Mercurial > hg > graal-compiler
annotate src/share/vm/memory/cardTableRS.cpp @ 7199:cd3d6a6b95d9
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author | twisti |
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date | Fri, 30 Nov 2012 15:23:16 -0800 |
parents | da91efe96a93 |
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rev | line source |
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0 | 1 /* |
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2 * Copyright (c) 2001, 2012, 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) : | |
342
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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) { |
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167 // Cannot yet substitute active_workers for n_par_threads |
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168 // in the case where parallelism is being turned off by |
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169 // setting n_par_threads to 0. |
0 | 170 _is_par = (SharedHeap::heap()->n_par_threads() > 0); |
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171 assert(!_is_par || |
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172 (SharedHeap::heap()->n_par_threads() == |
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173 SharedHeap::heap()->workers()->active_workers()), "Mismatch"); |
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174 } |
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175 |
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176 bool ClearNoncleanCardWrapper::is_word_aligned(jbyte* entry) { |
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177 return (((intptr_t)entry) & (BytesPerWord-1)) == 0; |
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178 } |
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179 |
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180 void ClearNoncleanCardWrapper::do_MemRegion(MemRegion mr) { |
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181 assert(mr.word_size() > 0, "Error"); |
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182 assert(_ct->is_aligned(mr.start()), "mr.start() should be card aligned"); |
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183 // mr.end() may not necessarily be card aligned. |
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184 jbyte* cur_entry = _ct->byte_for(mr.last()); |
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185 const jbyte* limit = _ct->byte_for(mr.start()); |
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186 HeapWord* end_of_non_clean = mr.end(); |
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187 HeapWord* start_of_non_clean = end_of_non_clean; |
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188 while (cur_entry >= limit) { |
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189 HeapWord* cur_hw = _ct->addr_for(cur_entry); |
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190 if ((*cur_entry != CardTableRS::clean_card_val()) && clear_card(cur_entry)) { |
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191 // Continue the dirty range by opening the |
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192 // dirty window one card to the left. |
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193 start_of_non_clean = cur_hw; |
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194 } else { |
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195 // We hit a "clean" card; process any non-empty |
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196 // "dirty" range accumulated so far. |
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197 if (start_of_non_clean < end_of_non_clean) { |
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198 const MemRegion mrd(start_of_non_clean, end_of_non_clean); |
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199 _dirty_card_closure->do_MemRegion(mrd); |
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200 } |
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201 |
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202 // fast forward through potential continuous whole-word range of clean cards beginning at a word-boundary |
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203 if (is_word_aligned(cur_entry)) { |
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204 jbyte* cur_row = cur_entry - BytesPerWord; |
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205 while (cur_row >= limit && *((intptr_t*)cur_row) == CardTableRS::clean_card_row()) { |
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206 cur_row -= BytesPerWord; |
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207 } |
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208 cur_entry = cur_row + BytesPerWord; |
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209 cur_hw = _ct->addr_for(cur_entry); |
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210 } |
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211 |
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212 // Reset the dirty window, while continuing to look |
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213 // for the next dirty card that will start a |
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214 // new dirty window. |
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215 end_of_non_clean = cur_hw; |
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216 start_of_non_clean = cur_hw; |
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217 } |
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218 // Note that "cur_entry" leads "start_of_non_clean" in |
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219 // its leftward excursion after this point |
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220 // in the loop and, when we hit the left end of "mr", |
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221 // will point off of the left end of the card-table |
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222 // for "mr". |
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223 cur_entry--; |
0 | 224 } |
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225 // If the first card of "mr" was dirty, we will have |
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226 // been left with a dirty window, co-initial with "mr", |
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227 // which we now process. |
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228 if (start_of_non_clean < end_of_non_clean) { |
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229 const MemRegion mrd(start_of_non_clean, end_of_non_clean); |
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230 _dirty_card_closure->do_MemRegion(mrd); |
0 | 231 } |
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232 } |
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233 |
0 | 234 // clean (by dirty->clean before) ==> cur_younger_gen |
235 // dirty ==> cur_youngergen_and_prev_nonclean_card | |
236 // precleaned ==> cur_youngergen_and_prev_nonclean_card | |
237 // prev-younger-gen ==> cur_youngergen_and_prev_nonclean_card | |
238 // cur-younger-gen ==> cur_younger_gen | |
239 // cur_youngergen_and_prev_nonclean_card ==> no change. | |
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240 void CardTableRS::write_ref_field_gc_par(void* field, oop new_val) { |
0 | 241 jbyte* entry = ct_bs()->byte_for(field); |
242 do { | |
243 jbyte entry_val = *entry; | |
244 // We put this first because it's probably the most common case. | |
245 if (entry_val == clean_card_val()) { | |
246 // No threat of contention with cleaning threads. | |
247 *entry = cur_youngergen_card_val(); | |
248 return; | |
249 } else if (card_is_dirty_wrt_gen_iter(entry_val) | |
250 || is_prev_youngergen_card_val(entry_val)) { | |
251 // Mark it as both cur and prev youngergen; card cleaning thread will | |
252 // eventually remove the previous stuff. | |
253 jbyte new_val = cur_youngergen_and_prev_nonclean_card; | |
254 jbyte res = Atomic::cmpxchg(new_val, entry, entry_val); | |
255 // Did the CAS succeed? | |
256 if (res == entry_val) return; | |
257 // Otherwise, retry, to see the new value. | |
258 continue; | |
259 } else { | |
260 assert(entry_val == cur_youngergen_and_prev_nonclean_card | |
261 || entry_val == cur_youngergen_card_val(), | |
262 "should be only possibilities."); | |
263 return; | |
264 } | |
265 } while (true); | |
266 } | |
267 | |
268 void CardTableRS::younger_refs_in_space_iterate(Space* sp, | |
269 OopsInGenClosure* cl) { | |
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270 const MemRegion urasm = sp->used_region_at_save_marks(); |
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271 #ifdef ASSERT |
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272 // Convert the assertion check to a warning if we are running |
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273 // CMS+ParNew until related bug is fixed. |
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274 MemRegion ur = sp->used_region(); |
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275 assert(ur.contains(urasm) || (UseConcMarkSweepGC && UseParNewGC), |
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276 err_msg("Did you forget to call save_marks()? " |
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277 "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in " |
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278 "[" PTR_FORMAT ", " PTR_FORMAT ")", |
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279 urasm.start(), urasm.end(), ur.start(), ur.end())); |
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280 // In the case of CMS+ParNew, issue a warning |
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281 if (!ur.contains(urasm)) { |
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282 assert(UseConcMarkSweepGC && UseParNewGC, "Tautology: see assert above"); |
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283 warning("CMS+ParNew: Did you forget to call save_marks()? " |
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284 "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in " |
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285 "[" PTR_FORMAT ", " PTR_FORMAT ")", |
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286 urasm.start(), urasm.end(), ur.start(), ur.end()); |
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287 MemRegion ur2 = sp->used_region(); |
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288 MemRegion urasm2 = sp->used_region_at_save_marks(); |
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289 if (!ur.equals(ur2)) { |
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290 warning("CMS+ParNew: Flickering used_region()!!"); |
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291 } |
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292 if (!urasm.equals(urasm2)) { |
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293 warning("CMS+ParNew: Flickering used_region_at_save_marks()!!"); |
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294 } |
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295 ShouldNotReachHere(); |
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296 } |
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297 #endif |
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298 _ct_bs->non_clean_card_iterate_possibly_parallel(sp, urasm, cl, this); |
0 | 299 } |
300 | |
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301 void CardTableRS::clear_into_younger(Generation* gen) { |
0 | 302 GenCollectedHeap* gch = GenCollectedHeap::heap(); |
303 // Generations younger than gen have been evacuated. We can clear | |
304 // card table entries for gen (we know that it has no pointers | |
305 // to younger gens) and for those below. The card tables for | |
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306 // the youngest gen need never be cleared. |
0 | 307 // There's a bit of subtlety in the clear() and invalidate() |
308 // methods that we exploit here and in invalidate_or_clear() | |
309 // below to avoid missing cards at the fringes. If clear() or | |
310 // invalidate() are changed in the future, this code should | |
311 // be revisited. 20040107.ysr | |
312 Generation* g = gen; | |
313 for(Generation* prev_gen = gch->prev_gen(g); | |
314 prev_gen != NULL; | |
315 g = prev_gen, prev_gen = gch->prev_gen(g)) { | |
316 MemRegion to_be_cleared_mr = g->prev_used_region(); | |
317 clear(to_be_cleared_mr); | |
318 } | |
319 } | |
320 | |
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321 void CardTableRS::invalidate_or_clear(Generation* gen, bool younger) { |
0 | 322 GenCollectedHeap* gch = GenCollectedHeap::heap(); |
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323 // For each generation gen (and younger) |
0 | 324 // invalidate the cards for the currently occupied part |
325 // of that generation and clear the cards for the | |
326 // unoccupied part of the generation (if any, making use | |
327 // of that generation's prev_used_region to determine that | |
328 // region). No need to do anything for the youngest | |
329 // generation. Also see note#20040107.ysr above. | |
330 Generation* g = gen; | |
331 for(Generation* prev_gen = gch->prev_gen(g); prev_gen != NULL; | |
332 g = prev_gen, prev_gen = gch->prev_gen(g)) { | |
333 MemRegion used_mr = g->used_region(); | |
334 MemRegion to_be_cleared_mr = g->prev_used_region().minus(used_mr); | |
335 if (!to_be_cleared_mr.is_empty()) { | |
336 clear(to_be_cleared_mr); | |
337 } | |
338 invalidate(used_mr); | |
339 if (!younger) break; | |
340 } | |
341 } | |
342 | |
343 | |
344 class VerifyCleanCardClosure: public OopClosure { | |
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345 private: |
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346 HeapWord* _boundary; |
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347 HeapWord* _begin; |
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348 HeapWord* _end; |
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349 protected: |
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350 template <class T> void do_oop_work(T* p) { |
0 | 351 HeapWord* jp = (HeapWord*)p; |
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352 assert(jp >= _begin && jp < _end, |
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353 err_msg("Error: jp " PTR_FORMAT " should be within " |
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354 "[_begin, _end) = [" PTR_FORMAT "," PTR_FORMAT ")", |
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355 _begin, _end)); |
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356 oop obj = oopDesc::load_decode_heap_oop(p); |
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357 guarantee(obj == NULL || (HeapWord*)obj >= _boundary, |
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358 err_msg("pointer " PTR_FORMAT " at " PTR_FORMAT " on " |
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359 "clean card crosses boundary" PTR_FORMAT, |
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360 (HeapWord*)obj, jp, _boundary)); |
0 | 361 } |
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362 |
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363 public: |
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364 VerifyCleanCardClosure(HeapWord* b, HeapWord* begin, HeapWord* end) : |
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365 _boundary(b), _begin(begin), _end(end) { |
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366 assert(b <= begin, |
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367 err_msg("Error: boundary " PTR_FORMAT " should be at or below begin " PTR_FORMAT, |
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368 b, begin)); |
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369 assert(begin <= end, |
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370 err_msg("Error: begin " PTR_FORMAT " should be strictly below end " PTR_FORMAT, |
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371 begin, end)); |
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372 } |
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373 |
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374 virtual void do_oop(oop* p) { VerifyCleanCardClosure::do_oop_work(p); } |
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375 virtual void do_oop(narrowOop* p) { VerifyCleanCardClosure::do_oop_work(p); } |
0 | 376 }; |
377 | |
378 class VerifyCTSpaceClosure: public SpaceClosure { | |
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379 private: |
0 | 380 CardTableRS* _ct; |
381 HeapWord* _boundary; | |
382 public: | |
383 VerifyCTSpaceClosure(CardTableRS* ct, HeapWord* boundary) : | |
384 _ct(ct), _boundary(boundary) {} | |
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385 virtual void do_space(Space* s) { _ct->verify_space(s, _boundary); } |
0 | 386 }; |
387 | |
388 class VerifyCTGenClosure: public GenCollectedHeap::GenClosure { | |
389 CardTableRS* _ct; | |
390 public: | |
391 VerifyCTGenClosure(CardTableRS* ct) : _ct(ct) {} | |
392 void do_generation(Generation* gen) { | |
393 // Skip the youngest generation. | |
394 if (gen->level() == 0) return; | |
395 // Normally, we're interested in pointers to younger generations. | |
396 VerifyCTSpaceClosure blk(_ct, gen->reserved().start()); | |
397 gen->space_iterate(&blk, true); | |
398 } | |
399 }; | |
400 | |
401 void CardTableRS::verify_space(Space* s, HeapWord* gen_boundary) { | |
402 // We don't need to do young-gen spaces. | |
403 if (s->end() <= gen_boundary) return; | |
404 MemRegion used = s->used_region(); | |
405 | |
406 jbyte* cur_entry = byte_for(used.start()); | |
407 jbyte* limit = byte_after(used.last()); | |
408 while (cur_entry < limit) { | |
409 if (*cur_entry == CardTableModRefBS::clean_card) { | |
410 jbyte* first_dirty = cur_entry+1; | |
411 while (first_dirty < limit && | |
412 *first_dirty == CardTableModRefBS::clean_card) { | |
413 first_dirty++; | |
414 } | |
415 // If the first object is a regular object, and it has a | |
416 // young-to-old field, that would mark the previous card. | |
417 HeapWord* boundary = addr_for(cur_entry); | |
418 HeapWord* end = (first_dirty >= limit) ? used.end() : addr_for(first_dirty); | |
419 HeapWord* boundary_block = s->block_start(boundary); | |
420 HeapWord* begin = boundary; // Until proven otherwise. | |
421 HeapWord* start_block = boundary_block; // Until proven otherwise. | |
422 if (boundary_block < boundary) { | |
423 if (s->block_is_obj(boundary_block) && s->obj_is_alive(boundary_block)) { | |
424 oop boundary_obj = oop(boundary_block); | |
425 if (!boundary_obj->is_objArray() && | |
426 !boundary_obj->is_typeArray()) { | |
427 guarantee(cur_entry > byte_for(used.start()), | |
428 "else boundary would be boundary_block"); | |
429 if (*byte_for(boundary_block) != CardTableModRefBS::clean_card) { | |
430 begin = boundary_block + s->block_size(boundary_block); | |
431 start_block = begin; | |
432 } | |
433 } | |
434 } | |
435 } | |
436 // Now traverse objects until end. | |
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437 if (begin < end) { |
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438 MemRegion mr(begin, end); |
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439 VerifyCleanCardClosure verify_blk(gen_boundary, begin, end); |
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440 for (HeapWord* cur = start_block; cur < end; cur += s->block_size(cur)) { |
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441 if (s->block_is_obj(cur) && s->obj_is_alive(cur)) { |
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442 oop(cur)->oop_iterate_no_header(&verify_blk, mr); |
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443 } |
0 | 444 } |
445 } | |
446 cur_entry = first_dirty; | |
447 } else { | |
448 // We'd normally expect that cur_youngergen_and_prev_nonclean_card | |
449 // is a transient value, that cannot be in the card table | |
450 // except during GC, and thus assert that: | |
451 // guarantee(*cur_entry != cur_youngergen_and_prev_nonclean_card, | |
452 // "Illegal CT value"); | |
453 // That however, need not hold, as will become clear in the | |
454 // following... | |
455 | |
456 // We'd normally expect that if we are in the parallel case, | |
457 // we can't have left a prev value (which would be different | |
458 // from the current value) in the card table, and so we'd like to | |
459 // assert that: | |
460 // guarantee(cur_youngergen_card_val() == youngergen_card | |
461 // || !is_prev_youngergen_card_val(*cur_entry), | |
462 // "Illegal CT value"); | |
463 // That, however, may not hold occasionally, because of | |
464 // CMS or MSC in the old gen. To wit, consider the | |
465 // following two simple illustrative scenarios: | |
466 // (a) CMS: Consider the case where a large object L | |
467 // spanning several cards is allocated in the old | |
468 // gen, and has a young gen reference stored in it, dirtying | |
469 // some interior cards. A young collection scans the card, | |
470 // finds a young ref and installs a youngergenP_n value. | |
471 // L then goes dead. Now a CMS collection starts, | |
472 // finds L dead and sweeps it up. Assume that L is | |
473 // abutting _unallocated_blk, so _unallocated_blk is | |
474 // adjusted down to (below) L. Assume further that | |
475 // no young collection intervenes during this CMS cycle. | |
476 // The next young gen cycle will not get to look at this | |
477 // youngergenP_n card since it lies in the unoccupied | |
478 // part of the space. | |
479 // Some young collections later the blocks on this | |
480 // card can be re-allocated either due to direct allocation | |
481 // or due to absorbing promotions. At this time, the | |
482 // before-gc verification will fail the above assert. | |
483 // (b) MSC: In this case, an object L with a young reference | |
484 // is on a card that (therefore) holds a youngergen_n value. | |
485 // Suppose also that L lies towards the end of the used | |
486 // the used space before GC. An MSC collection | |
487 // occurs that compacts to such an extent that this | |
488 // card is no longer in the occupied part of the space. | |
489 // Since current code in MSC does not always clear cards | |
490 // in the unused part of old gen, this stale youngergen_n | |
491 // value is left behind and can later be covered by | |
492 // an object when promotion or direct allocation | |
493 // re-allocates that part of the heap. | |
494 // | |
495 // Fortunately, the presence of such stale card values is | |
496 // "only" a minor annoyance in that subsequent young collections | |
497 // might needlessly scan such cards, but would still never corrupt | |
498 // the heap as a result. However, it's likely not to be a significant | |
499 // performance inhibitor in practice. For instance, | |
500 // some recent measurements with unoccupied cards eagerly cleared | |
501 // out to maintain this invariant, showed next to no | |
502 // change in young collection times; of course one can construct | |
503 // degenerate examples where the cost can be significant.) | |
504 // Note, in particular, that if the "stale" card is modified | |
505 // after re-allocation, it would be dirty, not "stale". Thus, | |
506 // we can never have a younger ref in such a card and it is | |
507 // safe not to scan that card in any collection. [As we see | |
508 // below, we do some unnecessary scanning | |
509 // in some cases in the current parallel scanning algorithm.] | |
510 // | |
511 // The main point below is that the parallel card scanning code | |
512 // deals correctly with these stale card values. There are two main | |
513 // cases to consider where we have a stale "younger gen" value and a | |
514 // "derivative" case to consider, where we have a stale | |
515 // "cur_younger_gen_and_prev_non_clean" value, as will become | |
516 // apparent in the case analysis below. | |
517 // o Case 1. If the stale value corresponds to a younger_gen_n | |
518 // value other than the cur_younger_gen value then the code | |
519 // treats this as being tantamount to a prev_younger_gen | |
520 // card. This means that the card may be unnecessarily scanned. | |
521 // There are two sub-cases to consider: | |
522 // o Case 1a. Let us say that the card is in the occupied part | |
523 // of the generation at the time the collection begins. In | |
524 // that case the card will be either cleared when it is scanned | |
525 // for young pointers, or will be set to cur_younger_gen as a | |
526 // result of promotion. (We have elided the normal case where | |
527 // the scanning thread and the promoting thread interleave | |
528 // possibly resulting in a transient | |
529 // cur_younger_gen_and_prev_non_clean value before settling | |
530 // to cur_younger_gen. [End Case 1a.] | |
531 // o Case 1b. Consider now the case when the card is in the unoccupied | |
532 // part of the space which becomes occupied because of promotions | |
533 // into it during the current young GC. In this case the card | |
534 // will never be scanned for young references. The current | |
535 // code will set the card value to either | |
536 // cur_younger_gen_and_prev_non_clean or leave | |
537 // it with its stale value -- because the promotions didn't | |
538 // result in any younger refs on that card. Of these two | |
539 // cases, the latter will be covered in Case 1a during | |
540 // a subsequent scan. To deal with the former case, we need | |
541 // to further consider how we deal with a stale value of | |
542 // cur_younger_gen_and_prev_non_clean in our case analysis | |
543 // below. This we do in Case 3 below. [End Case 1b] | |
544 // [End Case 1] | |
545 // o Case 2. If the stale value corresponds to cur_younger_gen being | |
546 // a value not necessarily written by a current promotion, the | |
547 // card will not be scanned by the younger refs scanning code. | |
548 // (This is OK since as we argued above such cards cannot contain | |
549 // any younger refs.) The result is that this value will be | |
550 // treated as a prev_younger_gen value in a subsequent collection, | |
551 // which is addressed in Case 1 above. [End Case 2] | |
552 // o Case 3. We here consider the "derivative" case from Case 1b. above | |
553 // because of which we may find a stale | |
554 // cur_younger_gen_and_prev_non_clean card value in the table. | |
555 // Once again, as in Case 1, we consider two subcases, depending | |
556 // on whether the card lies in the occupied or unoccupied part | |
557 // of the space at the start of the young collection. | |
558 // o Case 3a. Let us say the card is in the occupied part of | |
559 // the old gen at the start of the young collection. In that | |
560 // case, the card will be scanned by the younger refs scanning | |
561 // code which will set it to cur_younger_gen. In a subsequent | |
562 // scan, the card will be considered again and get its final | |
563 // correct value. [End Case 3a] | |
564 // o Case 3b. Now consider the case where the card is in the | |
565 // unoccupied part of the old gen, and is occupied as a result | |
566 // of promotions during thus young gc. In that case, | |
567 // the card will not be scanned for younger refs. The presence | |
568 // of newly promoted objects on the card will then result in | |
569 // its keeping the value cur_younger_gen_and_prev_non_clean | |
570 // value, which we have dealt with in Case 3 here. [End Case 3b] | |
571 // [End Case 3] | |
572 // | |
573 // (Please refer to the code in the helper class | |
574 // ClearNonCleanCardWrapper and in CardTableModRefBS for details.) | |
575 // | |
576 // The informal arguments above can be tightened into a formal | |
577 // correctness proof and it behooves us to write up such a proof, | |
578 // or to use model checking to prove that there are no lingering | |
579 // concerns. | |
580 // | |
581 // Clearly because of Case 3b one cannot bound the time for | |
582 // which a card will retain what we have called a "stale" value. | |
583 // However, one can obtain a Loose upper bound on the redundant | |
584 // work as a result of such stale values. Note first that any | |
585 // time a stale card lies in the occupied part of the space at | |
586 // the start of the collection, it is scanned by younger refs | |
587 // code and we can define a rank function on card values that | |
588 // declines when this is so. Note also that when a card does not | |
589 // lie in the occupied part of the space at the beginning of a | |
590 // young collection, its rank can either decline or stay unchanged. | |
591 // In this case, no extra work is done in terms of redundant | |
592 // younger refs scanning of that card. | |
593 // Then, the case analysis above reveals that, in the worst case, | |
594 // any such stale card will be scanned unnecessarily at most twice. | |
595 // | |
596 // It is nonethelss advisable to try and get rid of some of this | |
597 // redundant work in a subsequent (low priority) re-design of | |
598 // the card-scanning code, if only to simplify the underlying | |
599 // state machine analysis/proof. ysr 1/28/2002. XXX | |
600 cur_entry++; | |
601 } | |
602 } | |
603 } | |
604 | |
605 void CardTableRS::verify() { | |
606 // At present, we only know how to verify the card table RS for | |
607 // generational heaps. | |
608 VerifyCTGenClosure blk(this); | |
609 CollectedHeap* ch = Universe::heap(); | |
610 | |
611 if (ch->kind() == CollectedHeap::GenCollectedHeap) { | |
612 GenCollectedHeap::heap()->generation_iterate(&blk, false); | |
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613 _ct_bs->verify(); |
0 | 614 } |
615 } | |
616 | |
617 | |
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618 void CardTableRS::verify_aligned_region_empty(MemRegion mr) { |
0 | 619 if (!mr.is_empty()) { |
620 jbyte* cur_entry = byte_for(mr.start()); | |
621 jbyte* limit = byte_after(mr.last()); | |
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622 // The region mr may not start on a card boundary so |
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623 // the first card may reflect a write to the space |
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624 // just prior to mr. |
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625 if (!is_aligned(mr.start())) { |
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626 cur_entry++; |
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627 } |
0 | 628 for (;cur_entry < limit; cur_entry++) { |
629 guarantee(*cur_entry == CardTableModRefBS::clean_card, | |
630 "Unexpected dirty card found"); | |
631 } | |
632 } | |
633 } |