord*)p; |
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364 assert(jp >= _begin && jp < _end, |
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365 err_msg("Error: jp " PTR_FORMAT " should be within " |
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366 "[_begin, _end) = [" PTR_FORMAT "," PTR_FORMAT ")", |
8668 | 367 jp, _begin, _end)); |
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368 oop obj = oopDesc::load_decode_heap_oop(p); |
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369 guarantee(obj == NULL || (HeapWord*)obj >= _boundary, |
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370 err_msg("pointer " PTR_FORMAT " at " PTR_FORMAT " on " |
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371 "clean card crosses boundary" PTR_FORMAT, |
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372 (HeapWord*)obj, jp, _boundary)); |
0 | 373 } |
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374 |
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375 public: |
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376 VerifyCleanCardClosure(HeapWord* b, HeapWord* begin, HeapWord* end) : |
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377 _boundary(b), _begin(begin), _end(end) { |
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378 assert(b <= begin, |
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379 err_msg("Error: boundary " PTR_FORMAT " should be at or below begin " PTR_FORMAT, |
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380 b, begin)); |
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381 assert(begin <= end, |
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382 err_msg("Error: begin " PTR_FORMAT " should be strictly below end " PTR_FORMAT, |
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383 begin, end)); |
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384 } |
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385 |
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386 virtual void do_oop(oop* p) { VerifyCleanCardClosure::do_oop_work(p); } |
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387 virtual void do_oop(narrowOop* p) { VerifyCleanCardClosure::do_oop_work(p); } |
0 | 388 }; |
389 | |
390 class VerifyCTSpaceClosure: public SpaceClosure { | |
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391 private: |
0 | 392 CardTableRS* _ct; |
393 HeapWord* _boundary; | |
394 public: | |
395 VerifyCTSpaceClosure(CardTableRS* ct, HeapWord* boundary) : | |
396 _ct(ct), _boundary(boundary) {} | |
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397 virtual void do_space(Space* s) { _ct->verify_space(s, _boundary); } |
0 | 398 }; |
399 | |
400 class VerifyCTGenClosure: public GenCollectedHeap::GenClosure { | |
401 CardTableRS* _ct; | |
402 public: | |
403 VerifyCTGenClosure(CardTableRS* ct) : _ct(ct) {} | |
404 void do_generation(Generation* gen) { | |
405 // Skip the youngest generation. | |
406 if (gen->level() == 0) return; | |
407 // Normally, we're interested in pointers to younger generations. | |
408 VerifyCTSpaceClosure blk(_ct, gen->reserved().start()); | |
409 gen->space_iterate(&blk, true); | |
410 } | |
411 }; | |
412 | |
413 void CardTableRS::verify_space(Space* s, HeapWord* gen_boundary) { | |
414 // We don't need to do young-gen spaces. | |
415 if (s->end() <= gen_boundary) return; | |
416 MemRegion used = s->used_region(); | |
417 | |
418 jbyte* cur_entry = byte_for(used.start()); | |
419 jbyte* limit = byte_after(used.last()); | |
420 while (cur_entry < limit) { | |
421 if (*cur_entry == CardTableModRefBS::clean_card) { | |
422 jbyte* first_dirty = cur_entry+1; | |
423 while (first_dirty < limit && | |
424 *first_dirty == CardTableModRefBS::clean_card) { | |
425 first_dirty++; | |
426 } | |
427 // If the first object is a regular object, and it has a | |
428 // young-to-old field, that would mark the previous card. | |
429 HeapWord* boundary = addr_for(cur_entry); | |
430 HeapWord* end = (first_dirty >= limit) ? used.end() : addr_for(first_dirty); | |
431 HeapWord* boundary_block = s->block_start(boundary); | |
432 HeapWord* begin = boundary; // Until proven otherwise. | |
433 HeapWord* start_block = boundary_block; // Until proven otherwise. | |
434 if (boundary_block < boundary) { | |
435 if (s->block_is_obj(boundary_block) && s->obj_is_alive(boundary_block)) { | |
436 oop boundary_obj = oop(boundary_block); | |
437 if (!boundary_obj->is_objArray() && | |
438 !boundary_obj->is_typeArray()) { | |
439 guarantee(cur_entry > byte_for(used.start()), | |
440 "else boundary would be boundary_block"); | |
441 if (*byte_for(boundary_block) != CardTableModRefBS::clean_card) { | |
442 begin = boundary_block + s->block_size(boundary_block); | |
443 start_block = begin; | |
444 } | |
445 } | |
446 } | |
447 } | |
448 // Now traverse objects until end. | |
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449 if (begin < end) { |
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450 MemRegion mr(begin, end); |
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451 VerifyCleanCardClosure verify_blk(gen_boundary, begin, end); |
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452 for (HeapWord* cur = start_block; cur < end; cur += s->block_size(cur)) { |
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453 if (s->block_is_obj(cur) && s->obj_is_alive(cur)) { |
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454 oop(cur)->oop_iterate_no_header(&verify_blk, mr); |
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455 } |
0 | 456 } |
457 } | |
458 cur_entry = first_dirty; | |
459 } else { | |
460 // We'd normally expect that cur_youngergen_and_prev_nonclean_card | |
461 // is a transient value, that cannot be in the card table | |
462 // except during GC, and thus assert that: | |
463 // guarantee(*cur_entry != cur_youngergen_and_prev_nonclean_card, | |
464 // "Illegal CT value"); | |
465 // That however, need not hold, as will become clear in the | |
466 // following... | |
467 | |
468 // We'd normally expect that if we are in the parallel case, | |
469 // we can't have left a prev value (which would be different | |
470 // from the current value) in the card table, and so we'd like to | |
471 // assert that: | |
472 // guarantee(cur_youngergen_card_val() == youngergen_card | |
473 // || !is_prev_youngergen_card_val(*cur_entry), | |
474 // "Illegal CT value"); | |
475 // That, however, may not hold occasionally, because of | |
476 // CMS or MSC in the old gen. To wit, consider the | |
477 // following two simple illustrative scenarios: | |
478 // (a) CMS: Consider the case where a large object L | |
479 // spanning several cards is allocated in the old | |
480 // gen, and has a young gen reference stored in it, dirtying | |
481 // some interior cards. A young collection scans the card, | |
482 // finds a young ref and installs a youngergenP_n value. | |
483 // L then goes dead. Now a CMS collection starts, | |
484 // finds L dead and sweeps it up. Assume that L is | |
485 // abutting _unallocated_blk, so _unallocated_blk is | |
486 // adjusted down to (below) L. Assume further that | |
487 // no young collection intervenes during this CMS cycle. | |
488 // The next young gen cycle will not get to look at this | |
489 // youngergenP_n card since it lies in the unoccupied | |
490 // part of the space. | |
491 // Some young collections later the blocks on this | |
492 // card can be re-allocated either due to direct allocation | |
493 // or due to absorbing promotions. At this time, the | |
494 // before-gc verification will fail the above assert. | |
495 // (b) MSC: In this case, an object L with a young reference | |
496 // is on a card that (therefore) holds a youngergen_n value. | |
497 // Suppose also that L lies towards the end of the used | |
498 // the used space before GC. An MSC collection | |
499 // occurs that compacts to such an extent that this | |
500 // card is no longer in the occupied part of the space. | |
501 // Since current code in MSC does not always clear cards | |
502 // in the unused part of old gen, this stale youngergen_n | |
503 // value is left behind and can later be covered by | |
504 // an object when promotion or direct allocation | |
505 // re-allocates that part of the heap. | |
506 // | |
507 // Fortunately, the presence of such stale card values is | |
508 // "only" a minor annoyance in that subsequent young collections | |
509 // might needlessly scan such cards, but would still never corrupt | |
510 // the heap as a result. However, it's likely not to be a significant | |
511 // performance inhibitor in practice. For instance, | |
512 // some recent measurements with unoccupied cards eagerly cleared | |
513 // out to maintain this invariant, showed next to no | |
514 // change in young collection times; of course one can construct | |
515 // degenerate examples where the cost can be significant.) | |
516 // Note, in particular, that if the "stale" card is modified | |
517 // after re-allocation, it would be dirty, not "stale". Thus, | |
518 // we can never have a younger ref in such a card and it is | |
519 // safe not to scan that card in any collection. [As we see | |
520 // below, we do some unnecessary scanning | |
521 // in some cases in the current parallel scanning algorithm.] | |
522 // | |
523 // The main point below is that the parallel card scanning code | |
524 // deals correctly with these stale card values. There are two main | |
525 // cases to consider where we have a stale "younger gen" value and a | |
526 // "derivative" case to consider, where we have a stale | |
527 // "cur_younger_gen_and_prev_non_clean" value, as will become | |
528 // apparent in the case analysis below. | |
529 // o Case 1. If the stale value corresponds to a younger_gen_n | |
530 // value other than the cur_younger_gen value then the code | |
531 // treats this as being tantamount to a prev_younger_gen | |
532 // card. This means that the card may be unnecessarily scanned. | |
533 // There are two sub-cases to consider: | |
534 // o Case 1a. Let us say that the card is in the occupied part | |
535 // of the generation at the time the collection begins. In | |
536 // that case the card will be either cleared when it is scanned | |
537 // for young pointers, or will be set to cur_younger_gen as a | |
538 // result of promotion. (We have elided the normal case where | |
539 // the scanning thread and the promoting thread interleave | |
540 // possibly resulting in a transient | |
541 // cur_younger_gen_and_prev_non_clean value before settling | |
542 // to cur_younger_gen. [End Case 1a.] | |
543 // o Case 1b. Consider now the case when the card is in the unoccupied | |
544 // part of the space which becomes occupied because of promotions | |
545 // into it during the current young GC. In this case the card | |
546 // will never be scanned for young references. The current | |
547 // code will set the card value to either | |
548 // cur_younger_gen_and_prev_non_clean or leave | |
549 // it with its stale value -- because the promotions didn't | |
550 // result in any younger refs on that card. Of these two | |
551 // cases, the latter will be covered in Case 1a during | |
552 // a subsequent scan. To deal with the former case, we need | |
553 // to further consider how we deal with a stale value of | |
554 // cur_younger_gen_and_prev_non_clean in our case analysis | |
555 // below. This we do in Case 3 below. [End Case 1b] | |
556 // [End Case 1] | |
557 // o Case 2. If the stale value corresponds to cur_younger_gen being | |
558 // a value not necessarily written by a current promotion, the | |
559 // card will not be scanned by the younger refs scanning code. | |
560 // (This is OK since as we argued above such cards cannot contain | |
561 // any younger refs.) The result is that this value will be | |
562 // treated as a prev_younger_gen value in a subsequent collection, | |
563 // which is addressed in Case 1 above. [End Case 2] | |
564 // o Case 3. We here consider the "derivative" case from Case 1b. above | |
565 // because of which we may find a stale | |
566 // cur_younger_gen_and_prev_non_clean card value in the table. | |
567 // Once again, as in Case 1, we consider two subcases, depending | |
568 // on whether the card lies in the occupied or unoccupied part | |
569 // of the space at the start of the young collection. | |
570 // o Case 3a. Let us say the card is in the occupied part of | |
571 // the old gen at the start of the young collection. In that | |
572 // case, the card will be scanned by the younger refs scanning | |
573 // code which will set it to cur_younger_gen. In a subsequent | |
574 // scan, the card will be considered again and get its final | |
575 // correct value. [End Case 3a] | |
576 // o Case 3b. Now consider the case where the card is in the | |
577 // unoccupied part of the old gen, and is occupied as a result | |
578 // of promotions during thus young gc. In that case, | |
579 // the card will not be scanned for younger refs. The presence | |
580 // of newly promoted objects on the card will then result in | |
581 // its keeping the value cur_younger_gen_and_prev_non_clean | |
582 // value, which we have dealt with in Case 3 here. [End Case 3b] | |
583 // [End Case 3] | |
584 // | |
585 // (Please refer to the code in the helper class | |
586 // ClearNonCleanCardWrapper and in CardTableModRefBS for details.) | |
587 // | |
588 // The informal arguments above can be tightened into a formal | |
589 // correctness proof and it behooves us to write up such a proof, | |
590 // or to use model checking to prove that there are no lingering | |
591 // concerns. | |
592 // | |
593 // Clearly because of Case 3b one cannot bound the time for | |
594 // which a card will retain what we have called a "stale" value. | |
595 // However, one can obtain a Loose upper bound on the redundant | |
596 // work as a result of such stale values. Note first that any | |
597 // time a stale card lies in the occupied part of the space at | |
598 // the start of the collection, it is scanned by younger refs | |
599 // code and we can define a rank function on card values that | |
600 // declines when this is so. Note also that when a card does not | |
601 // lie in the occupied part of the space at the beginning of a | |
602 // young collection, its rank can either decline or stay unchanged. | |
603 // In this case, no extra work is done in terms of redundant | |
604 // younger refs scanning of that card. | |
605 // Then, the case analysis above reveals that, in the worst case, | |
606 // any such stale card will be scanned unnecessarily at most twice. | |
607 // | |
608 // It is nonethelss advisable to try and get rid of some of this | |
609 // redundant work in a subsequent (low priority) re-design of | |
610 // the card-scanning code, if only to simplify the underlying | |
611 // state machine analysis/proof. ysr 1/28/2002. XXX | |
612 cur_entry++; | |
613 } | |
614 } | |
615 } | |
616 | |
617 void CardTableRS::verify() { | |
618 // At present, we only know how to verify the card table RS for | |
619 // generational heaps. | |
620 VerifyCTGenClosure blk(this); | |
621 CollectedHeap* ch = Universe::heap(); | |
622 | |
623 if (ch->kind() == CollectedHeap::GenCollectedHeap) { | |
624 GenCollectedHeap::heap()->generation_iterate(&blk, false); | |
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625 _ct_bs->verify(); |
0 | 626 } |
627 } | |
628 | |
629 | |
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630 void CardTableRS::verify_aligned_region_empty(MemRegion mr) { |
0 | 631 if (!mr.is_empty()) { |
632 jbyte* cur_entry = byte_for(mr.start()); | |
633 jbyte* limit = byte_after(mr.last()); | |
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634 // The region mr may not start on a card boundary so |
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635 // the first card may reflect a write to the space |
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636 // just prior to mr. |
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637 if (!is_aligned(mr.start())) { |
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638 cur_entry++; |
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639 } |
0 | 640 for (;cur_entry < limit; cur_entry++) { |
641 guarantee(*cur_entry == CardTableModRefBS::clean_card, | |
642 "Unexpected dirty card found"); | |
643 } | |
644 } | |
645 } |